Growth cones, dying axons, and developmental fluctuations in the fiber population of the cat's optic nerve

Bioinspired micro- and nano-structured neural interfaces

The development of a functional nervous system requires neurons to interact with and promptly respond to a wealth of biochemical, mechanical and topographical cues found in the neural extracellular matrix (ECM). Among these, ECM topographical cues have been found to strongly influence neuronal function and behavior. Here, we discuss how the blueprint of the architectural organization of the brain ECM has been tremendously useful as a source of inspiration to design biomimetic substrates to enhance neural interfaces and dictate neuronal behavior at the cell-material interface. In particular, we focus on different strategies to recapitulate cell-ECM and cell-cell interactions. In order to mimic cell-ECM interactions, we introduce roughness as a first approach to provide informative topographical biomimetic cues to neurons. We then examine 3D scaffolds and hydrogels, as softer 3D platforms for neural interfaces. Moreover, we will discuss how anisotropic features such as grooves and fibers, recapitulating both ECM fibrils and axonal tracts, may provide recognizable paths and tracks that neuron can follow as they develop and establish functional connections. Finally, we show how isotropic topographical cues, recapitulating shapes, and geometries of filopodia- and mushroom-like dendritic spines, have been instrumental to better reproduce neuron-neuron interactions for applications in bioelectronics and neural repair strategies. The high complexity of the brain architecture makes the quest for the fabrication of create more biologically relevant biomimetic architectures in continuous and fast development. Here, we discuss how recent advancements in two-photon polymerization and remotely reconfigurable dynamic interfaces are paving the way towards to a new class of smart biointerfaces forin vitroapplications spanning from neural tissue engineering as well as neural repair strategies.

Complement Targets Newborn Retinal Ganglion Cells for Phagocytic Elimination by Microglia

Microglia play important roles in shaping the developing CNS, and at early stages they have been proposed to regulate progenitor proliferation, differentiation, and neuronal survival. However, these studies reveal contradictory outcomes, highlighting the complexity of these cell-cell interactions. Here, we investigate microglia function during embryonic mouse retina development, where only microglia, progenitors, and neurons are present. In both sexes, we determine that microglia primarily interact with retinal neurons and find that depletion of microglia via conditional KO of the Csf1 receptor results in increased density of retinal ganglion cells (RGCs). Pharmacological inhibition of microglia also results in an increase in RGCs, with no effect on retinal progenitor proliferation, RGC genesis, or apoptosis. We show that microglia in the embryonic retina are enriched for phagocytic markers and observe engulfment of nonapoptotic Brn3-labeled RGCs. We investigate the molecular pathways that can mediate cell engulfment by microglia and find selective downregulation of complement pathway components with microglia inhibition, and further show that C1q protein marks a subset of RGCs in the embryonic retina. KO of complement receptor 3 (CR3; Itgam), which is only expressed by microglia, results in increased RGC density, similar to what we observed after depletion or inhibition of microglia. Thus, our data suggest that microglia regulate neuron elimination in the embryonic mouse retina by complement-mediated phagocytosis of non-apoptotic newborn RGCs.SIGNIFICANCE STATEMENT Microglia are emerging as active and important participants in regulating neuron number in development, during adult neurogenesis, and following stem cell therapies. However, their role in these contexts and the mechanisms involved are not fully defined. Using a well-characterized in vivo system, we provide evidence that microglia regulate neuronal elimination by complement-mediated engulfment of nonapoptotic neurons. This work provides a significant advancement of the field by defining in vivo molecular mechanisms for microglia-mediated cell elimination. Our data add to a growing body of evidence that microglia are essential for proper nervous system development. In addition, we elucidate microglia function in the developing retina, which may shed light on microglia involvement in the context of retinal injury and disease.

Differences Exist in the Left and Right Sciatic Nerves of Naïve Rats and Cats

The sciatic nerve of rats and cats is commonly used in experimental models of peripheral nerve injury and repair, as well as experiments involving peripheral nerve electrode implantation. In such experiments, morphometric parameters from the implanted nerve are commonly evaluated and compared to control values obtained from the contralateral nerves. However, this may not be an appropriate approach as differences may naturally exist in the structure of the two nerves owing to developmental or behavioral asymmetry. Additionally, in the cat, baseline values for standard morphometric parameters are not well established. In this study, we characterized fascicle area, fiber count, fiber density, fiber packing, mean g-ratio, and fiber diameter distributions in the rat and cat, as well as investigated the potential for naturally occurring sided differences in these parameters in both species. We also investigated whether animal age or location along the nerve influenced these parameters. We found that sided or left/right leg differences exist in some parameters in both the rat and the cat, calling into question the validity of using the contralateral nerve as a control. We also found that animal age and location along the nerve can make significant differences in the parameters tested, establishing the importance of using control nerves from age- and behaviorally matched animals whose morphometric parameters are collected and compared from the same location.

Ontogenetic cell death and phagocytosis in the visual system of vertebrates

Programmed cell death (PCD), together with cell proliferation, cell migration, and cell differentiation, is an essential process during development of the vertebrate nervous system. The visual system has been an excellent model on which to investigate the mechanisms involved in ontogenetic cell death. Several phases of PCD have been reported to occur during visual system ontogeny. During these phases, comparative analyses demonstrate that dying cells show similar but not identical spatiotemporally restricted patterns in different vertebrates. Additionally, the chronotopographical coincidence of PCD with the entry of specialized phagocytes in some regions of the developing vertebrate visual system suggests that factors released from degenerating cells are involved in the cell migration of macrophages and microglial cells. Contradicting this hypothesis however, in many cases the cell corpses generated during degeneration are rapidly phagocytosed by neighboring cells, such as neuroepithelial cells or Müller cells. In this review, we describe the occurrence and the sites of PCD during the morphogenesis and differentiation of the retina and optic pathways of different vertebrates, and discuss the possible relationship between PCD and phagocytes during ontogeny.

Model of fasciculation and sorting in mixed populations of axons

We extend a recently proposed model [Chaudhuri et al., Europhys. Lett. 87, 20003 (2009)] aiming to describe the formation of fascicles of axons during neural development. The growing axons are represented as paths of interacting directed random walkers in two spatial dimensions. To mimic turnover of axons, whole paths are removed and new walkers are injected with specified rates. In the simplest version of the model, we use strongly adhesive short-range inter-axon interactions that are identical for all pairs of axons. We generalize the model to adhesive interactions of finite strengths and to multiple types of axons with type-specific interactions. The dynamic steady state is characterized by the position-dependent distribution of fascicle size and fascicle composition. With distance in the direction of axon growth, the mean fascicle size and emergent time scales grow monotonically, while the degree of sorting of fascicles by axon type has a maximum at a finite distance. To understand the emergence of slow time scales, we develop an analytical framework to analyze the interaction between neighboring fascicles.

Cell proliferation and cytoarchitectural remodeling during spinal cord reconnection in the fresh-water turtle Trachemys dorbignyi

In fresh-water turtles, the bridge connecting the proximal and caudal stumps of transected spinal cords consists of regenerating axons running through a glial cellular matrix. To understand the process leading to the generation of the scaffold bridging the lesion, we analyzed the mitotic activity triggered by spinal injury in animals maintained alive for 20-30 days after spinal cord transection. Flow cytometry and bromodeoxyuridine (BrdU)-labeling experiments revealed a significant increment of cycling cells around the lesion epicenter. BrdU-tagged cells maintained a close association with regenerating axons. Most dividing cells expressed the brain lipid-binding protein (BLBP). Cells with BrdU-positive nuclei expressed glial fibrillary acidic protein. As spinal cord regeneration involves dynamic cell rearrangements, we explored the ultra-structure of the bridge and found cells with the aspect of immature oligodendrocytes forming an embryonic-like microenvironment. These cells supported and ensheathed regenerating axons that were recognized by immunocytological and electron-microscopical procedures. Since functional recovery depends on proper impulse transmission, we examined the anatomical axon-glia relationships near the lesion epicenter. Computer-assisted three-dimensional models revealed helical axon-glial junctions in which the intercellular space appeared to be reduced (5-7 nm). Serial-sectioning analysis revealed that fibril-containing processes provided myelinating axon sheaths. Thus, disruption of the ependymal layer elicits mitotic activity predominantly in radial glia expressing BLBP on the lateral aspects of the ependyma. These cycling cells seem to migrate and contribute to the bridge providing the main support and sheaths for regenerating axons.

