Optic chiasm and infundibular decussation sites in the developing rat diencephalon are defined by glial raphes expressing p35 (lipocortin 1, annexin I)

Diversity in mammalian chiasmatic architecture: ipsilateral axons are deflected at glial arches in the prechiasmatic optic nerve of the eutherian Tupaia belangeri

Permanent ipsilaterally projecting axons approach the chiasmatic midline in rodents but are confined to lateral parts of the optic chiasm in marsupials. Hence, principally different mechanisms were thought to underlie axon pathway choice in eutherian (placental) and marsupial mammals. First evidence of diversity in eutherian chiasmatic architecture came from studies in the newborn and adult tree shrew Tupaia belangeri (Jeffery et al. [1998] J. Comp. Neurol. 390:183-193). Here, as in marsupials, ipsilaterally projecting axons do not approach the midline. The present study aims to clarify how the developing tree shrew chiasm is organized, how glial cells are arranged therein, and the extent to which the tree shrew chiasm is similar to that of marsupials or other eutherians. By using routinely stained serial sections as well as immunohistochemistry with antibodies against glial fibrillary acidic protein, vimentin, and medium-molecular-weight neurofilament protein, we investigated chiasm formation from embryonic day 18 (E18) to birth (E43). From E22 onward, ipsilaterally projecting axons diverged from contralaterally projecting axons in prechiasmatic parts of the optic nerve. They made sharp turns when arriving at glial arches found at the transition from the optic nerve to the chiasm. Thus, during the ingrowth period of axons, Tupaia belangeri and marsupials have specialized glial arrays in common, which probably help to deflect ipsilaterally projecting axons to lateral parts of the chiasm. Our observations provide new evidence of diversity in eutherian chiasmatic architecture and identify Tupaia belangeri as an appropriate animal model for studies on the mechanisms underlying axon guidance in the developing chiasm of higher primates.

Annexin A1 in the brain--undiscovered roles?

Annexin A1 (ANXA1) is an endogenous protein known to have potent anti-inflammatory properties in the peripheral system. It has also been detected in the brain, but its function there is still ambiguous. In this review, we have, for the first time, collated the evidence currently available on the function of ANXA1 in the brain and have proposed several possible mechanisms by which it exerts a neuroprotective or anti-neuroinflammatory function. We suggest that ANXA1, its small peptide mimetics and its receptors might be exciting new therapeutic targets in the management of a wide range of neuroinflammatory diseases, including stroke and neurodegenerative conditions.

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.

Structure of longitudinal brain zones that provide the origin for the substantia nigra and ventral tegmental area in human embryos, as revealed by cytoarchitecture and tyrosine hydroxylase, calretinin, calbindin, and GABA immunoreactions

In a previous work, mapping early tyrosine hydroxylase (TH) expressing primordia in human embryos, the tegmental origin of the substantia nigra (SN) and ventral tegmental area (VTA) was located across several neuromeric domains: prosomeres 1-3, midbrain, and isthmus (Puelles and Verney, [1998] J. Comp. Neurol. 394:283-308). The present study examines in detail the architecture of the neural wall along this tegmental continuum in 6-7 week human embryos, to better define the development of the SN and VTA. TH-immunoreactive (TH-IR) structures were mapped relative to longitudinal subdivisions (floor plate, basal plate, alar plate), as well as to radially superposed strata of the neural wall (periventricular, intermediate, and superficial strata). These morphologic entities were delineated at each relevant segmental level by using Nissl-stained sections and immunocytochemical mapping of calbindin, calretinin, and GABA in adjacent sagittal or frontal sections. A numerous and varied neuronal population originates in the floor plate area, and some of its derivatives become related through lateral tangential migration with other neuronal populations born in distinct medial and lateral portions of the basal plate and in a transition zone at the border with the alar plate. Some structural differences characterize each segmental domain within this common schema. The TH-IR neuroblasts arise predominantly within the ventricular zone of the floor plate and, more sparsely, within the adjacent medial part of the basal plate. They first migrate radially from the ventricular zone to the pia and then apparently move laterally and slightly rostralward, crossing the superficial stratum of the basal plate. Several GABA-IR cell populations are present in this region. One of them, which might represent the anlage of the SN pars reticulata, is generated in the lateral part of the basal plate.

