Ann Jervie Sefton

The morphology, number, distribution and central projections of class I retinal ganglion cells in albino and hooded rats

Class I retinal ganglion cells have been identified in wholemounts of rat retinae following injections of horseradish peroxidase (HRP) into retino-recipient nuclei. Class I cells are characterized by relatively large somata, 3-7 fairly frequently branching large-gauge primary dendrites and relatively thick axons. Cells with a very similar morphology have been visualized in the ganglion cell layer of retinal wholemounts using a neurofibrillar stain. The size of the somata and dendritic trees of Class I cells is affected by the density of all classes of ganglion cells: both somata and dendritic trees of Class I cells located in the region of peak density are smaller than those located in medium- and low-density ganglion cell regions. The mean numbers of Class I ganglion cells labelled following massive injections of HRP into retino-recipient nuclei were 876 (in albino rats) and 944 (in hooded rats), while the mean number of cells stained with the neurofibrillar method in albino retinae was 791. Thus, with the total number of positively identified retinal ganglion cells being 110,000-115,000 [Potts et al., 1982; Perry et al., 1983], Class I cells in both strains of rat constitute less than 1% of all retinal ganglion cells. Nevertheless the dendritic fields of Class I cells cover the entire retina. Although Class I cells are distributed relatively evenly across the retina, the density is slightly greater in the lower temporal retina where the bulk of the ipsilaterally projecting fibres originates. While Class I cells represent up to 10% of ipsilaterally projecting retinal ganglion cells in both strains of rat, fewer Class I cells project ipsilaterally in albinos than in hooded rats. All contralaterally projecting Class I cells appear to send branching axons to the superior colliculus and dorsal lateral geniculate nucleus. Class I cells represent a larger proportion of the ganglion cells projecting to the dorsal lateral geniculate nucleus (4-5%) than that of ganglion cells projecting to the superior colliculus (about 1%). The morphology, numbers, distribution and the pattern of the central projections of Class I retinal ganglion cells in rats suggest that they are likely to be homologues of the alpha-type ganglion cells distinguished in carnivores.

Loss of axons from the optic nerve of the rat during early postnatal development

The number of axons in the optic nerve of the newborn rat has been compared with the number present in the adult animal. Nerves taken from animals on the day of birth contain 242,000 +/- 29,000 (S.D.) fibres (n = 5). By the sixth postnatal day, the number of axons has fallen to the stable values of adults (99,000 +/- 3700, n = 8). Thus development of the rats visual system during the first 5 days of life is associated with a loss of 60% of the axons present in the optic nerve at birth. Counts made on the remaining nerve after enucleation of one eye suggest that the presence of retino-retinal axons during the first 5 postnatal days cannot account for all of this reduction.

A correlation of receptive field properties with conduction velocity of cells in the rat's retino-geniculo-cortical pathway.

Summary1.The receptive field properties and responses to electrical stimulation of 126 P-cells recorded from the dorsal lateral geniculate nucleus (LGNd) were studied in the hooded rat.2.Eighty-five cells had a concentric (Kuffler, 1953) receptive field organisation (46 off-centre on-surround; 39 on-centre off-surround). Of the remaining cells 29 had co-extensive on/off excitatory discharge regions, nine had on-centres with suppressive surrounds and two cells gave on-responses but had no suppressive surround. One cell was identified as suppressed-by-contrast.3.On the basis of the battery of tests developed for the identification of cell types in the cats retina and LGNd, 35 of the cells with a Kuffler-type receptive field organisation were identified as Y-like. The majority of the remaining cells, both concentric and others, reminded us of the different subclasses of W-cells of the cat. Nine concentric cells in most of the tests exhibited X-like properties.4.All of the Y-like cells were driven by relatively fast conducting retinal ganglion cell axons, comprising the t1 conduction velocity group. The majority of the remaining cells were driven by slower axons comprising t2 or t3 conduction velocity groups.5.Thus, in the rat, as in other mammalian species studied so far, there is a correlation between the conduction velocity groups in the retino-geniculo-cortical pathway and the functional groups based on the cells’ receptive field properties. There seem to be functional equivalents of the cats Y- and W-cell classes but evidence for a distinct X-like class of cells is lacking.

Cortical projections to visual centres in the rat: An HRP study

We have investigated the relationships of the visual cortex to other visual centres in the rat: namely the lateral geniculate nucleus, the visually responsive part of the thalamic reticular nucleus and the superior colliculus. We injected horseradish peroxidase iontophoretically so as to restrict the injectate to each of the regions, and reacted sections using 3 different procedures. Areas 17, 18 and 18a project to both dorsal and ventral lateral geniculate nucleus as well as to the visually responsive part of the thalamic reticular nucleus and superior colliculus. Pyramidal cells in lamina VI project to the dorsal lateral geniculate nucleus and to the thalamic reticular nucleus, whereas cells of origin of the projection to the superior colliculus lie in lamina V; cells in lamina V also project to ventral lateral geniculate nucleus. The implications of these findings are discussed, particularly in terms of the functional relationships between the visual cortex, lateral geniculate nucleus and visual thalamic reticular nucleus.

