Postnatal gliogenesis in the nerve fiber layer of the rabbit retina: an autoradiographic study

Cell Proliferation in Retinal Transplants

The MIB-1 antibody against a nuclear protein Ki-67 was used to study the proliferation of cells in the rabbit retinal transplants. Fragmented pieces of embryonic day 15 rabbit retinas were transplanted into the subretinal space of adult rabbits and allowed to survive for different times. Fragmented donor tissue starts organizing in rosettes 1 day after transplantation. The transplanted cells continue to proliferate in the host eye and their pattern of proliferation resembles that of normal developing retina, suggesting that the factors responsible for the proliferation pattern are preserved after transplantation. The dividing cells in metaphase line up in the luminal layers of the rosettes. Certain cells become postmitotic in the regions corresponding to the inner retina first, followed by the cells in the luminal layers of rosettes. Cells in the regions between the rosettes, corresponding to the inner nuclear layer, presumably the Müller cells, proliferate significantly for the equivalent age of postnatal day 2. Few cells in these regions proliferate for at least the equivalent age of postnatal day 11 in transplants. There is a layer of nonproliferating, degenerating cells in the transplant situated close to the host retina. However, some cells in this layer, situated at the host-graft interface, proliferate. These cells proliferate for a long time possibly indicating gliosis.

Postnatal development of somatostatin 2A (sst2A) receptors expression in the rabbit retina

In the retina, somatostatin (SRIF) acts as a neuromodulator by interacting with specific SRIF subtype (sst) receptors. Aim of this investigation was to determine the cellular localization of the sst2A receptor isoform in the postnatal rabbit retina. Receptor immunoreactivity was localized using the antiserum K-230, directed to the C-terminus of the human sst2A receptor. In the postnatal rabbit retina, sst2A receptors were abundantly expressed without significant regional differences. They were localized predominantly to rod bipolar cells, identified with a protein kinase C (PKC) antibody, to amacrine cells, some of which also containing tyrosine hydroxylase (TH), and to presumed rare horizontal cells. Quantitative analysis showed that sst2A-immunoreactive (-IR) bipolar and amacrine cells reached their maximum density and absolute number at the time of eye opening, when the expression pattern of sst2A receptors was similar to that in adult retinas. In the adult retina, 68% of the PKC-IR rod bipolars and 34% of the TH-IR amacrine cells were observed to also express sst2A receptors. The appearance of sst2A receptor immunolabeling prior to eye opening and the developmental profile of sst2A receptor expression are compatible with a role of SRIF in the maturation of retinal circuitries. The partial expression of sst2A receptors in PKC-IR rod bipolar cells and in TH-IR amacrine cells may suggest some type of heterogeneity within these cell populations.

Glial, vascular, and neuronal cytogenesis in whole-mounted cat retina

A method was developed for detecting cytogenesis in retinal whole-mount preparations by bromodeoxyuridine (BrdU) immunohistochemistry. Because BrdU is a nonspecific marker that labels all cells in the S phase of the cell cycle, it is ideally combined with other cell-specific markers to study the cytogenesis of specific cell types. Double-label protocols to visualize mitotically active astrocytes and cells associated with the forming vasculature have been developed and applied to the retina. This approach revealed that, during normal development of the kitten retina, vascular mitogenesis occurs predominantly in the ganglion cell and nerve fiber layers, where the inner retinal plexus is formed by a process involving transformation of mesenchymal precursor cells and division of vascular endothelial cells. The peak density of vascular mitogenesis moved in a central-to-peripheral manner and was associated with the leading edge of the forming capillary plexus. A small number of dividing vascular endothelial cells was also associated with angiogenesis, the process responsible for the formation of the outer retinal plexus, vessels at the area centralis, and the radial peripapillary capillaries. Cytogenesis associated with astrocytes occurred in the ganglion cell and nerve fiber layers but was apparent predominantly at or close to the optic nerve head. Confirming earlier studies, neuronal mitogenesis was shown to occur predominantly at the ventricular zone, first at the area centralis and spreading peripherally with increasing maturity. A second region of neuronal cytogenesis, at the subventricular zone, was also apparent. Tissue hyperoxia decreased the rate of vasculogenic cell division but had no apparent effect on neurogenic or astrocytic cell division. Four distinct zones of cell generation were therefore identified within the retina, each associated with either glial, vascular, or neuronal cytogenesis. Thus, BrdU immunohistochemistry in whole-mounted retinal preparations offers a fast and reliable alternative to [3H]thymidine autoradiography for the study of the topography of cytogenesis during development.

