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

Dendritic plasticity in the early postnatal feline retina: quantitative characteristics and sensitive period

Retinal lesions were made in kittens between 3 and 60 days postnatal age and in adult cats. After postlesion survival times ranging from 4 to 11 months the dendritic morphology of retinal ganglion cells was revealed by retrograde labeling with horseradish peroxidase or with neurofibrillar staining techniques. After retinal lesions on the third postnatal day changes of dendritic morphology were observed in retinal ganglion cells adjacent to regions of retrograde degeneration. Originating from eccentrically positioned somata the dendritic fields extended into the regions that were free of neighboring cells. The dendrites oriented toward the ganglion-cell-free region were elongated and thicker than normal. The density of dendrites per unit area was increased in this part of the dendritic trees. Lesions on the 20th, 38th, and 56th postnatal days elicited increasingly weaker changes of dendritic morphology. The sensitive period for the type of dendritic plasticity described ends between 40 and 60 days postnatally.

Changing distribution of retinal ganglion cells during area centralis and visual streak formation in the marsupial Setonix brachyurus

Injections of the axonal marker horseradish peroxidase (HRP) were made into optic tracts and/or visual centres of Setonix brachyurus, a small wallaby (quokka) during development and in adults. Distributions of HRP-labelled and unlabelled cells in the retinal ganglion cell layer were estimated from sections or wholemounts counterstained with cresyl violet. Between 20 and 40 days postnatal we did not observe an area centralis or visual streak in either the labelled or unlabelled cell populations. These regional specialisations arose between 60 and 80 days within the labelled cell population, while unlabelled cells remained approximately evenly distributed. Our findings suggest there is no 'hidden' area centralis and visual streak within the evenly distributed total cell population of the retinal ganglion cell layer at stages before cells can be classified on morphological grounds.

Functional development of the visual system in normal and protein deprived rats. II. Morphometric and biochemical studies on adult optic nerve

The optic nerve of normal (C) and protein deprived (PD) adult rats was examined by morphometry and biochemistry. The mean cross-sectional area of the optic nerve was reduced by 15% and the number of axons per unit area increased by 17% in the PD rats. Calibre spectrum analysis of axons revealed a reduction in median diameter from 0.49 micron in controls to 0.45 micron in PD rats. The number of axons with a diameter larger than 1 micron was reduced by 35% in PD rats. These reductions were probably due to a general reduction in size, since the calculated total number of axons in the optic nerve was almost identical in C and PD rats (126 X 10(3) and 124 X 10(3), respectively). The increased packing density of axons in the nerve was not only due to thinner axons. The biochemical measurements showed a marked reduction in myelin basic protein in the optic nerves of PD rats, without an alteration in the composition of the total protein. This confirms the persistent hypomyelination which has been reported previously in other malnutrition models. The possible relations between the structural and biochemical changes affecting optic nerve fibres and physiological findings on cortical visual evoked response and on optic nerve in vitro in PD rats are discussed.

Effect of neonatal optic nerve transection on some classes of amacrine cells in the rat retina

An optic nerve section of the right eye of rat pups was carried out and the retina of the left and right eyes analyzed eight weeks later. Immunocytochemical studies for the localization of tyrosine-hydroxylase, choline acetyltransferase and substance P in amacrine cells revealed no qualitative differences in the distribution of the cell bodies or dendrites for the right and left retinas. Biochemical analysis showed a higher level of choline acetyltransferase, dopamine and glutamate decarboxylase in the right than in the left retina, though the glutamate decarboxylase difference was statistically insignificant. The biochemical difference is thought to reflect the differences in the protein or wet weight content of the retinas due to degeneration of the ganglion cells. It is concluded that destruction of the ganglion cells has no obvious effect upon the development or survival of some classes of amacrine cells.