Distinct roles for laminin globular domains in laminin alpha1 chain mediated rescue of murine laminin alpha2 chain deficiency

Background

Laminin α2 chain mutations cause congenital muscular dystrophy with dysmyelination neuropathy (MDC1A). Previously, we demonstrated that laminin α1 chain ameliorates the disease in mice. Dystroglycan and integrins are major laminin receptors. Unlike laminin α2 chain, α1 chain binds the receptors by separate domains; laminin globular (LG) domains 4 and LG1-3, respectively. Thus, the laminin α1 chain is an excellent tool to distinguish between the roles of dystroglycan and integrins in the neuromuscular system.

Methodology/Principal Findings

Here, we provide insights into the functions of laminin α1LG domains and the division of their roles in MDC1A pathogenesis and rescue. Overexpression of laminin α1 chain that lacks the dystroglycan binding LG4-5 domains in α2 chain deficient mice resulted in prolonged lifespan and improved health. Importantly, diaphragm and heart muscles were corrected, whereas limb muscles were dystrophic, indicating that different muscles have different requirements for LG4-5 domains. Furthermore, the regenerative capacity of the skeletal muscle did not depend on laminin α1LG4-5. However, this domain was crucial for preventing apoptosis in limb muscles, essential for myelination in peripheral nerve and important for basement membrane assembly.

Conclusions/Significance

These results show that laminin α1LG domains and consequently their receptors have disparate functions in the neuromuscular system. Understanding these interactions could contribute to design and optimization of future medical treatment for MDC1A patients.

Sympathetic preganglionic neurons project to superior cervical ganglion via C7 spinal nerve in pup but not in adult rats

We investigated the distribution of sympathetic preganglionic fibers in each spinal nerve of the brachial plexus, and its correlation with presence of Horner's syndrome in the pup and adult rats. According to surgical intervention to the C7-T1 spinal nerves in the right side, rats of 7 days postnatal (P7), P14 and adulthood (24 for each age group) were subdivided into four subgroups of six each, respectively, i.e., C7 or C8 or T1 spared subgroup--where C7 or C8 or T1 alone was kept intact with avulsion of the other two spinal nerves and division of the sympathetic chain caudal to the stellate ganglion; C7-T1 avulsed subgroup--where C7-T1 were all avulsed but the sympathetic chain kept intact. Fluoro-Gold (FG) was injected bilaterally into the superior cervical ganglion (SCG) for labeling of sympathetic preganglionic neurons (SPNs). Furthermore, Horner's syndrome was examined after avulsion of different spinal nerves for P14 and adult rats. In C7 spared subgroups, FG-labeled neurons accounted averagely for 16.9% in P7, 13.5 in P14 and 1.0 in adult rats, and difference was statistically significant between P7 and adults (Z=-2.9, P=0.004), P14 and adults (Z=-2.9, P=0.004). When both C7 and C8 were avulsed, Horner's syndrome was more prone to be produced in pups than in adults (chi(2)=4.2, P=0.04). These results indicate that some SPNs project to SCG via C7 in the pup, but this pathway disappears during postnatal development. It suggests that in newborns with brachial plexopathy, presence of Horner's syndrome may be correlated with avulsion of C7.

Morphology and cellular interactions of growth cones in the developing corpus callosum

Previous studies of growth cones in invertebrates have shown that they become larger and more complex when changing direction in response to cell-specific contacts (Bentley and Caudy, '83; Raper et al., '83b; Caudy and Bentley, '86). In pathways of the vertebrate nervous system, analogous regions, termed "decision regions," have been identified in which axons change direction and their growth cones become more elaborate than when tracking along straight trajectories (Tosney and Landmesser, '85a; Bovolenta and Mason, '87). In order to assess the generality of these principles to the mammalian CNS, we studied the morphology of growth cones and their interactions with the environment in the developing corpus callosum. Given the straight pathway that callosal axons could use to navigate across the callosum, one might predict that later arriving axons would extend on those growing out earlier and that therefore, by analogy with previous studies, many growth cones would have simple tapered morphologies. Surprisingly, however, virtually all growth cones in the callosal white matter, regardless of age or position, were complex with broad lamellipodial veils and/or numerous, often lengthy filopodia. Only growth cones entering the cortical target were consistently smaller. As seen in the EM, the predominant elements in the callosal pathway are other axons and growth cones; we found no evidence for specialized contacts. These results suggest that there is no specific decision region for the fiber population as a whole; rather it is possible that in this mammalian CNS pathway individual growth cones respond independently to molecular cues broadly distributed in the callosum.

Abnormal growth of the corticospinal axons into the lumbar spinal cord of the hyt/hyt mouse with congenital hypothyroidism

Thyroid hormone deficiency may cause severe neurological disorders resulting from developmental deficits of the central nervous system. The mutant hyt/hyt mouse, characterized by fetal-onset, life-long hypothyroidism resulting from a point mutation of the thyroid-stimulating hormone receptor of the thyroid gland, displays a variety of abnormalities in motor behavior that are likely associated with dysfunctions of specific brain regions and a defective corticospinal tract (CST). To test the hypothesis that fetal and neonatal hypothyroidism cause abnormal CST development, the growth of the CST was investigated in hypothyroid hyt/hyt mice and their euthyroid progenitors, the BALB/cByJ mice. Anterograde labeling with biotinylated dextran amine demonstrated a decrease in the number of CST axons in the hyt/hyt mouse at the first lumbar level at postnatal day (P) 10. After retrograde tracing with fast blue (FB), fewer FB-labeled neurons were found in the motor cortex, the red nucleus, and the lateral vestibular nucleus of the hyt/hyt mouse. At the fourth lumbar level, the hyt/hyt mouse also showed smaller CST cross-sectional areas and significantly lower numbers of unmyelinated axons, myelinated axons, and growth cones within the CST during postnatal development. At P10, the hyt/hyt mouse demonstrated significantly lower immunoreactivity of embryonic neural cell adhesion molecule in the CST at the seventh cervical level, whereas the expression of growth-associated protein 43 remained unchanged. Our study demonstrated an abnormal development of the CST in the hyt/hyt mouse, manifested by reduced axon quantity and retarded growth pattern at the lumbar spinal cord.

Early development of the optic nerve in the turtle Mauremys leprosa

We show the distribution of the neural and non-neural elements in the early development of the optic nerve in the freshwater turtle, Mauremys leprosa, using light and electron microscopy. The first optic axons invaded the ventral periphery of the optic stalk in close relationship to the radial neuroepithelial processes. Growth cones were thus exclusively located in the ventral margin. As development progressed, growth cones were present in ventral and dorsal regions, including the dorsal periphery, where they intermingled with mature axons. However, growth cones predominated in the ventral part and axonal profiles dorsally, reflecting a dorsal to ventral gradient of maturation. The size and morphology of growth cones depended on the developmental stage and the region of the optic nerve. At early stages, most growth cones were of irregular shape, showing abundant lamellipodia. At the following stages, they tended to be larger and more complex in the ventral third than in intermediate and dorsal portions, suggesting a differential behavior of the growth cones along the ventro-dorsal axis. The arrival of optic axons at the optic stalk involved the progressive transformation of neuroepithelial cells into glial cells. Simultaneously with the fiber invasion, an important number of cells died by apoptosis in the dorsal wall of the optic nerve. These findings are discussed in relation to the results described in the developing optic nerve of other vertebrates.

The postnatal development of the optic nerve of a reptile (Vipera aspis): A quantitative ultrastructural study

The number of axons in the optic nerve of the ovoviviparous reptile Vipera aspis was estimated from electron micrographs taken during the first 5 weeks of postnatal life. One to two days after birth, the optic nerve contains about 170,000 fibres, of which about 9% are myelinated. At the end of the fifth postnatal week, the number of optic fibres has fallen to about 100,000, of which about 42% are myelinated. This fibre loss continues after the fifth postnatal week, since in the adult viper the nerve contains about 60,000 fibres, of which 85% are myelinated; overall, about 65% of the optic nerve fibres present at birth disappear before the number of axons stabilises at the adult level. This study shows, for the first time, that the mode of development of the visual axons of reptiles is not that of anamniote vertebrates but similar to that of birds and mammals.

