Endothelial cell proliferation in the choriocapillaris during human retinal differentiation

BACKGROUND

Differentiation patterns of the neural retina and its retinal vasculature are not well matched. The foveal region differentiates first, however the central retina is not vascularised until late in gestation. The authors explored the hypothesis that higher rates of endothelial cell proliferation in the choriocapillaris of the central retina might compensate for the slow growth of central retinal vessels, providing supplementary nutrients to the region during the early stages of neuronal maturation.

METHODS

Frozen sections of five human fetal eyes (14-18.5 weeks' gestation), were examined for Ki-67 and CD34 immunoreactivity using confocal microscopy. Measurements of choriocapillaris area and the number of proliferating choroidal endothelial cells were used to calculate the rate of choroidal endothelial proliferation at five different chorioretinal locations.

RESULTS

The choriocapillaris area is consistently greater in the foveal region than at other locations and increases progressively with age. A higher rate of endothelial cell proliferation was found in parts of the choriocapillaris associated with the undifferentiated (proliferating) neural retina, compared with the differentiated, central region.

CONCLUSION

The findings suggest that mechanisms regulating proliferation and growth of the choroidal vasculature are independent of differentiation in the neural retina, and are thus profoundly different from mechanisms that regulate formation of the retinal vasculature.

Muller cell expression of glutamate cycle related proteins and anti-apoptotic proteins in early human retinal development

AIMS

The distribution of glutamate cycle related proteins (glutamine synthetase (GS) and GLAST) and anti-apoptotic proteins (Bcl-2 and Bcl-X) was investigated in Müller cells during early human retinal development, relative to the onset of expression of synaptophysin, a presynaptic vesicle protein.

METHODS

Using frozen sections of human fetal eyes (13-22 weeks gestation) (n = 10), Bcl-2, Bcl-X, GS, GLAST, and synaptophysin immunoreactivities (IR) were imaged using fluorescence microscopy and plotted as a function of eccentricity from the incipient fovea. Frozen sections of adult human retina (n = 4) were immunolabelled with antibodies to Bcl-2 and Bcl-X.

RESULTS

Müller cell immunoreactivity for GS, GLAST, and Bcl-2 was initially detected in the incipient fovea, and then at more peripheral locations with increasing age. Synaptophysin-IR appeared earlier than all other target proteins. Within the synaptophysin-IR region, mature (differentiated) Müller cells expressed both Bcl-2 and Bcl-X-IR from 13 weeks gestation, ahead of GS-IR and GLAST-IR that were first seen at 14 weeks gestation. Additionally, from as early as 13 weeks gestation, ganglion cells and immature neuronal progenitor cells across the entire retina expressed Bcl-2-IR and Bcl-X-IR, respectively. In adult retina, ganglion cells and some bipolar cells expressed Bcl-X but not Bcl-2.

CONCLUSION

Müller cells express Bcl-2 and Bcl-X after synaptogenesis has commenced, but before the onset of GS and GLAST expression, suggesting a protective role for these proteins in Müller cells during the onset of glutamatergic transmission in early human retinal development.

Development of the primate retinal vasculature

Human and macaque retinae have similar retinal vascular anatomy. The general features of the retinal vascular anatomy of these two primates have much in common with more widely studied animal models such as rat and cat. However, primates are unique amongst mammals in having a region in temporal retina specialized for high visual acuity, which includes the fovea centralis (or 'fovea'). Several features distinguish the fovea from other parts of the retina, including a very high local density of cone photoreceptors, a high density of inner retinal cells during development, and an absence of retinal blood vessels. The retinal vascular complex comprises a number of cell types, in addition to vascular endothelial cells, including pericytes, microglia, astrocytes-none of which is intrinsic to the retina. In addition, amacrine-like cells make bouton-like associations with retinal vessels and may be involved in the autoregulation of blood flow. During development endothelial cells 'invade' the retina, accompanied by a population of microglial cells; glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes are also seen associated with the developing vasculature, and are in advance of the vascular front by a few hundred microns. Recent findings indicate that astrocytes at the vascular front proliferate in response to factors released by endothelial cells, including leukemia inhibitory factor. Better understood is the role of GFAP-immunoreactive astrocytes just in advance of the developing vessels. These astrocytes are sensitive to hypoxia and in response release vascular endothelial growth factor (VEGF) which in turn promotes the migration, differentiation and proliferation of vascular endothelial cells. This hypoxia/VEGF-mediated process of migration, proliferation and differentiation appears common to the retinae of a variety of species, including human. However, in human and macaque retina, different mechanisms appear to govern the development of the retinal vessels growing along the horizontal meridian of the retina towards the central area, which contains the fovea. Despite the relatively advanced state of differentiation and maturation of cells in the central area compared with the periphery, the growth of retinal vessels into the central area has been described as 'retarded', and the incidence of cell proliferation associated with these vessels is lower than in peripheral vessels. Furthermore, neither retinal vessels nor their accompanying astrocytes grow into a circumscribed region which, at a later stage, develops into the foveal depression. These observations suggest that molecular markers define the foveal region and inhibit cell proliferation and vascular growth at the fovea and, perhaps, along the horizontal meridian. The findings also suggest that at the fovea, the retina is adapted morphologically to its blood supply, since in the vicinity of the fovea, the development of retinal vessels is retarded or inhibited. The limitations on vascularization of central retina has implications for its vulnerability to degenerative changes, as seen in age-related macular degeneration.

Immunological and aetiological aspects of macular degeneration

Aetiological and immunological aspects of AMD, a leading cause of blindness in Western countries, have been reviewed. Developmental studies suggest that anatomical features unique to the fovea result in a critical relationship between metabolic demand and blood supply at the macula, which is maintained throughout life. Recent studies show a sufficient degree of consistency in the link between smoking and both dry and wet AMD to regard it as causative. Dry AMD is considered to be the natural endstage of the disease; epidemiological and morphological studies point to choroidal vascular atrophy as the causative event and it is suggested that signals associated with acute vascular compromise lead to the development of subretinal neovascularisation. The relationship between sub-pigment epithelial deposits, including basal laminar deposit, and the pathogenesis of AMD is examined. Much of the literature is consistent with a choroidal origin for the constituents of drusen. The blood-retinal barrier preserves the physiological environment of the neural retina and limits inflammatory responses. The factors, including cytokines, adhesion molecules and the presence of resident immunocompetent cells (microglia), which determine the immune status of the retina are considered. Historical descriptions of the involvement of inflammatory cells are provided, evidence implicating inflammation in the pathogenesis of AMD involving macrophages, giant cells and microglia has been derived from observations of human and animal subretinal neovascular lesions. The role of humoral factors such as anti-retinal autoantibodies and acute phase proteins together with clinical observations has been surveyed. Taken together these data demonstrate the involvement of both cellular and humoral immunity in the pathogenesis of AMD. It remains to be determined to what degree the influence of immunity is causative or contributory in both wet and dry AMD, however, the use of anti-inflammatory agents to ameliorate the condition further indicates the existence of an inflammatory component.