Development of cortical afferents and cortico-tectal efferents of the mammalian (rat) primary visual cortex

At the time when the fibres from the striate cortex (area 17) begin to innervate the superficial layers of the superior colliculus of the young rat (postnatal days 4 and 5) a high degree of specificity in the organization of this newly formed cortico-tectal projection is already apparent. Thus, in young rats, as in adult mammals of virtually all species studied so far, the somata of cortico-tectal neurones are confined to lamina V of the ipsilateral cortex. However, this high degree of laminar (radial) specificity in young animals is accompanied by a substantial degree of exuberance as indicated by a tangential distribution of the cortico-tectal cells which is wider than that in the adult. The exuberant projections are pruned during the second postnatal week. The cortico-cortical associational and commissural fibres start to enter the grey matter of the rat striate cortex after postnatal day 7. Again a high degree of specificity in the laminar distribution of those newly established projections is apparent. However, the cortico-cortical projection, at the time when cortico-cortical fibres enter the cortical laminae, is clearly exuberant since the tangential spread of cortical cells projecting to the striate cortex is wider than that in the adult. Pruning of these excessive projections takes place some time after postnatal day 14. It is believed that understanding the mechanism(s) underlying the development of connections of the rat visual cortex might be of general importance in understanding developmental abnormalities in the pattern of interconnections of the visual cortices of other mammalian orders.

Transient populations of presumptive macrophages in the brain of the developing hamster, as indicated by endocytosis of blood-borne horseradish peroxidase

During postnatal development, clusters of cells associated with the mononuclear phagocytic system appear within the white matter of rodents and cats. We studied the distribution and morphology of these cells in the hamster's brain during the first 2 weeks after birth. In animals of different ages, horseradish peroxidase was injected into the heart. After 3-6 h survival, the animals were perfused with aldehydes and had their brains removed, cut and reacted. In another series, fixed brain sections from horseradish peroxidase-injected and non-injected animals were reacted for a non-specific esterase expressed by monocytes and macrophages. The horseradish peroxidase reaction-product was seen throughout the nervous tissue at the first postnatal day, appearing more concentrated in certain brain sectors from postnatal day 3 through 10, to finally become restricted to a few regions at postnatal day 16. Horseradish peroxidase-labeled cells appeared in increasing numbers from postnatal day 1 to 8, decreasing thereafter to disappear completely at postnatal day 16. Some labeled cells were roundish or elliptical with few, if any, processes; others had several clearly detectable processes. Horseradish peroxidase-labelled cells formed clusters within the dorsal subventricular zone, dorsal cortical white matter, corpus callosum and several other prosencephalic fiber tracts. The morphology of esterase-reactive cells was less clearly outlined but their distribution and relative density correlated with those of horseradish peroxidase-labeled cells. Also, many horseradish peroxidase-labeled cells were esterase-positive in most clusters. We conclude that (1) some cells in the developing brain selectively endocytose and accumulate blood-borne horseradish peroxidase in their cytoplasm, (2) these cells do not appear to be neurons but a particular cell type associated to the mononuclear phagocytic system and (3) they cluster transiently in particular sectors of the cortical and subcortical white matter during the first 2 weeks after birth.

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.

Evidence for differential effects of terminal and dendritic competition upon developmental neuronal death in the retina

Retinal ganglion cells with ipsilaterally projecting axons were labelled with horseradish peroxidase injected unilaterally along the optic pathway in adult rats. Unoperated controls were compared with three groups of animals operated at birth, given (a) contralateral enucleation, (b) contralateral lesion to the optic tract or (c) both lesions simultaneously. The numbers of ipsilaterally projecting cells were increased in all three operated groups, presumably because of a reduction in natural neuronal death following diminished terminal and dendritic competition. The pattern of increase of labelled cell density varied with the type of lesion: enucleation led to a major increase within lower temporal retina; optic tract lesion caused its major increase in upper temporal retina, centred at the location of the area centralis; and the double lesion combined both effects above. The distribution of cell-body sizes was differentially affected by the lesions: enucleation led to a shift in the distribution towards the small cell side of the spectrum, when compared with the controls; optic tract lesion shifted the distribution towards the large cell side of the spectrum, but only outside the temporal crescent; and the double lesion led to a shift towards small cells within the temporal crescent and towards large cells outside the crescent, again combining the effects of the single lesions. Large alpha-like neurones with ipsilateral axons were common in the nasal retina of both groups given optic tract lesions but they were rare in the nasal retina of unoperated and, especially, of enucleated rats. The limits of the temporal crescent were unchanged, notwithstanding the large numbers of cells outside the crescent in operated rats. It is suggested that postnatal competitive interactions at the level of terminals and of dendrites control natural neuronal death in the rat retina with different requirements regarding retinal topography and ganglion cell types. The postnatal regulation of neuronal numbers is not responsible for the generation of the nasotemporal division but may be involved in the development of differential distributions of specific ganglion cell types across the retina.

