Distribution of transferrin binding protein immunoreactivity in the chicken central and peripheral nervous systems

Transferrin binding protein (TfBP) is a glycoprotein originally purified from chicken oviduct that exhibits transferrin binding activity. Recent work has shown that TfBP is a post-translationally modified form of the heat shock protein (HSP108), the avian homologue of a glucose regulated protein, GRP94. The function of this protein, however, has not yet been clearly defined. Antiserum to TfBP was found to selectively stain oligodendrocytes of the avian brain. In this study, we further describe these oligodendrocytes and other cell types positive to anti-TfBP in the chick nervous system. In accordance with previous studies, the most prominent cell type that labels with antiserum to TfBP is the oligodendrocyte. At the electron microscopic level, the immunoreactive product is confined to the perinuclear cytoplasm and fine processes of the oligodendrocytes, whereas myelin and axoplasm are devoid of staining. The immunoreactive product is found both in the cytoplasmic matrix and bound to the endoplasmic reticulum and plasma membrane, suggesting that TfBP may have properties of both a soluble and an integral membrane protein. There is great variability in the number of TfBP-oligodendrocytes in different areas of the central nervous system (CNS); large numbers of cells are associated with the white matter regions and are found in the myelinated tracts, whereas few cells are present in the gray matter regions. In the retina, TfBP is localized specifically in the cells that are morphologically oligodendrocytic and is present in the optic nerve fiber layer and the ganglion cell layer. Obvious staining is also seen in the Bergmann glial cells of the cerebellum and in the Schwann cells of the sciatic nerve. Furthermore, the choroid plexus cells similarly exhibit a strong reaction. The association of TfBP in these specific cell types responsible for myelination and sequestering iron and transferrin implies that TfBP may be involved in myelination and iron metabolism of the chick nervous system, perhaps through a role in transferrin concentration in these cells. A putative role of TfBP, as HSP108, is considered. J. Comp. Neurol. 260-271, 1997. © 1997 Wiley-Liss, Inc.     

Oligodendroglia in the avian retina: immunocytochemical demonstration in the adult bird.

Immunohistochemical techniques were used in conjunction with an avian-specific probe for oligodendrocyte (OLG) marker, the antibody for transferrin binding protein (TfBP), to study the characteristics and distribution of OLGs in the retina of chickens and quails. For comparison, other antibodies such as myelin basic protein, Rip, and those for labeling Müller cells and microglia were used. A large population of OLGs was found to be distributed throughout the retina, with the distinct pattern of a central-to-peripheral gradient. It was possible to detect a spectrum of OLG morphology that bore a resemblance to the subtype of the mammalian central nervous system. In addition to these mature OLGs, limited numbers of TfBP-positive (TfBP(+)) cells with the morphology of immature OLGs were found in the immediate vicinity of the optic head. The majority of OLGs appeared in the ganglion cell layer throughout the retina, whereas OLGs in the nerve fiber layer were seen mainly in the central zone of the retina, near the optic nerve head. Double-labeling experiments showed that OLGs were associated with myelin only in the central region, where the majority of retinal OLGs occurred, but not toward the periphery of the retina. The present study is the first comprehensive analysis of the morphological features and spatial distribution of OLGs in the adult avian retina and provides in vivo evidence for the existence of a substantial population of both mature and immature OLGs in the retina of adult birds. The putative functions of TfBP(+) OLGs including myelination and the tropic role of the ganglion cells are discussed in conjunction with the physical properties of TfBP and structural characteristics of the avascular retina of birds.     

Intraorbital transection of the rabbit optic nerve: consequences for ganglion cells and neuroglia in the retina.