Developmental expression of protein kinase C immunoreactivity in rod bipolar cells of the rabbit retina

Rod bipolar cells constitute the second-order neuron in the rod pathway. Previous investigations of the rabbit retina have evaluated the development of other components of the rod pathway, namely the dopaminergic and AII amacrine cell populations. To gain further insights into the maturation of this retinal circuitry, we studied the development of rod bipolar cells, identified with antibodies directed to the alpha isoform of protein kinase C (PKC), in the rabbit retina. Lightly immunostained PKC-immunoreactive (IR) somata are first observed at postnatal day (PND) 6 in the distal inner nuclear layer (INI.). Immunostaining is also observed in the outer plexiform layer (OPL), indicating the presence of PKC-IR dendrites. PKC-IR axons are present in the INL oriented toward the inner plexiform layer (IPL). Several of them terminate with enlarged structures resembling growth cones. At PND 8, some immunostained terminal bulbs, characteristic of rod bipolar cells, are detected in the proximal IPL. PKC-IR cells at PND 11 (cye opening) display stronger immunostaining and more mature characteristics than at earlier ages. The dendritic arborizations of these cells in the OPL and their axon terminals in the IPL attain mature morphology at later ages (PND 30 or older). The density of PKC-IR cells shows a peak at PND 11 followed by a drastic decrease up to adulthood. The total number of PKC-IR cells increases from PND 6 to PND 11 and then it remains almost unchanged until adulthood. The mosaic of PKC-IR cells is nonrandom in some retinal locations at PND 6, but the overall regularity index at PND 6 is lower than at older ages. The present data provide a comprehensive evaluation of the development of rod bipolar cells in the postnatal rabbit retina and are consistent with those previously reported for dopaminergic and AII amacrine cell populations, indicating that different components of the rod pathway follow a similar pattern of maturation, presumably allowing the rod pathway to be functional at eye opening.

Differentiation and maturation of rabbit retinal oligodendrocyte precursor cells in vitro

The differentiation of oligodendrocytes from undifferentiated progenitor cells was studied in cultures obtained from the postnatal rabbit retina. 'Sandwich' cultures were established by turning the coverslips with adhering cells up-side down about 24 h after seeding. As a result O4-positive oligodendrocyte progenitors stop dividing and differentiate. Within 6 days in vitro they form extensive membranous sheets and acquire myelin associated glycoprotein (MAG), proteolipid protein (PLP), and myelin basic protein (MBP). O4-/MBP-positive oligodendrocytes and vimentin-positive/GFAP-negative Müller cells (a kind of modified astrocyte type in the retina), which are also present in these cultures, occupy distinct territories in vitro. When oligodendrocyte precursors were seeded onto a preformed Müller cell feeder layer they prefer to settle on the Müller cell free substrate poly-L-lysine, develop numerous processes but no membranous sheets and fail to acquire detectable amounts of MBP. In addition, culturing Müller cells and oligodendrocytes within the same medium, but without direct contact to each other, oligodendrocyte precursor cells fail to express MBP. The Müller cell factor(s) responsible for this interaction remains to be determined.

Growth factor effects on the proliferation of different retinal glial cells in vitro

Vascularized mammalian retinae contain two distinct neuroglial cells types, radially oriented Müller cells and astrocytes, which are located in the nerve fiber layer. These cell types derive from different precursor cells and proliferate during ontogenesis at distinct schedules. The aim of the present study was to disclose whether growth factors, which are known to interfere with the development of neuroglial cells in the central nervous system, like basic and acidic fibroblast growth factor (aFGF and bFGF), epidermal growth factor (EGF) and platelet-derived growth factor, have similar or distinct effects on the proliferative capacity of retinal astrocytes and Müller cells. These questions were tested by applying growth factors to cultured astrocytes and Müller cells from early postnatal rabbit retina. Proliferating cells were identified by double labeling experiments combining cell type specific markers with bromodeoxyuridine immunocytochemistry and [3H]thymidine incorporation experiments, respectively. In addition, we used the anatomical advantage of the rabbit retina. Its peripheral part is astroglial cell-free. Cultures prepared from this part of the retina (P-cultures) contain Müller cells, microglial cells and neurons, while cultures from the 'central part', the medullary rays (MR) region contain, in addition, astrocytes and oligodendrocytes. Our studies show that Müller cell proliferation is stimulated by EGF in a dose dependent manner, while astrocyte proliferation is stimulated by aFGF and bFGF. The proliferation of O4-positive glial precursor cells is stimulated by aFGF, bFGF and platelet-derived growth factor, but not by EGF. Microglial cells, which are a minor population in these cultures, do not respond to either of these factors.

