Morphology of congenital microphthalmia in chicks (Gallus gallus)

Abstract

This study examines the morphology of sporadic congenital microphthalmia in 1-day-old chicks, with particular emphasis on the neural retina. On the basis of the size of the eyeball it is possible to classify microphthalmia into two groups, severe and mild. In severe microphthalmia (less than 5 mm in equatorial diameter), the eyeball is severely malformed, but in most cases it shows evidence of an organized neural retina. Although ganglion cells and an optic nerve head are present in a small proportion of these retinae, we could not trace an optic nerve projection to the brain. These results indicate that some ganglion cells are able to be sustained after the period of naturally occurring cell death, suggesting either that those ganglion cells have established some contact with the central nervous system or that the presence of their axons in a rudimentary optic nerve is adequate for survival. In mild microphthalmia (greater than 5 mm in equatorial diameter), the most consistent abnormality is a defect in the pecten, which together with other abnormalities such as orbital cysts and colobomas indicates that the major abnormality occurs in the region of the choroid fissure. Associated with these defects are abnormalities within the ganglion cell layer. In some cases the number of ganglion cells was reduced, and in others the numbers of both ganglion and displaced amacrine cells were reduced. Unexpectedly, there were localized regions completely devoid of cells in the ganglion cell layer. The timing of the congenital defect may provide some clue as to the presence of a critical period in which displaced amacrine cells are formed or are sensitive to events related to ganglion cell loss.

Effects of Maternal Hyperthermia on the Developing Guinea‐pig Eye

“…It therefore appears that the critical event is significant cell loss that is not able to be reconstituted at later stages. In severe congenital microphthalmia, there is evidence of some organization of the neural retina even when a central connection is apparently absent (Ehrlich et al, 1989). This is consistent with the events occurring in heat-induced micrencephaly, where cell loss in neuroblast precursors can be up to 40% (Upfold et al, 1989) and some eye development has always been observed in guinea-pigs.…”