Astrocytes and blood vessels define the foveal rim during primate retinal development

PURPOSE

To investigate the relationship between development of the perifoveal blood vessels and formation of the foveal depression.

METHODS

Retinal sections and flatmounts from monkeys aged between fetal day (Fd)80 and 2 years of age were double labeled using antisera to CD31 or von Willebrand factor to detect vascular endothelial cells and antiserum to glial fibrillary acidic protein to detect astrocytes. Sections were studied by fluorescence or confocal microscopy.

RESULTS

From Fd88 to 115, vessels on the horizontal meridian were found only at the level of the ganglion cell layer (GCL)-inner plexiform layer (IPL) border where they form the ganglion cell layer plexus (GCP). Stellate astrocytes accompany GCP vessels and extend closer to the fovea than vessels. The foveal avascular zone was present within the GCP at Fd101, and at Fd105 a shallow foveal depression encircled by the GCP was present. The GCP foveal margin had the same dimensions as the adult foveal pit. Both blood vessels and astrocytes were excluded from the emerging fovea throughout development. After Fd140, capillary plexuses in the outer retina anastomosed with the GCP on the foveal slope to form a perifoveal plexus, but this plexus did not mature until a month or more after birth. After Fd142, astrocytes rapidly disappeared from the GCP and most of central retina.

CONCLUSIONS

An avascular area is outlined by the GCP before the foveal pit begins to form, suggesting that molecular factors in this region exclude both vessels and astrocytes. These factors may also guide neuronal migration to form the pit. Because the perifoveal plexus is formed during late gestation, both capillary growth and foveal development may be affected adversely by prematurity.

Triamcinolone acetonide modulates permeability and intercellular adhesion molecule-1 (ICAM-1) expression of the ECV304 cell line: implications for macular degeneration

Whilst animal studies and a pilot clinical trial suggest that intravitreal triamcinolone acetonide (TA) may be useful in the treatment of age-related macular degeneration (AMD), its mode of action remains to be fully elucidated. The present study has investigated the capacity of TA to modulate the expression of adhesion molecules and permeability using a human epithelial cell line (ECV304) as a model of the outer blood-retinal barrier (BRB). The influence of TA on the expression of ICAM-1 and MHC-I was studied on resting and phorbol myristate acetate (PMA)- or interferon-gamma (IFN-gamma)- and/or tumour necrosis factor-alpha (TNF-alpha)-activated cells using flow cytometry and immunocytochemistry. Additionally, ECV304 cells were grown to confluence in uncoated Transwell chambers; transepithelial resistance (TER) across resting and PMA-activated cells was monitored. TA significantly decreased the paracellular permeability of ECV304 cells and down-regulated ICAM-1 expression, consistent with immunocytochemical observations. PMA-induced permeability changes were dose-dependent and TA decreased permeability of both resting and PMA-activated monolayers. MHC-I expression by ECV304 cells however, was not significantly affected by TA treatment. The modulation of TER and ICAM-1 expression in vitro correlate with clinical observations, suggesting re-establishment of the BRB and down-regulation of inflammatory markers are the principal effects of intravitreal TA in vivo. The results further indicate that TA has the potential to influence cellular permeability, including the barrier function of the retinal pigment epithelium (RPE) in AMD-affected retinae.

The human hyaloid system: cell death and vascular regression

The present study had investigated the roles of apoptosis and necrosis in the regression of the human fetal hyaloid vasculature. Normal human fetal hyaloid specimens (n = 67) ranging from 10 to 20 weeks' gestation were studied. Specimens were either immunolabeled with anti-von Willebrand factor and major histocompatibility complex class I antibodies or investigated using the terminal-deoxyribonucleotidyl transferase-mediated dUTP-biotin DNA nick-end labeling technique. A fluorescent DNA-binding dye acridine orange/ethidium bromide mixture was also applied to unfixed flat mounts of hyaloid vasculature and some specimens were processed for transmission electron microscopy. Vascular regression including cell loss in the connecting vessels, stretching and thinning of the vasa hyaloidea propria, tunica vasculosa lentis and the pupillary membrane was clearly evident after 13 weeks' gestation. Cresyl violet staining revealed condensed cells and pyknotic bodies throughout the hyaloid system; cell death occurred either in single cells or along small capillary segments associated with vascular regression. Acridine orange/ethidium bromide staining showed DNA condensation at early and late stages of cell death. Similarly, DNA nick-end labeling was positive in endothelial cells, pericytes and vessel and non-vessel associated hyalocytes. The observation of hyalocytes juxtaposed to cytolysed endothelial cells may indicate a role for these cells in vascular regression. Features of apoptosis were more evident during early vascular regression whilst necrosis was increasingly evident at later stages.

Astrocyte proliferation during development of the human retinal vasculature

Wholemounts of human fetal retinas were labeled with antibodies to Ki67 or proliferating cell nuclear antigen, to map the distribution of proliferating cells in the developing primary vasculature and neural retina. Double labeling was used to determine the relative proportions of endothelial cells (CD34), astrocytes (glial fibrillary acidic protein - GFAP) and microglia (major histocompatability complex class II) associated with the developing vessels. The differentiated region of neural retina (cold spot) was 3.5 mm(2)at 15 weeks gestation (WG), centred on the incipient fovea, and increased in size with age to 80.5 mm(2)by 23-24 WG. Ki67 immunoreactive cells were distributed throughout the developing vasculature at all ages. The mean density of dividing cells in the neural retina increased with gestational age from 146 mm(-2)at 15 WG, to 624 mm(-2)at 23-24 WG. By 20 WG proliferation in the vasculature overlapped the margins of the cold spot, which was almost completely vascularized by 23-24 WG, except for a narrow strip on the horizontal meridian, which included the incipient fovea. Counts of CD34/Ki67 immunoreactive cells indicated that 15-52% of proliferations in the developing vasculature at 18 WG are endothelial cells. In contrast, in the fellow retina 65-85% cells were Ki67/GFAP immunoreactive, indicating proliferation of astrocytes in situ. No dividing microglia were observed. The findings suggest that large numbers of proliferating astrocytes accompany the developing vessels as they migrate across the primate retina.

