Visual pigment and visual receptor cells in fetal and adult sheep.

The visual pigments, and the structure of the visual cells, were investigated by spectrophotometry and by light and electron microscopy in fetal and adult sheep. The rhodopsin system in adult sheep closely resembles that of cattle. The absorbance maxima of rhodopsin, lumirhodopsin, and metarhodopsin I are at 498, 490, and 480 nm, respectively. The estimated molar absorbance coefficient at the wavelength of maximum absorption of rhodopsin is 40,000M-1 . cm-1. Rhodopsin was detected from a fetal age of 85 days (term at 145 days). Partial bleaching of extracts from fetal eyes (95, 105, and 115 days) did not demonstrate cone pigments, although cones were present in fair numbers at a fetal age of about 105 days. The time course of rhodopsin formation between 85 and 140 days resembles a growth curve. The amount of rhodopsin shortly before birth (140 days) is about 0.6 times that in the adult. The number and dimensions of rod outer segments as well as packing of the discs were studied structurally and related to the rhodopsin content. A fairly good correlation was found at the earliest stages (95 and 105 days gestation age), when rhodopsin concentration was very low and rod outer segments were few and small, as well as at the latest stage (140 days). At 115 days the rhodopsin content observed by spectrophotometry was less than that indicated by the outer segment volume, probably mainly due to the outer segment discs and possibly to the rhodopsin molecules being less tightly packed than at 140 days.