Cell death during differentiation of the retina in the mouse.

A reproducible pattern of cell death associated with differentiation of the retina in mice was analyzed quantitatively by microscopy. Cell death occurs primarily during the first 2 weeks after birth and is essentially complete by the end of the third week. Death of individual cells involves nuclear condensation and pyknosis (apoptosis), followed by phagocytosis of the cellular remains by adjacent cells or motile phagocytes. From birth through 4 days, an increasing incidence of cell death is observed among ventricular cells. Ganglion cell degeneration is prominent during the first 11 days, peaking on days 2-5. Many presumptive amacrine cells die within the inner plexiform and inner nuclear layers, particularly between 3 and 8 days. Among adjoining bipolar and Müller cells, degeneration reaches a peak at 8-11 days. On day 5, formation of the outer plexiform layer separates the rods into two groups. Rod nuclei situated on the inner side of that layer immediately move across it to enter the outer nuclear layer, but numerous cells die during nuclear migration. Sporadic death of rods continues during the following 2 weeks. Cell death associated with cell differentiation (histogenetic death) is considered to represent a normal developmental process. Possible mechanisms resulting in cell degeneration are discussed. It is suggested that genetically regulated cell death serves to fine-tune neuronal networks during the terminal stages of development.