Cell death in the mammalian visual system during normal development: II. Superior colliculus

Lamina specific postnatal development of PKCγ interneurons within the rat medullary dorsal horn

Protein kinase C gamma (PKCγ) interneurons, located in the superficial spinal (SDH) and medullary dorsal horns (MDH), have been shown to play a critical role in cutaneous mechanical hypersensitivity. However, a thorough characterization of their development in the MDH is lacking. Here, it is shown that the number of PKCγ-ir interneurons changes from postnatal day 3 (P3) to P60 (adult) and such developmental changes differ according to laminae. PKCγ-ir interneurons are already present at P3-5 in laminae I, IIo, and III. In lamina III, they then decrease from P11-P15 to P60. Interestingly, PKCγ-ir interneurons appear only at P6 in lamina IIi, and they conversely increase to reach adult levels at P11-15. Analysis of neurogenesis using bromodeoxyuridine (BrdU) does not detect any PKCγ-BrdU double-labeling in lamina IIi. Quantification of the neuronal marker, NeuN, reveals a sharp neuronal decline (∼50%) within all superficial MDH laminae during early development (P3-15), suggesting that developmental changes in PKCγ-ir interneurons are independent from those of other neurons. Finally, neonatal capsaicin treatment, which produces a permanent loss of most unmyelinated afferent fibers, has no effect on the development of PKCγ-ir interneurons. Together, the results show that: (i) the expression of PKCγ-ir interneurons in MDH is developmentally regulated with a critical period at P11-P15, (ii) PKCγ-ir interneurons are developmentally heterogeneous, (iii) lamina IIi PKCγ-ir interneurons appear less vulnerable to cell death, and (iv) postnatal maturation of PKCγ-ir interneurons is due to neither neurogenesis, nor neuronal migration, and is independent of C-fiber development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 102-119, 2017.

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.

Cytoarchitectural disruption of the superior colliculus and an enlarged acoustic startle response in the Tuba1a mutant mouse

The Jenna mutant mouse harbours an S140G mutation in Tuba1a that impairs tubulin heterodimer formation resulting in defective neuronal migration during development. The consequence of decreased neuronal motility is a fractured pyramidal cell layer in the hippocampus and wave-like perturbations in the cerebral cortex. Here, we extend our characterisation of this mouse investigating the laminar architecture of the superior colliculus (SC). Our results reveal that the structure of the SC in mutant animals is intact; however, it is significantly thinner with an apparent fusion of the intermediate grey and white layers. Birthdate labelling at E12.5 and E13.5 showed that the S140G mutation impairs the radial migration of neurons in the SC. A quantitative assessment of neuronal number in adulthood reveals a massive reduction in postmitotic neurons in mutant animals, which we attribute to increased apoptotic cell death. Consistent with the role of the SC in modulating sensorimotor gating, and the circuitry that modulates this behaviour, we find that Jenna mutants exhibit an exaggerated acoustic startle response. Our results highlight the importance of Tuba1a for correct neuronal migration and implicate postnatal apoptotic cell death in the pathophysiological mechanisms underlying the tubulinopathies.

Pax7 is requisite for maintenance of a subpopulation of superior collicular neurons and shows a diverging expression pattern to Pax3 during superior collicular development

Background

Pax7 encodes a transcription factor well-established as an important determinant of mesencephalic identity and superior collicular development. Pax7 mutant mice, however, present with no obvious morphological impairments to the superior colliculus. This finding is paradoxical and has been attributed to functional redundancy afforded by its paralogue Pax3. Here we utilise Pax7 mutant mice to investigate the precise role of this important developmental regulator during superior collicular development and neuronal specification/differentiation. We also assess its spatiotemporal relationship with Pax3 during embryonic development.

Results

Analysis of the superior colliculus of Pax7 mutant and wildtype mice at a variety of developmental timepoints revealed that whilst correct initial specification is maintained, a subpopulation of dorsal mesencephalic neurons is lost at early postnatal stages. Moreover, a comparative analysis of embryonic Pax3 and Pax7 expression profiles indicate that Pax3 expression overlaps extensively with that of Pax7 initially, but their expression domains increasingly diverge as development progresses, coinciding spatiotemporally with neuronal differentiation and maturation of the tissue. Furthermore, Pax3 expression is perturbed within the CNS of embryonic Pax7 mutant mice.

Conclusion

In summary, these results demonstrate that during superior collicular development, Pax7 is required to maintain a subpopulation of dorsal, mesencephalic neurons and partially regulates, spatiotemporally, Pax3 expression within the CNS. The differential nature of Pax7 and Pax3 with respect to neuronal differentiation may have implications for future stem cell therapies aimed at exploiting their developmental capabilities.

Downregulation of glial scarring after brain injury: the effect of purine nucleoside analogue ribavirin

The weak regenerative capacity of self-repair after injury to the adult brain is caused by the formation of glial scar due to reactive astrogliosis. In the present study the beginning of reactive astrogliosis in the adult, as shown immunocytochemically by upregulation of glial fibrillary acidic protein (GFAP) and vimentin, was seen two days after the left sensorimotor cortex lesion, being maximal during the first two weeks and declining by 30 days after the lesion. This was accompanied by intensive glial scarring. Conversely, after the neonatal lesion a lack of gliotic scar was seen until 30 days postsurgery, although the pattern of GFAP and vimentin expression during recovery period was the same. The aim of the study was to define an appropriate therapeutic intervention that could modulate astrocyte proliferation and diminish glial scar formation after adult brain lesion. For this purpose the effects of an antiproliferative agent, the purine nucleoside analogue ribavirin was examined. It was shown that daily injection of ribavirin for 5 and 10 days considerably decreased the number of reactive astrocytes, while slight GFAP labeling was restricted to the lesion site. Obtained results show that ribavirin treatment downregulates the process of reactive astrogliosis after adult brain injury, and thus may be a useful approach for improving neurological recovery from brain damage.

Graded ephrin-A2 expression in the developing hamster superior colliculus

During development, ephrin gradients guide retinal ganglion cell axons to their appropriate topographic locations in the superior colliculus (SC). Expression of ephrin-A2, assessed immunohistochemically in the developing hamster SC, revealed a rostral(low) to caudal (high) gradient that is most prominent at postnatal days P4 and P7 when topography is established. Double-labelling immunohistochemistry for ephrin-A2 and cell specific markers revealed that ephrin-A2 is expressed exclusively by a subset of neurons. The expression pattern has implications for mechanisms underlying establishment of topography during development and following injury.

NMDA antagonists in the superior colliculus prevent developmental plasticity but not visual transmission or map compression

Partial ablation of the superior colliculus (SC) at birth in hamsters compresses the retinocollicular map, increasing the amount of visual field represented at each SC location. Receptive field sizes of single SC neurons are maintained, however, preserving receptive field properties in the prelesion condition. The mechanism that allows single SC neurons to restrict the number of convergent retinal inputs and thus compensate for induced brain damage is unknown. In this study, we examined the role of N-methyl-D-aspartate (NMDA) receptors in controlling retinocollicular convergence. We found that chronic 2-amino-5-phosphonovaleric acid (APV) blockade of NMDA receptors from birth in normal hamsters resulted in enlarged single-unit receptive fields in SC neurons from normal maps and further enlargement in lesioned animals with compressed maps. The effect was linearly related to lesion size. These results suggest that NMDA receptors are necessary to control afferent/target convergence in the normal SC and to compensate for excess retinal afferents in lesioned animals. Despite the alteration in receptive field size in the APV-treated animals, a complete visual map was present in both normal and lesioned hamsters. Visual responsiveness in the treated SC was normal; thus the loss of compensatory plasticity was not due to reduced visual responsiveness. Our results argue that NMDA receptors are essential for map refinement, construction of receptive fields, and compensation for damage but not overall map compression. The results are consistent with a role for the NMDA receptor as a coincidence detector with a threshold, providing visual neurons with the ability to calculate the amount of visual space represented by competing retinal inputs through the absolute amount of coincidence in their firing patterns. This mechanism of population matching is likely to be of general importance during nervous system development.

