The electron microscope reveals that development of the hyaloid vessels is characterized by both increasing complexity of the structure of the vessel wall and increased differentiation and complexity of those components making up the vessel wall. Most early hyaloid capillaries are simple endothelial tubes with very irregular outlines. With development, hyaloid vessels become more rounded in contour and more thick-walled as a layer of pericytes appears in the wall. The pericytes become delimited on their outer side by a definite basement membrane. A layer of cytoplasmic filaments then differentiates along the inner aspect of the pericytes, and a basement membrane is formed between the pericytes and endothelial cells, enveloping the pericytes between leaflets of basement membrane. This gradual envelopment of pericytes in basement membrane is similar to that seen in the development of retinal vessels.
Jack RL. Ultrastructural Aspects of Hyaloid Vessel Development. Arch Ophthalmol. 1972;87(4):427-437. doi:10.1001/archopht.1972.01000020429013