Apoptosis during development of the human retina: relationship to foveal development and retinal synaptogenesis

Apoptosis in the ganglion cell (GCL) and inner nuclear (INL) layers of human fetal retinae aged 14-35 weeks of gestation (WG) was investigated in relation to synaptogenesis and foveal depression formation. Terminal transferase dUTP-biotin nick end labeling (TUNEL) was used to identify apoptosis, and synapse development was demonstrated by synaptophysin immunoreactivity (-IR). The distribution of apoptotic cells and synaptophysin-IR was studied as a function of eccentricity. Between 14 and 23-24 WG in the GCL, rates of apoptosis were relatively low in central retina. A shallow fovea was detected at 23-24 WG. In the central GCL, the rate of apoptosis was 0.21% of viable cells compared with a higher incidence of 0.79-1.64% peripherally. Apoptosis in the INL was 2-8 times greater than that in the GCL. At 14-15 WG, peak death occurred at the incipient fovea; however, by 20 WG the distribution was bimodal, with peaks at more eccentric locations on either side of the incipient fovea with increasing age. Approximately 90% of INL apoptotic cells were in the middle and outer regions, suggesting that bipolar cells formed the majority of dying neurons. Synaptophysin-IR was present in cones, bipolar cells, and processes in the inner and outer plexiform layers at the incipient fovea at 14 WG and spread peripherally with increasing age. The peripheral margin of synaptophysin-IR coincided with areas of peak INL apoptosis. This pattern suggests that bipolar cell elimination is associated with the onset of synaptogenesis. Apoptosis in the GCL and INL is not a significant factor in foveal depression morphogenesis.

The human hyaloid system: cellular phenotypes and inter-relationships

We have investigated the expression of leucocyte markers, phenotypic characteristics and cellular relationships of the normal human fetal hyaloid vasculature using immunohistochemistry, light and electron microscopy. Antibodies against von Willebrand Factor, alpha-smooth muscle actin, glial fibrillary acidic protein, vimentin, major histocompatibility complex classes-I and -II, CD45 (leucocyte-common antigen) and calcitonin gene-related peptide were used to identify the cellular constituents of the hyaloid vasculature in whole mounts. Additional morphological features were described at the ultrastructural level. Endothelial cells throughout the hyaloid system were immunoreactive to von Willebrand Factor and major histocompatibility complex class-I antibodies. Pericytes were immunoreactive to alpha-smooth muscle actin antibody; labeled cells were distributed along large branches of the hyaloid artery, vasa hyaloidea propria, tunica vasculosa lentis and pupillary membrane but no immunoreactivity was detected on small connecting capillaries. Vessel and non-vessel-associated hyalocytes on the hyaloid artery, vasa hyaloidea propria, tunica vasculosa lentis, pupillary membrane and vitreous were immunoreactive to major histocompatibility complex classes-I and -II, CD45 and calcitonin gene-related peptide antibodies. Anti-glial fibrillary acidic protein reactivity was detected on Bergmeister's papilla but not on the hyaloid artery. Cells immunoreactive for vimentin were present throughout the hyaloid vasculature including small connecting capillaries. Ultrastructural observations of the hyaloid vasculature revealed junctional complexes, including zonulae adherens, macula adherens and possible zonulae occludens, between adjacent endothelial cells. Fenestrae were not observed in the gestational ages included in the present study. The use of whole mounts in conjunction with specific antisera has provided novel immunohistochemical definitions of the structure and cellular constituents of the human hyaloid. The results indicate that hyalocytes are a heterogeneous population of leucocyte-lineage cells.

Isolation, culture and characteristics of human foetal and adult retinal pigment epithelium

BACKGROUND

Current methods for the isolation and culture of adult retinal pigment epithelium (RPE) provide successful primary culture. However, most methods result in contamination with other cell types and low cell yields. The isolation and culture of human foetal RPE presents further problems associated with the limited size of the eye cup and adherence among cells. Reliable methods are necessary for the culture of human RPE and subsequent functional studies.

METHODS

The present procedure is based on mechanical peeling of the whole RPE layer under the dissecting microscope. Dissected pieces are subsequently explanted to a 35 mm culture dish and are cultured with Dulbecco's modified Eagle's medium containing 10% foetal bovine serum. Peroxidase immunohistochemical methods were used to investigate cell phenotypes.

RESULTS

Primary cultures were obtained within 10-14 days with high yields, good viability and purity in subsequent culture. Cultured cells were vimentin and cytokeratin positive and CD31 negative.

CONCLUSIONS

This mechanical dissection technique is recommended for the isolation of foetal and young adult RPE cells, while the enzyme digestion method is preferred for aged adult tissue.

Modulation of the resistance of a human endothelial cell line by human retinal glia

BACKGROUND

Human umbilical endothelial cells were used to model the vascular component of the blood-retinal barriers and to examine the capacity of glial cultures to modulate endothelial cell resistivity in vitro.

METHODS

Endothelial cell resistivity was monitored with and without cocultured human retinal glia. Immunohistochemistry indicated that both macroglia and microglia were present in one culture, while only macroglia were detectable in the second culture.

RESULTS

Both cocultures produced increased resistivity in the target endothelial cells; however, a further significant increase in resistivity was noted with the glial coculture containing microglia. The results suggest that the presence of microglia significantly increases the capacity of astrocytes and Müller cells to modulate endothelial cell resistivity.

Ontogeny of the primate fovea: a central issue in retinal development

The formation of the primate fovea has fascinated a substantial number of histologists, pathologists, ophthalmologists and physiologists for more than a century. In this article, using data from the literature as well as our own observations, we identify events which we believe are crucial in this process and present a developmental neurobiologist's view of the formation of the primate fovea. The fovea is a region of the retina specialized for diurnal, high acuity functions which require a high spatial density of cone photoreceptors as well as a large number of inner retinal cells in order to establish the distinct retinofugal pathways (ganglion cell axons) receiving from individual cones in the foveal cone mosaic. A unique feature of the fovea is the displacement of cells connected to the foveal cones onto the rim of the fovea. It is generally believed that this displacement counteracts the problems caused by the scattering of the incoming light by cells and blood vessels of the inner retina. We believe that one of the crucial events in the formation of the primate fovea is the early centripetal migration of photoreceptors towards the central area (centripetal displacement). This process, initiated early in development, continues throughout intrauterine life until some months or years postnatal. We propose that the displacement of cells from the inner layers is related to the earlier developmental accumulation of photoreceptors and inner retinal cells centrally. This, we propose, leads to metabolic "starvation" of the inner retina, resulting from the complete absence of retinal vessels from the vicinity of the incipient fovea. It is suggested that these factors in turn trigger centrifugal displacement of inner retinal cells towards the encroaching perifoveal capillary network and lead to the formation of the foveal depression.