Neurogenesis and cell death in the ganglion cell layer of vertebrate retina

The correct formation of all central nervous system tissues depends on the proper balance of neurogenesis and developmental cell death. A model system for studying these programs is the ganglion cell layer (GCL) of the vertebrate retina because of its simple and well-described structure and amenability to experimental manipulations. The GCL contains approximately equal numbers of ganglion cells and displaced amacrine cells. Ganglion cells are the first or among the first cells born in the retina in all the studied vertebrates. Neurogenesis and cell death have been studied extensively in the GCL of various amniotes (rodents, chicks, and monkeys) and anamniotes (fish and frogs), and the two processes highlight developmental differences between the groups. In amniotes, neurogenesis occurs during a defined period prior to birth/hatch or the opening of the eyes, whereas in anamniotes, neurogenesis extends past hatching into adulthood-sometimes for years. Roughly half of GCL neurons die during development in amniotes, whereas developmental cell death does not occur in the GCL neurons of anamniotes. This review discusses the spatial and temporal patterns of neurogenesis, cell death, and possible explanation of cell death in the GCL. It also examines markers widely used to distinguish between ganglion cells and displaced amacrine cells, and methods employed to birth date neurons.

Developmental changes in the fibre population of the optic nerve follow an avian/mammalian-like pattern in the turtle Mauremys leprosa

The changes in the axon and growth cone numbers in the optic nerve of the freshwater turtle Mauremys leprosa were studied by electron microscopy from the embryonic day 14 (E14) to E80, when the animals normally hatch, and from the first postnatal day (P0) to adulthood (5 years on). At E16, the first axons appeared in the optic nerve and were added slowly until E21. From E21, the fibre number increased rapidly, peaking at E34 (570,000 fibres). Thereafter, the axon number decreased sharply, and from E47 declined steadily until reaching the mature number (about 330,000). These observations indicated that during development of the retina there was an overproduction and later elimination of retinal ganglion cells. Growth cones were first observed in the optic nerve at as early as E16. Their number increased rapidly until E21 and continued to be high through E23 and E26. After E26, the number declined steeply and by E40 the optic nerve was devoid of growth cones. These results indicated that differentiation of the retinal ganglion cells occurred during the first half of the embryonic life. To examine the correlation between the loss of the fibres from the optic nerve and loss of the parent retinal ganglion cells, retinal sections were processed with the TUNEL technique. Apoptotic nuclei were detected in the ganglion cell layer throughout the period of loss of the optic fibres. Our results showed that the time course of the numbers of the fibres in the developing turtle optic nerve was similar to those found in birds and mammals.

Development of the corticospinal tract in the mouse spinal cord: A quantitative ultrastructural analysis

The growth of corticospinal tract (CST) axons was studied quantitatively at the 7th cervical (C7) and the 4th lumbar (L4) spinal segments in the balb/cByJ mice at the ages of postnatal day (P) 0, 2, 4, 6, 8, 10, 14, and 28. The cross-sectional area of the CST increased progressively with time. Unmyelinated axons, the most prominent CST element during early development, reached maximum at C7 and L4 on P14. Two phases of increase in the number of unmyelinated axons were observed at C7, while only one surge of axonal outgrowth was found at the L4 level. Pro-myelinated axons, defined as axons surrounded by only one layer of oligodendrocytic process, were first seen at P2 and P4 in the C7 and the L4 level, respectively, followed by a dramatic increase in the number of myelinated axons from P14 onwards at both spinal levels. Myelination of the CST axons occurred topographically in a dorsal-to-ventral pattern. The number of growth cones increased rapidly at the C7 level to reach its maximum at P4, while those at L4 increased steadily to the peak at P10. Growth cones with synapse-like junctions were occasionally observed in the growing CST. Degenerating axons and growth cones partly accounted for the massive axon loss at both spinal segments during CST development. Overall, the mouse CST elements changed dynamically in numbers during postnatal development, suggesting a vigorous growing and pruning activity in the tract. The mouse CST also showed a similar growth pattern to that of the rat CST.

Variations in the architecture and development of the vertebrate optic chiasm

At the vertebrate optic chiasm there is major change in fibre order and, in many animals, a separation of fibres destined for different hemispheres of the brain. However, the structure of this region is not uniform among all species but rather shows marked variations both in terms of its gross architecture and the pathways taken by different fibres. There also are striking differences in the developmental mechanisms sculpting this region even between closely related animals. In spite of this, recent studies have provided strong evidence for a remarkable degree of conservation in the molecular nature of the guidance signals and regulatory genes driving chiasmatic development. Here differences and similarities in chiasmatic organisation and development between separate groups of animals will be reviewed. While it may not be possible to ascribe a single set of factors that are universal components of the vertebrate chiasm, there are both strikingly similar elements as well as diverse features to the development, organisation and architecture of this region. This review aims to highlight key issues in the organisation and development of the vertebrate optic chiasm with a focus on comparing and contrasting the data that has been gleaned to date from different vertebrate groups.

Cell birth and death in the mouse retinal ganglion cell layer

Here we describe quantitatively the birth and death of the two separate populations of neurons, ganglion cells and displaced amacrine cells, in the mouse retinal ganglion cell layer (GCL). The two cell types, which are roughly equally numerous, were distinguished pre- and postnatally by labeling the ganglion cells retrogradely with fluorescent dye. Embryos were labeled cumulatively with bromodeoxyuridine (BrdU) delivered by an osmotic minipump implanted in the mother; cell birth dates were established as having occurred before or after pump implantation. Early cohorts (GCL cells born before embryonic day [E] 11.8 and E12.8) were 98+/-1.1% and 99+/-0.2% ganglion cells (mean+/-SEM), respectively, and a late cohort (born after E15.8) was 97+/-1.2% displaced amacrines. Thus birth date was a strong predictor of a GCL cell's ultimate identity. Cell death in each cohort was estimated by counting cells at different time points (soon after the cohort was produced and later) and subtracting the later from the earlier number. This method avoids the problem of simultaneous birth and death that has plagued many of the earlier attempts to assess cell death. Negligible numbers died during the first week after a cell's birthday. The amount of cell death differed in the two cohorts; 48.5+/-15% and 29.0+/-12.4% in early and late, respectively, and most of it was postnatal. These findings disagree sharply with an earlier conclusion that ganglion cells die within 5 days of their birthdays or not at all.

Development of On and Off retinal pathways and retinogeniculate projections

A fundamental functional feature of the visual system, one recognized in the very first electrophysiological retinal recordings ever made, is that some cells respond to light increments (On cells) while others are activated by light decrements (Off cells). The circuitry underlying On and Off responses in the mature retina have been well-established. In particular, it is known that the dendrites of On- and Off-center retinal ganglion cells (RGCs) stratify in different sublamina of the inner plexiform layer (IPL), where they are innervated by spatially segregated On- and Off-cone bipolar cell inputs. Also, segregated into On and Off sublaminae of the IPL are the processes of starburst amacrine cells. In some species (notably ferret and mink) the retinogeniculate projections are also segregated into sublayers of the dorsal lateral geniculate nucleus (dlgn). The mature organizational features summarized above arise gradually during the course of normal development. Thus, the dendrites of immature RGCs initially ramify throughout the IPL before becoming stratified into On or Off sublamina. This developmental event is regulated by the release of glutamate by developing bipolar cells. Treating the developing retina with the glutamate analog 2-amino-4-phosphonobutyric acid (APB) has been found to prevent the stratification of RGC dendrites. In the mature retina APB binds with mGluR6 receptors expressed by On cone and rod bipolar cells which hyperpolarizes these retinal interneurons and blocks their release of glutamate. The effects of short-term APB treatment are reversible by subsequent normal visual experience, while those of long-term treatment appear to be permanent. At the time that developing RGCs are multistratified they respond to both light onset as well as light offset, suggesting that these neurons are initially functionally innervated by On as well as Off-cone bipolar cells. In the dark-adapted state, On-Off responses of immature multistratified RGCs are completely blocked by APB, while at maturity only On responses are APB-sensitive. This suggests that an APB-resistant Off pathway (possibly from rods to Off-cone bipolar cells) is formed relatively late in development, after RGCs attain their stratified state. In contrast to the activity-regulated refinement of stratified On and Off RGCs, the segregated ingrowth of On- and Off-cone bipolar cells occurs in a highly specific manner, and is not dependent on the presence of either RGCs or cholinergic amacrine cells. It is suggested that the directed ingrowth of bipolar cell axons is guided by molecular cues expressed in the extracellular matrix whose identity is yet to be established. There is also evidence that the later formation of segregated On and Off retinogeniculate projections in the ferret is regulated by an activity-dependent Hebbian type mechanism. Blockade of RGC discharges as well as NMDA receptors in the dlgn perturbs the formation of such segregated inputs. Moreover, On and Off RGCs show distinct correlated firing patterns during the developmental period when the intermingled projections of these neurons are being sorted into sign specific sublayers. Collectively, the available evidence indicates that different developmental mechanisms operate on the different components of retinal and retinogeniculate On and Off pathways to attain the segregated state characteristic of the mature visual system.