Postnatal alterations of GABA receptor profiles in the rat superior colliculus

Midbrain sections taken from Sprague-Dawley rats of varying ages within the first four postnatal weeks were used to determine, immunocytochemically, putative changes of GABA(A) receptor beta2/3 subunits, GABA(B) receptor (R1a and R1b splice variants), and GABA(C) receptor rho1 subunit expression and distribution in the superficial, visual layers of the superior colliculus. Immunoreactivity for the GABA(A) receptor beta2/3 subunits was found in the superficial grey layer from birth. The labelling changed with age, with an overall continuous reduction in the number of cells labelled and a significant increase in the labelling intensity distribution (neuropil vs soma). Further analysis revealed an initial increase in the labelling intensity between postnatal days 0 and 7 in parallel with an overall reduction of labelled neurones. This was followed by a significant decrease in labelling intensity distribution between postnatal days 7 and 16, and a subsequent increase in intensity between postnatal days 16 and 28. The labelling profiles for GABA(B) receptors (R1a and R1b splice variants) and GABA(C) receptors (rho1 subunit) showed similar patterns. Both receptors could be found in the superficial layers of the superior colliculus from birth, and the intensity and distribution of labelling remained constant during the first postnatal month. However, the cell body count showed a significant decrease between postnatal days 7 and 16. These changes may be related to the time-point of eye opening, which occurred approximately two weeks after birth. For all three receptor types, the cell body count remained constant after postnatal day 16. By four weeks of age, there was no significant difference between the cell numbers obtained for the different receptors. Both GABA itself and neurofilament labelling were also obtained in the superficial superior colliculus at birth. Neurofilament, although found at birth, showed very little ordered arrangement until 16days after birth. When slices were double labelled for GABA(C) receptors and neurofilament, some overlap was observed. Double labelling for the presynaptic protein synaptophysin and GABA(C) receptors showed proximity in some places, indicative of a partly synaptic location of GABA(C) receptors. When GABA(C) and GABA(A) receptors were labelled simultaneously, some but not all neurones showed immunoreactivity for both receptor types. In conclusion, all three GABA receptor types were found to be present in the superior colliculus from birth, and all show some form of postnatal modification, with GABA(A) receptors demonstrating the most dramatic changes. However, GABA(B) and GABA(C) receptors are modified significantly around the onset of input-specific activity. Together, this points towards a contribution of the GABAergic system to processes of postnatal maturation in the superficial superior colliculus.

Rescue of the limb deformity in hammertoe mutant mice by retinoic acid-induced cell death

Retinoids, used therapeutically primarily in the treatment of skin disorders, are potent teratogens. Several craniofacial, neural tube, and limb defects derive from a selective increase in cell death by retinoic acid in sites of spontaneous programmed cell death. Previously we showed that programmed cell death in the limb was apoptotic, and that the webbing of the foot of the Hammertoe mutant mouse correlates with diminished cell death in these regions of webbing. We therefore examined the effect of the induction of cell death by retinoic acid in normal and mutant limbs. Here we report that exogenously administered retinoic acid enhances cell death in the interdigital and marginal regions of the limb. This cell killing is apoptotic by several criteria. We also report that retinoic acid induces cell death in areas of the Hammertoe limb that display a suppression of cell death during development. This induction of cell death ameliorates the mutant phenotype. These results establish that a genetic defect in cell death can be modified by retinoic acid. Retinoic acid, therefore, may be a signal involved in the regulation of cell death during normal limb development. However, neither the effect of retinoic acid on cell death nor the defect of cell death in Hammertoe correlates with an altered expression pattern of the homeobox-containing Msx genes, the retinoic acid receptor beta gene, or the ability of endogenous retinoic acid to bind its receptors. We conclude that retinoic acid may influence pattern formation and cell death through an indirect mechanism.

Genesis and fate of the perireticular thalamic nucleus during early development

A striking feature of the internal capsule during early development is that it is full of small neurones. Later, this group of neurones, called the perireticular thalamic nucleus, appears to have reduced in size, and only a few scattered cells are seen. In an effort to understand better the developmental history of the perireticular nucleus this study examines: i) the period of cell generation in the nucleus, ii) the magnitude of cell loss in the nucleus, and iii) the subsequent fate of cells in the nucleus during development. The perireticular cells are generated very early in development, being among the first generated in the thalamus (rats: E13-14; cats: E21-30). In rats, the first perireticular cells are generated at about the same developmental stage as the first subplate cells, which are among the first generated cells of the cortex: in cats, the first perireticular cells are generated well before those in the subplate (E24-30). In rats, the number of perireticular cells during developmental peaks at P5 (approximately 30,000) and then declines sharply (approximately 98%) by P15 (approximately 750), when adult-like patterns are seen. This dramatic loss of perireticular cells is due to both cell death and a migration of cells into the adjacent globus pallidus. The majority of the perireticular cells which migrate into the globus pallidus, however, are likely to die also. The presence of pyknotic profiles (indicators of dying cells) in the rat perireticular nucleus points to cell death as a contributor to the reduction in cell number during development. In this study, a period of relatively high pyknotic profile incidence (number of pyknotic cells per 1,000 "living" cells) is recorded in the perireticular nucleus over a 5 day period, from P2 to P7 (13.5-15.5). Similar values and patterns are recorded in the reticular nucleus and globus pallidus, except that in these structures, a period of relatively high pyknotic profile incidence (15-20) occurs over a shorter period (3 days; P2-5). Previous studies have suggested that some perireticular cells migrate into and settle within the adjacent globus pallidus. This study, with the use of long-term survivals after tracer injections in rats, shows that none (or very few) of these perireticular cells which migrate into the globus pallidus survive into more mature postnatal stages. Tracer (biotinylated dextran) was injected into the sensory nuclei of the dorsal thalamus at early stages (P7) and the rats were allowed to survive for either a day thereafter (to P8) or until well after the period of cell death was complete (to P16 or P21). In the short-term survivals (to P8), there are many dextran-labelled cells seen in the globus pallidus and in the perireticular nucleus. In the long-term survivals (to P16 or P21), by contrast, there are no dextran-labelled cells apparent in the globus pallidus or in the perireticular nucleus. It is likely that these cells in the globus pallidus, as with those in the perireticular nucleus, undergo cell death during development.

Pre- and postnatal expression of amino acid neurotransmitters, calcium binding proteins, and nitric oxide synthase in the developing superior colliculus

Neurons within the superior colliculus (SC) contain a variety of neurochemicals, including the amino acid neurotransmitters GABA and glutamate, the calcium binding proteins calbindin and parvalbumin, and the neuromodulator nitric oxide. We have examined the development of expression of these substances using antibody immunocytochemistry. These results are summarized in Fig. 10. GABA and calbindin are expressed very early in development, at a time when cells are still dividing and migrating from the subventricular zone. The expression of both GABA and CB is maximal at around E40-46, the age at which these cells have just established their adult lamination and extrinsic afferents have begun to grow into the tectum. GABA and CB likely play diverse roles during this stage of development, including the regulation of intracellular calcium during cell migration and neurite outgrowth. Glutamate is expressed somewhat later in development while parvalbumin immunoreactivity does not appear until shortly after birth. These two substances continue to increase in density throughout the period of postnatal growth, at a time when synapse formation and evoked electrical activity are beginning to develop. Both PV and glutamate may be involved in one or both of these activity-dependent processes. Nitric oxide synthase (NOS) is expressed at different times in different cell groups. NOS appears very early in prenatal development in cells within the SVZ and in the deep gray layer of SC. On the other hands, cells within the intermediate gray layer of SC do not express NOS until shortly before birth. The igl cells that express NOS at this age are clustered neurons similar to those that project to the CFR in the adult. NOS expression occurs in these cells at precisely the time when axons begin to form patches that innervate these clusters. Based upon this temporal correlation, we hypothesize that nitric oxide may regulate synapse formation in this cell group.