Development of the human retinal vasculature: cellular relations and VEGF expression

We have investigated the relationships of the cellular constituents of the retinal vasculature--astrocytes, microglia and pericytes--to the differentiating endothelium in human fetal retina. The vascular endothelium was stained using NADPH-diaphorase histochemistry in 12 human fetal retinae ranging in gestational age from 15-22 weeks. Specimens were double labeled using antibodies against glial fibrillary acid protein, alpha smooth muscle actin, or major histocompatibility complex class II antigens to label astrocytes, contractile cells and microglia, respectively. In addition, specimens of 12, 14, 16 and 20 weeks gestation were hybridized in situ for VEGF expression. In retinal wholemounts the vascularized area comprised four lobes that converged on the optic disc. The vascular network was more dense in the temporal lobes than in the nasal lobes, and different growth patterns were evident. Astrocytes were distributed in two layers--one associated with the optic axons and a deeper layer associated with the developing vessels. In retinae younger than 20 weeks, astrocytes in the deep layer were only loosely associated with the developing vessels and extended as far as 150 microns ahead of the most peripheral vessels. A closer register between retinal vessels and the distribution of astrocytes was evident in the nasal region of retinas older than 20 weeks. In situ hybridization demonstrated expression of VEGF mRNA in the vascular layer, superficial to the ganglion cell layer, at the margins of the vascularized zone. Differences were evident in the density of astrocyte coverage of developing vessels and in the extent of VEGF expression in different regions of the retina: the relationship of these differences to differentiation gradients in the neural retina is discussed. Intensely immunoreactive microglia were observed in the vascular layer, associated with the vascular endothelium as far as the most peripheral loops, but not beyond. Alpha smooth muscle actin-containing cells covered the proximal parts of large arteries, but not corresponding veins; they were absent from arterial side-arm branches, as well as the newly formed and small diameter vessels in the age range studies. The results suggest that microglia, contractile cells and astrocytes have distinct temporo-spatial relationships to the differentiating vascular endothelium in human retinas and that VEGF expression at the vascular front, presumably by astrocytes, is associated with the spread of the retinal vasculature, as described in other species.

Modulation of major histocompatibility complex class II expression in retinas with age-related macular degeneration

PURPOSE

To investigate antigenic and morphologic features of microglial and vascular elements in the neural retina associated with age-related macular degeneration (ARMD) compared with those features in age-matched normal and young adult retinas.

METHODS

Adult eyes (n = 97) were classified histopathologically into normal and ARMD-associated groups. Peroxidase imunohistochemical examination of retinal flatmounts was used to visualize major histocompatibility complex class II (MHC-II) immunoreactivity; the intensity and distribution of labeling were quantified by image analysis. In histochemical investigation, reduced nicotinamide-adenine dinucleotide phosphate diaphorase and glial fibrillary acidic protein or MHC-II double labeling were used to detect vascular changes in some preparations.

RESULTS

An increase in the proportion of the retina (percentage of total area) expressing MHC-II immunoreactivity was observed in age-matched retinas compared with that seen in normal retinas. A significant increase (P < 0.05) in the percentage of area immunoreactive for MHC-II was observed, primarily on vascular elements, in retinas with incipient ARMD compared with the area affected in the age-matched group. Increased MHC-II immunoreactivity on vessels in the normal-aged group observed with confocal microscopy was associated with irregularities in the organization of astrocytes. Hypertrophy of retinal microglia was also apparent, although the intensity of microglial MHC-II immunoreactivity was not significantly different between groups.

CONCLUSIONS

The results indicate that an increase in MHC-II immunoreactivity on retinal vascular elements is associated with normal aging. A further increase in MHC-II immunoreactivity on vascular elements and morphologic changes in microglia was associated with incipient ARMD. Immunologic responses in neural retinal microglia and vascular elements appear to be related to early pathogenetic changes in retinal pigment epithelium pigmentation and drusen formation.

Development of microglial topography in human retina

The development of microglial topography in wholemounts of human retina has been examined in the age range 10-25 weeks gestation (WG) using histochemistry and immunohistochemistry for CD45 and major histocompatibility complex class II antigens. Microglia were present in three planes corresponding to the developing nerve fibre layer/ganglion cell layer, the inner plexiform layer and the outer plexiform layer. Distribution patterns of cells through the retinal thickness and across the retinal surface area varied with gestational age. Microglia were elongated in superficial retina, large and ramified in the middle plane, and small, rounded and less ramified in deep retina. Intensely labeled, rounded profiles seen at the pars caeca of the ciliary processes, the retinal margin and at the optic disc may represent precursors of some retinal microglia. At 10 WG, the highest densities of microglia were present in middle and deep retina in the far periphery and at the retinal margin, with few superficial microglia evident centrally at the optic disc. At 14 WG, high densities of microglia were apparent superficially at the optic disc; microglia of middle and deep retina were distributed at more central locations although continuing to concentrate in the retinal periphery. Microglia appear to migrate into the developing human retina from two mains sources, the retinal margin and the optic disc, most likely originating from the blood vessels of the ciliary body and iris, and the retinal vasculature, respectively. The data suggest that the development of microglial topography occurs in two phases, an early phase occurring prior to vascularization, and a late phase associated with the development of the retinal vasculature.

Ontogeny and cellular expression of MHC and leucocyte antigens in human retina

We have investigated the ontogeny of MHC class I, class II, CD45, and macrophage antigens in whole mounts of normal human fetal retina at 10-25 weeks gestation (WG) using monoclonal antibodies and immunogold histochemistry. MHC class I antigens were expressed on retinal vascular endothelial cells and provided a useful marker of vessel organization from 14-25 WG. Microglial cells expressed immunoreactivity to MHC class I, class II, and CD45 antigens from 10 WG (pre-vascularization) and macrophage S22 (Mac S22) antigen from 14 WG (post-vascularization), although none of the antigens tested were detected on neuronal or macroglial elements. Microglia expressing MHC, CD45, and macrophage antigens occurred in both ramified and rounded forms with no close correlation being observed between morphology and antigenicity. The numbers of immunoreactive cells labeled with each of the four markers increased steadily throughout gestation in all specimens studied. Equivalent numbers of microglia expressed MHC class I, class II, and CD45 antigens in retinae at similar gestational ages; however, our data indicate that microglia expressing Mac S22 antigen comprise approximately 40% or less of the population of MHC and CD45-immunoreactive cells during development. Topographical analyses suggest that MHC class I, class II, and CD45-positive microglia enter the retina from both the peripheral retinal margin and the optic disc from at least 10 WG; Mac S22-positive cells appear in association with the development of the retinal vasculature and enter the retina via the optic disc after 14 WG.