Quantitative and morphological studies on developing optic axons in normal and enucleated albino rats

SummaryIn the albino rat, the number of optic axons increases from 400 on embryonic day 15 to reach a peak of 240000 at birth, before declining to adult numbers (100000) by postnatal day 5. Throughout the period of loss of axons there are few signs of degeneration in the optic nerve, which does not change its diameter: the decrease in density of axons is matched by an increase in the cross-sectional area of individual axons. Myelination of the initially non-myelinated axons starts on day 5, when axonal numbers stabilize. Following neonatal removal of one eye, fewer axons than normal are present in the contralateral optic nerve up to day 5. The axons removed by enucleation may be retino-retinal axons, representing up to 40% of the 83000 fibres lost between postnatal days 2 and 5. There is no increase in the numbers of optic axons in the remaining nerve in adult animals; this appears to be due to the small absolute numbers of ipsilateral axons saved by enucleation. After enucleation, axons remain clear and undergo a “watery” degeneration after initially swelling, and the removal of degenerative products is accomplished within four days.

The origin and development of retinal astrocytes in the mouse

SummaryAstrocytes, a class of glia which appear in the mammalian retina late in development, have been postulated either to originatein situ from Müller cells or extra-retinally from the optic stalk epithelium, only subsequently invading the eye. The site of origin and the developmental characteristics of retinal astrocytes were examined in the mouse, a species not previously studied for this purpose. Sections of normal eyes and stalks at different ages were examined. Cells positive for glial fibrillary acidic protein (GFAP) were first observed at post-conceptional day 17 at the optic disc end of the stalk. From this site, the GFAP-positive cells migrated into and across the retina at a rate of ∼290 μm per day, reaching its edge by post-conceptional day 28. While migrating across the retina, the astrocytes progressively increased in size and morphological complexity, observations confirmed by measurement of their fractal dimension. Over the same period, a wave of differentiation swept along the stalk in the cranial direction. Further evidence that retinal astrocytes are born outside the retina emerged when foetal hemiretinae with or without optic stalks were explanted to the chorioallantoic membrane of the chick. When examined one to twelve days later, no expiant cultured without the optic stalk contained GFAP-positive astrocytes, while expiants with the stalk left attached contained relatively normal numbers of astrocytes. We observed, using fluorescence confocal microscopy, that retinal astrocytes in the mouse as in the rat, associate predominantly with blood vessels, not axonal bundles. It was of interest to determine whether this class of glia is essential to the normal cytoarchitectural development of the neural retina. Morphological analysis of the expiants revealed no observable differences in cytoarchitecture or in the timing of developmental events between retinae maturing with or without astrocytes. It was therefore concluded that astrocytes may not be essential to the normal structural development of the murine retina.

The innervation of the Lateral Geniculate Nucleus and Anterior Colliculus in the rat

Abstract In the rat, electrophysiological studies support the hypothesis that all or most of the Optic Nerve (ON) fibres supplying Lateral Geniculate Nucleus (LGN) bifurcate to supply Anterior Colliculus (AC). Three similar groups of fibres are activated in ON by stimulating either in AC or in LGN. Occlusion studies showed that the majority of ON fibres tested either from LGN or AC were made refractory by a prior stimulus applied to the other nucleus. Field responses and single units in LGN were excited either by AC or ON stimuli; patterns of activity were similar in each case. Conduction velocities of fibres supplying the LGN were calculated to be 18 m/sec, and 7.3 m/sec and 3.2 m/sec for the fastest fibres of the three groups and conduction velocities of branches from LGN to AC were calculated to be 5.8 m/sec, 3.9 m/sec and 2.2 m/sec.

The retinal location and fate of ganglion cells which project to the ipsilateral superior colliculus in neonatal albino and hooded rats

We have studied the organization of the ipsilateral retinocollicular pathway in neonatal rats by injecting the enzyme horseradish peroxidase (HRP) into the superior colliculus within 24 h of birth and later examining the location of labelled cells in the contralateral and ipsilateral retinae. One day after HRP injection, regardless of the location of the injection site in the superior colliculus, the great majority (over 80%) of ipsilaterally projecting cells was located in the lower peripheral retina. Five days after injection into the posterior pole of the superior colliculus (which in adult animals does not receive input from the ipsilateral retina), there were very few labelled cells in the ipsilateral retina, but labelled cells were quite numerous in the appropriate part of the contralateral retina. These results suggest that in the neonatal rat the great majority of ipsilaterally projecting retinal ganglion cells lie in the same part of the retina as do ipsilaterally projecting cells in the adult, but that many of those cells which project to inappropriate parts of the superior colliculus die by the fifth postnatal day.

Electrical activity of lateral geniculate nucleus and optic tract of the rat

Abstract In the rat, the field responses of the lateral geniculate nucleus and optic tract were recorded after electrical stimulation of the optic nerve. The presence of three groups of fibres in the optic tract is suggested. The responses of the lateral geniculate nucleus to increasing strength of stimulus, the recovery of excitability, the activity during repetitive stimulation and the conduction velocities in the optic tract are described.

Relation of the parabigeminal nucleus to the superior colliculus and dorsal lateral geniculate nucleus in the hooded rat

SummaryThe retino-recipient layers of the superior colliculus project predominantly to the dorsal and ventral divisions of the ipsilateral parabigeminal nucleus, while receiving an input chiefly from the medial division of the contralateral nucleus. A variety of retrograde tracing techniques was used to confirm that there is a projection from the medial division of the parabigeminal nucleus to the contralateral dorsal lateral geniculate nucleus in normal adult hooded rats. Some parabigeminal cells branch to supply both dorsal lateral geniculate nucleus and retino-recipient layers of the superior colliculus.