Apoptosis in the developing CNS

In this review, apoptosis during normal development of the CNS and abnormal apoptosis inducing hydrocephaly and arhinencephaly will be discussed. As the prominent sites of apoptosis during normal development of the CNS, we focused on the area of fusion of the neural plate to form the neural tube, the developing rhombomeres, and neuronal loss in the CNS during embryogenesis and postnatal development. As examples of abnormal apoptosis inducing abnormal brain morphogenesis, we will discuss genetically induced arhinencephaly and hydrocephaly. It was suggested that apoptosis of the precursor mitral cells in the anlage of the olfactory bulb was induced by non-innervation of olfactory neurons, and apoptosis of the precursor neurons in the pyriform cortex was induced by the non-innervation caused by the death of mitral cells in the mutant arhinencephalic mouse brain (Pdn/Pdn). Thus, sequential apoptosis of the precursor neurons and sequential manifestation of the brain abnormalities were proposed in arhinencephalic mutant mouse embryos and also in the arhinencephalic brains induced experimentally by fetal laser surgery exo utero. Meanwhile, it was speculated that the Gli3 gene, mutation of which is responsible for the arhinencephaly in Pdn/Pdn mice, might play a role in mesenchymal programmed cell death during development.

Effects of eye enucleation on NADPH-diaphorase positive neurons in the superficial layers of the rat superior colliculus

Dihydronicotinamide adenine-dinucleotide phosphate diaphorase (NADPH-d) positive neurons in the superficial layers of superior colliculus (SC) were studied in the adult rat after eye enucleation at postnatal day 5 (P5). Bilaterally, NADPH-d histochemistry revealed either weakly or intensely labeled neurons. In the SC contralateral to the enucleation, the volume of superficial layers decreased significantly, whereas the total number of NADPH-d positive neurons was only slightly reduced, thus resulting in an increased cell density. Bilaterally, the number of NADPH-d positive neurons was around 20% of Nissl-stained neurons. While the number of neurons which were weakly positive for NADPH-d was unchanged contralateral to the enucleation (thus resulting in a significant increase in their percentage on the overall NADPH-d population), the number of intensely labeled neurons decreased by 30%. Intensely labeled neurons were classified with respect to cell size and dendritic distribution. Some (126) were reconstructed and analyzed on the computer, in order to quantitate morphological differences in dendritic distribution in the denervated and control SC. The percent of neurons which could be assigned to some classes (marginal, stellate, narrow field vertical and wide field vertical) was reduced contralateral to the enucleation. In addition, vertically-oriented neurons (narrow field vertical, wide field vertical and pyriform) showed a significant decrease in soma size, dendritic length and number of branch points. And finally, the overall orientation of dendrites on narrow and wide field vertical neurons was more dispersed, when compared to the control colliculus. Thus, P5 eye enucleation affects the adult morphology of NADPH-d positive neurons in the superficial layers of the rat SC, resulting in increased cell density, changed relative number of cells in each morphological type, and altered soma size, dendritic length and orientation in specific neurons.

Development of corticotectal synaptic terminals in the cat: a quantitative electron microscopic analysis

We studied the development of corticotectal synaptic terminal boutons and synapses by making injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into area 17 of visual cortex in kittens ranging from newborn to 12 weeks of age and in adults. The location and extent of the injection site, and labeled corticotectal axon terminals in the superficial layers of the superior colliculus were demonstrated histochemically with the cobalt-glucose oxidase diaminobenzidine reaction. During the first 2 weeks after birth, the majority of labeled profiles resembled axonal growth cones, or structures intermediate in morphology between growth cones and synaptic terminals, while very few corticotectal axon terminals forming well-defined synaptic contacts were observed. Labeled synaptic terminals in kittens at 1 and 2 weeks of age were small, contained very few synaptic vesicles, which were usually restricted to the contact zone, and exhibited few mitochondria. By 4 and 6 weeks after birth, a well-developed population of synaptic terminals was established; however, growth cones and intermediate profiles were still numerous. At 8 weeks of age synaptic terminals were morphologically mature, and growth cone-like profiles were no longer observed. To study quantitative changes in synapse development we used the disector method to obtain unbiased estimates of the density and number of corticotectal synaptic terminals and synapses; both the density and number of terminals and synapses increased steadily throughout postnatal development. These results suggest that the corticotectal projection develops by the progressive elaboration of synapses, as opposed to synapse overproduction and subsequent elimination.

Numbers of neurons in the developing principal sensory nucleus of the trigeminal nerve: enhanced survival of early-generated neurons over late-generated neurons

The overproduction and subsequent death of neurons is a common phenomenon in the developing vertebrate central nervous system (CNS). We tested the hypothesis that the survival of a neuronal subpopulation is related to its time of origin. Neuronal survival was examined in a well-defined CNS structure, the principal sensory nucleus of the trigeminal nerve (PSN) of the rat. The changes in the total number of PSN neurons and in the numbers of early- and late-generated neurons (i.e., neurons heavily labeled by a single injection of [3H] thymidine on G12 or G14, respectively), between gestational day (G) 16 and postnatal day (P) 10 were determined. The total number of neurons in the PSN rose prenatally to a maximum of 40,600 on G18.5. The increase in neuronal number correlates to the period of migration. More than half of the neurons that successfully migrated to the PSN were lost by P10. The patterns for the changes in the numbers of early- and late-generated neurons were similar; however, there were significant differences between the two subpopulations. The maximum number of early-generated neurons (4,250) was attained on G18.2 and subsequently 58.9% of these neurons were lost. In contrast, the maximum number of late-generated neurons (5,050) was attained on G20.0 and 66.6% of these neurons were lost by P10. Therefore, it appears that the survivability of early generated neurons is greater than for late-generated neurons. This enhanced survivability presumably results from a competitive advantage that early-generated neurons have for forming synapses or gaining access to trophic factor(s) that are in limited supply.

Changes in synaptic density after developmental compression or expansion of retinal input to the superior colliculus

The retinal projection to the superior colliculus can be made abnormally dense by inducing a "compressed" retinal projection into a subnormal tectal volume, or abnormally sparse by monocular enucleation early in development. Any or all of the features of cell number, axonal arbor, dendritic arbor, and synaptic density could potentially be adjusted to compensate for such variations in the convergence of one cell population on another. We have examined the consequences of neonatal partial tectal ablation or monocular enucleation for synaptic length, density, and relative numbers of synapse classes in the superficial gray layer of the hamster superior colliculus. Monocular enucleation resulted in a reduction of synaptic density in the superficial gray layer of the colliculus ipsilateral to the remaining eye. This decrease in density was entirely accounted for by a reduction of the number of synapses with round vesicles, large asymmetric terminal specializations, and pale mitochondria characteristic of retinocollicular terminals (RLP synapses). There was no compensatory increase in any other synaptic class. RLP synapses were larger in monocular enucleates. Partial tectal ablation had no effect on synaptic density, nor on the relative proportions of different synaptic types. Synapses of the RLP class were slightly smaller than normal. These results suggest that synaptic density is normally at a maximum that cannot be altered by increases in potential input. However, density may be reduced by decreasing the number of inputs. Terminal classes do not appear to compete with each other within the collicular volume, suggesting that postsynaptic cells controls both the classes and numbers of their potential inputs.