Human retinal microglia: expression of immune markers and relationship to the glia limitans

The immunoreactivity, morphology and relationship to the glia limitans of microglia were investigated in flatmounts and sections of normal human retina, using immunogold histochemistry, electron microscopy (EM), and antibodies directed against CD45, major histocompatability complex class I (MHC-I), MHC-II, and human macrophage antigens. Immunoreactivity was evident for all antibodies tested, including MHC-I, which labeled both microglia and retinal vascular endothelium. Most consistent labeling was obtained using antibodies to CD45, MHC-II, and anti-human macrophage (S22) antigen. Immunoreactive cells were seen in the perivascular space (perivascular cells), where they were closely adherent to the vessel profile, and in the retinal parenchyma (microglia). Some parenchymal microglia were also vessel associated and by EM were seen to be closely related to the glia limitans (paravascular microglia). Paravascular microglia were shown by optical densitometry, to express higher levels of MHC antigens than neighboring, non-vessel associated, parenchymal microglia. In addition, paravascular microglia were macrophage (S22) antigen positive, while other parenchymal microglia did not express macrophage antigens. Quantitative data indicate that similar populations of microglia are immunoreactive to CD45, MHC-I, and MHC-II, while relatively few microglia (approximately 10%) are immunoreactive for human macrophage (S22) antigens, supporting previous suggestions that microglia are a heterogeneous population.

Immunohistochemical and topographic studies of dendritic cells and macrophages in human fetal cornea

PURPOSE

To investigate the distribution and phenotype of major histocompatibility complex (MHC) class II-positive dendritic cells and macrophages in normal human fetal cornea in the age range 10 to 25 weeks gestation.

METHODS

Peroxidase and gold immunohistochemistry were used to visualize MHC class II and macrophage antigen (S22) immunoreactive cells. Cell distributions were analyzed quantitatively, and topographic maps were produced.

RESULTS

Immunoreactive cells, concentrated centrally, were present at 10 weeks gestation in the corneal epithelium and stroma. Average densities increased steadily up to 25 weeks gestation. Two morphologic forms of MHC class II and S22 immunoreactive cells were observed--large, dendritiform cells and small, rounded cells with short processes. Electron microscopy revealed that most MHC class II-positive cells were morphologically consistent with previous ultrastructural descriptions of corneal Langerhans cells. Immunoreactive cells were more numerous in immunogold-labeled specimens than in peroxidase-labeled specimens of similar ages. However, quantitative analysis of both techniques revealed that S22-positive cells comprised 30% to 50% of MHC class II-positive cells.

CONCLUSIONS

This study provides a detailed description of heterogeneous populations of MHC class II and S22 immunoreactive cells in the human fetal cornea. In contrast to the adult cornea, which is typically devoid of MHC class II-positive cells, immunoreactive cells in the fetal cornea are concentrated centrally and increase in density up to at least 25 weeks gestation. These results indicate that reduction in Langerhans cell numbers to adult levels must occur after 25 weeks gestation. The presence of dendritic cells and macrophages in the fetal cornea has important implications for the understanding of corneal immunology.

Modulation of MHC class II expression in the absence of lymphocytic infiltrates in Alzheimer's retinae

This study describes the expression of MHC class II antigens in retinal flat mounts from normal donors and patients with Alzheimer's disease (AD). We confirm previous observations of MHC class II immunoreactivity on microglia in normal retinae, while observing insignificant levels of reactivity on endothelial cells (EC). A significantly increased level of MHC class II expression was detected in AD retinae. This increased immunoreactivity was found to occur in the absence of lymphocytic infiltrates, suggesting that the pathogenesis of AD in the retina may be distinct from that reported to occur in some regions of the brain. MHC class II expression, measured using computerized optical densitometry, appeared to be increased principally as a result of induced MHC class II immunoreactivity on EC. Ramified microglia and perivascular macrophages, although hypertrophied, appeared to show unchanged levels of MHC class II expression. These findings are consistent with earlier suggestions that both aberrant MHC class II expression and suppressor activity of resident macrophages may restrict immune responses.

NADPH-diaphorase histochemistry reveals cone distributions in adult human retinae

NADPH-diaphorase histochemistry was used to identify and analyse the topography of cones in adult human retinae (in the age range 44 to 68 years). Retinae were treated for NADPH-diaphorase reactivity, present in vascular walls, amacrine cells and cone and rod outer segments; a subpopulation of less intensely reactive profiles, morphologically resembling cones, possibly represents the blue cone population. Regularly spaced pairs of cones (one intensely labelled and one weakly labelled cone) were also apparent throughout the retina, and were most common along the horizontal meridian, particularly towards the periphery. The diameters and distributions of labelled cone outer segments were assessed using image analysis. Cone density in the adult retina ranged from 2000 per mm2 in the temporal periphery, to 82,000 to 120,000 per mm2 at the fovea centralis. Distribution patterns confirmed the presence of a cone streak, extending from the foveal region into nasal retina, but no evidence of superior-inferior asymmetry was detected.

Human retinal microglia express phenotypic characteristics in common with dendritic antigen-presenting cells

Neural tissue has been considered to be immunologically privileged and major histocompatibility complex (MHC) class II antigens not expressed in normal human brain grey matter and retina. In the present study we compare phenotypic characteristics of human retinal microglia and dendritic Langerhans cells, including their morphologies and distribution, MHC class II and CD45 antigen expression and nucleotidase reactivity. Levels of class II expression were measured using optical densitometry in combination with standard immunohistochemical techniques applied to retinal flatmounts. The results indicate that ramified retinal microglia have features in common with dendritic antigen presenting cells of cornea and conjunctivum, including the constitutive expression of MHC class II antigens.

Evidence of photoreceptor migration during early foveal development: a quantitative analysis of human fetal retinae

We have analyzed aspects of photoreceptor topography in wholemounts of human fetal retinae in the age range 13-24 weeks of gestation. Fetal retinae were stained with cresyl violet and the sizes and packing densities of rods and cones analyzed in the conventional manner. Cones and rods were present within a differentiating region, free of mitotic figures and approximately centered on the putative fovea, represented by the foveal cone mosaic. At 13 weeks of gestation the foveal cone mosaic was clearly differentiated, cone nuclei reaching a packing density of 14,200 per mm2; a small number of rods were present in the immediately adjacent region. The packing densities of both rods and cones in these regions gradually increased and the area of the foveal cone mosaic gradually decreased throughout the age range sampled, although individual variations were evident. By 24 weeks of gestation, cone density was approximately 38,000 per mm2 in the foveal cone mosaic. The maximum rod density observed was 59,200 per mm2 in the region surrounding the foveal cone mosaic in a specimen of 20-21 weeks of gestation. In all specimens, maximum cone density occurred within the foveal cone mosaic and gradually declined towards the periphery of the differentiating region; a pronounced inverse relationship between cone soma diameter and packing density was also observed. The evidence strongly suggests that both rods and cones migrate centripetally, that is towards the center of the developing fovea, from early in development, possibly from the time that they first differentiate. The implications of these findings for foveal development are discussed.

Antibodies to human leucocyte antigens indicate subpopulations of microglia in human retina

Monoclonal antibodies to human leucocyte antigens, including anti-CD45 and anti-CD68, have been used to describe microglia in flatmounts of normal adult human retina for the first time. Anti-CD45 (the leucocyte common antigen) intensely labeled large numbers of cells in a regular distribution across the retina; anti-CD68 and anti-macrophage antibodies labeled fewer cells with distinctive morphologies, suggesting the presence of subpopulations of microglia in the human retina expressing leucocyte antigens.

