Morphology and permeability properties of blood capillaries in extraocular muscles of macaque monkeys.

Extrinsic eye muscles are very different from other skeletal muscles with regard to the morphology of their fibers and response to drugs. In addition, they are provided with an unusually rich blood supply. Since it had been previously reported that the capillaries of extraocular muscles are, at least in part, of the fenestrated type, a feature unusual for skeletal muscles, we have analyzed the morphology of these vessels in macaque monkeys and tested their permeability properties with horseradish peroxidase (HRP). Our study of the blood capillaries in the superior rectus muscle and levator palpebrae superioris in Macaca mulatta and M. fascicularis has demonstrated that these vessels are morphologically similar to the capillaries of other skeletal muscles. Furthermore, when HRP is introduced intravenam, it is transported out of the vessels by a vesicular transcellular mechanism. When HRP is injected in the interstitial spaces of the muscles, it is returned to the lumen by an identical vesicular transcellular transport. Thus, a bidirectional movement of macromolecules can take place across the walls of these vessels, and such a movement is not different from that previously reported for other skeletal muscles.