New cell surface marker of the rat floor plate and notochord

Regionally expressed cell surface molecules are thought to mediate contact-dependent interactions that regulate pattern formation and axon pathfinding in the developing vertebrate central nervous system (CNS). We recently isolated monoclonal antibody (mAb) CARO 2 through a screen for positional markers in the developing rat CNS. Between embryonic day (E)11.5 and E13, mAb CARO 2 specifically labels both the floor plate and notochord in the developing spinal cord. In contrast to the distribution of several well-characterized ventral midline markers, mAb CARO 2 labeling is restricted to the apical portion of the floor plate and the outer surface of the notochord. The anterior limit of mAb CARO 2 immunoreactivity corresponds to the midbrain/hindbrain border. Floor plate labeling persists throughout embryogenesis, whereas notochord labeling is not detectable after E13. During later stages of embryonic development (E16-E20) apically restricted floor plate labeling is present only in the rostral spinal cord. In postnatal rats, mAb CARO immunoreactivity is not present in any region of the CNS. Immunoblot analyses show that mAb CARO 2 recognizes an epitope on a 28-kD protein that is enriched in the floor plate, transiently expressed during embryogenesis, and membrane-associated. Consistent with the latter result, mAb CARO 2 labels the surfaces of floor plate cells. These findings suggest that the CARO 2 antigen is a new cell surface marker of the floor plate and notochord which may participate in neural cell patterning and/or axon guidance in the developing rat spinal cord.

First evidence of diversity in eutherian chiasmatic architecture: tree shrews, like marsupials, have spatially segregated crossed and uncrossed chiasmatic pathways

In the optic chiasm of mammals, axons either cross the midline to the opposite side of the brain or remain uncrossed. In the eutherian species studied to date, uncrossed axons in the caudal nerve are found in all regions. In the chiasm, they are dispersed through the hemichiasm, with many axons approaching the midline and then turning back to enter the same side of the brain as the originating eye. In marsupials, by contrast, uncrossed axons never approach the midline; instead, they remain grouped in the lateral nerve and chiasm. The impression gained from these data is that there is a major difference in chiasmatic architecture between eutherian and marsupial mammals. Therefore, the mechanisms by which axons choose their route through the chiasm was also thought to differ between the two major groups of mammals. However, the present study shows that the chiasm of a highly visual eutherian mammal, the tree shrew, is similar to that found in marsupials, with uncrossed axons confined to lateral regions and not approaching the midline. However, unlike marsupials, in the tree shrew, optic fascicles in the chiasm are often separated by thick collagen bundles. It is probable that the chiasmatic structure described to date for eutherian mammals is not ubiquitous, as was previously thought, and theories explaining the mechanisms by which axons chose their route through the chiasm during development will have to be expanded.

Molecular cloning of a new intermediate filament protein expressed by radial glia and demonstration of alternative splicing in a novel heptad repeat region located in the carboxy-terminal tail domain

In the present study we describe the molecular cloning of transitin, formerly named EAP-300. We show that transitin is an intermediate filament protein with a core domain most closely resembling nestin and tanabin. Transitin also contains a novel heptad amino acid repeat domain, comprising multiple leucine zipper repeats, located in its tail region. Based on these structural motifs we propose that a novel intermediate filament protein that is transiently expressed by radial glia during CNS development has been identified. We also show the existence of splice variants of transitin with splicing occurring in the novel heptad repeat domain to give rise to transitin isoforms that lack this heptad repeat. By in situ hybridization analysis we show that transitin mRNA is expressed by midline radial glial structures, by several axon commissures, and by Bergmann glia of the developing cerebelium. Based on the structural properties of the transitin protein, and expression of its mRNA, we suggest that transitin is a new member of the intermediate filament gene superfamily that is transiently expressed by radial glia.

