Ultrastructure of the hyaloid vasculature in primates.

The histology and ultrastructure of the hyaloid vascular system was studied in 16 monkeys, ages 85 days in utero to 3 years old. The well-developed hyaloid artery is identifiable ultrastructurally as an arteriole consisting of a nonfenestrated intima, a multilayered smooth muscular media, a connective tissue adventitia, and a perivascular sheath. By the fourth week after birth, the vessel walls appear hyalinized and acellular; the lumen is occluded by a thrombus. The time of the complete disappearance of the hyaloid artery varied considerably in individual monkeys; remnants of the hyaloid artery were often present on the disc (Bergmeister's papilla) in adult life. The vasa hyaloidea propria and tunica vasculosa lentis are small branches of the hyaloid artery which fill the primary vitreous. Ultrastructurally, they are Type A-1-alpha capillaries having a nonfenestrated endothelium, incomplete pericyte layer, and basement membranes surrounding each. Fluorescein angiography of several newborn monkeys revealed nonleakage of dye from the hyaloid artery and its branches, thus correlating with the presence of endothelial tight functions.