Rabbit retinae were stained with antibodies to glial fibrillary acidic protein (GFAP) at various times up to 5 months after complete unilateral intraorbital optic nerve transection, which is known to induce degeneration of ganglion cell axons and perikarya in the retina. A transient immunoreactivity for GFAP was observed in Müller glial cells that normally lack this marker. Müller-cell GFAP immunoreactivity became detectable 4 days after the lesion, but Müller cells were no longer labeled 3 months later. GFAP-labeled astrocytes located in the nerve fiber layer showed no change in immunoreactivity at any stage after transection. Application of horseradish peroxidase to the left and right superior colliculus of a rabbit whose optic nerve had been transected unilaterally 2 years before confirmed the completeness of the transection. Yet electron microscopy showed the presence of some healthy-looking ganglion cell axons in the lesioned retina, although these cells were deprived of their target. Labeling retinal wholemounts with neurofilament antibodies confirmed the presence of some ganglion cell axons and perikarya in the retina more than 2 years after transection. The course of these axons suggested that they were remnants of axons. Using antibodies to galactocerebroside (GC) we found that, as in the normal rabbit, these persisting ganglion cell axons were myelinated in the medullary rays. Although many ganglion cell axons had disappeared after 2 years, the number of neuroglial cells (including astrocytes and oligodendrocytes) present in the medullary ray region was not altered. The cell bodies of some oligodendrocytes were covered with a myelin sheath, an aberrant feature not seen normally.     

A quantitative comparison between the ganglion cell populations and axonal outflows of the visual streak and periphery of the rabbit retina

A vertical density profile of the ganglion cells 2 mm temporal of the optic nerve head in the rabbit retina has been produced by counting somata in the cresyl-violet-stained, ganglion cell layer of a flat-mounted retina. Somata classified as ganglion cells were characterized by obvious Nissl staining in an extensive cytoplasm and typically had diameters greater than 9 μm. The accuracy of the profile, and thus of the classification criteria, has been substantiated by electron micrographic determination of the numbers of ganglion cell axons arising within local regions of known area on the same retina This study indicates that Vaney and Hughes' estimate ('76) of 547,100 presumed ganglion cells in the rabbit retina should be changed to 373,500 ganglion cells. The latter value is within the statistical error of their optic nerve count of 394,000 fibers The mean diameter of ganglion cells 6 mm from the visual streak in the inferior periphery (density: 550 cells/mm²) was 28% greater than that of cells on the peak of the streak (density: 5,400 cells/mm²), although the form of the ganglion cell diameter distribution did not change markedly with eccentricity. The increase in the mean size of ganglion cells in the periphery appeared to be approximately matched by an increase in the size of their axons. Larger axons became myelinated farther from the edge of the myelinated band than did smaller axons Within the ganglion cell layer there was another population of cells which were quite distinct from the obvious neuroglia: Their nuclei were similar to those of the larger ganglion cells and many appeared to have Nissl granules within their limited cytoplasm. About half of this heterogeneous population was classified as “coronate cells,” which were characterized by the partial nuclear encapsulation of their eccentric cytoplasm.     

Retinal axon regeneration in the lizard Gallotia galloti in the presence of CNS myelin and oligodendrocytes

Retinal ganglion cell (RGC) axons in lizards (reptiles) were found to regenerate after optic nerve injury. To determine whether regeneration occurs because the visual pathway has growth-supporting glia cells or whether RGC axons regrow despite the presence of neurite growth-inhibitory components, the substrate properties of lizard optic nerve myelin and of oligodendrocytes were analyzed in vitro, using rat dorsal root ganglion (DRG) neurons. In addition, the response of lizard RGC axons upon contact with rat and reptilian oligodendrocytes or with myelin proteins from the mammalian central nervous system (CNS) was monitored. Lizard optic nerve myelin inhibited extension of rat DRG neurites, and lizard oligodendrocytes elicited DRG growth cone collapse. Both effects were partially reversed by antibody IN-1 against mammalian 35/250 kD neurite growth inhibitors, and IN-1 stained myelinated fiber tracts in the lizard CNS. However, lizard RGC growth cones grew freely across oligodendrocytes from the rat and the reptilian CNS. Mammalian CNS myelin proteins reconstituted into liposomes and added to elongating lizard RGC axons caused at most a transient collapse reaction. Growth cones always recovered within an hour and regrew. Thus, lizard CNS myelin and oligodendrocytes possess nonpermissive substrate properties for DRG neurons—like corresponding structures and cells in the mammalian CNS, including mammalian-like neurite growth inhibitors. Lizard RGC axons, however, appear to be far less sensitive to these inhibitory substrate components and therefore may be able to regenerate through the visual pathway despite the presence of myelin and oligodendrocytes that block growth of DRG neurites. GLIA 22:61–74, 1998. © 1998 Wiley-Liss, Inc.     