Postnatal development of tyrosine hydroxylase immunoreactive amacrine cells in the rabbit retina: II. Quantiative analysis

Tyrosine hydroxylase (TH)-immunoreactive (IR) amacrine cells of the rabbit retina mature during the first four postnatal weeks, and their cellular development is described in the preceding paper (Casini, G., and N.C. Brecha, J. Comp. Neurol. 326:283-301, 1992). The present investigation is a quantitative analysis of the postnatal development of the TH-IR amacrine cell population. TH-IR amacrine cells gradually increase in size from birth (soma area of 44.7 +/- 12.4 microns2, mean +/- standard deviation) to adulthood (144.2 +/- 28.0 microns2). Cell density slightly increases from postnatal day (PND) 0 (41.9 +/- 9.5 cells/mm2) to PND 6 (47.2 +/- 7.2 cells/mm2), then markedly decreases from PND 6 to adulthood (17.8 +/- 5.3 cells/mm2) as a consequence of retinal growth. TH-IR cell number almost doubles from PND 0 (about 4,100 cells/retina) to adulthood (about 7,850 cells/retina). The increase in the total number of TH-IR amacrine cells can be explained by the generation of new TH-IR cells in the inner nuclear layer, a delay in the expression of the TH phenotype after neurogenesis by cells committed to be dopaminergic, or the acquisition of this dopaminergic phenotype by uncommitted cells. The development of the TH-IR amacrine cell mosaic was assessed by an evaluation of the distribution of nearest neighbor distances of TH-IR cells. There is a poor correlation between this distribution and a theoretical nonrandom distribution before PND 12. After this age, the nearest neighbor distance distribution shifts towards a nonrandom distribution, and is similar to that of the TH-IR amacrine cell population in the adult retina. The establishment of the TH-IR amacrine cell population mosaic is likely to be achieved through different interacting events, including intrinsic (e.g., genetic) factors, environmental influences, and nonuniform retinal growth. Overall, the population parameters analyzed in the present study approach adult values about the time of eye opening (PND 12) and they reach adult values by PND 26.

Postnatal development of tyrosine hydroxylase immunoreactive amacrine cells in the rabbit retina: I. Morphological characterization

The present and accompanying (Casini, G., and N.C. Brecha, J. Comp. Neurol. 326:302-313, 1992) papers investigate the postnatal development of tyrosine hydroxylase (TH)-immunoreactive (IR) amacrine cells in the rabbit retina. This study is focused on a detailed analysis of the patterns of cellular growth and differentiation of TH-IR amacrine cells, which serve as a model to gain insights into the mechanisms underlying developmental changes associated with the maturation of amacrine cells. Faintly staining TH-IR neurons are present in the proximal inner nuclear layer of newborn retinas. They are characterized by a large nucleus and usually a single primary process lacking varicosities. At postnatal day (PND) 6, TH-IR processes display more complex morphological characteristics, including a few varicosities, and second- and third-order ramifications. Growth cones are often seen. At PNDs 10 and 12 (eye opening), TH-IR cells have general morphological characteristics similar to adult TH-IR amacrines. They display 2-5 primary processes, which start forming a complex network of fibers in lamina 1 of the inner plexiform layer (IPL). TH-IR processes are also present in lamina 3 and rarely in lamina 5 of the IPL. Many fibers ending in growth cones are observed. In addition, very rare, thin TH-IR fibers are present in the outer plexiform layer. At PND 19, TH-IR fibers form a complex, dense network in lamina 1 of the IPL, and loose networks in laminae 3 and 5. Growth cones are not observed at this age. At PND 26, a few "ring-like" structures formed by TH-IR fibers in lamina 1 of the IPL are observed for the first time. In adult retinas, the "ring-like" structures are more numerous than at PND 26. A second, rare type of TH-IR cell ("type B") is encountered in all retinal regions beginning at PND 10. These cells are characterized by weak immunostaining and a small soma size. The present findings show that a significant differentiation of TH-IR neurons occurs during the first 10-12 PNDs. Eye opening is an important period for the maturation of TH-IR amacrines and, more generally, for the maturation of the IPL.