Sex differences in the anteroventral periventricular nucleus of the preoptic area and in the related effects of androgen in prenatal rats

The density of cells in the anteroventral periventricular nucleus (AVPv) of the preoptic area of female rats was greater than that of males on day 21 of gestation. It appears that this difference between the sexes is caused by the action of androgen since the density of cells in the AVPv of female fetuses fell to the density in males when the mother received injections of testosterone propionate (TP) on days 14-18 of gestation. Pycnotic cells were more frequently found in oil-treated control males and TP-treated female fetuses than in control female fetuses. This result suggests that the prenatal injections of TP enhanced the rate of degeneration of the cells in the AVPv.

Brain macrophages and microglia respond differently to lesions of the developing and adult visual system

Traumatic injury in the brain usually results in rapid degeneration of neuronal elements and a response by peripherally derived macrophages (brain macrophages, BMOs) and resident microglia. One intriguing result of lesions performed in the developing brain as compared to lesions of the mature brain is the faster resolution of the cellular debris and the absence of significant scarring. The purpose of this study was to examine the response of BMOs to induced cell death distant to the lesion site and to investigate possible differences in the responding phagocytic populations (BMOs versus microglia) following lesions in neonates and adults. Ablation of the visual cortex at birth results in very rapid retrograde degeneration and removal of neurons of the dorsal lateral geniculate nucleus (dLGN) within a few days. Lesions to the visual cortex of adult rats also induce neurons within the dLGN to die, but these cells do so over a much more protracted time course. Utilizing differences in morphology and immunocytochemical staining with the monoclonal antibodies ED1 and OX-42 to distinguish between BMOs and microglia, we found that in the developing CNS, BMOs are signalled rapidly and specifically to the location of induced cell death. Microglia are not involved in this response. As might be expected, the temporal response in the adult is much more protracted. In contrast to the developing brain, microglia and not macrophages are the predominant responding cell class after the adult lesion. The data suggest that these are distinct populations of phagocytic cells that respond to brain damage during development and in the adult, which may be critical in modulating the resolution and growth response after injury.

Generation and death of cells in the dorsal lateral geniculate nucleus and superior colliculus of the wallaby, Setonix brachyurus (quokka)

To study postnatal cell generation in primary visual centres of the quokka, tritiated thymidine was injected into pouch-young aged postnatal day (P)1-P85. Brains were examined at P100, just before eye-opening, when primary visual projections are essentially mature. Neurons in the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) were generated at P1-P10 and P1-P18 respectively. Peak numbers of labelled cells were seen at P3 and P5 in the dLGN and SC. Cell death was assessed in the dLGN and SC of young aged P10-P150. Low numbers of dying cells were seen in the dLGN throughout this period, with a small peak at P85. A more substantial peak of cell death was seen in the SC, also at P85. In the quokka, the time interval between the peaks of cell generation and of cell death in the dLGN and SC is 70-80 days, considerably longer than the interval of 40 days between birth and death of retinal cells.

Topographic organization of the retinocollicular projection in the neonatal rat

The topographic order of the retinocollicular projection in the rat was examined from birth until maturity. Small, localized deposits of rhodamine-filled latex microspheres were placed into the superior colliculus at different locations. To minimize labeling fibers of passage deposit sites were typically, although not exclusively, placed into the caudal-lateral pole of the colliculus. Examination of the area and density of labeled cells in the retinae of these animals led to the following conclusions: (1) At each age examined, the location of the majority of labeled cells was observed to be in appropriate topographic register with the deposit site in the superior colliculus. (2) Confirming the work of previous investigators, errors in topographic projection were observed. These were present in both the contralateral and ipsilateral retinae and decreased with increasing postnatal age. The mature pattern was present by P10. (3) Quantitatively, the number of retinal ganglion cells terminating nontopographically within the colliculus constituted a relatively minor proportion of the total number of labeled cells in both retinae. It is concluded that the majority of the retinal ganglion cells make topographically appropriate terminations within the superior colliculus during development.

Heterogeneity in the distribution and morphology of microglia in the normal adult mouse brain

We have examined the distribution of microglia in the normal adult mouse brain using immunocytochemical detection of the macrophage specific plasma membrane glycoprotein F4/80. We were interested to learn whether the distribution of microglia in the adult brain is related to regional variation in the magnitude of cell death during development and resulting monocyte recruitment, or whether the adult distribution is influenced by other local microenvironmental cues. We further investigated the possibility that microglia are sensitive to their microenvironment by studying their morphology in different brain regions. Microglia are present in large numbers in all major divisions of the brain but are not uniformly distributed. There is a more than five-fold variation in the density of immunostained microglial processes between different regions. More microglia are found in gray matter than white. Particularly, densely populated areas include the hippocampus, olfactory telencephalon, basal ganglia and substantia nigra. In comparison, the less densely populated areas include fibre tracts, cerebellum and much of the brainstem. The cerebral cortex, thalamus and hypothalamus have average cell densities. There was no simple relationship between the amount of developmental cell death and the adult distribution of microglia. An estimate of the total number of microglia in the adult mouse brain, 3.5 x 10(6), is comparable to that found in the liver on a weight for weight basis. However, microglia possess up to twice the surface area of membrane of Kupffer cells, the large resident macrophages of the liver. The proportion of cells that were microglia varied from 5% in the cortex and corpus callosum, to 12% in the substantia nigra. Microglia vary in morphology depending on their location. They were broadly classified into three categories. Compact cells are rounded cells, sometimes with one or two short thick limbs, bearing short processes ("bristles"). They resemble Kupffer cells of the liver and are found exclusively in sites lacking a blood-brain barrier. Longitudinally branched cells are found in fibre tracts and possess several long processes which are usually aligned parallel to, or more occasionally perpendicular to, the longitudinal axis of the nerve fibres. Radially branched cells are found throughout the neuropil. They can be extremely elaborate and there is wide variation in the length and complexity of branching of the processes. There was no evidence of monocyte-like cells in the adult CNS. The systematic variation in microglial morphology provides further evidence that these cells are sensitive to their microenvironment.

Neuronal death in the developing sexually dimorphic periventricular nucleus of the preoptic area in the female rat: effect of neonatal androgen treatment

Neonatal treatment of female rats with androgen decreases the nuclear volume of the anteroventral periventricular nucleus of the preoptic area (AVPv-POA). In order to examine the effect of androgen on the neural substrates in the developing AVPv-POA which show a sexual dimorphism in nuclear volume, the cell death pattern in the AVPv-POA was compared between normal females and androgenized females. Wistar female rats were treated with 50 micrograms of testosterone propionate (TP) for 5 days from the day of birth. Degenerating cells (pyenotic cells) and normal cells were counted in every third section from days 1 to 13 of life. The rate of pycnotic cells to 1000 cells in TP-treated females sacrificed at days 7 and 10 was significantly higher than that in normal females. These results may suggest that neonatal androgen regulates neuronal death in the AVPv-POA, decreasing the number of neurons in the nucleus.