Rat optic nerve: disruption of gliogenesis with 5-azacytidine during early postnatal development

The normal sequence of gliogenesis in the rat optic nerve was disrupted by neonatal treatment with the mitotic inhibitor 5-azacytidine (5-AZ). This protocol caused a marked reduction in the number of glial cells, especially oligodendrocytes, seen in 11- and 14-day-old animals. Myelin formation was also greatly reduced in animals of this age compared to controls, but optic nerve axons appeared to be well preserved. Electrophysiological studies demonstrated similar excitability properties and activity-evoked [K+] changes in normal and 5-AZ-treated nerves prior to 5 days of age. However, in older nerves there were striking changes in the compound action potential and activity-dependent K+ accumulation in 5-AZ-treated nerves compared to controls. This simple model of disrupted central nervous system gliogenesis should prove useful in analyzing a variety of questions regarding neuroglial interactions including the role of glial cells in ionic homeostasis of brain extracellular space.

RNA content of normal and axotomized retinal ganglion cells of rat and goldfish

The responses of rat and goldfish retinal ganglion cells to axotomy were examined by a quantitative cytochemical method for RNA and by morphometric measurement 1-60 (rat) and 3-90 (goldfish) days after interruption of one optic nerve or tract intracranially. Unoperated control animals were studied also. The RNA content of axotomized neurons of rat fell 7-60 days postoperatively. Additionally, atrophy of the axotomized somas occurred. Over time, neuronal atrophy approximately paralleled the loss of RNA, and mean cell area and RNA content were reduced by about 25% 60 days after axotomy. Incorporation of 3H-uridine by axotomized neurons declined also. Axotomized retinal ganglion cells of goldfish behaved differently from those of the rat and showed increases in RNA content, most conspicuously 14-60 days postoperatively. Enlargement of axotomized fish neurons occurred but was less proportionately than concomitant increases in RNA content. The nonaxotomized ganglion cells of goldfish displayed statistically significant increases in size and RNA content 14-49 days after unilateral optic nerve or tract lesions. In contrast, alterations in rat retinal ganglion cells contralateral to interruption of one optic nerve were of limited and questionable significance. The contrasting reactions to axotomy by the retinal ganglion cells of these two vertebrates, one of which regenerates optic axons and one of which does not, may support the proposition that the somal response to axon injury has an important bearing upon the success or failure of CNS regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical localization of macrophages and microglia in the adult and developing mouse brain

Macrophages and microglia in the developing and adult mouse brain have been identified by immunohistochemical localization of the macrophage-specific antigen F4/80 and monoclonal antibodies to the FcIgG1/2b (2.4G2) and type-three complement (Mac-1) receptors. In the adult mouse there are two classes of F4/80-positive cells; those associated with the choroid plexus, ventricles and leptomeninges and the microglia. The cells bearing Fc and complement receptors are indistinguishable, by their morphology and distribution, from those revealed by F4/80. During development macrophages invade the brain and can be followed through a series of transitional forms as they differentiate to become microglia. Macrophage invasion occurs when naturally dying cells are observed in large numbers and this is consistent with the idea that dying neurons and axons provide a stimulus for macrophage infiltration. Our results provide strong support for the hypothesis that the microglia are derived from monocytes and show that microglia possess receptors which would allow them to play a part in the immune defence of the nervous system.