Genesis of neurons in the retinal ganglion cell layer of the monkey

We have analyzed the genesis of various neuronal classes and subclasses in the ganglion cell layer of the primate retina. Neurons were classified according to their size and the time of their origin was determined by pulse labeling with 3H-thymidine administered to female monkeys 38 to 70 days pregnant. All offspring were sacrificed postnatally, and their retinas processed for autoradiography. The somata of cells in the retinal ganglion cell layer generated on embryonic day (E) 38 ranged from 9 to 14 microns in diameter. Between E40 and E56, the minimum soma diameter remained around 8-9 microns, while the maximum gradually increased to 22 microns. As a consequence, the means of the distributions of labeled cells also increased with age, from 11.8 microns diameter for cells generated on E38 to 14.6 microns diameter at E56. Over this period the percentage of labeled cells in the 10.5-16.5 microns and greater than 16.5 microns diameter range gradually increased. The proportion of the labeled cells in the less than 10.5 microns diameter range decreased from E38 to E45, but subsequently increased rapidly. At the end of neurogenesis in the retinal ganglion cell layer, around E70, most labeled cells were considerably smaller (7-9 microns) than those generated earlier. Our results indicate that within the ganglion cell layer of the macaque, neurons of small caliber are generated first, followed successively by medium sized cells. Large, putative P alpha cells are generated late. The production between E56 and E70 of cells with the smallest somata suggests that the last-generated neurons in the ganglion cell layer are predominantly displaced amacrine cells. Within the same sector of retina, different classes of neurons in the ganglion cell layer of the rhesus monkey appear to have a sequential schedule of production.

Development of the rabbit retina: II. Müller cells

Müller (glial) cells of the rabbit retina were stained with antibodies against the intermediate filament protein vimentin in retinal wholemounts from various developmental stages. Both the density of stained profiles and the mean diameter of these profiles were measured, with the microscope focus in the inner plexiform layer of the retinae. Within this retinal layer, every Müller cell possesses one stout vitread process; thus counts of the stained profiles allow an estimation of their number. After postnatal day (P) 9, the total number of stained cells was slightly above 4 million per retina; for the adult rabbit retina, this agrees well with earlier data obtained by our group based on another method, as well as with published data from other groups. We suggest that after P 9, only Müller cells are stained, and this population is numerically stable. In contrast, neonatal retinae contained significantly more stained profiles. This indicates that either the total number of Müller cells is reduced by "physiological cell death" or that additional cells are stained neonatally. We discuss why we favour the second possibility. After P 9, two peculiarities occur in the Müller cell population: (1) their density decreases gradually, to a greater extent in the retinal periphery than in the center (i.e., in the "visual streak"), and (2) Müller cell diameters increase, again more in the periphery than in the center. We argue that differential retinal expansion leads to dispersion of the pre-existing cell population and allows for widening of the Müller cell processes. We conclude that Müller cells can be used postnatally in the rabbit retina as "landmarks" of expansion.

Development of the rabbit retina. IV. Tissue tensility and elasticity in dependence on topographic specializations

A method is introduced for the quantification of specific compliance and the elasticity of small pieces of living retinal tissue. These pieces are fixed at their margins by means of tissue glue, and loaded with a small iron spherule the bending force of which can be gradually enhanced by the action of an electromagnet. Retinal bending caused by such calibrated forces is measured by a horizontal light microscope, and used for estimations of specific compliance and elasticity of the tissue. Three different particular regions of the rabbit retina--periphery, visual streak, and (prospective) medullary rays--were tested at several post-natal developmental stages. From very early stages on (day 2 p.p.) up to adulthood the peripheral retina was found to be significantly more tensile than the two other central regions. This can be shown to depend greatly on the thickness of the tissue which is lower in the retinal periphery. During early post-natal development, all retinal regions except the (prospective) medullary rays become thinner. The tensility of the tissue increases, with the exception of the medullary rays which reduce their compliance strongly. In the adult retina, however, the tensility of all retinal regions is reduced as compared with the neonatal tissue. This seems to be caused by a constant gradual increase of the elasticity of the retina during development which, in turn, may be caused by several developmental parameters, e.g. the formation of synapses, the outgrowth of glial side branches ensheathing neighbouring neuronal cells, or a reduction in extracellular clefts. It is proposed that these differences in tensility between different retinal regions, may be the cause for differential retinal expansion driven by the intraocular pressure. Thus, simple mechanical features of the tissue may contribute to the formation of important topographic specializations of the retina, e.g. the visual streak as the site of highest visual acuity.