Genetic control of interconnected neuronal populations in the mouse primary visual system

Proliferation and survival of different cell types is thought to be modulated by cell interactions during development that achieve numerical and functional balance. We tested the precision of coregulation of numbers of neurons, glial cells, and endothelial cells in the dorsal lateral geniculate nucleus (LGN) in 58 isogenic strains of mice. We acquired matched counts of retinal ganglion cells (RGCs) in these strains and tested the precision of numerical matching between retina and LGN. Cells were counted using unbiased counting protocols and tissue from the Mouse Brain Library (www.mbl.org). Classification criteria were assessed using immunohistochemical criteria. The LGN contains an average of 17,000 neurons, 12,000 glial cells, and 10,000 endothelial cells. Variation around these means is typically twofold, and cell ratios vary widely. Strain differences in LGN volume correlate moderately well with glial cell number (r = 0.69) and less well with RGC number (r = 0.35) and with LGN neuron number (r = 0.32). Populations of LGN neurons and glial cells correlate only modestly (r = 0.44; p < 0.01). The single most surprising and unequivocal finding was the lack of any detectable correlation between populations of LGN neurons and RGCs, a correlation of merely 0.01 across 56 strains. In contrast, RGC number correlates significantly with LGN glial cell number, a surprising twist on the numerical matching hypothesis (r = 0.33; p < 0.01). We conclude that numbers of these two functionally coupled neuron populations are modulated over a wide range by independent genetic and developmental mechanisms.

TrkB receptor signaling regulates developmental death dynamics, but not final number, of retinal ganglion cells

We investigated the effects of endogenous neurotrophin signaling on the death-survival of immature retinal ganglion cells (RGCs) in vivo. Null mutation of brain-derived neurotrophic factor [(BDNF) alone or in combination with neurotrophin 4 (NT4)] increases the peak rate of developmental RGC death as compared with normal. Null mutation of NT4 alone is ineffective. Null mutation of the full-length trkB (trkBFL) receptor catalytic domain produces a dose-dependent increase in the peak RGC death rate that is negatively correlated with retinal levels of trkBFL protein and phosphorylated (activated) trkBFL. Depletion of target-derived trkB ligands by injection of trkB-Fc fusion protein into the superior colliculus increases the peak rate of RGC death compared with trkA-Fc-treated and normal animals. Adult trkBFL+/- mice have a normal number of RGCs, despite an elevated peak death rate of immature RGCs. Thus, target-derived BDNF modulates the dynamics of developmental RGC death through trkBFL activation, but BDNF/trkB-independent mechanisms determine the final number of RGCs.

Axonal sprouting in the optic nerve is not a prerequisite for successful regeneration

Axonal sprouting, the production of axons additional to the parent one, occurs during optic nerve regeneration in goldfish and the frog Rana pipiens, with numbers of regenerate axons exceeding normal values four- to sixfold (Murray [1982] J. Comp. Neurol. 209:352-362; Stelzner and Strauss [1986] J. Comp. Neurol. 245:83-103). To determine whether axonal sprouting is a prerequisite for regeneration, the frog Litoria moorei was examined, a species that undergoes successful optic nerve regeneration but with a different time course compared with R. pipiens. Sprouting was assessed, as in goldfish and R. pipiens, from electron microscopic counts between the lesion and chiasm. However, disconnected axons that persist after axotomy would have falsely elevated the counts. The suspected overlap of these two axon populations was confirmed by labeling regenerate axons anterogradely with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) and disconnected ones retrogradely with DiA (4-4-dihexadecylaminostyrl 1-N methylpyridinium iodide). Numbers of disconnected axons were estimated after preventing regeneration and subtracted from numbers in regenerate nerves. Throughout, the total number of regenerate axons was approximately one third lower than normal (P < 0.05) supporting a previous finding of minimal axonal sprouting in L. moorei (Dunlop et al. [2002] J. Comp. Neurol. 446:276-287). The validity of the subtractive electron microscopic method was confirmed by retrograde labeling to estimate numbers of retinal ganglion cells whose axons had crossed the lesion; values were approximately one third lower than normal. The data suggest that sprouting is not essential for either axon outgrowth or topographic map refinement.

A taxonomy for artificial embryogeny

A major challenge for evolutionary computation is to evolve phenotypes such as neural networks, sensory systems, or motor controllers at the same level of complexity as found in biological organisms. In order to meet this challenge, many researchers are proposing indirect encodings, that is, evolutionary mechanisms where the same genes are used multiple times in the process of building a phenotype. Such gene reuse allows compact representations of very complex phenotypes. Development is a natural choice for implementing indirect encodings, if only because nature itself uses this very process. Motivated by the development of embryos in nature, we define artificial embryogeny (AE) as the subdiscipline of evolutionary computation (EC) in which phenotypes undergo a developmental phase. An increasing number of AE systems are currently being developed, and a need has arisen for a principled approach to comparing and contrasting, and ultimately building, such systems. Thus, in this paper, we develop a principled taxonomy for AE. This taxonomy provides a unified context for long-term research in AE, so that implementation decisions can be compared and contrasted along known dimensions in the design space of embryogenic systems. It also allows predicting how the settings of various AE parameters affect the capacity to efficiently evolve complex phenotypes.

The Effects of Prenatal Intracranial Infusion of Tetrodotoxin on Naturally Occurring Retinal Ganglion Cell Death and Optic Nerve Ultrastructure

In the developing vertebrate nervous system, cell death is known to play an important role in determining final neuron number. Retinal ganglion cells in the cat's visual system undergo a massive elimination by cell death during the prenatal period between E44 (age of embryo in days) and birth (= E65). We have examined whether neural activity contributes to ganglion cell death by infusing tetrodotoxin (TTX), a blocker of the voltage-sensitive sodium channel. TTX was infused intracranially via osmotic minipumps implanted in utero at E42. The effects of the TTX treatment on ganglion cell death and optic nerve ultrastructure were examined at either E49 or E57 by electron microscopy and quantitative analysis of optic axon number. The numbers of optic nerve axons counted in the optic nerves of animals after either 1 or 2 weeks of TTX treatment were not significantly different from the counts in normal animals at comparable ages: E49 TTX-3.2 x 105; E48 normal-3.3 x 105; E57 TTX-2.1 x 105; E59 normal-2.4 x 105. These results suggest that retinal ganglion cells cannot be rescued from death by blockade of neural activity central to the optic chiasma. However, the ultrastructure of optic nerves following 2 weeks of TTX infusion was quite abnormal. The usual packaging of axons into fascicles by glia was disrupted by the presence of many pale, organelle-poor processes that were about 10 times larger in their cross-sectional areas than axons in either normal or TTX-treated nerves. Examination of these processes in serial transverse or in longitudinal electron microscope (EM) sections of the nerve revealed that they were most likely glial in origin. The ultrastructural organization of the optic nerve following 1 week of TTX treatment was normal, indicating that this effect on glial ultrastructure is either cumulative or delayed in onset. These results suggest that while the conduction of action potentials to the terminals of retinogeniculate axons may not play a significant role in regulating ganglion cell number prenatally, it may affect the normal maturation of optic nerve glia.