Astroglial differentiation in the opossum superior colliculus

Glial markers, namely, vimentin, glial fibrillary acidic protein (GFAP), and glycogen, as well as accumulation of axon-borne horseradish peroxidase (HRP), were used to visualize radial glial cells in the developing opossum superior colliculus (SC) and to follow changes in young astrocytes of the superficial layers. Vimentin, GFAP, and glycogen are relatively abundant in elements of the median ventricular formation (MVF), which persists at least as late as weaning time, i.e., postconception day 103, postnatal day 90 (PND90). Radial profiles and end-feet in the remaining collicular sectors (main radial system, MRS) are also vimentin-positive but show little or no glycogen or anti-GFAP staining. The numeric density of MRS profiles is very high at the final stages of neuronal migration (PND12) but falls to vestigial numbers by PND 56-60. Antivimentin staining and filling of MRS profiles by axon-borne HRP disappear in parallel. Before total regression of MRS profiles, young astrocytes of the superficial gray layer exhibit a transiently high GFAP expression that is not found in those of the subjacent layers. The results suggest that 1) radial glia at or near the collicular midline are well equipped for a mechanical supportive role, and their abundant glycogen accumulation may reflect their eventual transformation in cells with high glycolytic metabolism, including tanycytes; 2) in most collicular sectors, some radial glia cells persist for long periods after cessation of neuronal migration and may interact with afferent fibers coursing through the superficial neuropil; 3) radially oriented astrocytes of the superficial gray layer exhibit a transiently high GFAP expression that is temporally correlated with late transformations of the retinocollicular projections.

Transient projections from the lateral geniculate to the posteromedial lateral suprasylvian visual cortex in kittens

The postnatal maturation of the projection from the lateral geniculate nucleus to the posteromedial lateral suprasylvian visual cortex (PMLS) was studied with injections of fluorescent dyes into the PMLS at various postnatal ages. Labeled neurons projecting to the PMLS were present in all laminae of the ipsilateral lateral geniculate on the day of birth. However, there was a conspicuous change in the distribution of labeled geniculo-PMLS neurons by 11 days of age: now very few labeled neurons were present in lamina A, indicating a loss of geniculo-PMLS connections. The loss of connections began at the peripheral margins of lamina A and proceeded through other laminae toward laminae C1-3. By adulthood, labeled geniculo-PMLS neurons were largely confined to laminae C1-3; they were never observed in lamina A or A1 and were rarely observed in lamina C. To determine whether the lateral geniculate neurons survived after their projections to PMLS were lost, injections of fast blue were made at 1 or 2 days postnatally and the animals were allowed long postinjection survival times. Labeled neurons were found in all lateral geniculate laminae, thereby indicating that for many neurons the loss of connections could be attributed to a loss of their axon collaterals rather than to the death of the neurons themselves. After injections of fast blue into the PMLS and diamidino yellow dihydrochloride into area 17 shortly after birth, many double-labeled neurons were present in all laminae, indicating that they have collaterals to both targets. Thus, the survival of many of the geniculo-PMLS neurons contributing to the transient geniculo-PMLS projection seems to be due to sustaining collateral projections to area 17 or other cortical targets.

Cell death in the developing retinal ganglion cell layer of the wallaby Setonix brachyurus

The distribution and number of dying cells in the developing retinal ganglion cell layer of the wallaby Setonix brachyurus were assessed by using cresyl violet stained tissue. The density of dying cells has been expressed per 100 live cells for the entire retinal surface, data being presented as a grid of 500 micron squares. For statistical analysis, retinae were divided into 8 regions; dorsal, ventral, nasal, and temporal quadrants, each further divided into center and periphery. This method allowed comparison of the extent of cell death at different retinal locations as the high density area centralis of live cells developed temporal to the optic disk from 60 days onward. Between 30 and 70 days, dying cells were seen across the entire retina; beyond 100 days very few were seen. Initially, there was a significantly higher incidence of dying cells in the central retina compared to the periphery, whereas from 50 days this situation was reversed. Analysis of the central retina before and during area centralis formation consistently indicated a significantly lower number of dying cells per 100 live cells in temporal compared to other retinal quadrants. This differential pattern suggests that cell death lowers live cell densities less in the emerging area centralis than elsewhere, and therefore must play a part in establishing live cell density gradients. However, we cannot exclude the possibility that other factors are also instrumental. Indeed, factors such as areal growth (Beazley et al., in press) presumably operate at later stages since live cell density gradients continue to be accentuated even after cell death is complete. Numbers of dying cells peaked by 50 days, reaching approximately 1% of the live cell population. At this stage, counts were also maximal for live cells with values up to 30% above the adult range.

Do early lesions affect cell death in the central nervous system? A study on the effects of early cerebellar hemispherectomy in rats

Cell death patterns in the lateral and interposed nuclei were compared in control rats and rats in whom a unilateral cerebellar hemispherectomy was performed at day 2 of life. Both groups were studied between days 2 and 20 of life. Pyknotic cells and live neuronal and glial cells were counted from Nissl stained sections. After correction of these values, pyknotic to live cell ratios were calculated. In the lateral nucleus of normal rats, around 14-28 pyknotic cells per 1,000 live cells occurred from day 2 to day 12. Thereafter this value decreased, and from day 16 less than 3 pyknotic/1,000 live cells were observed. In the interposed nuclei, 18-28 pyknotic cells/1,000 live cells occurred at day 2, and from this age onward values gradually decreased. At day 20 values ranged around 1.6/1,000. After unilateral cerebellar hemispherectomy, values in both nuclei began to decrease as early as from day 8. Results from the present study strongly suggest that these cells are prevented from dying because they find an aberrant synaptic target in the ipsilateral red nucleus. Our results demonstrate that early lesions interfere with the regulation of fundamental processes of neuro-ontogeny.

Role of target tissue in regulating the development of retinal ganglion cells in the albino rat: effects of kainate lesions in the superior colliculus

Kainic acid or ibotenic acid was injected unilaterally into the major target regions of the axons of retinal ganglion cells--the superior colliculus (SC) or dorsal lateral geniculate nucleus (DLG)--of rat pups ranging in age from postnatal day 0 to postnatal day 10 (P0 - P10). While the collicular or geniculate neurons within the injection site died within 48 hours of the injection, damage to axons and terminals of extrinsic origin within the injected region was not apparent. The neuronal degeneration induced by the neurotoxins, observed at both the light and electron microscopic levels, resembled the neuronal degeneration that occurs in the colliculus during normal development. Macrophages were identified in the regions containing degenerating cells. Two to three weeks after the injections of neurotoxin, massive injections of the enzyme, horseradish peroxidase (HRP), were made into the retinorecipient nuclei. After about 24-hour survival time the numbers of retinal ganglion cells were estimated by counting the number of neurons containing HRP reaction products in sample areas distributed in a regular rectangular array across the entire retinal surface. In the animals in which the neurotoxin was injected into the SC during the first 4 postnatal days, there was a substantial reduction (on average 41.5%; the range: 27.5-65.5%) in the normal number (mean value of 113,000--Potts et al.: Dev. Brain Res. 3:481-486, '82) of retinal ganglion cells surviving the period of "naturally occurring ganglion cell death" in the retinae contralateral to the injected SC. By contrast, injections of neurotoxins into the DLG and/or the optic tract of newborn rats did not result in a significant reduction in the numbers of retinal ganglion cells surviving the period of naturally occurring ganglion cell death. The period of sensitivity of retinal ganglion cells to the injection of neurotoxin into the colliculi extends from birth to about the end of the first postnatal week; the greatest sensitivity seems to be restricted to the first 3-4 postnatal days. In the retinae in which the total number (and density) of ganglion cells was substantially reduced by the selective destruction of their target cells, the centro-peripheral difference in the somal diameters of the ganglion cells (apparent in normal animals) was abolished, both amongst the whole population of ganglion cells and amongst the ganglion cells with the largest somata, relatively thick axons, and large-gauge primary dendrites (Class I cells). The number and distribution of the Class I cells in the depleted retinae were, however, unaltered.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of cell number in the developing visual system. II. Effects of partial tectal ablation

The effects of potential excess innervation on cell survival in the superior colliculus and related structures during the period of normally occurring cell death was examined. A unilateral, partial lesion of the superficial layers of the superior colliculus on the day of birth, which results in a compression of the retinotectal map into the remaining area, was the manipulation used to produce the potential excess innervation. Cell density was reduced in the tectal fragment early in development, consistent with hyperinnervation, but had returned to normal by the end of the period of normally occurring cell death. The overall incidence of cell degeneration in the remaining partial colliculus was not different from the undamaged contralateral colliculus or from normal, though there was evidence of a transitory depression and later elevation of cell loss. Cell loss in the retina contralateral to the lesion was increased in the late part of the period of normal cell loss and there were fewer cells in the retinal ganglion cell layer at maturity. The amount of the cell loss in the retina was small compared to the amount of target removal. These results suggest that the survival of neurons with branching axons does not sensitively reflect target availability.