Glia, neurons, and axon pathfinding during optic chiasm development

The importance of vision in the behavior of animals, from invertebrates to primates, has led to a good deal of interest in how projection neurons in the retina make specific connections with targets in the brain. Recent research has focused on the cellular interactions occurring between retinal ganglion cell (RGC) axons and specific glial and neuronal populations in the embryonic brain during formation of the mouse optic chiasm. These interactions appear to be involved both in determining the position of the optic chiasm on the ventral diencephalon (presumptive hypothalamus) and in ipsilateral and contralateral RGC axon pathfinding, development events fundamental to binocular vision in the adult animal.

Annexin IV is a marker of roof and floor plate development in the murine CNS

Midline structures, such as the notochord and floor plate, are crucial to the developing central nervous system (CNS). Previously, we demonstrated that annexin IV is an excellent marker of midline structures. In the present study, we explore the possible role of annexin IV in development of the CNS midline. Using immunocytochemistry with an antibody to annexin IV, we have elucidated the temporal and spatial expression of this molecule. Annexin IV is present in the notochord at embryonic day (E) 8.5, prior to its expression in any structures within the neural tube. Subsequently, annexin IV is expressed by floor plate cells at E9.5. Annexin IV is also expressed in the roof plate, but not until E10.5. To determine if normal morphogenesis of these midline structures is essential for annexin IV expression, we analyzed two strains of mutant mice that have defective formation of either the floor or the roof plate. In Danforth's short-tail mice, the floor plate is absent from the caudal spinal cord, and annexin IV immunopositivity disappears at the level where the floor plate is missing. In curly tail mutant mice, there can be a failure of the neural tube to close, and in these regions there is no annexin IV expression in presumptive roof plate cells. Finally, annexin IV immunolabeling is present from the caudal spinal cord, through the brainstem up to the diencephalon and lamina terminalis. Thus, annexin IV is an excellent marker for differentiated midline cells, is temporally and spatially correlated with development of the floor and roof plates, and is expressed in a rostral-caudal manner that supports the hypothesis that the floor plate extends the full length of the original neural tube.

Retinal axon divergence in the optic chiasm: midline cells are unaffected by the albino mutation

The visual pathway in albino animals is abnormal in that there is a smaller number of ipsilaterally projecting retinal ganglion cells. There are two possible sites of gene action that could result in such a defect. The first site is the retina where the amount of pigmentation in the retinal pigment epithelium is correlated with the degree of ipsilateral innervation (La Vail et al. (1978) J. Comp. Neurol. 182, 399–422). The second site is the optic chiasm, the site of retinal axon divergence. We investigated these two possibilities through a combination of in vivo and in vitro techniques. Our results demonstrate that the growth patterns of retinal axons and the cellular composition of the optic chiasm in albino mice are similar to those of normally pigmented mice, consistent with the albino mutation exerting its effects in the retina, and not on the cells from the chiasmatic midline. We directly tested whether the albino mutation affects the chiasm by studying ‘chimeric’ cultures of retinal explants and chiasm cells isolated from pigmented and albino mice. Crossed and uncrossed axons from pigmented or albino retinal explants display the same amount of differential growth when grown on either pigmented or albino chiasm cells, demonstrating that the albino mutation does not disrupt the signals for retinal axon divergence associated with the albino optic chiasm. Furthermore, in vitro, a greater proportion of albino retinal ganglion cells from ventrotemporal retina, origin of uncrossed axons, behave like crossed cells, suggesting that the albino mutation acts by respecifying the numbers of retinal ganglion cells that cross the chiasmatic midline.