Glutamate-mediated responses in developing retinal ganglion cells

Glutamate has been suggested to regulate the development of retinal ganglion cells, but little is known about the functional properties of glutamate receptors during ontogeny of these neurons. Using whole-cell and outside-out patch-clamp recordings, we have characterized the pharmacological, rectification, and kinetic properties of ionotropic glutamate receptors in ganglion cells isolated from fetal and postnatal cat retinas. The fetal cells were studied at embryonic day 33 (E33) to E38, before significant outgrowth of their dendritic processes and prior to the formation of synaptic contacts in the inner plexiform layer. In several respects, the functional properties of early fetal ganglion cells were found to be remarkably similar to those of postnatal cells. In both age groups, glutamate and AMPA produced fast desensitizing currents, kainate yielded large steady-state currents, while applications of NMDA resulted in multiple channel openings. The shapes and amplitudes of these glutamate-gated currents were also similar and the current-voltage relations were nearly linear, with reversal potentials near 0 mV. Moreover, the dose-response curves (to kainate) were virtually identical in the fetal and postnatal neurons. The proportion of neurons responsive to NMDA and non-NMDA agonists was nearly the same in both age groups. This early functional expression of glutamate receptors cannot be involved in the transmission of electrical information in the developing retina because at this stage few ganglion cells are capable of generating action potentials (Skaliora et al., 1993). It is suggested that the early activation of NMDA and non-NMDA receptors in fetal ganglion cells may regulate the outgrowth and stabilization of dendritic processes in these neurons. Our data also revealed some differences in the responses of fetal and postnatal cells to glutamate and its agonists. Thus, the unitary NMDA conductance was found to decrease with age, while the rate of glutamate receptor desensitization increased with age. Also, while virtually all postnatal cells responded to glutamate, the proportion of fetal cells that manifested glutamate-mediated responses was lower. These maturational changes presumably allow retinal ganglion cells to integrate synaptic inputs for the transmission of electrical signals to the visual centers of the brain.

Architecture of the optic chiasm and the mechanisms that sculpt its development

At the optic chiasm the two optic nerves fuse, and fibers from each eye cross the midline or turn back and remain uncrossed. Having adopted their pathways the fibers separate to form the two optic tracts. Research into the architecture and development of the chiasm has become an area of increasing interest. Many of its mature features are complex and vary between different animal types. It is probable that numerous factors sculpt its development. The separate ganglion cell classes cross the midline at different locations along the length of the chiasm, reflecting their distinct periods of production as the chiasm develops in a caudo-rostral direction. In some mammals, uncrossed axons are mixed with crossed axons in each hemi-chiasm, whereas in others they remain segregated. These configurations are the product of different developmental mechanisms. The morphology of the chiasm changes significantly during development. Neurons, glia, and the signals they produce play a role in pathway selection. In some animals fiber-fiber interactions are also critical, but only where crossed and uncrossed pathways are mixed in each hemi-chiasm. The importance of the temporal dimension in chiasm development is emphasized by the fact that in some animals uncrossed ganglion cells are generated abnormally early in relation to their retinal location. Furthermore, in albinos, where many cells do not exit the cell cycle at normal times, there are systematic chiasmatic abnormalities in ganglion cell projections.

Comparative retinal ganglion cell and optic nerve morphology

The optic nerve is divided in four regions: intraocular, intraorbital, intracanalicular, and intracranial. The vertebrate retinal ganglion cells are classified by morphology, physiology and soma size. Species differences and similarities occur with retinal ganglion cells. Alpha retinal ganglion cells have large somata, large dendritic fields, large-diameter axons, and are most dense in the peripheral retina. Beta retinal ganglion cells have smaller diameter somata, smaller dendritic fields, small diameter axons, and predominate in the central retina. Gamma retinal ganglion cells are a heterogenous class of cells and have small diameter axons, and slow axon conduction velocities. The spatial distribution and organization of the retinal ganglion cells extends retinotopically through the nerve fiber layer, optic nerve, optic chiasm, optic tract, lateral geniculate nucleus, and visual cortex. The retinal nerve fiber layer thickness decreases from the optic disk toward the periphery of the retina. The retrobulbar optic nerve axon counts and axon density vary by species, with larger nerves having higher axon counts. Decussation of the optic nerve axons at the optic chiasm varies with 100% decussation in most birds and fish, 65% in cats, 75% in dogs, 80-90% in large animals, and 50% in primates. Centrifugal axons also occur in the optic nerve and may represent a method by which the brain can influence retinal activity.

Development of retinal ganglion cell structure and function

In this review, we summarize the main stages of structural and functional development of retinal ganglion cells (RGCs). We first consider the various mechanisms that are involved in restructuring of dendritic trees. To date, many mechanisms have been implicated including target-dependent factors, interactions from neighboring RGCs, and afferent signaling. We also review recent evidence showing how rapidly such dendritic remodeling might occur, along with the intracellular signaling pathways underlying these rearrangements. Concurrent with such structural changes, the functional responses of RGCs also alter during maturation, from sub-threshold firing to reliable spiking patterns. Here we consider the development of intrinsic membrane properties and how they might contribute to the spontaneous firing patterns observed before the onset of vision. We then review the mechanisms by which this spontaneous activity becomes correlated across neighboring RGCs to form waves of activity. Finally, the relative importance of spontaneous versus light-evoked activity is discussed in relation to the emergence of mature receptive field properties.

A POU domain transcription factor-dependent program regulates axon pathfinding in the vertebrate visual system

Axon pathfinding relies on the ability of the growth cone to detect and interpret guidance cues and to modulate cytoskeletal changes in response to these signals. We report that the murine POU domain transcription factor Brn-3.2 regulates pathfinding in retinal ganglion cell (RGC) axons at multiple points along their pathways and the establishment of topographic order in the superior colliculus. Using representational difference analysis, we identified Brn-3.2 gene targets likely to act on axon guidance at the levels of transcription, cell-cell interaction, and signal transduction, including the actin-binding LIM domain protein abLIM. We present evidence that abLIM plays a crucial role in RGC axon pathfinding, sharing functional similarity with its C. elegans homolog, UNC-115. Our findings provide insights into a Brn-3.2-directed hierarchical program linking signaling events to cytoskeletal changes required for axon pathfinding.

The course of later generated axons in the developing optic nerve of the chick embryo. A morphometric electron microscopic study

The topographic position of growth cones (GCs) shows the course of ingrowing axons within the optic nerve and allows to draw conclusions with respect to the fiber order in this pathway. Therefore, the topographic distribution and frequency of GCs as well as the proximal and distal axon shaft segments were studied within cross-sections of the distal, middle, and prechiasmatic part of the nerve of 3-8-day-old embryos using electron microscopy. The ingrowth of GCs was not confined to a particular region. Initially, GCs were found near the ventral periphery. With increasing age, simultaneous ingrowth occurred within an area that expanded dorsally. In parallel, GCs also occurred in dorsal regions and eventually in the dorsal periphery. GCs intermingled everywhere with more mature axon profiles. However, youngest profiles predominated ventrally, oldest dorsally. Hence, maturity increased from ventral to dorsal. This indicated that the time of arrival of axons and the topographic position in the cross-section correlated significantly. It is concluded that axons are chronotopically organized, but in a probabilistic sense. The predominant ingrowth of axons in the ventral part may be associated largely with the first wave of neurogenesis of retinal ganglion cells. The ingrowth in dorsal regions of the cross section may be related to later generated axons that enter the nerve following older axons of the same retinal sector as well as axons of neighboring ganglion cells which continue to leave the mitotic cycle while the front of neurogenesis has spread into the periphery.

Retinal mosaics: new insights into an old concept

It has been known since the middle of the 19th century that different neuronal types are distributed across the retinal surface in non-random arrays: indeed, these arrays, called 'mosaics', have long been considered to be a fundamental feature of retinal organization. However, until recently, little was known about how such mosaics are established during development. In the hope of stimulating further research, this article reviews the current status of three very different approaches to this intriguing general problem. The first postulates arrays of molecular markers, which are produced by specific cell types shortly after their final mitotic divisions and could be influential in the differentiation of other cell types. The second invokes a tangential dispersion of differentiating cells to generate spatial order, either while these cells are still migrating or soon after they reach their laminar destinations. The third involves the elimination of wrongly positioned cells through the process of naturally occurring cell death.