Control of cell number in the developing visual system. III. Effects of visual cortex ablation

The effect of unilateral deletion of the visual cortex on early cell death and eventual cell number in various structures of the visual system was examined. At minimum, this manipulation potentially provides excess retinal afference to the superior colliculi, partially denervates the superior colliculi, reduces normal retinal terminal area and opens up potential target space for the retina and superior colliculus in those areas where they share terminal space with the visual cortex. All layers of the superior colliculus, bilaterally, showed an initial decrease in the rate of cell death relative to normal followed by an increase in cell death rates. No change in the number or distribution of cells in the retinal ganglion cell layer resulted despite a substantial loss of retinal terminal area, and a substantial alteration of the pattern of retinal central termination. These results are interpreted as evidence for two stages in normally occurring cell death, a first in which axons compete to colonize any available terminal space, and a second in which axon-to-target specificity must be matched. These results also provide evidence that the amount of target required for neuron survival is clearly variable.

Control of cell number in the developing visual system. I. Effects of monocular enucleation

Monocular enucleation of hamsters on the day of birth caused an increase in cellular degeneration and a corresponding loss of cells in the dorsal lateral geniculate nucleus contralateral to the enucleation over the first 12 postnatal days. The superficial layers of the contralateral superior colliculus showed a similar increase in cell degeneration, except rostrally where the remaining ipsilateral projection is found. No changes in degeneration were found in either the ipsi- or contralateral ventral lateral geniculate nuclei, the intermediate and deep layers of the superior colliculus, or in the dorsal lateral geniculate and superficial superior colliculus ipsilateral to the enucleation, even though all were denervated to some degree. The disparities in the incidence of degenerating cells normally seen in the central and peripheral regions of the superior colliculus and dorsal lateral geniculate were preserved following the monocular enucleation. The incidence of degenerating cells in early development correlates well with known alterations in adult cell number. Only major denervations of retinal targets appear to be adequate to produce measurable changes in early cellular degeneration.

Development of cell and fiber lamination in the mouse superior colliculus

The emergence of laminar organization in the superior colliculus was investigated in the mouse with several anatomical methods, including tritiated-thymidine autoradiography, Golgi impregnation, and general stains for cell bodies and for fibers. The sequence of neurogenesis, cell migration, and early morphological differentiation of neurons was shown to exhibit a discontinuity between the lower and upper divisions (i.e., between the deep and intermediate "gray" and "white" layers and the superficial "gray" and "white" layers). These events proceed in an inside-out order within the lower division, but the same events within the upper division commence in advance of the completion of this progression. Thus, peak generation times for layers of the lower division proceed from (embryonic day) E11 to E13 and for the upper division from E12 to E13. Cell migration, as monitored with tritiated-thymidine labelling, reflects closely the pattern of cytogenesis. This is most clearly evident on E15 when a population of E11-labelled cells is divided into superficial and deep layers (the strata superficiale and profundum--SS and SP) by the interposition of E13-labelled cells at an intermediate level (stratum intermedium--SI). A contingent of the latter cells continue their migration and join their predecessors within the SS on E17, a time point when cell migrations are largely complete. Paralleling this sequence of arrival of neurons and the formation of three primary layers, both the time course of accumulation of fiber fascicles and the early morphological differentiation of neurons in the interval from E13 to E17 tends to proceed from SP to SS and from SS to SI. Thus, the transverse fiber system and large multipolar neurons of SP develop in advance of the longitudinal fiber system and vertically oriented neurons of SS, which in turn develop precociously with respect to the longitudinal fibers and medium-sized multipolar neurons of SI. In contrast, later events of differentiation that underly a major radial growth and an architectonic sublamination of the primary strata proceed in a simpler inside-out sequence from E17 to (postnatal day) P6. The major morphogenetic events underlying the establishment of statification in the colliculus appear to involve the operation of relatively independent programs of assembly for the two basic subdivisions. It is probable that selective cell-cell interactions contribute to the delivery of concurrently generated neurons to different laminae as well as to the deployment of axons in a manner that respects laminar boundaries.

Identification of cells undergoing physiological neuronal death in the neonatal rat brain by the Fink-Heimer method

There are certain limitations in identifying cells undergoing physiological neuronal death (PND) by conventional histological methods. Here, the Fink-Heimer method for impregnation of degenerating axons and terminals was successfully employed in selectively visualizing PND in the neonatal rat brain. The cell bodies thus observed were in close correlation with the known distribution and time-course of PND. In addition, massive occurrences of PND were observed in the periaqueductal gray, inferior colliculus, hypothalamus, and the rostral portion of the hippocampus, findings hitherto undescribed. Thus, the Fink-Heimer method provides a useful adjunct to the study of PND in the neonatal period.

Cell generation, death, and retinal growth in the development of the hamster retinal ganglion cell layer

During the early postnatal period in the hamster, the retinal ganglion cell layer grows, establishes its central connections, and undergoes substantial cell loss. In this study, we describe the development of the retinal ganglion cell layer with particular attention to the creation of local specializations in cell density. Changes in the number and spatial distribution of cells identified by a single 3H thymidine injection were examined through the period of maximal cell loss (postnatal days 4-10) and at adulthood. The cells of the retinal ganglion cell layer are generated from embryonic day 10 to postnatal day 3. Overall, cell number in the ganglion cell layer increases by approximately 108,000 cells (223%) from postnatal day 1 to 5, because of continued migration of cells generated prenatally. Cell number decreases from postnatal day 5 to 10 (25%), coincident with the presence of degenerating cells. Cell type is correlated with day of generation: the largest cells, all having retinal ganglion cell morphology, are generated on embryonic days 10 and 11; intermediate-sized cells predominantly of ganglion cell morphology on embryonic day 12; and smaller cells of displaced amacrine or glial cell morphology thereafter. At adulthood, the hamster retina shows a streaklike elevation of cell density through central retina. However, at the time of maximal cell number (postnatal day 5), cell density is uniform across the retina. During the period of cell degeneration, cells are lost in greater relative numbers from the retinal periphery. This cell loss occurs principally from the first-generated cells (embryonic days 10 and 11), as shown by both changes in the distribution of labeled cells and by the spatial pattern of labeled degenerating cells. From postnatal day 10 to adulthood, relative cell density continues to decline in the periphery of the retina, thus suggesting that differential growth completes the production of the adult cell density distribution.