The development of the optic nerve in rodents

In mammals, normal visual function depends upon both the retinotopic organization of visual nuclei and their interconnections. We have investigated in rodents some developmental mechanisms contributing to this organization. On embryonic day 14, in the rat, retinal ganglion cells first project axons through glial channels on the retinal surface before reaching the optic stalk. We suggest that the sequence in which axons enter the stalk (central before peripheral) and their prominent fasciculation impose some retinotopic order amongst the emerging optic nerve fibres. At birth (embryonic day 21) there are over 240 000 axons in the optic nerve, all non-myelinated. However, within one week, the number falls to the adult value (100 000) and myelination, complete in the adult, commences. Axons lost include some which misproject--to the opposite eye or to inappropriate parts of central visual regions. The number of surviving retinal ganglion cells depends on the amount of appropriate target tissue available. It is well established that removal of one eye early in development increases the survival of axons in the remaining optic nerve. However, in a group of adult mice with congenital unilateral anophthalmia, we counted only 21 000 optic axons in the remaining nerve compared with the 31 000 in normal mice of the same strain. Degenerating axons were observed, suggesting that the defect is not a simple developmental failure, but is associated with active degenerative processes throughout life.

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

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

Lesion-induced myelin formation in the retina

In the normal rat retina ganglion cell axons are not myelinated until they enter the optic nerve. After a lesion to the retina made via the sclera and choroid, Schwann cells invade the retina and myelinate ganglion cell axons. The lesion-induced myelin formation is most conspicuous in animals operated between the day of birth and 20 days of age. A lesion to the retina made from the vitread surface does not produce Schwann cell invasion. We suggest that the Schwann cells migrate into the retina from extraocular structures via the sclera. These observations provide a valuable system for the study of interactions between CNS axons and Schwann cells.

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.

Changes in the numbers of optic nerve fibers during late prenatal and postnatal development in the albino rat

We have estimated from electron micrographs the numbers of axons in the optic nerves of a series of albino rats at 8 different ages ranging from embryonic day (E)18 through postnatal day 28. The number of axons was found to reach a maximum of about 325,000 (324,790 +/- 38,589) on E20, and to decline to about 275,000 (273,744 +/- 20,973) by the day of birth. By the middle of the second postnatal week, the number was further reduced to a stable figure of just over 100,000 (105,809 +/- 7,610). This represents a loss of two-thirds of the axons from the peak value at E20, and suggests that there is a comparable degree of cell death among the retinal ganglion cells. The reduction in the number of optic nerve fibers is not affected significantly by the removal of the opposite eye at birth.

Growth and restriction of the ipsilateral retinocollicular projection in the opossum

The distribution of optic nerve fibers and terminals in the superior colliculus (SC) was followed throughout its development in pouch young opossums in order to establish the normal sequence of events leading to the formation of mature patterns. Up to 7 days of life in the pouch, labeled fibers can be followed only as far as the rostral aspect of the optic tract. The earliest evidence for crossed retinal projections in the SC is found at 10 days of age. In parasagittal sections, the label extends along the rostrocaudal tectal axis from the rostral border to the presumptive caudal pole of the SC. Unequivocal evidence for ipsilateral retinocollicular projection is found at 15 days extending to all but the caudal 5th of the rostrocaudal extent of the SC. The projections from both eyes overlap extensively in the SC at 22 days and after this age significant changes occur, mostly at the ipsilateral side: a sub-pial tier of fine label develops excluding both rostral and caudal collicular poles; a deeper tier of coarse label extends from the rostral to the caudal pole and a third, patchy tier of label is found at the prospective strata griseum superficiale and griseum intermediate. By 47 and 60 days the tangential distribution of the projections is virtually indistinguishable from the adult pattern although laminar segregation does not seem as sharp as in the adult. Comparisons of the changeable patterns of ipsilateral retinocollicular projections from 22 to 34 days with the invariant, aberrant pattern in adult animals submitted to uniocular enucleation at either age suggests that the preservation of a juvenile pattern does not provide a comprehensive explanation for the formation of aberrant projections.