Enhanced axonal regeneration following combined demyelination plus schwann cell transplantation therapy in the injured adult spinal cord

We have treated spinal cord injured rats with demyelination plus Schwann cell transplantation and assessed neurite outgrowth in a quantifiable model of axonal regeneration. Axonal injuries of differing severity were induced in the dorsal funiculus of adult rats using a micromanipulator-controlled Scouten knife. Demyelinated regions were produced so as to overlap with the injury site by the injection of galactocerebroside antibodies plus complement one segment cranial to the axonal injury site. Schwann cells were isolated from the sciatic nerve, expanded in vitro, and transplanted into the injury site 1 day later. Animals were killed after an additional 7 days. Schwann cells were evenly distributed throughout the region of demyelination, which extended 6-7 mm cranial to the axonal injury site. The severity of axonal injury was quantified by counting degenerate axons in transverse resin sections. The degree of axonal regeneration was assessed by an electron microscopic analysis of growth cone frequency and distribution relative to the site of axonal injury. Quantification of growth cones at a distance from the site of axonal injury indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of severed axons; the ratio of growth cones to severed axons was increased by 26.5% in demyelinated plus transplanted animals compared to demyelinated animals without a transplant. Furthermore, only the demyelinated plus transplanted animals contained growth cones associated with myelin in white matter immediately outside of the region of complete demyelination. Growth cones were absent in transplanted-only animals at a distance from the site of axonal injury. These findings indicate that combined demyelination plus Schwann cell transplantation therapy enhances axonal regeneration following injury and suggests that growth cones are able to overcome myelin-associated inhibitors of neurite outgrowth in the presence of trophic support.

Ultrastructural observations on the expression of axonin-1: implications for the fasciculation of sensory axons during axonal outgrowth into the chick hindlimb

To help understand how axons interact as they grow into the developing chick hindlimb, we used electron microscopy in conjunction with immunoperoxidase staining for the cell adhesion molecule axonin-1 to label sensory axons. The results showed that sensory axons travel together in bundles, tightly apposed to one another. In contrast, motoneuron axons are more widely spaced, although motoneuron axons situated at the perimeter of sensory axon bundles are found in close contact with neighboring sensory axons. Sensory growth cones and lamellipodia tend to be located centrally within the bundles, with several lamellipodia typically being found stacked together. Strikingly, regions of close axonal apposition are accompanied by axonin-1 expression, suggesting that such contacts are indeed adhesive. Taken together, these observations suggest that groups of sensory axons of a similar age grow together, with some of the older sensory axons fasciculating along motoneuron axons and younger sensory axons later fasciculating along older sensory axons. Axons situated at the periphery of sensory bundles are typically partly labelled, such that axonin-1 is expressed on membranes apposing other labelled axons but not on those facing unlabelled axons or unlabelled Schwann cells. Thus, axonin-1 appears to become redistributed within the membranes of axons growing into the limb, as it does on cultured neurons. In contrast, the neuron-glia cell adhesion molecule (NgCAM), which binds heterophilically to axonin-1, appears uniformly distributed on even those axons that would have an asymmetric distribution of axonin-1. Thus, the localization of axonin-1 strongly suggests that it plays an important role in sensory axon fasciculation, but the relative contributions of its interactions with various potential ligands are unclear. Finally, we found that some sensory growth cones have lamellipodia that are spread over considerable expanses. This suggests that although fasciculation is important in sensory axon guidance, sensory axons may also explore the local environment.

Apoptosis in developing retinal tissue

The mechanisms of apoptosis are strongly dependent on cell-cell interactions typical of organized tissues. Experimental studies of apoptosis using a histotypical preparation of retinal explants are reported in the present article. We found that various characteristics of apoptosis are selectively associated with retinal cell death depending on cell type, stage of maturation, and means of induction of apoptosis. Among these were: (1) the requirements of protein synthesis; (2) the role of cAMP; (3) the expression of certain apoptosis-associated proteins; and (4) the sensitivity to excitotoxicity, modulation of protein phosphatases and calcium mobilization. Dividing cells undergo apoptosis in response to several inducers in specific phases of the cell cycle, and in distinct regions within their pathway of interkinetic nuclear migration. Recent post-mitotic cells are selectively sensitive to apoptosis induced by blockade of protein synthesis, while both proliferating and differentiated cells are more resistant. We also studied the association of several proteins, some of which play critical roles in the cell cycle, with both differentiation and apoptosis in the retinal tissue. Detection of cell cycle markers did not support the hypothesis that retinal cells re-enter the cell cycle on their pathway to apoptosis, although some proteins associated with cell proliferation re-appeared in degenerating cells. The transcription factors c-Jun, c-Fos and c-Myc were found associated with apoptosis in retinal cells, but their sub-cellular location in apoptotic bodies is not consistent with their canonical functions in the control of gene expression. The bifunctional redox factor/AP endonuclease Ref-1 and the transcription factor Max are associated with progressive cell differentiation, and both are down-regulated during cell death in the retina. The data suggest that Ref-1 and Max may normally function as negative modulators of retinal apoptosis. The results indicate that nuclear exclusion of transcription factors and other important control proteins is a hallmark of retinal apoptosis. Histotypical explants may be a choice preparation for the experimental analysis of the mechanisms of apoptosis, in the context both of cell-cell interactions and of the dynamic behavior of developing cells within the organized retinal tissue.

Prenatal development of retinogeniculate axons in the macaque monkey during segregation of binocular inputs

In the fetal monkey the projections from the two eyes are initially completely intermingled within the dorsal lateral geniculate nucleus (DLGN) before separating into eye-specific layers (). To assess the cellular basis of this developmental process, we examined the morphological properties of individual retinogeniculate axons in prenatal monkeys of known gestational ages. The period studied spanned the time from when binocular overlap has been reported to be maximum, circa embryonic (E) day 77 through E112, when the segregation process is already largely completed in the caudal portion of the nucleus. Retinogeniculate fibers were labeled by making small deposits of DiI crystals into the fixed optic tract. After adequate time was allowed for diffusion of the tracer, fibers were visualized by confocal microscopy, and morphometric measures were made from photomontages. This revealed that retinogeniculate fibers in the embryonic monkey undergo continuous growth and elaboration during binocular overlap and subsequent segregation. Importantly, very few side-branches were found along the preterminal axon throughout the developmental period studied. Thus, restructuring of retinogeniculate fibers does not underlie the formation of eye-restricted projections in the primate. Rather, the results support the hypothesis that binocular segregation in the embryonic monkey is caused by the loss of retinal fibers that initially innervate inappropriate territories ().

Cell production and cell death in the generation of variation in neuron number

Retinal ganglion cell numbers in adult mice vary from 40,000 to 80, 000. Much of this variation and the prominent bimodality of strain averages are generated by allelic variants at the neuron number control 1 (Nnc1) locus on chromosome 11. The Nnc1 locus may modulate either ganglion cell production or the severity of ganglion cell death. Here we have determined what the relative contributions of these two processes are to variation in adult cell number by estimating total ganglion cell production in 10 strains of mice (A/J, BALB/cJ, BXD32, C57BL/6J, CAST/Ei, CARL/ChGo, CE/J, C3H/HeSnJ, DBA/2J, and LP/J). These strains have adult populations that range from 45,000 to 76,000 (data available at http://qtl.ml.org). We estimated cell production by counting ganglion cell axons after ganglion cell neurogenesis but before the onset of significant cell death. Total cell production ranges from 131,000 to 224,000, and most of the variation in adult ganglion cell number is explained by this significant variation in cell production. In contrast, the percentage of cell death is relatively uniform in most strains (approximately 69% cell loss). The exceptions are BXD32, a strain that has an extremely high adult cell population, and Mus caroli (CARL/ChGo), a wild southeast Asian species that is distantly related to laboratory strains. In BXD32 and M. caroli, approximately 62% of the population dies. Our analysis indicates that substitutions of single alleles at the Nnc1 locus are responsible for production differences of approximately 8000 ganglion cells.

