Calretinin immunoreactivity in the developing retina of sharks: Comparison with cell proliferation and GABAergic system markers

The calcium-binding protein calretinin (CR) has been widely used as a marker of neuronal differentiation. In the present study we analyzed the distribution of CR-immunoreactive (CR-ir) elements in the embryonic and postembryonic retina of two elasmobranchs, the lesser spotted dogfish (Scyliorhinus canicula) and the brown shyshark (Haploblepharus fuscus). We compared the distribution of CR with that of a proliferation marker (the proliferating cell nuclear antigen, PCNA) in order to investigate the time course of CR expression during retinogenesis and explored the relationship between CR and glutamic acid decarboxylase (GAD), the synthesizing enzyme of the gamma-aminobutyric acid (GABA), which has been reported to play a role in shark retinogenesis. The earliest CR immunoreactivity was concurrently observed in subsets of: a) ganglion cells in the ganglion cell layer; b) displaced ganglion cells in the inner plexiform layer and inner part of the inner nuclear layer (INLi); c) amacrine cells in the INLi, and d) horizontal cells. This pattern of CR distribution is established in the developing retina from early stage 32, long after the appearance of a layered retinal organization in the inner retina, and coinciding with photoreceptor maturation in the outer retina. We also demonstrated that CR is expressed in postmitotic cells long after they have exited the cell cycle and in a subset of GABAergic horizontal cells. Overall our results provide insights into the differentiation patterns in the elasmobranch retina and supply further comparative data on the development of CR distribution in the retina of vertebrates. This study may help in understanding the possible involvement of CR in aspects of retinal morphogenesis.     

Cognin distribution during differentiation of embryonic chick retinal cells in vitro

Differentiation of individual retina neurons is closely linked to development of retina function. This differentiation may be intrinsic to the cell or determined by the position of the cell within the developing tissue. Retina cognin, a cell-cell recognition protein, which may itself mediate positiondependent cell interactions in vivo exhibits a characteristic change in distribution during embryonic chick development. Cognin is progressively lost from the outer retina in a manner which appears position-dependent. ¹⁰ We asked if this change in cognin distribution was actually position-dependent or intrinsic to the retina cells. Neural retina cells from 8-day-old chick embryos were cultured in vitro. Continued differentiation of the cultured cells was demonstrated by neurite outgrowth and characteristic increases in choline acetyltransferase and glutamic acid decarboxylase activity. In such cultures, the characteristic developmentally related disappearance of retina cognin occurred as in vivo. This indicated that this aspect of retina neuronal differentiation was independent of position within the tissue and likely intrinsic to individual cells after 8 days of embryonic development.     

Cell generation, death, and retinal growth in the development of the hamster retinal ganglion cell layer

During the early postnatal period in the hamster, the retinal ganglion cell layer grows, establishes its central connections, and undergoes substantial cell loss. In this study, we describe the development of the retinal ganglion cell layer with particular attention to the creation of local specializations in cell density. Changes in the number and spatial distribution of cells identified by a single 3H thymidine injection were examined through the period of maximal cell loss (postnatal days 4-10) and at adulthood. The cells of the retinal ganglion cell layer are generated from embryonic day 10 to postnatal day 3. Overall, cell number in the ganglion cell layer increases by approximately 108,000 cells (223%) from postnatal day 1 to 5, because of continued migration of cells generated prenatally. Cell number decreases from postnatal day 5 to 10 (25%), coincident with the presence of degenerating cells. Cell type is correlated with day of generation: the largest cells, all having retinal ganglion cell morphology, are generated on embryonic days 10 and 11; intermediate-sized cells predominantly of ganglion cell morphology on embryonic day 12; and smaller cells of displaced amacrine or glial cell morphology thereafter. At adulthood, the hamster retina shows a streaklike elevation of cell density through central retina. However, at the time of maximal cell number (postnatal day 5), cell density is uniform across the retina. During the period of cell degeneration, cells are lost in greater relative numbers from the retinal periphery. This cell loss occurs principally from the first-generated cells (embryonic days 10 and 11), as shown by both changes in the distribution of labeled cells and by the spatial pattern of labeled degenerating cells. From postnatal day 10 to adulthood, relative cell density continues to decline in the periphery of the retina, thus suggesting that differential growth completes the production of the adult cell density distribution.