Figure 3.

Various electron photomicrographs, from the brains of 12-day-old hamster embryos, showing various aspects of the perforation of the cerebral cortex external glial limiting membrane by pial capillaries. (A) Photomicrograph (4 μm-scale) showing that the cerebral cortex delimited by the external glial limiting membrane (EGLM) composed of glial endfeet (G) covered by basal lamina material a several pial capillaries above it. The capillaries (*) are composed of endothelial cells separated by junctions (arrows) and their size varies, including very small (possibly growing) capillaries. One of the capillary has established direct contact with the cortex EGLM, with fusion of both vascular and glial basal laminae, and some endothelial cell filopodia (PF, long fine arrows) that already have perforated through the EGLM and penetrate into the nervous tissues. This pial capillary is a growing one with numerous internal and external filopodia as well as a sliding endothelial cells over its wall. (B) Detail of endothelial cell filopodia establishing early contacts with the cortex EGLM. (C) Detail of the entrance of an endothelial cell filopodium (PF) into the nervous tissue, through the fused vascular and glial basal laminae, with the formation of a funnel (white arrow) that remains open to the meningeal interstitium. (D) Low-power view showing a pial capillary (*) entrance into the nervous tissue through the EGLM (arrows) with the formation of the Virchow-Robin Compartment (V-RC) around it, and the penetration of a meningeal cell (P) into the extravascular space. The growing sprout of the perforating vessels is composed of several advancing endothelial cells. (E) High-powered view (4 μm scale) of a pial perforating capillary (*) showing its entrance into the cortex (curved arrows) and the formation of the extravascular V-RC (V-RC) around it. The continuous incorporation of additional glial endfeet (G) to the V-RC outer wall seems as an extension the surface EGLM that keep the vessels extrinsic (outside) of the nervous tissue. The entrance of a pericytes (P) from the meningeal space into the extravascular V-RC is also shown. The V-RC remains open to the meningeal interstitium and function as the brain's sole drainage (prelymphatic) system.