The changing pattern of fibre bundles that pass through the optic chiasm of mice

The organization of retinofugal fibres in the developing and adult mouse has been studied with transmission electron microscopy, autoradiography and the Bodian silver method. It has previously been shown that all retinal ganglion cell axons are in glial-wrapped bundles in the developing and adult optic nerve, but are not in similar bundles close to the chiasm. In the embryonic mouse this region shows a transition in glial morphology from an interfascicular to a radial type and here retinofugal fibres begin to form a new order related to their age. Growth cones become concentrated at the pial surface of the juxtachiasmatic nerve and older fibres are restricted to deeper regions. This same age-related order is also evident in the optic tract. However, the age-related order is lost within the chiasm, where growth cones, young and old fibres are again mingled in distinct bundles as they cross the mid-line. This study is particularly concerned with the structure of the mid-line bundles. These fibre bundles cross each other at right angles, and are recognizable in fetal and adult mice. In the adult, monocular injections of H3 proline followed by autoradiographic study show that the individual mid-line bundles are monocular and that they fuse again, losing the fascicular structure as they leave the chiasm and enter the tract. In the fetus and in the adult, the bundles generally lack a complete glial wrapping so that growth cones can lie in intimate contact with two crossing bundles, one coming from the left eye, the other from the right. The interesting question about the mechanisms that keep growth cones from entering the wrong bundles when they are in this position remains to be addressed.

A quantifiable model of axonal regeneration in the demyelinated adult rat spinal cord

Strategies to increase the extent of axonal regeneration in the adult CNS must address an array of intrinsic and environmental factors which influence neuritic outgrowth. In order to develop an in vivo model of axonal regeneration in which potential therapies may be assessed, we have quantified growth cones within demyelinated regions in the dorsal funiculus of the spinal cord, following a discrete axotomy. Demyelinated lesions were produced by the intraspinal injection of galactocerebroside antibodies plus serum complement proteins. Axonal integrity was not compromised by the demyelination protocol. Axonal injury was induced at the caudal extent of the demyelinated region using a micromanipulator-controlled Scouten knife. The severity of axonal injury was varied in different animals at the time of surgery and was quantified 8 days later by counting degenerate axons in transverse 1-microm resin sections. Evidence of axonal regeneration within these animals was assessed by an electron microscopic analysis of growth cone frequency and position relative to the site of axotomy. Growth cones were identified within the region of demyelination only; no growth cones were identified within the dorsal column white matter adjacent to the demyelinated region, or rostral or caudal to the region of demyelination, or in animals with an injury but no demyelination. Quantification of growth cones within regions of demyelination indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of axons severed. These findings indicate that demyelination facilitates axonal regeneration in the adult rat CNS and illustrate a quantifiable method of assessing axonal regeneration.

Reduced size of retinal ganglion cell axons and hypomyelination in mice lacking brain-derived neurotrophic factor

While brain-derived neurotrophic factor (BDNF) delays the death of axotomized retinal ganglion cells in rodents, it is unclear if it affects any aspect of the normal development of these cells. Here we examined the optic nerve of bdnf-/- mice. Axonal numbers were normal, but their diameter, as well as the proportion of myelinated axons, was reduced at postnatal day 20 (P20). In contrast, the facial nerve was not hypomyelinated. Expression levels of mRNAs coding for the myelin proteins PLP and MBP were substantially reduced in the hippocampus and cortex at P20, but not in the sciatic nerve. Intraventricular injections of BDNF into the ventricles of wild-type mice at P10 and P12 up-regulated expression of PLP in the hippocampus at P14. These results indicate a role of BDNF, discussed as indirect, in the control of myelination in the central nervous system.

Postnatal development of the pulmonary neuroepithelial bodies in various animal species

Various aspects of the postnatal development of intrapulmonary neuroepithelial bodies (NEB) were quantified in guinea-pigs, rabbits, cats, rats and hamsters. The highest densities of NEB were found at birth, especially in species with very immature neonates. Postnatally this density decreased, most probably by lung expansion and growth. The number of corpuscular cells per NEB generally did not change during postnatal development. Likewise, the volume density of their secretory dense cored vesicles remained unchanged. On the other hand, in most species and especially in those with very immature neonates, the number of intracorpuscular NEB nerve endings increased during postnatal development. At the same time, the number of afferent nerve endings increased at the expense of the efferent ones. We argue that NEB serve a dual function: endocrine and chemoreceptor. The endocrine function, dependent on the number of cells and their content of secretory vesicles, is already well developed at birth and does not mature further. The chemoreceptor function, dependent on the innervation, shows considerable maturation in the postnatal period.

Dendritic development of retinal ganglion cells after prenatal intracranial infusion of tetrodotoxin

The dendritic form of a cell may be established by many factors both intrinsic and environmental. Blockade of action potentials along the course of axons and in their postsynaptic targets dramatically alters the development of axonal morphology. The extent to which blockade of target cell activity retrogradely alters the dendritic morphology of the presynaptic cells is unknown. To determine whether the establishment of dendritic form by developing retinal ganglion cells depends on activity within their targets, the sodium channel blocker, tetrodotoxin (TTX), was administered via minipumps to the diencephalon of cat fetuses from embryonic day 43 (E43) to E57. At E57 retinae were removed and living retinal ganglion cells injected in vitro with Lucifer yellow to reveal their dendritic morphology. In the TTX-treated animals both alpha and beta types of retinal ganglion cells were present, as were putative gamma cells. Overall, the dendrites of retinal ganglion cells in TTX-treated animals appeared qualitatively and quantitatively similar to those of untreated animals. The only significant change in the TTX-treated cases was a small increase in the number of dendritic spines on the non-beta cells. These results indicate that the acquisition of basic dendritic form of developing ganglion cells is not influenced by the action potential activity within their targets, and that it is also independent of the terminal branching patterns of their axons.

The human fetal retinal nerve fiber layer and optic nerve head: a DiI and DiA tracing study

The organization of the primate nerve fiber layer and optic nerve head with respect to the positioning of central and peripheral axons remains controversial. Data were obtained from 32 human fetal retinae aged between 15 and 21 weeks of gestation. Crystals of the carbocyanine dyes, DiI or DiA, and fluorescence microscopy were used to identify axonal populations from peripheral retinal ganglion cells. Peripheral ganglion cell axons were scattered throughout the vitreal-scleral depth of the nerve fiber layer. Such a scattered distribution was maintained as the fibers passed through the optic nerve head and along the optic nerve. There was a rough topographic representation within the optic nerve head according to retinal quadrant such that both peripheral and central fibers were mixed within a wedge extending from the periphery to the center of the nerve. There was no indication that the fibers were reorganized in any way as they passed through the optic disc and into the nerve. The present results suggest that any degree of order present within the fiber layer and optic nerve is not an active process but a passive consequence of combining the fascicles of the retinal nerve fiber layer. Optic axons are not instructed to establish a retinotopic order and the effect of guidance cues in reordering fibers, particularly evident prechiasmatically and postchiasmatically, does not appear to be present within the nerve fiber layer or optic nerve head in humans.

Changing course of growing axons in the optic chiasm of the mouse

The distribution of retinofugal fibres has been studied by electron microscopy throughout the extent of the developing mouse optic nerve and chiasm at embryonic day (E) 16, in order to determine the course of fibre growth. Growth cones and mature axons, which are randomly distributed in bundles in the extracranial optic nerve, segregate in the juxtachiasmatic optic nerve. Here, growth cones accumulate in subpial regions amongst the endfeet of radial glia, whereas axons lie in the depths of the nerve. Surprisingly, however, growth cones move away from this region toward the ventricular zone in the lateral and midline parts of the chiasm, only to return to subpial regions once more before entering the optic tract, where fibres are again in an age-related order. Superficially, mature axons mingle with growth cones in the chiasm and near the beginning of the optic tract, suggesting that the age-related order begins to be reestablished before growth cones enter the tract. Deep and superficial regions of the pathway were examined in different planes of section. Specialised membrane relationships between retinofugal fibres and radial glial cells were also studied in deep and superficial regions of the lateral part of the chiasm. In addition, the distribution of retinofugal fibre bundles in the adult mouse was looked at by using light microscopy. The changing fibre positions noted in the embryo are maintained in the adult.