The structure of the terminal arborizations of physiologically identified retinal ganglion cell Y axons in the kitten

Retinal ganglion cell (r.g.c.) axons (n = 17) in the optic tract of 4-5 week-old kittens and adult cats (n = 4, this study, n = 27 from other reports) were studied both physiologically and morphologically. Axons were initially classified during extracellular recording with a battery of physiological tests that included Fourier analysis of the response to a sinusoidally counterphased sine-wave grating. Y axons had a significant second harmonic response component (greater than twice the fundamental) present independent of the spatial phase position of the grating. These axons were then recorded from intracellularly and subsequently filled ionophoretically with horseradish peroxidase (HRP). The HRP filled the axons' terminal arborizations in the dorsal lateral geniculate nucleus (l.g.n.). The innervation pattern and and structure of the terminal arborizations of the kitten r.g.c. Y axons were compared to those of the adult. The kitten Y axons innervated the l.g.n. in a pattern similar to that of the adult (individual branches from a single axon always innervated lamina A or A1 and may also have innervated lamina C, the medial interlaminar nucleus (m.i.n.) and/or sent branches that coursed medial to the l.g.n.). Fourteen of seventeen of these Y axons in the kitten innervated either of the A-laminae heavily (greater than 200 terminal boutons per axon). The remaining three r.g.c. Y axons in the kitten had only small arborizations within lamina A (less than fifty terminal boutons per axon) but heavily innervated lamina C. The structure of the terminal boutons on the kitten r.g.c. Y axons was highly variable when compared to axons of adult cats. Some of the boutons were spherical or crenulated as in the adult. Many others had filopodia and growth cone-like terminals with fine extensions. This variable maturation of terminal boutons was seen both between axons and on individual axons. The number of boutons on the kitten r.g.c. Y axons in the A-laminae was significantly less than that of adult Y axons. The mean numbers of boutons per axon were 476 and 1553 in the kittens and adult cats, respectively (P less than 0.001, Mann-Whitney U test). The width of the terminal arborization of individual Y axons in the A-laminae of the kittens was considerably smaller than in adult cats (mean widths of the terminal arborizations are 192 and 293 micron in the kittens and adult cats, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)

Ganglion cells in retinae transplanted to newborn rats

Cells projecting out of retinal transplants placed over the tectum of newborn rats were studied by labelling with horseradish peroxidase 1 month or more after transplantation. Using this technique, it was found that only cells with the dendritic characteristics of ganglion cells were labelled and, furthermore, that the major classes of ganglion cells seen in normal retinae were also present in the transplants. The cell body size histograms of ganglion cells in normal and transplanted retinae compared closely with each other. Dendritic trees were closely confined by the limits of the inner plexiform layer, and if that layer was folded or distorted, they were themselves frequently abnormal. While axons usually coursed over the surface of the retinal transplants, they quite often followed an anomalous course crossing the individual layers. It appears, therefore, that this transplantation procedure has relatively little impact on the ability of ganglion cells to develop many of their characteristic morphological features. Whether the different functional responses of the various ganglion cell classes are also preserved after transplantation is a matter for further investigation.

Normal postnatal development of retinogeniculate axons and terminals and identification of inappropriately-located transient synapses: electron microscope studies of horseradish peroxidase-labelled retinal axons in the hamster