NADPH-diaphorase reactivity in adult and developing cat retinae

We have examined the distribution and size of nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase reactivity in adult and developing cat retinae. From late gestation E (embryonic day) 58 to adulthood, NADPH-diaphorase reactivity was detected in amacrine cells with somata located in the inner nuclear layer (INL) and ganglion cell layer (GCL) and in processes spreading in the middle strata of the inner plexiform layer (IPL). Reactivity was also present in small rounded profiles located in the outer plexiform layer (OPL) and thought to be cone pedicles. The number of NADPH-diaphorase reactive cells present in adult retinae was about 40,000, 75% of these somata were located in the GCL, the remainder in the INL. At birth, however, there was more than double this number of labelled somata (85,000), the total gradually declining to reach adult values by P (postnatal day) 25. This loss of NADPH-diaphorase reactive somata may be partly explained by natural cell death (apoptosis) or by loss of the active diaphorase from the cells. The density distributions of NADPH-diaphorase reactive cells in the INL and GCL of retinal wholemounts reached maxima in regions slightly inferior to the area centralis at all ages studied. The principal topographical difference between adult and developing retinae was that the density gradient of NADPH-diaphorase reactive cells was steeper in adults than at younger ages. During early development, the somal and dendritic field diameters of NADPH-diaphorase reactive cells at the area centralis were about the same size as those in the periphery; by adulthood, cells in the periphery were larger. The change in the somal diameter gradient apparently emerged because of a reduction in somal size of the centrally located cells. The change in the dendritic diameter gradient emerged because of a greater growth of peripheral cells as compared to central cells. We suggest that NADPH-diaphorase may have a role in the formation of synapses in the developing IPL.

A distinctive soma size gradient among catecholaminergic neurones of human retinae

We have examined the soma diameters and distribution of catecholaminergic (CA) cells in human retinae, by using an antibody to tyrosine hydroxylase (TH), the rate limiting enzyme in the production of catecholamines. TH-immunoreactivity was detected in two classes of cells (CA1 and CA2 cells). CA1 cells had relatively large somata (mean diameter 14 microns) located in either the inner nuclear layer (INL) or in the ganglion cell layer and extensive dendrites spreading into the other strata of the inner plexiform layer (IPL). CA2 cells had smaller, weakly labelled somata (mean diameter 9.6 microns) located principally in the inner regions of the INL and weakly labelled dendrites extending into the IPL. The mean density of CA2 cells in the far retinal periphery was approximately 38/mm2. The number of CA1 cells averaged approximately 15,600 per retina, with a mean density of 16/mm2. The density distribution of CA1 cells closely paralleled the distribution of ganglion cells, their density peaking at the foveal rim, with an area of relatively high density extending horizontally from the macula region toward the nasal margin (along the visual streak). A distinctive gradient was detected among the soma diameters of CA1 cells: they were largest in the mid-periphery, in a visual streak-like configuration around the optic disk. This gradient of soma size among CA cells closely corresponds to the density distribution of the rod photoreceptors in human retinae.

NADPH-diaphorase neurones of human retinae have a uniform topographical distribution

We have examined the morphology and distribution of neurones that contain nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase in human retinae. NADPH-diaphorase reactivity was observed in three different classes of amacrine cells (ND1, ND2, ND3 cells) and in the cone photoreceptors. ND1 cells had relatively large somata (mean, 12.3 microns) located in the inner nuclear layer (INL) and in the ganglion cell layer (GCL). Their dendrites were often strongly labeled and spread into either the middle or outer strata of the inner plexiform layer (IPL). The somata of ND2 cells were medium-sized (mean, 8.2 microns) and located in the INL and in the GCL; their dendrites were usually beaded and often spread in either the middle or outer strata of the IPL. ND3 cells had small, round somata (mean, 5.2 microns) located in either the INL or GCL, and were without labeled processes. The total number of NADPH-diaphorase cells (all classes) was estimated at 118,000, with a mean density of about 100/mm2. The most striking feature of NADPH-diaphorase cells in humans was that their distribution was relatively uniform across the retina, with no evidence of a peak in density at the foveal rim.

Angiogenesis in normal human retinal development: the involvement of astrocytes and macrophages

Recent studies have suggested a role for mononuclear phagocytes series (MPS) cells in neovascularisation associated with retinal pathology and experimentally induced subretinal neovascularisation. The present study is concerned with the normal development of the human retinal vasculature. Morphological details are provided of developing vascular structures including the formation of tight junctions and canalisation of angioblast cords. The relationships of astrocytes and pericytes to developing structures and the presence of a perivascular collagenous matrix are described. Ultrastructural and histochemical analyses reveal an association between MPS cells and developing vascular structures. It is suggested that MPS cells may influence angiogenesis in normal retinal development, as well as in retinal pathology.

Early differentiation of ganglion, amacrine, bipolar, and Muller cells in the developing fovea of human retina

We examined the differentiation and maturation of neurons and glia of the inner nuclear layer (INL) and ganglion cell layer (GCL) in the retina of a human fetus of 15 weeks gestation. Serial, ultrathin sections were cut from a resin-embedded specimen from the posterior pole of the retina. The region of the putative fovea was defined by the absence of rod photoreceptors from the outer nuclear layer; only sections through the putative fovea were studied. Cell somata were classified on the basis of morphological criteria and, through the analysis of serial sections, morphological characteristics of the cell processes were established. In the inner plexiform layer (IPL), the types of synapses were analysed. The majority of cells in the INL and GCL were differentiated and could be identified. Ganglion cell somata were observed in the GCL and INL. Of 186 somata analysed in the INL, 66 were Muller cells, 21 amacrine cells, and 2 ganglion cells; a further 7 cells were classified as either amacrine or ganglion. Bipolar cells were thought to comprise the majority of the remaining 90 somata, but these could not be positively identified, as it was not possible to trace bipolar cell axons to their cell bodies deep in the INL. A detailed description of the morphological characteristics of the identified cells and their processes, and of the axonal processes of bipolar cells, is provided. Puncta adherentia and other simple intercellular junctions were commonly seen in the IPL and involved all cell types. Amacrine cell synapses and immature, monad bipolar cell synapses were common within the IPL. Dyad bipolar synapses were uncommon at this stage of development. A possible sequence of synaptogenesis in the IPL is discussed.