The aberrant retino-retinal projection during optic nerve regeneration in the frog. I. Time course of formation and cells of origin

We have reported previously that during optic nerve regeneration in Rana pipiens, axons are misrouted into the opposite nerve and retina. In the present investigation we have examined the time course of formation of these “misrouted” axons and their cells of origin. The right eye of 31 frogs was injected with ³H-proline at various times after right optic nerve crush. In every frog examined 2 weeks and later after nerve crush, the distribution of autoradiographic label indicated that axons from the right eye had grown into the left optic nerve at the chiasm. The amount of label increased from 2 weeks to reach a maximum at 6 weeks where the entire left nerve was filled with silver grains. At 5 to 6 weeks after crush, laboled axons were found within the ganglion cell fiber layer (GCFL) of the retina of the opposite eye for a maximum distance of 2.3 mm from the optic disc. In frogs examined at intervals later than 6 weeks after crush, the amount of label within the left eye and nerve progressively decreased, indicating a gradual disappearance of the misrouted axons. Studies using anterograde transport of horseradish peroxidase (HRP) after nerve injection confirmed these autoradiographic findings. The position of ganglion cells in the right eye whose axons were misrouted to the left eye was determined by retrograde transport of HRP. Five or 6 weeks after crushing the right optic nerve, the left eye was injected with HRP and labled ganglion cells were found throughout the right eye retina. The largest percentage of labeled cells was found within the ventral half of the retina, particularly within the temporal quadrant, and nearly all of the labeled cells were found in more peripheral portions of the retina. Since few retino-retinal axons are found during normal development, the present results show that the factors guiding regenerating axons in the adult frog differ substantially from those present during development.     

Immunocytochemical study with an anti-transferrin binding protein serum: a marker for avian oligodendrocytes

We have investigated immunocytochemically the localization of a transferrin binding protein (TfBP) in adult CNS of avian and mammalian species using a polyclonal antibody raised against the protein purified from hen oviduct membranes (αOV-TfBP). TfBP has recently been shown to be HSP108. An overall strong immunoreactivity was revealed in most parts of the avian brains, especially in the white matter. The main immunoreactivity originated in small, intensively reacting cells interpreted as oligodendrocytes. The density of TfBP-labeled oligodendrocytes of the avian brains was generally proportional to the degree of myelination. There were no marked differences in TfBP-immunostaining pattern between avian species (chick, pigeon and lovebird). On the other hand, in rat, rabbit and cat brains we could not find any TfBP-immunoreactivity. Immunoelectron microscopy has further revealed that TfBP is present in the light and medium types of oligodendrocytes which are known to have high metabolic activities. TfBP reaction product was homogeneously dispersed throughout the perinuclear cytoplasm and fine processes of oligodendrocytes. The intracytoplasmic organelles such as mitochondria and Golgi apparatus were devoid of reaction product. The presence of TfBP in oligodendrocytes implies that this protein may play an important role in transferrin-mediated iron metabolism in the CNS. The complete lack of cross-reactivity between αOV-TfBP and mammalian tissues suggests that there is species variability in TfBP structure. We conclude that this chick TfBP antiserum will prove useful in studies of oligodendrocytes and myelination in the avian CNS.     

Giant neural systems in the inner retina and optic nerve of small whales

Giant neural cell systems (dendrites, cell bodies, and axons) are present among more usual structures in the retina and optic nerve of the small whale (dolphin) Tursiops truncatus retina. Giant cell body dimensions range up to 75 μm in diameter. Nuclei of the cells are frequently larger (>20 μm) than nearby ganglion, bipolar, and receptor cell bodies. The presence of the giant cell system and giant elements in the nerve fiber layer agree with the unusually broad fiber spectrum of the dolphin optic nerve where more than 6% of the axons are >15 μm in diameter. Smaller axons in the size distribution are typical of dimensions found in terrestrial mammals. The axon estimate totaled 157,000 per optic nerve. The giant cell-axon systems of the whale retina may be a unique expression of the large ganglion cell-axon (transient or “Y” functional unit) systems recently identified in terrestrial mammals.     

