Ultrastructure of Precapillary Sphincters and the Neurovascular Unit

Abstract

Neurons communicate with vasculature to regulate blood flow in the brain. The cells that maintain this are collectively named the neurovascular unit (NVU). This communication, known as neurovascular coupling, is thought to involve astrocytes or molecules that can pass through the astrocytic endfeet. However, the exact mechanism is still unclear. Using large 3D electron microscopy datasets, we can now study the entire NVU in context. In this study, I provide evidence for the role of precapillary sphincters as a hub for neurovascular coupling and endothelial transcytosis, as well as the role of collagen synthesized by fibroblasts in strengthening first-order capillaries. I also show how astrocytic endfeet form a barrier for fluid flow and how the microvasculature of the cortex is not innervated but is surrounded by a surprising organization of parenchymal neuronal processes around penetrating arterioles and arterial-end capillaries in both mouse and human brains.

Ultrastructure of the brain waste-clearance pathway

“…Microglia may phagocytose and subsequently break down metabolic waste within their lysosomes, while astrocytes may play a role in transporting waste out of the parenchyma. As previously described (Grubb 2022), astrocytic endfeet overlap tightly, making a paracellular route between astrocytic endfeet seem unlikely for waste clearance. Instead, waste clearance may occur through vesicular transport via the microtubule network (Figure 5f) and exocytosis into either the PVS (Figure 5C) or, for glia limitans astrocytes, into the subpial space.…”

“…This work is based on the MICrONS dataset provided by the Allen institute (Consortium et al 2021), as described previously (Bonney et al 2022;Grubb 2022). The dataset comprises an approximately 1 mm 3 chunk of the mouse (male P75-87) visual cortex, including parts of the primary visual cortex and higher visual areas for all cortical layers except extremes of L1.…”

“…However, the even distribution of macrophages could optimize the wastefiltering effect of phagocytosing macrophages. Perivascular fibroblasts generate collagen fibrils that span the PVS (Grubb 2022). These fibrils connect mural cells with astrocytic endfeet and may explain why the astrocyte-endfoot tube follows PA constrictions during sleep (Bojarskaite et al 2022), causing perivascular pumping (Mestre et al 2018).…”

“…The glymphatic system, which involves the perivascular pumping of CSF, also plays an important role in metabolic waste clearance(Rasmussen et al 2022). The neurovascular unit (NVU) is a key player in this system(Bojarskaite et al 2022; Fultz et al 2019; Grubb 2022; Grubb and Lauritzen 2019; Grubb, Lauritzen, and Aalkjær 2021), but its exact involvement is still unclear. Moreover, functional hyperemia, which increases blood flow to meet neuronal demands, may have an additional purpose, such as aiding in the removal of metabolic waste.…”

“…Leptomeningeal fibroblasts are characterized by their squamous appearance, often displaying a discshaped nucleus (Figure 1d and e). These cells contain a low number of lysosomes and phagosomes and possess a primary cilium and fibropositors (Grubb 2022). In the MICrONS dataset, fibroblasts can also be found surrounding pial and parenchymal blood vessels, along with collagen fibrils that wrap around the blood vessels and traverse the PVS or SAS to adhere to the astroglial basement membrane, as I recently described (Grubb 2022).…”