Vascular Approach and Contact with the CNS Surface
Embryonic pial vessels are separated from the CNS surface by pial cells, tissue space, collagen fibers and by their corresponding basal laminae (Figs 2.6, 2.7, 2.9). Occasionally, some pial vessels approach and establish direct contact with the surface of the CNS (Figs 2.6, 2.7, 2.9). The endothelium of these glia-touching vessels becomes parallel to the surface of the CNS and their corresponding basal laminae establish direct contacts at some points (Fig 2.6, insert). The only appreciable separation between these vessels and the surface of the CNS is that of their corresponding basal laminae (Fig 2.6, insert). The leading endothelial cells of these glia-touching vessels produce numerous filopodia, which project both inside and outside of vessel lumina (Figs 2.6, 2.7, 2.9). Some of the outside projecting filopodia perforate through the vascular basal lamina and establish direct contact with that of the surface of the CNS (Fig 2.6, insert). This type of vascular contact with the surface of the CNS is considered to be a prerequisite for its subsequent perforation.
Endothelial Filopodia Perforation of CNS Surface
The actual perforation of the CNS surface is carried out by the endothelial filopodia of glia-touching pial vessels (Figs 2.7, 2.9). This type of filopodial perforation occurs through areas in which the vascular and the CNS basal laminae are in contact (Fig 2.7). The perforating endothelial filopodia seem to be able to disintegrate (digest) both basal laminae and to pass through them into the nervous tissue (Fig 2.7). The filopodia enter the CNS tissue usually between adjacent glial endfeet (Figs 2.7, 2.9). The penetrating filopodia advance freely into the nervous substance and are deprived of recognizable basal lamina. This type of perforation can be carried out by several filopodia arriving from the leading endothelial cell or cells of the glia-touching capillary. The glial endfeet between the perforating filopodia often undergo swelling, vacuolization and their membrane disintegrate with the formation of myelin figures (Marin-Padilla 1985b). The endothelial filopodia of growing embryonic capillaries are able to perforate through anatomical barrier and to cause focal disintegration of the membrane of the glial endfeet of the CNS surface. Although the nature of this active process remains unknown, proteolytic enzymes, possibly produced by the endothelial filopodia, could participate in it (Ausprunk 1979). Although filopodia have been described in the leading endothelium of growing capillaries in the CNS (Klosovskii 1963, Bär and Wolff 1972, Wolff et al. 1975, Press 1977) and in a variety of experimental situations (Schoefl 1963, Ausprunk and Folkman 1977, Ausprunk 1979, Madri et al. 1983, Sholley et al. 1984) their possible significance and functional role in angiogenesis have been inadequately investigated.
At the site of the perforation the vascular and the CNS basal laminae fuse together around the perforating filopodia (Figs 2.7, 2.9). Their fusion creates a central opening through which the leading filopodia and subsequently the entire endothelial cell or cells are able to penetrate into the nervous tissue. The fusion of both basal laminae thus establishes anatomical continuity between the vessel wall and the surface of the CNS. The fusion of both basal laminae also establishes a shallow “pial-funnel” around the perforating vessels (Figs 2.7–2.9). This embryonic pial-funnel will play a significant role in the establishment of the VRC (Figs 2.8, 2.9).
Fig 2.6 Ultrastructural composition and organization of the pial vascular plexus and upper region of the cerebral cortex of a 12 day old hamster embryo. The pial vessels (*) are of differing calibers and are composed of several endothelial cells joined by tight junctions (arrows). The pial vessels are separated from each other and from the cortical surface by pial cells (F), intercellular spaces and fine collagen fibers. The marginal glia (G) covered by the CNS external basal lamina represents an anatomical barrier which must be perforated by the pial vessels. The pial vessel illustrated near the center of the figure approaches the cortical surface and its leading endothelial cells (1 and 2) have filopodia which project inside and outside its lumen. Some of these filopodia have established contact with the cortical surface. The endothelium of these glial-touching pial vessels becomes parallel to the cortical surface (insert) and the vascular and CNS basal laminae establish contacts at some points. Some filopodia from this glia-touching pial vessel (insert arrows) have perforated through the vascular basal lamina and have established direct contact with that of the cortical surface. This type of contact between the vascular and the CNS external basal laminae is considered to be a prerequisite for the subsequent perforation of its surface by the pial vessels. Few primitive neurons (N) are recognized in layer I. (From Marin-Padilla, M.: J. comp. Neurol. 241: 237–249, 1985), x5500.
Fig 2.7 Detail of the perforation of the cortical surface by the leading filopodium (arrow head) of a glia-touching pial vessel (*). The filopodium has perforated the cortical surface between two adjacent glia endfeet (G). The vascular and the CNS external basal laminae fuse around the perforating filopodium creating an opening through which whole endothelial cells eventually penetrate into the nervous tissue. The endothelial cell (E) of the upper pial vessel shows the prominent granular endoplasmic reticulum filled with dense material which characterize those of growing capillaries. Also illustrated are the processes of pial cells (F) and the tight junctions (arrows) of the pial vessels. (From Marin-Padilla, M.: J. comp. Neurol. 241: 237–249, 1985), x7000.
Fig 2.8 Detail of the ultrastructure of the newly formed intracortical capillary depicted in the left lower corner insert. Proliferation of penetrating endothelial cells (insert) results in the in situ formation of new intracortical vessels. At the entrance of the vessel a shallow pial-funnel (arrows) is formed between the fused vascular and CNS external basal laminae. This pial-funnel will elongate accompanying the newly formed intracortical capillary into the nervous tissue. It represents an embryonic Virchow-Robin compartment (VRC) and contains cytoplasmic processes of pial cells, fine collagen fibers and intercellular tissue spaces around the perforating vessel. The embryonic VRC and its vessels become separated from the nervous tissue by new glial processes (G) which become continuous with those of the marginal glia of the cortical surface. This newly formed glial wall is covered by new basal lamina material which becomes continuous with that of the CNS surface. (From Marin-Padilla, M.: J. comp. Neurol. 241: 237–249, 1985), x10000.
Fig 2.9 Schematic drawings illustrating the three fundamental stages of the vascular perforation of the surface of the embryonic cerebral cortex (G) by pial vessels (P). The following stages (from left to right) are illustrated: a) the early endothelial filopodia perforations; b) the endothelial cell perforation and proliferation with the in situ formation of a new intracortical vessel; and c) the establishment of the Virchow-Robin compartment (VRC). The fusion of the vascular and CNS external basal laminae around the perforating vessel and its participation in the formation of both the pial-funnel and the embryonic Virchow-Robin are also illustrated. The composition and organization of the embryonic VRC viewed in longitudinal and transverse (thick arrow) perspectives are also illustrated. The embryonic VRC represents a perivascular tissue space (ICS) formed between the vascular and the CNS basal laminae (BL). It contains the perforating vessel (E) with paravascular cells (P) enclosed within its basal