Topographic analysis of the retinal ganglion cell layer and optic nerve in the sandlance Limnichthyes fasciatus (Creeiidae,Perciformes)

The sandlance or tommy fish Limnichthyes fasciatus (Creeiidae, Perciformes) is a tiny species that lives beneath the sand with only its eyes protruding and is found throughout the Indopacific region. The retina of the sandlance possesses a deep convexiclivate fovea in the central fundus of its minute eye (1.04 mm in diameter). A Nissl-stained retinal whole mount in which the pigment epithelium had been removed by osmotic shock was used to examine the retinal topography of the ganglion cell layer. There was a foveal density of between 13.0 × 10⁴ cells per mm² (S.D. ± 1.8 × 10⁴ cells per mm²), counted in the retinal whole mount, and 15.0 × 10⁴ cells per mm², counted in transverse sections, which diminished to a peripheral density of 4.5 × 10⁴ cells per mm² (S. D. ± 0.8 × 10⁴ cells per mm²). The total population of axons within the optic nerve was assessed by electron microscopy. Optic axon densities ranged from 2 × 10⁶ axons per mm² in the caudal apex to over 16 × 10⁶ axons per mm² within a specialized region of unmyelinated axons in the rostral apex. The topography of the proportion of unmyelinated axon population (26%) follows closely that of the total population of optic nerve axons. There was a total of 104,452 axons within the optic nerve compared with 102,918 cells within the retinal ganglion cell layer. A close relationship is revealed between ganglion cell soma areas and axon areas where the organization in the optic nerve and retina may reflect some functional retinotopicity.     

Regeneration of the newt retina: Order of appearance of photoreceptors and ganglion cells

The adult newt regenerates a functional retina following removal or destruction of the original retina. We studied the order of appearance of cell types in the regenerating retina by using immunohistochemical techniques. An antibody that recognizes the alpha subunit (260 kDa) of voltage-dependent Na⁺ channels was found to label a 255-kDa band in Western blots of crude membrane fractions from the normal retina. Cryosections of normal retina revealed intense Na⁺ channel immunoreactivity in somata and axons of ganglion cells, weaker immunoreactivity in somata of amacrine cells, and no immunoreactivity in the inner plexiform layer. In the same sections, immunoreactivity to a monoclonal antibody (RB-1) specific to newt cones was intense in the photoreceptor layer. In regenerating retinas, double staining with the Na⁺ channel antibody as a possible marker of ganglion cells and RB-1 antibody first revealed immunoreactive cells at the intermediate stage (three to five cells thick), which does not exhibit segregated synaptic layers. Na⁺ channel-immunoreactive ganglion cells appeared before the RB-1-immunoreactive photoreceptors. Because ganglion cells also appear before photoreceptor cells in normal development, common mechanisms may control both the generation and the regeneration of the newt retina. J. Comp. Neurol. 396:267–274, 1998. © 1998 Wiley-Liss, Inc.     

Myelination of the rat retina by transplantation of oligodendrocytes into 4-day-old hosts

