The human choriocapillaris: evidence for an intrinsic regulation of the endothelium?

The aim of the present study was to describe the ultrastructure of particular cells observed in the microvascular bed of the healthy human choroid, in close relation to the wall of the microvessels and resembling the periadventitial cells of other vascular areas of the human body. Serial sections of 12 fresh human eyes were studied by transmission electron microscopy. In all the eyes, the sections were obtained by cutting from the same zones (inner and outer choroid at the posterior pole of the eye). Standard techniques were used for transmission electron microscopy. Round cell bodies were found in the inner choroid at the posterior pole of the eye, mainly located in the intercapillary connective tissue. The cells were composed of an electron-transparent cytoplasm containing a few small mitochondria, and a dilated smooth surface of endoplasmic reticulum, at some points continuous with the nuclear membrane. These cells showed processes forming contact with the capillary wall. Some of these processes extended to the elastic layer of Bruch's membrane, but none had contact with the retinal pigment epithelium. A thin basement membrane surrounded both the cell bodies and processes. We believe that these cells are special cells resembling some type of periendothelial cells also localized in other microvascular districts of the human body. The close topographic correlation with the endothelial cells seems to indicate that these special cells play a role in the intrinsic control of proper endothelial functions.