Axons from the eyes reach the dorsal lateral geniculate nucleus of the hamster at birth and both crossed and uncrossed axons spread throughout the nucleus within which they overlap extensively between postnatal days 2-6, before segregating to terminate in different parts of the nucleus by days 8-10 [So, Schneider and Frost (1978) Brain Res. 142, 343-352]. We have labelled retinal axons and their terminations between the day of birth (day 0) and day 6 by injecting one eye with horseradish peroxidase a few hours prior to sacrifice. Labelled profiles were then systematically sought, identified and their position determined, by electron microscope study of large frontal thin sections of both dorsal lateral geniculate nuclei. Labelled crossed and a few labelled uncrossed axons were present at day 0 and became progressively more common over the following few days; appropriately-located labelled uncrossed axons and terminals in the centromedial part of the nucleus (future ipsilateral sector) were relatively less common than labelled crossed axons in the ventrolateral part of the nucleus (part of the future contralateral sector), particularly between days 0 and 3. Synaptic contacts established by such labelled axons were characterized by predominantly electron-lucent spherical presynaptic vesicles and a prominent postsynaptic density. At day 4, labelled uncrossed axons made synaptic contact in the future contralateral sector (which is devoid of uncrossed input after days 8-10) and a few crossed axons made synaptic contacts in the future ipsilateral sector (devoid of crossed input after days 8-10). Such terminals and their synaptic contacts, were identical to appropriately-located ones in the same material. Inappropriately-located terminals were not found in the future contralateral sector at day 6, or in adults. No specialized contacts were observed between inappropriately-located axons or terminals and either other axon terminals or glial cell processes. Thus, during the development of the hamster retinogeniculate projection, inappropriately-located axons establish transient synaptic contacts with geniculate cells, and these contacts are lost as the segregated adult pattern of projections is established.(ABSTRACT TRUNCATED AT 400 WORDS)

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 X 10(4) in Sebastiscus and 87 X 10(4) in Navodon, whereas the total number of optic nerve fibers was about 35 X 10(4) and 70 X 10(4), 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Is Thy-1 expressed only by ganglion cells and their axons in the retina and optic nerve?

The distribution of Thy-1 in the retina and optic nerve has been examined immunohistochemically, and compared to that of the astrocytic marker glial fibrillary acidic protein. The axons and cell bodies of ganglion cells were found to be Thy-1 positive as were processes within the inner plexiform layer. Transection of the optic nerve in the neonatal rat results in the rapid degeneration of the ganglion cells but some Thy-1 staining remains in the inner plexiform layer. We have estimated using an immunoassay of normal and optic nerve transected retinae that about 70% of the Thy-1 in the retina is on ganglion cells and their axons and the remainder is on cells which contribute processes to the inner plexiform layer, presumably amacrine, bipolar or Müller cells. In the optic nerve the Thy-1 was found to be limited to the fascicles of optic nerve fibres and the intrafascicular spaces, containing astrocytes and their processes, were not stained. Axotomy of the adult nerve, which produced axonal degeneration and astrocytic proliferation, led to a loss of over 95% of the Thy-1 from the nerve. We found no evidence that the astrocytes of the retina or optic nerve were Thy-1 positive in normal animals or during degeneration.

Regressive events in neurogenesis

The development of most regions of the vertebrate nervous system includes a distinct phase of neuronal degeneration during which a substantial proportion of the neurons initially generated die. This degeneration primarily adjusts the magnitude of each neuronal population to the size or functional needs of its projection field, but in the process it seems also to eliminate many neurons whose axons have grown to either the wrong target or an inappropriate region within the target area. In addition, many connections that are initially formed are later eliminated without the death of the parent cell. In most cases such process elimination results in the removal of terminal axonal branches and hence serves as a mechanism to "fine-tune" neuronal wiring. However, there are now also several examples of the large-scale elimination of early-formed pathways as a result of the selective degeneration of long axon collaterals. Thus, far from being relatively minor aspects of neural development, these regressive phenomena are now recognized as playing a major role in determining the form of the mature nervous system.

