Chronotopographical distribution patterns of cell death and of lectin‐positive macrophages/microglial cells during the visual system ontogeny of the small‐spotted catshark Scyliorhinus canicula

The patterns of distribution of TUNEL‐positive bodies and of lectin‐positive phagocytes were investigated in the developing visual system of the small‐spotted catshark Scyliorhinus canicula, from the optic vesicle stage to adulthood. During early stages of development, TUNEL‐staining was mainly found in the protruding dorsal part of the optic cup and in the presumptive optic chiasm. Furthermore, TUNEL‐positive bodies were also detected during detachment of the embryonic lens. Coinciding with the developmental period during which ganglion cells began to differentiate, an area of programmed cell death occurred in the distal optic stalk and in the retinal pigment epithelium that surrounds the optic nerve head. The topographical distribution of TUNEL‐positive bodies in the differentiating retina recapitulated the sequence of maturation of the various layers and cell types following a vitreal‐to‐scleral gradient. Lectin‐positive cells apparently entered the retina by the optic nerve head when the retinal layering was almost complete. As development proceeded, these labelled cells migrated parallel to the axon fascicles of the optic fiber layer and then reached more external layers by radial migration. In the mature retina, lectin‐positive cells were confined to the optic fiber layer, ganglion cell layer and inner plexiform layer. No evident correlation was found between the chronotopographical pattern of distribution of TUNEL‐positive bodies and the pattern of distribution of lectin‐labelled macrophages/microglial cells during the shark′s visual system ontogeny.