Localized vascular regression during limb morphogenesis in the chicken embryo: II. Morphological changes in the vasculature

The regression of blood vessels in the distal wing bud of chicken embryos from stages 19 to 31 was examined by light and electron microscopy. The vessels were double-labelled by an injection of Monastral blue B (MB) to label the regressing endothelial cells, followed 6-48 hours later with an injection of India ink which marked the lumens of patent vessels. Prior to stage 26 the vessels contained only India ink since the endothelial cells were not phagocytic at this stage. Vessels at stage 26 or later were often double-labelled, with MB sequestered in the endothelial cell cytoplasm and India ink in the vessel lumens. After stage 27 cells not associated with lumens, but labelled with MB, were observed in areas undergoing vascular regression. Ultrastructural changes in the endothelial cells as the vessels regressed included formation of luminal and abluminal processes, long complex junctions, and vacuoles containing MB. In many involuting vessels the endothelial cells appeared normal even though the lumens were collapsed. Occasionally, isolated pyknotic cells were observed in regions that had been previously vascularized. At stage 31 cells in the developing cartilage had vacuoles containing MB. Our study suggests that blood vessels may disappear from the prechondrogenic zone of the distal wing bud by several mechanisms. These could include a type of cell death that does not elicit a cellular infiltrate, migration of the endothelial cells away from vascularized regions, and/or transdifferentiation into cells that resembled chondrocytes.

Development of the avian accessory optic system: effects of embryonic optic lesions

Representative cross-sections of the nuclei ectomammillaris (EM) from both normal and optically lesioned chick embryos (45 h of incubation, stage 12), were analyzed and compared on days 6, 8, 10, 12, 14 and 16 of incubation. An identifiable EM is clearly present at 8 days, in both normal and lesioned embryos, and increases in cell number and area up to embryonic day 12. However, embryos with partial or complete unilateral optic ablations demonstrate an apparent acceleration in cell death rate when compared with normals, from days 12-16, when a relatively mature and stable form of EM is apparently reached. Thus, early optic lesions do not affect the morphology of EM until day 12. These data also indicate that embryonic ipsilateral pathways to EM may persist and even expand when one eye primordium is removed or partially lesioned.

Postnatal development of the ipsilateral retinocollicular projection and the effects of unilateral enucleation in the golden hamster

Anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was used to study the normal development of the ipsilateral retinocollicular projection in golden hamsters, and to examine the effect of enucleation of the other eye at birth. In neonatal animals there were retinal fibers and sparsely distributed granular labeling in the superficial layers of the ipsilateral superior colliculus over its entire areal extent. Differences in the uncrossed projections of normal and enucleated animals first became clear at day 5. In normal animals, retinal fibers withdrew from the superficial layers of the superior colliculus, and the projection became concentrated in the stratum opticum, where denser clumps of label in the rostral part of the superior colliculus were first seen at day 5. In enucleated animals, the retinal projection persisted in the most superficial layers, and the density of labeling was higher than in normals. The very sensitive WGA-HRP technique showed retinal fibers extending to the caudal pole of the superior colliculus at all ages: even in normal animals more than 2 weeks of age some fibers reached as far as the inferior colliculus. When the shrunken size of the superior colliculus in the enucleated animals was taken into account, the total areal distribution of the ipsilateral projection was similar in normal animals and enucleates. The major difference between the two groups was in the higher density of ipsilateral labeling, especially in the caudal part of the superior colliculus, and in its more superficial laminar distribution in the enucleated animals.

Regional differences in normally occurring cell death in the developing hamster lateral geniculate nuclei

Normal cellular degeneration occurs in the lateral geniculate nuclei (LGN) of the hamster thalamus early in postnatal development. Degenerative debris can be observed in the ventral and dorsal nuclei at postnatal days 2-10 and is present in greater and more variable amounts in the ventral nucleus. Cell degeneration in the dorsal LGN is maximal at postnatal day 5, identical to the degeneration pattern of the hamster retina and superior colliculus, but shows a second peak at postnatal day 8 which may relate to the establishment of cortical connectivity. The incidence of degenerative debris is significantly higher in the peripheral margins of the dorsal nucleus, a pattern also seen in the retina and the superior colliculus, suggesting that a differential cell death may be involved in the formation of regional specializations in the visual system.

Neuronal death and synapse elimination in the olivocerebellar system. I. Cell counts in the inferior olive of developing rats

A transient multiple innervation of cerebellar Purkinje cells by climbing fibers has been described during postnatal development of the rat. The aim of the present study was to determine if the regression of redundant synapses is related to the loss of presynaptic cells in the inferior olivary nucleus (ION), which is the sole source of climbing fibers in rodents. To this end, the population size of the ION was evaluated by counting healthy cells of the four main subnuclei in rats from birth to adulthood. The cell population at birth was found to be very similar to that of the adult animal (27,655 versus 28,385), but a loss of 25% of the cells occurred in the first five days, presumably through their death since degenerating cells were observed over the same period. Although cell loss was found throughout the whole nucleus, it was more pronounced in the medial accessory olive. A subsequent apparent increase of the cell population was observed so that the adult value was again reached at 15 days. The evolution of the ION population is then characterized by a period of moderate cell death which takes place before the peak of polyneuronal innervation of Purkinje cells by olivary axons is attained. This strongly suggests that the removal of the redundant synaptic contacts established by climbing fibers onto Purkinje cells during development is caused by a progressive reduction of the branching of olivary axons rather than by degeneration of the presynaptic cells.

Ontogenetic development of serotoninergic neurons in the brain of a teleost, the three-spined stickleback. An immunohistochemical analysis

The ontogenetic development of serotoninergic neurons in the brain of the stickleback was investigated with the indirect immunocytochemical peroxidase-antiperoxidase technique, using a specific antibody to serotonin (5-hydroxytryptamine, 5-HT). Formation of neuronal populations takes place during embryonic development. By 80 h after fertilization, the first 5-HT perikarya have appeared in the ventricular zone of the hypothalamus (nucleus recessus lateralis) and the raphe region. At 108 h the first 5-HT perikarya can be observed in area praetectalis. At 118 h a transient group of 5-HT neurons appears rostral to the nucleus recessus lateralis, and at this same age the first 5-HT perikarya may be visualized in nucleus recessus posterioris. A group of 5-HT neurons appears in the dorsolateral tegmentum at 166 h (one day after hatching, which occurs at 120-144 h after fertilization). Differentiation of the neuronal populations, in terms of migration and formation of subdivisions, starts between 80 h and 94 h, and seems to be completed between 1 and 5 days after hatching. Raphe nuclei form an anterior group comprising nuclei raphe dorsalis, raphe medialis and a ventrolateral group, and a posterior group comprising a nucleus raphe pallidus/obscurus complex, a lateral nucleus reticularis paragigantocellularis and a ventromedial nucleus raphe magnus. The posterior and ventral raphe nuclei, which are well developed at the time of hatching, have not been visualized in the adult stickleback. While formation of 5-HT neuronal systems, as well as their primary efferent pathways, takes place during early ontogenetic development, the establishment of terminal areas and their subsequent differentiation apparently takes place during later ontogenetic stages. Most presumptive target areas are penetrated by 5-HT axons at hatching, although terminal formation does not seem to start until later. A considerable number of 5-HT neuronal groups present in the embryonic and newly hatched stickleback have not been visualized in the adult stickleback. This may be due to selective cell death, changes in transmitter phenotype or maturation of axonal transport processes during development.

Geniculocortical projection to layer I of area 17 in kittens: orthograde and retrograde HRP studies

Using the orthograde and retrograde HRP methods, the distribution of geniculocortical (G-C) afferent terminals in area 17 was examined in kittens and an adult cat. Following HRP injection in the entire dorsal lateral geniculate nucleus (LGNd), afferent terminals were labeled in layer I and deeper layers. The terminals in layer I in young kittens (less than 2 weeks old) were densely distributed throughout layer I, whereas in old kittens (over 1 month old) they were sparsely distributed in the outer half of layer I. When the HRP injection site was confined to the A laminae (lamina A and lamina A1), a few HRP-labeled terminals in layer I were seen in young kittens but not in older kittens or an adult cat. Following HRP injection in layer I of area 17 in young kittens, relatively large neurons were labeled retrogradely. The labeled neurons were located chiefly in the C complex and partly in the A laminae. The present experiments revealed that the G-C afferents to layer I of area 17 in young kittens originate from relatively large neurons which are located chiefly in the C complex and partly in the A laminae and that these afferents give dense terminals covering the entire layer I. These findings contrast with those obtained in adult cats, in which the G-C afferents to layer I originate exclusively from small neurons in the C complex and give sparse terminals to the outer half of layer I.