Oligodendrocyte transplantation into the retina enables us to investigate the early events in myelin formation in a new in vivo system. The axons of rat retinal ganglion cells are unmyelinated in the eye but should express a myelination initiation signal since they acquire myelin posterior to the globe. The lamina cribrosa may block the migration of oligodendrocytes from the optic nerve into the retina. Animals that lack a lamina cribosa such as the rabbit have myelinated retinas. We have bypassed the lamina cribrosa by using transplantation techniques and inserted freshly isolated syngeneic 3-week-old rat oligodendrocytes into the unmyelinated 4-day-old rat retina during the period of active optic nerve myelination. The animals are sacrificed at 1-week intervals for 8 weeks. The retinas are examined immunocytochemically for myelin with an antibody to myelin basic protein (MBP). MBP-positive cells are seen extending processes at 1 and 2 weeks. Three and four week retinas show the formation of thicker and longer myelin sheaths oriented along the same radial path as the retinal ganglion axons with maximal MBP staining intensity seen by 5 weeks. Transplanted retinas are negative when stained for P0, a Schwann cell antigen, ruling out Schwann cell myelination of our retinas. We have shown that rat cerebral oligodendrocytes survive, mature, and express a myelin-specific protein in the retinal environment in a pattern consistent with myelination of ganglion cell axons. Retinal transplantation provides a new in vivo model to study oligodendrocyte development and axonal-glial interactions, free from the difficulties inherent in culture systems.     

Transplantation of oligodendrocyte progenitor cells into the rat retina: Extensive myelination of retinal ganglion cell axons

In most mammals, retinal ganglion cell axons are unmyelinated in the retina. The same axons become myelinated in the optic nerve. Various studies suggest that retinal ganglion cell axons are also in principle, myelination competent intraretinally and that non-neuronal factors at the retinal end of the optic nerve prevent the migration of oligodendrocyte progenitor cells into the retina. To test this hypothesis directly, we injected oligodendrocyte progenitor cells into the retina of young postnatal rats. We observed massive myelination of ganglion cell axons in the retina 1 month after cell transplantation. Electron microscopic analysis revealed that intraretinal segments of ganglion cell axons were surrounded by central nervous system myelin sheaths with a normal morphology. Our results thus provide direct evidence for the myelination competence of the intraretinal part of rat retinal ganglion cell axons. © 1996 Wiley-Liss, Inc.     

Retinal ganglion cells in two teleost species,Sebastiscus marmoratus and Navodon modestus

Distribution patterns of ganglion cells in the retina were examined in Nissl-stained retinal whole mounts of Sebastiscus and Navodon. The existence of area centralis in the temporal retina in both species suggests binocular vision. In Navodon, another high density area was found in the nasal retina, and a dense band of ganglion cells was observed along the horizontal axis between the two high-density areas. There is an obvious trend for the ganglion cell size to increase as the density decreases. The total number of ganglion cells was estimated to be about 45 × 10⁴ in Sebastiscus and 87 × 10⁴ in Navodon, whereas the total number of optic nerve fibers was about 35 × 10⁴ and 70 × 10⁴, respectively. The retinal ganglion cells labeled with HRP were classified into six types according to such morphological characteristics as size, shape, and location of the soma as well as dendritic arborization pattern. Type I cells have a small round or oval soma in the ganglion cell layer and a small dendritic field in the inner plexiform layer. Type II cells are similar to type I cells, but the dendrites arborize more closely to the ganglion cell layer in the innermost region of the inner plexiform layer. Type III cells have a medium-sized round soma in the ganglion cell layer, and the dendrites extend in an extremely wide area in the inner plexiform layer with few branches. Type IV cells have a large soma which is located in the ganglion cell layer. Dendrites emanate from the soma in all directions, branching out several times within a rather small region in the innermost part of the inner plexiform layer. Type V cells have large somata of various shapes, usually dislocated to the inner plexiform or granular layer. The dendrites extend in every direction and occupy an extremely large area in the inner plexiform layer. Type VI cells have the largest somata, which are also dislocated to the inner plexiform or granular layer. Type VI cells have a characteristic triangular or fan-shaped dendritic field. Soma size and the axon diameter are intimately linked, that is, small somata of type I and II cells give off thin axons, and large somata of type V and VI give off thick axons. Medium-sized somata of type III cells or large somata of type IV cells, which have rather small dendritic fields, give off medium-sized axons. The histograms of the soma areas in the whole retina are quite similar to the histograms of the diameters of the optic nerve fibers.     

