Ultrastructure of precapillary sphincters and the neurovascular unit

Abstract

Neurons communicate with vasculature to regulate blood flow in the brain, a process maintained by the neurovascular unit (NVU). This interaction, termed neurovascular coupling, is believed to involve astrocytes or molecules capable of traversing the astrocytic endfeet. The precise mechanism, however, remains elusive. Using large 3D electron microscopy datasets, we can now study the entire NVU in context of vascular hierarchy. This study presents evidence supporting the role of precapillary sphincters as a nexus for neurovascular coupling and endothelial transcytosis. It also highlights the role of fibroblast-synthesized collagen in fortifying first-order capillaries. Furthermore, I demonstrate how astrocytic endfeet establish a barrier for fluid flow and reveal that the cortex's microvasculature is semicircled by an unexpected arrangement of parenchymal neuronal processes around penetrating arterioles and arterial-end capillaries in both mouse and human brains. These discoveries offer insights into the NVU's structure and its operational mechanisms, potentially aiding researchers in devising new strategies for preserving cognitive function and promoting healthy aging.

Microglia contact cerebral vasculature through gaps between astrocyte endfeet

“…Astrocyte endfeet ultrastructures have been previously described in detail. 4 (f) Representative example of a microglia nucleus (Nuc), cytoplasm (Cy) and lysosomes (Ly) in the Cortical MM^3 dataset. (g) 3D reconstruction of microglia in (f).…”

“…Arteriole and venule starting points were identified using annotations provided by previous studies (https://n9.cl/vid4i 4, 20 ). The colorized 3D segmentation of vasculature in Fig.…”

“…Microglia were identified as previously described. 4, 12 In brief, they were identified by their small soma size compared to other cell bodies, their distinct heterochromatin pattern, thin cytoplasm, prominent dark lysosomes (visible at low magnification), and an elongated irregular shape that often resembles a bean 22, 23 (Fig. 1(f-g)).…”

“…Oligodendrocyte progenitor cells (OPCs) are visually similar to microglia, but these were differentiated by the presence of primary cilia in OPCs 24 (see Supplementary Table 1 for examples), which are absent in microglia. 25 Endothelial cells were readily identified forming the inner lining of the vasculature 4 and pericytes were identified on the abluminal surface of endothelial cells on capillaries, with which they share the basement membrane. Both have protruding cell bodies and long processes.…”

“…2, 11 However, it is not yet clear if such discontinuities truly exist, or if they simply reflect astrocyte endfeet becoming so thin that they are difficult to discern, despite still being present. 4…”