Crossed and uncrossed retinal axons respond differently to cells of the optic chiasm midline in vitro

In mouse, retinal axon divergence takes place within a cellular specialization localized at the midline of the optic chiasm. To test whether the cells in this locus present cues for differential retinal axon growth, retinal explants were cocultured with cells dissociated from the chiasmatic midline, both taken from day 14-15 embryos, during the principal period of retinal axon divergence. Compared with crossed axons from other retinal regions, axons from ventrotemporal retina, the sole source of uncrossed axons, were shorter, more fasciculated, and fewer in number when growing on chiasm cells. Furthermore, uncrossed axons avoided clusters of chiasm neurons and glia having the composition and arrangement of the midline specialization, but crossed axons readily grew over them. In contrast to the clusters of chiasm cells, however, individual neurons and glia did not elicit differential retinal axon growth. These data demonstrate that cues for divergence derive from cells resident to the chiasm and suggest that cellular interactions among resident midline cells are required to produce these cues.

Platelet-derived growth factor receptors of mouse central nervous system cells in vitro

This study evaluates the distribution of receptors for platelet-derived growth factor (PDGF) on central nervous cells maintained in vitro using colloidal gold-labeled immunocytochemical markers at the electron microscopic level. Platelet-derived growth factor receptors were found to be sparsely distributed over the surface of type 1 astrocytes, apparent type 2 astrocytes, and neurons. Receptors appeared to be preferentially associated with filopodia-like extensions of the cell membrane. The existence of functional receptors was confirmed using the impermeant, water-soluble affinity cross-linking agent bis(sulfosuccinimidyl)suberate to covalently link radiolabeled PDGF to its receptor. The PDGF/receptor complexes could also be immunoprecipitated with the same antibody used in immunocytochemical experiments. The improved resolution of these techniques allows definitive identification of PDGF receptors on cultured mammalian central nervous system cells other than oligodendrocytes. These data expand the range of possible roles of PDGF during nervous system development. Receptors for PDGF are likely to play a key role in the differentiation of cells in the central nervous system.

Development of the chiasm of a marsupial, the quokka wallaby

We have previously shown that the mature optic chiasm of a marsupial is divided morphologically into three regions, two lateral regions in which ipsilaterally projecting axons are confined and a central region containing only contralaterally projecting axons. By contrast, in the chiasms of eutherian (placental) mammals studied to date, there is no tripartite configuration. Ipsilaterally and contralaterally projecting axons from each eye are mixed in the caudal nerve and in each hemichiasm and encounter axons from the opposite eye near the midline of the chiasm. Here, we show that, unlike eutherians, marsupials have astrocytic processes in high concentrations in lateral regions of the nerve and rostral chiasm. Early in development, during the period when optic axons are growing through the chiasm, many intrachiasmatic cells are seen with densities five to eight times higher in lateral than in central chiasmatic regions. Such cells continue to be added to all chiasmatic regions; later in development, considerably more are added centrally, as the chiasm increases in volume. In the mature chiasm, cell densities are similar in all regions. By contrast to the marsupial, cell addition in the chiasm of a placental mammal, the ferret, is almost entirely restricted to later developmental stages, after axons have grown through the chiasm, and there are no obvious spatial variations in the distribution of cells during the period examined. During development, similar to the adult marsupial, ipsilaterally projecting axons do not approach the chiasmatic midline but remain confined laterally. We propose that the cells generated early and seen in high densities in the lateral chiasmatic regions of the marsupial may play a role in guiding retinal axons through this region of pathway selection. These data suggest that there is not a common pattern of developmental mechanisms that control the path of axons through the chiasm of different mammals.

Lipocortin 1 in apoptosis: mammary regression

BACKGROUND

Enigmatically, degradation of debris generated in programmed cell death (apoptosis) elicits little inflammation. Having previously detected the upregulation of lipocortin 1 (LC1), a 35-kDa protein with anti-inflammatory and immuno-suppressive properties, at sites of non-inflammatory phagocytosis in the central nervous system (J Neurosci Res 36:491-500, 1993), we sought to determine if LC1 was involved in apoptosis.