Postnatal development of cat superior colliculus. I. Presumptive neurons, immature neurons and degenerating cells in the intermediate and deep layers

Presumptive neurons, immature neurons and degenerating structures were found in the intermediate and deep layers of the superior colliculus of normal cats over the age range 7-45 days postnatal. Electron microscopical analysis revealed these structures to be unexpectedly frequent in the early postnatal period. There were 34 immature neurons per 1000 neurons at 7-8 days and 78 per 1000 neurons at 14 days postnatal. Few were found in older cats. Two types of degenerative changes in the neurons were observed: nuclear and cytoplasmic. The number of degenerating structures to be unexpectedly frequent in the early postnatal period. There were 34 immature neurons per 1000 neurons at 7-8 days and 78 per 1000 neurons at 14 days postnatal. Few were found in older cats. Two types of degenerative changes in the neurons were observed: nuclear and cytoplasmic. The number of degenerating neurons was higher at 14 days (28 per 1000 neurons) than at 7-8 days (21 per 1000 neurons), but only a few were found at 30 and 45 days. Degenerative changes were noted in synaptic terminals at all ages studied. The greatest number of degenerating synaptic profiles were observed at 14 and 30 days. They were infrequent in younger or older animals. The number of presumptive neurons does not change significantly over 7-14 days. It is speculated that they persist as a reverse pool of neurons. The increasing number of immature neurons in the period between 7 and 14 days indicates that formation of new cells in the intermediate and deep layers of the superior colliculus continues into the early postnatal period. The degeneration of some neurons and synapses over the same developmental period suggest that the final adjustment of the neuronal composition of the superior colliculus occurs postnatally.

Transplantation of tectal tissue in rats. IV. Maturation of transplants and development of host retinal projection

We have examined the time course of maturation of embryonic tectal tissue transplanted to the midbrain region of newborn rats and studied the development of the host retinal projection to the grafts. Transplants were examined 2-19 days after transplantation. The morphology of developing transplants was studied using Holmes silver and neutral red stained material. Tectal transplants attained their mature morphology about 17 days after transplantation. The time course of tectal transplant maturation appeared to be similar to that of normal superior colliculus in situ. The development of the host retinal projection into the transplants was examined by injecting the host eyes with horseradish peroxidase (HRP) at various times after transplantation. Retinal fibers anterogradely labeled with HRP were first seen growing into the transplants 3-4 days after transplantation. Ingrowing fibers were always located close to the surface of the transplants. The rate of growth of optic axons into the grafts was estimated to be about 250 to 300 micron per day. Patch-like arborizations of retinal afferents were formed soon after innervation and the mature pattern of optic innervation was established by about two weeks. There was no evidence for an initial transitory phase in which the axons invaded the whole transplant. The development of the host retinal projection preceded morphological maturation of the transplants. The mode of ingrowth of retinal axons into tectal grafts was in many respects similar to the way optic fibers grow into the superior colliculus during normal in situ development. The transplant technique thus provides an opportunity to manipulate and analyze the factors which guide optic fiber growth in intact brains.

Postnatal development of corticotectal neurons in the kitten striate cortex: a quantitative study with the horseradish peroxidase technique

Postnatal development of striate cortical neurons projecting to the superior colliculus (SC) was studied in cats, ranging in age from newborn to adult, by injection of horseradish peroxidase (HRP) into the SC. At birth HRP-labelled cells were widely distributed throughout the cortex between the splenial and suprasylvian sulci, although a very rough topographic correspondence seemed to exist between the striate cortex and SC. The labelled cells were confined to layer V of the cortex, as in the adult. They were very densely packed and their somas were already pyramidal in shape although very slender. During the third to eighth days, apical dendrites of a substantial number of cells, mostly located in the upper bank of the splenial sulcus, were filled with HRP up to layer I and their somas were larger than those of cells located near the crown of the lateral and postlateral gyri. At the eighth day and thereafter, the distribution of the labelled cells across the visual cortex was not so widespread as that seen in the newborn kittens. The dendritic arborization pattern of labelled cells became nearly adultlike at the four week, and its full maturation was seen at the eighth week. A quantitative analysis of the cross-sectional areas of the cells and their packing density in layer V of the cortex revealed that (1) the size of cells increased very rapidly during the second week and became almost adultlike at the fifth week; (2) the density of cells reduced dramatically during the second week and thereafter at a low rate until the eighth week; and (3) the ratio of the labelled to unlabelled cells in layer V decreased remarkably also during the second week. These results suggest that an elimination of axon collaterals of corticotectal cells or their death may take place mostly during the second week of age, when eye-opening occurs in kittens. By comparison with previous data on functional development of the SC, it is also suggested that the maturation of visual response properties of SC neurons may depend on postnatal development of corticotectal cells.

Cell degeneration in early development of the forebrain and cerebellum

The hippocampus, dentate gyrus, cerebellum, amygdala and caudate were examined for degenerating cells in normal neonatal hamsters. Each structure was studied from postnatal day 5 (P5) to P 10. The cerebellum was also studied on P 12. This time span was chosen to coincide with cessation of migration and establishment of connectivity in these structures. Substantial numbers of pycnotic cells were found in all structures except the dentate gyrus. The timing and amount of cell death varied between structures. Both the caudate nucleus and the amygdala showed greater cell losses in their peripheral margins than in their centers. The deep half of the internal granule cell layer of the cerebellum showed greater cell loss than the superficial half on all postnatal days studied.

Neuronal death in the development and aging of the cerebral cortex of the mouse

The numbers of neurons and glial cells in the cerebral cortex of the mouse have been estimated during its whole life-span (5 to 720 days), taking into account both the cellular densities of several areas and the cortical volumes. The results clearly demonstrate that there is a massive neuronal loss in the cerebral cortex during early postnatal development, greater in layers II-IV than in layers V-VI. In contrast, aging is characterized by a discrete neuronal loss in the cerebral cortex, purely restricted to layers II-IV. The number of glial cells increases continuously from 5 to 720 days. We emphasize here the need to obtain volumetric measure together with cellular densities in order to get interpretable quantitative data on cellular death and proliferation.

Reorganization of trigeminal primary afferents following neonatal infraorbital nerve section in hamster

The infraorbital nerve was sectioned and the ipsilateral whisker follicles were cauterized in hamsters within 12 h of birth. Sixty to ninety days later application of HRP to the proximal stumps of the ipsilateral lingual, inferior alveolar, mylohyoid and auriculotemporal nerves resulted in increased numbers of labeled somata in trigeminal ganglion regions which contain primarily infraorbital cell bodies in normal animals. The labeled central processes of mandibular nerves also occupied portions of the brainstem trigeminal complex normally innervated by infraorbital axons. These findings represent the first anatomical demonstration of trigeminal primary afferent plasticity.

Local differences in the amount of early cell death in neocortex predict adult local specializations

The amount of early cell loss in five neocortical areas was inversely related to adult numbers of neurons in those areas. Differential cell death predicted particularly the thickness of the upper cortical laminae; it was not related to neuron numbers in the lower laminae. Cell loss thus determines some features of local neocortical differentiation.