Activity and the control of ganglion cell death in the rat retina

In newborn rats each retina projects principally to the contralateral superior colliculus, but there is also a sparse projection to the whole of the ipsilateral superior colliculus. During the first 2 weeks postnatally the ipsilateral projection normally becomes restricted to the rostromedial part of the superior colliculus. The restriction of this projection is due to the preferential death of ipsilaterally projecting retinal ganglion cells and is apparently the result of competition between optic fibers from the two eyes, since it can be prevented by enucleation of the opposite eye at birth. To determine if electrical activity plays a role in the normal restriction of the ipsilateral retinocollicular projection, the sodium channel-blocking agent tetrodotoxin was administered to one or both eyes during the first 2 weeks postnatally. Tetrodotoxin blockade of activity in one eye resulted in the persistence of a sparse projection from the opposite eye throughout the ipsilateral superior colliculus and the survival of a substantial number of the ipsilaterally projecting retinal ganglion cells in that eye that would normally have died. When both eyes were treated with tetrodotoxin no restriction of the ipsilateral projection was seen on either side. These findings suggest that the competition between retinal ganglion cell axons (either for terminal space or an essential trophic factor), which normally leads to retinal ganglion cell death and the restriction of the ipsilateral retinocollicular projection, is mediated in some way by electrical activity.

Binocular interaction in the fetal cat regulates the size of the ganglion cell population

During fetal development of the cat's visual system there is a marked overproliferation of optic nerve axons. In utero binocular interaction contributes to the severity of fiber loss since removal of an eye during gestation attenuates axon loss in the remaining optic nerve. The purpose of the present study was to determine whether this reduced loss of optic nerve fibers is due to a failure of retraction by supernumerary axon branches or to a reduction in ganglion cell death. To resolve this issue, we compared the number of ganglion cells and optic nerve fibers in adult cats which had one eye removed at known gestational ages. Retinal ganglion cells were backfilled with horseradish peroxidase and counts were made from retinal wholemounts. The axon complement was assessed with an electron microscopic assay. In the retinas of a normal cat we estimated 151,000 and 152,000 ganglion cells. The optic nerves of two other normal cats contained approximately 158,000 and 159,000 axons. In comparison, an animal enucleated on embryonic day 42 had 180,000 ganglion cells and 178,000 optic nerve fibers, while in an animal enucleated on embryonic day 51 the corresponding estimates were 182,000 and 190,000. The close agreement between cell and fiber counts indicates that axonal bifurcation does not contribute appreciably to the axon surplus in the optic nerve of prenatally enucleated cats. These results demonstrate that prenatal binocular interaction regulates the size of the mature retinal ganglion cell population.

Postnatal development of ipsilateral retino-geniculate projections in normal albino rats and the effects of removal of one eye at birth

The postnatal development of ipsilateral retinofugal projections to the lateral geniculate body in normal albino rats, and in rats unilaterally enucleated at birth has been examined. At postnatal ages ranging from 1 day to 6 months, horseradish peroxidase was injected into one eye of normal rats and into the remaining eye of neonatally enucleated animals. After approximately 20 hours, the animals were perfused, the brains sectioned and reaction product visualised using tetramethylbenzidine. Ipsilateral retinal projections to the lateral geniculate body in normal animals were extensive on postnatal day 1 and became reduced over the next few days, the adult pattern being established between days 9 and 12. In the enucleated group, the terminal fields of the ipsilateral projections to the lateral geniculate body from the remaining eye remained larger and displayed a greater density of terminal labelling than in age-matched controls. In addition, the ipsilateral terminal field in the dorsal lateral geniculate nucleus occupied a more lateral position than in control animals. These findings support previous suggestions that the abnormally large ipsilateral retino-fugal projections observed in adult rats following removal of one eye, at or close to, birth, result from a failure of the ipsilateral projection to become restricted and that terminal or preterminal sprouting of retinal axons may also make a small contribution to the formation of the exuberant ipsilateral projection.

Development and aging of the eye in mice with inherited optic nerve aplasia: histopathological studies

We have examined the morphological development of optic nerve aplasia in a subpopulation (10-20%) of anophthalmic mice (Strain ZRDCT -AN) that develop microphthalmia. During embryonic stages the optic fissure in microphthalmic mutants did not involute into the optic stalk. Even in the absence of a proper fissure, early differentiation of the various retinal elements was not disturbed. Subsequently, however, the optic nerve fibers failed to exit from the eye in their appropriate position. Secondary changes in the retina, probably resulting from a failure of optic axons to reach their central targets, were near total loss of ganglion cells and variable attenuation of the other nuclear and plexiform layers. Retinal rosettes were also commonly present.