METHODS

LC1 immunoreactivity in mammary glands of adult rats was quantified in situ using video microdensitometry before and during postlactational regression.

RESULTS

LC1 is present in the mammary ducts but is absent from the alveoli during lactation. One day after weaning, however, LC1 is detected in the lactiferous cells and, as apoptosis proceeds over the ensuing 4 days, total LC1 in the gland increases > 10-fold over resting levels. LC1 remains high in both the apoptotic cells and epithelial phagocytes through day 10, but the total LC1 per gland drops as the apoptotic cells are cleared.

CONCLUSIONS

Published experiments have shown that LC1 specifically binds Ca++ and phosphatidylserine, and that these affinities are modulated by tyrosine phosphorylation and cross-linking with transglutaminase. Thus, LC1 appears to be a candidate for several putative activities in apoptosis (e.g., phagocyte recognition via phosphatidylserine binding and/or buffering intracellular Ca++) in addition to its anti-inflammatory role.

The annexins: specific markers of midline structures and sensory neurons in the developing murine central nervous system

The annexins are a family of cytoplasmic proteins that have been shown to have numerous actions within a cell. Recent evidence suggests that at least one of these proteins plays a role in the development of the central nervous system (CNS). The present study examines the temporal expression and spatial distribution of annexins I, II, IV, V, and VI during development and at maturity in the murine CNS by immunocytochemical analysis. The results demonstrate that annexins I, II and IV exhibit clear immunolabeling in the murine CNS with distinct patterns of temporal and spatial expression. Annexin IV is the first annexin to be expressed on embryonic day (E) 9.5 while annexin I is the last to be expressed (E11.5). Annexins I, II and IV are found in the floor plate region, but to differing rostrocaudal extents. Annexin I has a very restricted distribution, only present in the midline raphe of the brainstem. Annexin II is present in the spinal cord, brainstem and mesencephalon. Annexin IV has the widest midline distribution, being observed in the floor and roof plates of the developing CNS. Additionally, antibodies against annexin II and IV immunolabel most dorsal root and sensory ganglion cells and their axons. During early postnatal development, immunolabeling with each antibody gradually disappears in many structures, and only first order sensory neurons and their fibers are immunopositive for annexins II and IV at weaning. Three functions of the annexins are suggested by the present findings: (1) to help establish the midline structures of the floor and roof plates, (2) to help direct the decussation of sensory fibers, and (3) to regulate some aspect of sensory neuron processing, such as signal transduction.

Glial domains and axonal reordering in the chiasmatic region of the developing ferret

This study has examined the developing glial architecture of the optic pathway and has related this to the changing organization of the constituent axons. Immunocytochemistry was used to reveal the distribution of glial profiles, and DiI was used to label either radial glial profiles or optic axons. Electron microscopy was used to determine the distribution of glial profiles, axons, growth cones, and wrists at different locations along the pathway. Three different glial boundaries were defined: Two of these are revealed as changes in the distribution of vimentin-immunoreactive profiles occurring in the prechiasmatic optic nerve and at the threshold of the optic tract, respectively, and one by the presence of glial fibrillary acidic protein (GFAP)-immunoreactive profiles at the chiasmatic midline. The latter, midline boundary may be related to the segregation of nasal from temporal optic axons. The boundary at the threshold of the optic tract coincides with the segregation of dorsal from ventral optic axons that emerges at this location in the pathway. The segregation of old from young optic axons is shown to occur only gradually along the pathway. Glial profiles are most frequent in the deeper parts of the tract, coursing parallel to the optic axons and orthogonal to their usual radial axis. These are suggested to arise from later-growing radial glial fibers that are diverted to grow amongst the older optic axons. Those glial profiles may subsequently impede axonal invasion, thus creating the chronotopic reordering by forcing the later-arriving axons to accumulate superficially.