Changing patterns of expression and subcellular localization of TrkB in the developing visual system

Neurotrophins play important roles in the survival, differentiation, and maintenance of CNS neurons. To begin to investigate specific roles for these factors in the mammalian visual system, we have examined the cellular localization of the neurotrophin receptor trkB within the developing cerebral cortex and thalamus of the ferret using extracellular domain-specific antibodies. At prenatal ages (gestation is 41 d), trkB-immunostained fibers were observed in the internal capsule and as two distinct fascicles within the intermediate zone of the cerebral cortex. The staining of these fiber tracts declined with increasing age, whereas soma and dendrite staining of cortical neurons was first evident in early postnatal life and increased during subsequent development. Staining of subplate neurons [by prenatal day 5 (P5)] was followed by staining of cortical layer 5 neurons (at P10). By P31, trkB immunoreactivity was particularly prominent in layers 3 and 5 but was absent from subplate neurons. Staining included cells, especially pyramidal neurons, in all cortical layers by P45, and this pattern was maintained into adulthood. The optic tract and fibers within the lateral geniculate nucleus (LGN) were also strongly trkB immunoreactive at prenatal ages. Cellular staining of a subset of LGN neurons, those within the C-layers and perigeniculate nucleus, was apparent by P10 and maintained until P45, when the adult pattern of highly trkB-immunoreactive neurons in all layers of the LGN first appeared. The pattern of trkB immunoreactivity suggests that specific subsets of cortical and thalamic neurons may respond to neurotrophins such as brain-derived neurotrophic factor and/or NT-4/5 at discrete developmental times and locations. The appearance of trkB on axon fibers early in development and then on cell bodies and dendritic processes later is consistent with roles for both long-range and local, including autocrine and/or paracrine, delivery of neurotrophins in cell survival and maturation.

Genetic and environmental control of variation in retinal ganglion cell number in mice

How much of the remarkable variation in neuron number within a species is generated by genetic differences, and how much is generated by environmental factors? We address this problem for a single population of neurons in the mouse CNS. Retinal ganglion cells of inbred and outbred strains, wild species and subspecies, and F1 hybrids were studied using an unbiased electron microscopic method with known technical reliability. Ganglion cell numbers among diverse types of mice are highly variable, ranging from 32,000 to 87,000. The distribution of all cases (n = 252) is close to normal, with a mean of 58,500 and an SD of 7800. Genetic factors are most important in controlling this variation; 76% of the variance is heritable and up to 90% is attributable to genetic factors in a broad sense. Strain averages have an unanticipated bimodal distribution, with distinct peaks at 55,500 and 63,500 cells. Three pairs of closely related strains have ganglion cell populations that differ by > 20% (10,000 cells). These findings indicate that different alleles at one or two genes have major effects on normal variation in ganglion cell number. Nongenetic factors are still appreciable and account for a coefficient of variation that averages approximately 3.6% within inbred strains and isogenic F1 hybrids. Age- and sex-related differences in neuron number are negligible. Variation within isogenic strains appears to be generated mainly by developmental noise.

Retinotopy of the human retinal nerve fibre layer and optic nerve head

The organisation of the primate nerve fibre layer and optic nerve head with respect to eccentricity or the positioning of central and peripheral axons remains controversial. Crystals of the carbocyanine dyes DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), or DiA (4-[4-didecylaminostryryl]-N-methylpridiniumiodide) were used to trace retinal ganglion cell axons within the nerve fibre layer, optic nerve head, and optic nerve. The present study demonstrated that peripheral retinal axons were scattered throughout the vitreal-scleral depth of the nerve fibre layer. This scattered distribution was maintained as the fibres passed through the optic nerve head and into the optic nerve. Axons of the arcuate bundles showed a bias towards the scleral portions of the nerve fibre layer and a variable degree of fibre scatter across the nerve fibre layer which was not as evident in labelling from other retinal regions. There was a rough topographic representation within the optic nerve head according to retinal circumference such that both peripheral and central fibres were mixed within a wedge extending from the periphery to the centre of the nerve. Foveal fibres occupied a large proportion of the temporal aspect of the optic nerve head and nerve, whereas fibres from areas temporal to the fovea appeared to be displaced to more superior and inferior regions. Consistent with the scleral bias seen in the retina, arcuate fibres maintained a peripheral position as they passed through the optic nerve head and occupied a more peripheral position in the nerve. The present results suggest that any degree of order present within the optic nerve is not an active process; optic axons are not instructed to establish a retinotopic order within the initial portions of the visual pathway.

The initial stages of development of the retinocollicular projection in the wallaby (Macropus eugenii): distribution of ganglion cells in the retina and their axons in the superior colliculus

The time course of ingrowth of retinal projections to the superior colliculus in the marsupial mammal, the wallaby (Macropus eugenii), was determined by anterograde labelling of axons from the eye with horseradish peroxidase, from birth to 46 days, when axons cover the colliculus contralaterally and ipsilaterally. The position of retinal ganglion cells giving rise to these projections over this period was determined in fixed tissue by retrograde labelling from the colliculus with a carbocyanine dye. Axons first reach the rostrolateral contralateral colliculus 4 days after birth and extend caudally and medially, reaching the caudal pole at 18 days and the far caudomedial pole at 46 days. The first contralaterally projecting cells are in the central dorsal and temporal retina, followed by cells in the nasal and finally the ventral retina. They are distributed closer to the periphery with increasing age. The first sign of a visual streak appears by 18 days. Axons reach the ipsilateral colliculus a day later than contralateral axons and come from a similar region of the retina. The sparser ipsilateral projection reaches the caudal and medial collicular margins by 46 days but by 16-18 days, ganglion cells giving rise to this transient projection are already concentrated in the temporoventral retina. The orderly recruitment of ganglion cells from retinotopically appropriate regions of the retina as axons advance across the contralateral colliculus suggests that the projection is topographically ordered from the beginning. The ipsilateral projection is less ordered as cells are located in the temporoventral crescent at a time when their axons are still transiently covering the colliculus prior to becoming restricted to the rostral colliculus. Features of mature retinal topography such as the visual streak and the location of ipsilaterally projecting cells begin to be established very early in development, before the period of ganglion cell loss and long before eye opening at 140 days.

Topographic organization in the retinocollicular pathway of the fetal cat demonstrated by retrograde labeling of ganglion cells

The topographic organization of the developing retinocollicular pathway was assessed by making focal deposits of a retrograde tracer (usually rhodamine latex beads) into the superficial layers of the superior colliculus of fetal cats at known gestational ages. Subsequently, the distributions of labeled cells in the contralateral and ipsilateral retinas were examined. At all stages of development, a high density of labeled cells was found in a delimited area (core region) of both retinas. The locations of the retinal regions containing the high density of labeled cells varied with the locus of the tracer deposit in the superior colliculus in a manner consistent with the topographic organization of the mature cat's retinocollicular pathway. Additionally, some labeled ganglion cells, considered to be ectopic, were found to be scattered throughout the contralateral and ipsilateral fetal retinas. Such ectopic cells were few in number throughout prenatal development. For every 100 cells projecting to the appropriate region of the colliculus, we estimate that less than one ganglion cell makes a gross projection error. The incidence of ectopic cells did not differ between the contralateral and ipsilateral retina, even though the overall density of crossed labeled cells was always greater than that of uncrossed labeled cells. In the youngest fetal animals, tracer deposits into the caudal portion of the superior colliculus resulted in a core region of labeled cells in the contralateral nasal retina as well as in the nasal ipsilateral retina. Such uncrossed nasal cells, not seen in more mature animals, appear to innervate the appropriate topographic location of the superior colliculus, but on the wrong side of the brain. Most likely, these uncrossed nasal ganglion cells contribute to the widespread distribution of the ipsilateral retinocollicular pathway observed in fetal cats after intraocular injections of anterograde tracers (Williams and Chalupa, 1982). Collectively, our findings demonstrate that the developing retinocollicular pathway of the fetal cat is characterized by a remarkable degree of topographic precision.