Development of neuroventricular cells into ocular pigment cells in embryonic chick eyes: morphometrical and electrophysiological observations.

The development of neuro-ventricular cells into pigment cells (outer layer of the optic cup) was studied morphometrically and electrophysiologically in eye primordia of chick embryos. The prospective pigment cells are homogeneous until days 4 to 5 1/2 of embryonic development. During this period, all cells are devoid of pigment and display a basic neuroventricular profile of both inward (probably sodium) and outward (potassium) currents evoked by depolarization. Pigment cell differentiation occurs rapidly between days 5 and 6. On day 6 most cells are pigmented and respond to depolarization with outward currents only. Inward currents were elicted only in a few pigmented cells. By contrast, the minority of unpigmented cells (or cells with faintly dark cytoplasmic inclusions, probably premelanosomes) still present on day six displayed the "immature" pattern of mixed inward and outward currents. We conclude, that the differentiation of neuroepithelial cells into pigment cells involves both the synthesis of pigment granula and the down-regulation of the expression of inward current-mediating ion channels in their membrane.