The human hyaloid system: cell death and vascular regression.

The present study had investigated the roles of apoptosis and necrosis in the regression of the human fetal hyaloid vasculature. Normal human fetal hyaloid specimens (n = 67) ranging from 10 to 20 weeks' gestation were studied. Specimens were either immunolabeled with anti-von Willebrand factor and major histocompatibility complex class I antibodies or investigated using the terminal-deoxyribonucleotidyl transferase-mediated dUTP-biotin DNA nick-end labeling technique. A fluorescent DNA-binding dye acridine orange/ethidium bromide mixture was also applied to unfixed flat mounts of hyaloid vasculature and some specimens were processed for transmission electron microscopy. Vascular regression including cell loss in the connecting vessels, stretching and thinning of the vasa hyaloidea propria, tunica vasculosa lentis and the pupillary membrane was clearly evident after 13 weeks' gestation. Cresyl violet staining revealed condensed cells and pyknotic bodies throughout the hyaloid system; cell death occurred either in single cells or along small capillary segments associated with vascular regression. Acridine orange/ethidium bromide staining showed DNA condensation at early and late stages of cell death. Similarly, DNA nick-end labeling was positive in endothelial cells, pericytes and vessel and non-vessel associated hyalocytes. The observation of hyalocytes juxtaposed to cytolysed endothelial cells may indicate a role for these cells in vascular regression. Features of apoptosis were more evident during early vascular regression whilst necrosis was increasingly evident at later stages. Copyright 2000 Academic Press.