Autoantibodies to retinal astrocytes associated with age-related macular degeneration

Sera from 128 patients with age-related macular degeneration (AMD) were examined and profiles of a variety of serum constituents, including immunoglobulins, alpha and beta globulins and autoantibodies, were tabulated. A similar series of tests were carried out on 20 control sera. The results indicate a higher incidence of serum abnormalities, particularly involving alpha-2 globulin, in patients with disturbance of pigmentation of the retinal pigment epithelium (RPE). The sera were further tested for the presence of autoantibodies with specificity for retinal tissue, and five major staining patterns were observed. Many sera produced patterns of labelling on human retina identical to that observed using labelled monoclonal anti-glial fibrillary acid protein (GFAP) antibodies, which are an established marker of retinal astrocytes. Although anti-retinal autoantibodies have been reported in association with a number of ocular pathologies, the observation of anti-astrocyte autoantibodies is new. Astrocytes are involved in the maintenance of the blood-retinal barrier (BRB) and also appear to be the facultative antigen-presenting cells of neural tissue. The present results indicate that the formation of anti-astrocyte autoantibodies may be an early feature of the pathogenesis of AMD.

Somatostatinergic neurones of the developing human and cat retinae

We have examined somatostatin-immunoreactive (S-IR) neurones in developing retinae of the human and cat. At 14 and 16 weeks' gestation (G14 and G16) in the human, S-IR cells were only found close to the putative fovea centralis, but by 18 weeks' gestation (G18), they were located in all retinal regions. By adulthood, the majority of S-IR cells were restricted to inferior retina. In the developing cat retina, two classes of S-IR cells were recognized. S1-IR cells were similar in morphology and distribution to adult cells: they had small round somata which were only found in inferior retina and gave rise to beaded processes which traversed the inner plexiform layer (IPL) and nerve fibre layer (NFL). S2-IR cells had larger somata located in the ganglion cell layer (GCL) and the label was compartmentalized within their cytoplasm. Most S2-IR cells had lost immunoreactivity by P (postnatal day) 25 and may have been alpha-ganglion cells transiently expressing somatostatin in association with their retention of plasticity into postnatal life.

Morphology of intraretinal new vessels in the PETH rat

The mature stages of retinal dystrophy in PETH rats are characterised by loss of the photoreceptor layer and invasion of the retinal pigment epithelium by new capillaries derived from the retinal vessels. The new capillaries are fenestrated where they are adjacent to the basement membrane of the retinal pigment epithelium and are surrounded by cells of the disrupted pigment epithelium, which follow the course of the capillaries into the inner retina. Abnormal basement membrane deposits are common within the retinal pigment epithelium.

Class-specific cell death shapes the distribution and pattern of central projection of cat retinal ganglion cells

The development of the nasotemporal division in cat retina was studied. We find that in the normally pigmented neonatal cat significant numbers of ganglion cells of all types in temporal retina project to the contralateral dorsal lateral geniculate nucleus (LGNd); far fewer cells in temporal retina project contralaterally to the LGNd in the normal adult. Thus, most of these cells must be eliminated during development. Experimental interruption of one optic tract in the neonate results in the retrograde degeneration of the ipsilaterally projecting ganglion cells in the temporal retina ipsilateral to the lesion. Consequent to the loss of the ipsilaterally projecting cells in this hemiretina, many of the ganglion cells projecting to the intact contralateral LGNd, which are normally eliminated, survive. Also, unlike in the normal cat, in which very few of the small ganglion cells in temporal retina project contralaterally to the thalamus, in optic tract sectioned (OTX) cats, significant numbers of the smallest ganglion cells in the temporal retina ipsilateral to the lesion project contralaterally to the intact thalamus. In order to make a quantitative comparison of the distributions of ipsilaterally and contralaterally projecting cells in the temporal retinae of normal cats, OTX cats, and neonatal kittens, it was necessary to determine the position of the vertical meridian in all animals. We defined the vertical meridian as the median edge (Stone, 1966). The median edge was determined from the distribution of the most nasally located, ipsilaterally projecting cells in temporal retina. The results indicate that the angle of the vertical meridian (median edge) with respect to the area centralis and optic disc is specified before birth and does not differ in normal cats, OTX cats, or neonatal kittens. Since the location of the vertical meridian does not change with age in postnatal life and is not affected by optic tract section, corresponding regions of retina in the different groups could be compared. A quantitative analysis of ganglion cell density in the temporal retina contralateral to the section, ipsilateral to the intact hemisphere, indicated that there was a reduction in the population of ipsilaterally projecting ganglion cells that was complementary to the abnormally large number of contralaterally projecting cells surviving in the temporal retina ipsilateral to the lesion.(ABSTRACT TRUNCATED AT 400 WORDS)

Patterns of cell death in the ganglion cell layer of the human fetal retina

The distribution of dying cells in the ganglion cell layer (GCL) of retinae from human fetuses has been analysed. Both whole-mounted and sectioned retinae have been studied. Results suggest that cells are lost from the GCL between weeks 14 and 30 of the gestation period, approximately. This period corresponds to the period during which axons are lost from the developing optic nerve. Cell loss is greatest between weeks 16 and 21 of the gestation period. The pattern of cell loss is nonuniform, and between weeks 16 and 24, the relative frequency of pyknotic cells (pyknotic cells:viable cells) in peripheral retina is considerably higher than in central retina. This pattern of cell loss predominates during the period in which a distinct centroperipheral gradient of cell densities emerges in the GCL of the human fetal retina (between 18 and 23 weeks gestation). It is suggested that the regional loss of ganglion cells may contribute to the formation of the cell density gradient.

Age-related macular degeneration: ultrastructural studies of the relationship of leucocytes to angiogenesis

We describe the ultrastructural features of subretinal neovascularisation associated with the pathogenesis of age-related macular degeneration (AMD). The choroidal origin of new vessels was confirmed, and ultrastructural details are presented. Serial sectioning of new vessels revealed a relationship between leucocytes and neovascular structures. The results are discussed in the context of the previously established role of leucocytes in angiogenesis. Our results provide circumstantial evidence, based on morphological observations, for the involvement of leucocytes in the promotion of neovascular proliferation and exudation from new vessels.

Cell death in the development of the human retina: phagocytosis of pyknotic and apoptotic bodies by retinal cells

Apoptosis is a natural form of cell death and has features in common with aspects of cell deletion observed in the course of human retinal development. In this report, we describe the occurrence of apoptotic cells in various layers of the developing retina. Pyknotic residues were observed within phagosomes of neighbouring retinal cells. Our observations imply that most of the debris resulting from cell death is taken up by adjacent tissue cells rather than by mononuclear phagocyte series cells (macrophages) during early stages of human retinal development.

Visual deprivation in infancy and childhood: clinical aspects

The effects of visual deprivation in 15 patients (14 children and one adult) were studied. The age of onset of deprivation and the significance of final visual outcome are discussed. Results indicate that a critically sensitive period in visual development occurs between approximately four months and three years of age. A period of plasticity, when the effects of deprivation are more responsive to therapy, follows. Age of onset of deprivation was found to be most important, but in addition loss of accommodation in association with deprivation may be a significant factor in the development of amblyopia.

