Arteriovenous anastomoses of the episcleral vasculature in the rabbit and rat eye.

PURPOSE

To obtain more information about the functional significance of arteriovenous anastomoses as regulatory elements, the authors investigated the topography and innervation of episcleral anastomoses in the rat and rabbit.

MATERIALS AND METHODS

The topography of arteriovenous anastomoses in the limbal and episcleral vasculature of the rat and rabbit eye was studied by scanning electron microscopy of vascular resin casts. The perivascular distribution of the following neuropeptides was investigated by fluorescence immunohistochemistry: neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), substance P (SP), and calcitonin gene-related peptide (CGRP).

RESULTS

The episcleral arteriovenous anastomoses connect arterioles directly with the episcleral venous plexus which also drains the aqueous humor. The nerve fiber plexus around the episcleral arteriovenous anastomoses is far more dense than around arteriovenous connections at the limbal arcades. NPY-, VIP-, SP-, and CGRP-immunoreactive fibers concentrate at the arteriolar segment of the episcleral arteriovenous anastomoses and are reduced at the venular segment. Still, numerous VIP-IR fibers are on the venous side of the rabbit episcleral anastomoses.

CONCLUSION

We conclude that the elaborated innervation of the episcleral anastomoses is a prerequisite for a subtle modulation of the blood flow and possibly of the aqueous humor outflow dynamics.