Astrocyte foot processes

Fig. 15.2 Diagram showing a transverse cross-section of a cerebral capillary. The endothelial cells, responsible for the main barrier properties of the blood-brain barrier are separated from the astrocyte foot processes, pericytes and occasional neurons by the basement membrane. All these components make up the blood-brain barrier.

Pericytes lie periendothelially on the abluminal side of the microvessels (Figure 15.3). A layer of basement membrane separates the pericytes from the endothelial cells and the astrocyte foot processes. Pericytes send out cell processes which penetrate the basement membrane and cover around 20-30% of the micro-vascular circumference [18]. Pericyte cytoplasmic projections encircling the endothelial cells provide both a vasodynamic capacity and structural support to the microvasculature. They bear receptors for vasoactive mediators such as catecholamines, endothelin-1, VIP, vasopressin and angiotensin II. Pericytes become mark-... 

Fig. 15.3 D iagram showing a longitudinal cross-section of the blood-brain barrier, with the brain capillary endothelial cells sealed by the tight junctions and surrounded by pericytes and astrocyte foot processes. These cellular components of the BBB are separated by a basement membrane.

Golden PL, Pardridge WM. P-glycopro-tein on astrocyte foot processes of unfixed isolated human brain capillaries. Brain Res 1999 819 143-146. 

The blood-brain barrier is markedly different from peripheral capillaries Peripheral capillaries are fenestrated with openings up to 50 nm wide. In contrast, cerebral endothelial cells are closely connected by tight junctions and zonulae occludentes, resulting in extremely high transendothelial resistances of up to 1500-2000 12 cm2 [16] (Figure 17.1). The capillaries are surrounded by a basal membrane enclosing intermittently pericytes, which have been postulated to be involved in host defense. The outer surface of the basement membrane is covered by astrocytic foot processes. Most likely, secretion of soluble growth factors by astrocytes plays an important role in endothelial cell differentiation. 

Adenosine, which crosses the BBB via the adenosine carrier, is converted to inosine via adenosine deaminase. This enzyme is localized in the astrocyte foot processes of the brain, with minimal localization in capillary endothelial cells. This astrocytic enzymatic barrier to adenosine movement into brain interstitial... 

Water moving from the blood into the brain through an intact BBB has to cross three membranes luminal and abluminal endothelial cell membranes, and the membrane of the astrocyte foot processes (Kimelberg, 2004 Tait et al., 2008). High density of AQP4 is present in the vascular-facing astrocytic membranes. Although... 

Transferrin synthesised by the oligodendrites in the brain will bind the majority of the iron that traverses the blood—brain barrier after the oxidation of the iron, possibly by a glycophosphoinositide-linked ceruloplasmin found in astrocytic foot processes that surround brain endothelial cells. Neurons acquire iron from diferric transferrin. However, the source of iron within microglia cells is unclear — other phagocytic cells such as macrophages, take up iron via transferrin receptors and release iron via ferroportin. 

A widespread early reponse by astrocytes is that they demonstrate swelling and cytoplasmic lucency within minutes of reduction in CBF.1416 However, there is increasing evidence that the microglial response (see below) precedes or at least parallels that of astrocytes. The most notable response by astrocytes occurs in perivascular foot processes (Figure 2.1), possibly related to the high concentration of transport systems in the membranes of these processes. Swollen astrocyte foot processes demonstrate a lucent cytoplasm lacking arty content of cytoplasmic... 

Astrocyte foot process Native Glucuronldated Siilfated Methylated... 

FIGURE 14.2 Schematic of the cross-sectional view of (a) a peripheral microvessel (the microvessel in nonbrain organs), and (b) the blood-brain barrier (BBB) or cerebral microvessel (the microvessel in the brain). In addition to other structures as in a peripheral microvessel, the BBB is wrapped by astrocyte foot processes (AS, green). BM, basement membrane (or basal lamina) EC, endothelial cell N, nucleus of endothelial cell P, pericytes SGL, surface glycocalyx layer TJ, tight junction. 

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