Human fetal optic nerve: overproduction and elimination of retinal axons during development

We have estimated the number of axons in the optic nerves of human fetuses ranging in gestational age from approximately 10 to 33 weeks. At 10-12 weeks of gestation there were an estimated 1.9 million axons in the optic nerve. A peak count of 3.7 million axons was obtained from a specimen of 16-17 weeks gestation. The estimated number of axons then declined, stabilizing at an estimated 1.1 million axons by about week 29 of gestation. This figure is in close agreement with an estimate of 1.1-1.3 million optic axons in the human adult optic nerve. The results indicate that at least 70% of optic axons generated during development of the primary visual pathway are lost during fetal life. Part of this loss probably occurs as a result of the refinement of the terminal distribution of ganglion cell projections within their target nuclei. The significance of the relatively prolonged period of axonal loss is discussed.

Retinal development in humans: the roles of differential growth rates, cell migration and naturally occurring cell death

The distribution of ganglion cells throughout the retinal ganglion cell layer is non-uniform in adult mammals. This paper reviews some of our data describing the development of retinal ganglion cell topography in the human fetus. Results indicated that early in the fetal period the distribution of cells in the ganglion cell layer is almost uniform, but by the end of gestation there is a gradient in cell density of about 10:1 (central:peripheral). Peripheral retina grows more rapidly than the central retina prior to about 23 weeks gestation, but this differential growth rate apparently has little effect on the development of a centro-peripheral density gradient. The gradient appears between about 18 and 30 weeks gestation, and during this period there appears to be a greater rate of cell death in the ganglion cell layer of the peripheral retina. Cell density at the developing fovea is less than the perifoveal cell density at all ages, suggesting that ganglion cells migrate from foveal into perifoveal regions throughout the fetal period.

Development of the human retina: patterns of cell distribution and redistribution in the ganglion cell layer

Neurogenesis in the ventricular layer and the development of cell topography in the ganglion cell layer have been studied in whole-mounts of human fetal retinae. At the end of the embryonic period mitotic figures were seen over the entire outer surface of the retina. By about 14 weeks gestation mitosis had ceased in central retina and differentiation of photoreceptor nuclei was evident within a well-defined area which constituted about 2% of total retina area. This area was approximately centered on the site of the putative fovea, identified by the exclusive development of cone nuclei at that location. The area of retina in which mitosis had ceased increased as gestation progressed. By mid-gestation mitosis in the ventricular layer occupied about 77% of the outer surface of the retina and by about 30 weeks gestation mitosis in the ventricular layer had ceased. Cell density distributions in the ganglion cell layer were nonuniform at all stages studied (14-40 weeks). Densities were highest at about 17 weeks gestation, and by mid-gestation the adult pattern of cell topography was present with maps showing elevated cell densities in posterior retina and along the horizontal meridian. Cell densities generally declined throughout the remainder of the gestation period, except in the posterior retina, where densities in the perifoveal ganglion cell layer remained high during the second half of gestation. There is a rapid decline in cell density in the foveal ganglion cell layer toward the end of gestation, and it is suggested that the persistence of high densities in the perifoveal region may be related to migration of cells away from the developing fovea. The total population of cells in the ganglion cell layer was highest (2.2-2.5 million cells) between about weeks 18 and 30 of gestation. After this the cell population declined rapidly to 1.5-1.7 million cells. It is suggested that naturally occurring neuronal death is largely responsible for this decline.

Ganglion cell topography in human fetal retinae

Whole-mounted human fetal retinae of gestational ages 14-40 weeks have been studied. These preparations clearly show the distribution of retinal ganglion cells or their precursors across the retina, and the pattern of the retinal vessels and vessel primordia. The ganglion cell layer is present at 14 weeks of gestation and distribution of cells in this layer (ganglion cell precursors) is at first uniform. Ganglion cell density gradients that foreshadow those seen in the adult retina become evident by about 20 weeks gestation. Both mature ganglion cells and precursor cells are present in retinae at about 24 weeks and precursor cells are still seen in the peripheral parts of the retina at about 30 weeks of gestation. The development of mature ganglion cells would appear to coincide with the establishment of retinal circulation, and proceeds in centrifugal sequence from central retinal.

The distribution of ipsilaterally and contralaterally projecting ganglion cells in the retina of the pigmented rabbit

The retinal distribution of ipsilaterally and contralaterally projecting ganglion cells has been determined in the rabbit using both degeneration and horseradish peroxidase tracing techniques. Contralaterally projecting ganglion cells are present throughout the retinas, while ipsilaterally projecting ganglion cells are confined to a 3.0-3.5 mm wide strip adjacent to the temporal retinal margin. Thus, in this temporal strip both ipsilaterally and contralaterally projecting cells intermingle, while at more nasal locations all ganglion cells project contralaterally. Each of the contra- and ipsilaterally projecting populations comprises ganglion cells with soma diameters representing the full range present in the rabbit retina. However, a relatively large proportion of the ipsilaterally projecting ganglion cells have large somata (greater than or equal to 20 micrometer). Large ganglion cells are most numerous in the rabbit's temporal retina and have previously been described as reaching their peak density at the large cell node, just above the temporal end of the visual streak (Provis 1979). The large cell node lies immediately temporal to the nasal border of the strip of retina in which ipsilaterally projecting cells are located. It is possible that this specialization in the region of retina which observes the binocular visual field plays a particular role in binocular vision for the rabbit.

The distribution and size of ganglion cells in the regina of the pigmented rabbit: a quantitative analysis

The distribution and soma diameters of retinal ganglion cells have been examined in whole mounted retinae of pigmented rabbits. Maps of the distribution of ganglion cells confirmed several features of earlier descriptions, but generally showed lower density values and yielded lower total ganglion cell counts (250,000-270,000). The maximum ganglion cell density encountered in each retina and its retinal location both varied between rabbits. As previously reported, the dominant feature of the rabbit's retina is a strongly developed visual streak (Hughes, '71) but some evidence of an area centralis-like specialization was found. This appears not as the area of peak gangion cell density, but as a concentration of large (greater than or equal to 20 micron in diameter) ganglion cells at the temporal end of the visual streak, 2-3 mm from the temporal margin of the retina. In one rabbit in which the optic tract was sectioned five months previously, the density distribution of large retinal ganglion cells in the retinae has been mapped. These maps indicate that the nasotemporal division for large ganglion cells in the rabbit retina is approximately centered on the area of their maximum density. It has previously been reported that in the cat the area centralis is characterized by an aggregation of smaller diameter ganglion cells of a particular functional type (Stone, '65, '78). It is possible that areas of retinal which subserve area centralis-like functions are represented not simply by localized increases in ganglion cell density, but by changes in the relative proportions of ganglion cell types, which are reflected in the changing relative densities of ganglion cell soma diameter groups.