Distribution and morphology of retinal ganglion cells in the Japanese quail

A ganglion cell density map was produced from the Nissl-stained retinal whole mount of the Japanese quail. Ganglion cell density diminished nearly concentrically from the central area toward the retinal periphery. The mean soma area of ganglion cells in isodensity zones increased as the cell density decreased. The histograms of soma areas in each zone indicated that a population of small-sized ganglion cells persists into the peripheral retina. The total number of ganglion cells was estimated at about 2.0 million. Electron microscopic examination of the optic nerve revealed thin unmyelinated axons to comprise 69% of the total fiber count (about 2.0 million). Since there was no discrepancy between both the total numbers of neurons in the ganglion cell layer and optic nerve fibers, it is inferred that displaced amacrine cells are few, if any. The spectrum in optic nerve fiber diameter showed a unimodal skewed distribution quite similar to the histogram of soma areas of ganglion cells in the whole retina. This suggests a close correlation between soma areas and axon diameters. Retinal ganglion cells filled from the optic nerve with horseradish peroxidase were classified into 7 types according to such morphological characteristics as size, shape and location of the soma, as well as dendritic arborization pattern. Taking into account areal ranges of somata of each cell type, it can be assumed that most of the ganglion cells in the whole retinal ganglion cell layer are composed of type I, II and III cells, and that the population of uniformly small-sized ganglion cells corresponds to type I cells and is an origin of unmyelinated axons in the optic nerve.     

Developmental appearance of oligodendrocytes in the embryonic chick retina

The axons of the optic nerve layer are known to be myelinated by oligodendrocytes in the chick retina. The development of the retinal oligodendrocytes has been studied immunohistochemically with antibodies against oligodendrocyte lineage: monoclonal antibodies O4 and O1, and an antibody against myelin basic protein. O4 positive (O4+) cells were first detected in the retina on the tenth day of incubation (embryonic day (E)10, stage 36). The labeled cells were located in the optic nerve layer close to the optic fissure. Most were unipolar in shape, extending a leading process with a growth cone toward the periphery of the retina. By E12, unipolar O4+ cells had spread to the middle of the retina. Many O4+ cells close to the optic fissure showed radial arrangement with extension of processes toward the inner limiting membrane. O1+ oligodendrocytes were first observed in the E14 retina positioned just above (interiorly to) retinal ganglion cells. These labeled cells extended fine processes in the optic nerve layer. Limited numbers of myelin basic protein-positive cells were present by E16 and located interiorly to the retinal ganglion cells. In addition to the oligodendrocyte in the optic nerve layer, a limited number of O4+ cells were observed in the inner nuclear layer by E14, and they became O1+ by E18. Furthermore, explant culture experiments showed E10 to be the youngest stage at which the retina contained oligodendrocyte precursors. An intraventricular injection of fluorescent dye 1,1′,dioctadecyl-3,3,3′,3-tetramethylindocarbocyanine perchlorate (DiI) at E6 yielded O4+/DiI+ cells in the retina at E10, which provided direct evidence to support migration of oligodendrocyte precursor into the retina. The present results demonstrated the sequential appearance of the cells of oligodendrocyte lineage and the detailed morphology of the developing oligodendrocytes in the retina. These morphologic features strongly suggested that retinal oligodendrocytes were derived from the optic nerve and spread by migration through the optic nerve layer. J. Comp. Neurol. 398:309–322, 1998. © 1998 Wiley-Liss, Inc.     

Axonal redirection at the dorsoventral intraretinal boundary

Cobaltous-lysine applied to the goldfish optic nerve backfilled retinal ganglion cells and their axons. Confined to the ventronasal and ventrotemporal retina was a small population of retinal ganglion cells whose axons traveled dorsally and parallel to the retinal margin. On reaching the boundary between dorsal and ventral retina, the axons arched, joined radially oriented bundles of axons, and traveled toward the optic disk. Control studies showed that the axons came from retinal ganglion cells rather than from retinopetal cells. The somatic area of retinal ganglion cells (RGCs) with circumferential axons was 30-50 microns, and was similar to that of average ganglion cells. The axons of these cells coursed between the optic fiber and ganglion cell layers or between the ganglion cell and inner plexiform layers. Many somata were displaced slightly toward the inner plexiform layer, but were not really displaced ganglion cells. The aberrant axonal trajectory may be related to the slightly displaced location of the cell. However, ganglion cells that are displaced to the edge of the inner nuclear layer usually have radially coursing axons. We digitized the coordinates of the bending points and the dorsoventral retinal boundary. On average, the bending points occurred within 100 microns of the dorsoventral retinal border. These findings suggest that some molecular, rather than mechanical, factor at the dorsoventral retinal boundary alters the course of the circumferential axons. Furthermore, because there are cells with circumferential axons throughout the ventral retina, the data imply that at least ventral RGC axons avoid mingling with the axons from dorsal RGCs.     