Organization of pioneer retinal axons within the optic tract of the rhesus monkey

Retinal ganglion cell axons must make a decision at the embryonic optic chiasm to grow into the appropriate optic tract. To gain insight into the cues that play a role in sorting out the crossed from the uncrossed optic axons, we investigated the sequence of their initial ingrowth in rhesus monkey embryos. Two carbocyanine dyes, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate and 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide, were placed, respectively, into the left and right retinas to identify the course of uncrossed and crossed retinal axons through the optic chiasm and tract. Our results show that at embryonic day 36 the most advanced retinal projections are uncrossed. At this age the leading crossed axons are just reaching the chiasmatic midline, whereas the uncrossed fibers have already entered the optic tract. This indicates that the pathfinding of these pioneer uncrossed fibers does not require the presence of retinal axons from the opposite eye. At subsequent stages of development (embryonic days 40 and 42) there is a clear partial segregation of the uncrossed and crossed retinal axons within the optic tract: the uncrossed-component course is in the deeper portion of the optic tract, whereas the crossed component lies in a more superficial region. Thus, the spatial organization of retinal axons within the primordial optic tract reflects the sequential addition of the uncrossed and crossed retinal fibers. The orderly and sequential ingrowth of these pioneer retinal axons indicates that specific chiasmatic cues are expressed early in development and that such pioneer fibers may serve as guides for the later-arriving retinal fibers.

Specific guidance and modulation of growth cone motility during in vivo development

The hemidecussation of retinal fibers that occurs in mammals offers the opportunity to study several aspects of growth cone guidance in a single model system. Recent studies suggest that growth cones of crossed and uncrossed retinal fibers respond in differential manners when they contact cells at the optic chiasm midline, and that such contact interactions are the main event involved in their divergence. Observations of the in situ behaviors of these growth cones disclose that their guidance in this decision region involves two different processes: an orientational response, mediated by the selective guidance of the filopodia of frowth cones away or towards the midline of the optic chiasm, and a dynamic response, in which growth cones go through cycles of advance and pauses while in the optic chiasm. We hypothesize that these two aspects of growth cone motility represent two different aspects of the biology of growth cones in response to extrinsic cues, which are both used in their guidance during development.

Lipocortin 1 immunoreactivity identifies microglia in adult rat brain

Immunohistochemical localization of two Ca(++)-binding proteins, Lipocortin 1 (LC1) and S100-beta, demonstrates two distinct classes of primitive glia in the floor plate of rat embryos. With proper fixation (formalin-lysine-periodate-acetic acid), dendritic glia in the CNS of adult rats also apparently stain for either LC1 or S100-beta in the ratio of 1:3. In order to further distinguish and identify these two glial classes, we have examined their population density, topography, and responses to localized neuron death. Neurons of the ipsilateral thalamus undergo apoptosis following cortical ablation; the contralateral thalamus serves as control. By eight days post-lesion, the number of LC1 cells in the ipsilateral thalamus has increased > 4-fold, the increase comprising primarily activated phagocytes adjacent to degenerating neurons. The S100-beta glia in the same region are virtual- ly indistinguishable from control; but background staining (apparently representing extra-cellular S100-beta) is increased. Thus, the responses of dendritic LC1 glia resemble these previously described for microglia and are quite different from the astrocytes identified by S100-beta immunoreactivity. Both dendritic and activated forms of LC1 glia stain with the microglial marker, Griffonia simplicifolia iso-lectin B4. However, before the correspondence of LC1 glia and microglia can be confirmed, two anomalies require resolution: (1) the LC1 glia are greater in number and more evenly distributed than microglia marked with other methods; (2) the dendritic LC1 glia apparently are progeny of primitive glia that form the midline raphe of the embryonic floor plate. The participation of LC1 glia in the removal of CNS debris supports the hypothesis that LC1 plays anti-inflammatory and/or immunosuppressive roles in phagocytes.