Extracellular perivascular connective tissue space in the medial terminal nucleus of the accessory optic system in rats

Summary Two types of capillaries were found in the medial terminal nucleus of the rat accessory optic system. Type one capillaries are surrounded by glial processes and lack a perivascular space, whereas the type two capillaries and arterioles show a distinct extracellular perivascular space often filled with collagen fibrils. An internal as well as an external basal lamina lines these spaces. No fenestration of the endothelium was observed.This study was supported by N. I. H. Grant RR-00165 to Yerkes Regional Primate Research Center, and N. I. H. Grants R01-EY 00565-03 and R01-EY 00638-02 to J. Tigges. The excellent technical assistance of Mrs. G. L. Luttrell is gratefully acknowledged. We thank Dr. W. K. O'Steen for providing the rats and Mr. F. H. Kiernan for photographic help. Special thanks are due to Mrs. B. A. Olberding, in charge of the maintenance of the electron microscope.