Quantitative analysis of the retinal ganglion cell layer and optic nerve of the barn owl Tyto alba

The visual capacity of the common barn owl (Tyto alba) was studied by quantitative analysis of the retina and optic nerve. Cell counts in the ganglion cell layer of the whole-mounted retina revealed a temporal area centralis with peak cell density of 12,500 cells/mm2 and a horizontal streak of high cell density extending from the area centralis into the nasal retina. Integration of the ganglion cell density map gave an estimated total of 1.4 million cells for the ganglion cell layer. Electron microscopy of a single, complete section of the optic nerve revealed a bimodal fiber diameter spectrum (modes at 0.3 and 0.9 microns; bin width = 0.2 microns), with diameters ranging from 0.15 microns (unmyelinated) to 6.05 microns (myelinated, sheath included). The total axon count for the optic nerve was estimated from sample counts to be about 680,000 axons (25% unmyelinated). Therefore, roughly half of the cells in the retinal ganglion cell layer do not send axons into the optic nerve. With certain assumptions, the data predict a visual spatial acuity for barn owls on the order of 8 cycles/degree, a value similar to the known behaviorally measured acuities of masked owls (10 cycles/degree) and domestic cats (6 cycles/degree).     

Postnatal changes in retinal ganglion cell and optic axon populations in the pigmented rat

The number of ganglion cells in the retina of the postnatal rat has been examined. We estimated both the number of axons in the optic nerve and the number of cells which can be retrogradely labelled with horseradish peroxidase from injections into the brain. In the retina of the newborn rat there are at least twice as many ganglion cells as in the adult rat. By retrograde labelling of the ganglion cells and following transection of their axons 24-48 hrs later we can find no evidence that ganglion cells withdraw their axon without degeneration of the patent cell body. We have found that the excess ganglion cells are lost over the first ten postnatal days and during this period we observe pyknotic nuclei in the ganglion cell layer. From our estimates of the total number of neurones in the ganglion cell layer and the number of ganglion cells found at different ages we conclude that the migration of amacrine cells into the ganglion cell layer occurs in the first five postnatal days.     

Microglial cell responses in the rabbit retina following transection of the optic nerve

The response of retinal microglial cells, which accompanies retrograde degeneration of ganglion cell axons and perikarya (induced by transection of the optic nerve), was studied in whole-mounted rabbit retinae labeled enzyme-histochemically for nucleoside diphosphatase (NDPase), which is a microglial cell marker. A few days after transection, the number of microglial cells/mm2, as well as their staining intensity, began to increase in the inner plexiform layer. The mosaic-like distribution of the star-shaped microglial cells present in the inner plexiform layer of a normal rabbit retina was preserved during ganglion cell degeneration. As in the normal retina, processes of individual cells never overlapped with those of neighboring cells in the inner plexiform layer because individual cells in the "degenerating" retina acquired shorter processes, i.e., the cells occupied a smaller territory compared to the normal retina. In the nerve fiber layer the number and staining intensity of NDPase-labeled microglial cell processes (most of which are aligned in parallel with degenerating ganglion cell axons) transiently increased and returned to normal values by 5 months post-transection. Microglial cells that are not detectably NDPase labeled in the outer plexiform layer of a normal rabbit retina acquire intense staining a few days after the nerve is cut. The functional significance of the increased NDPase activity in the plasma membrane of microglial cells during degeneration remains to be determined.