Microscopic investigation of the hyaloid vascular system in 5-month-old human fetuses.
Corrosion casting and light and scanning electron microscopy.
The hyaloid artery ramifies into a tuft of vasa hyaloidea propria, which communicates with the posterior portion of the tunica vasculosa lentis, characterized by a network of anastomosing vessels. They further pass to the lateral portion of the tunica, acquiring a nonanastomosing palisadelike array and drain into the vessels of the ciliary processes or, after bending over the edge of the developing iris, drain into the outer choriocapillaris. The tunica vasculosa lentis vessels also communicate with the pupillary membrane, a system of vascular arcades arranged in several interconnected tiers, supplied by the terminal branches of the long posterior ciliary arteries. In tunica vasculosa lentis, arterioles seem to pass directly into veins, without forming a capillary bed.
At the investigated developmental stage, the fully developed hyaloid system enters its subsequent involution, and the vessels nourishing the vitreous have already involuted. The system is generally similar to that observed in other mammals.