Astrocyte end-feet

Lo AC, Chen AY, Hung VK, Yaw LP, Fung MK, Ho MC, Tsang MC, Chung SS, Chung SK (2005) Endothelin-1 overexpression leads to further water accumulation and brain edema after middle cerebral artery occlusion via aquaporin 4 expression in astrocytic end-feet. J Cereb Blood Flow Metab 25 998-1011... 

Astrocytes have long been considered as structural support cells of the brain. The anatomy of brain microvasculature shows that astrocytic end feet constitute an envelope around blood vessels (Kacem et al., 1998). Astrocytic processes are positioned beneath the pial membrane and the ependymal surface and thereby segregate the CNS parenchyma from external environment (Figure 8.1). The cytoplasmic processes of astrocytes form a close network around the synaptic complex and maintain synaptic integrity (Newman, 2003). 

The BBB consists principally of concentric layers of endothelial cells, pericytes, basal membranes, and tightly apposed astrocytic end-feet,7 but it is the endothelium that is predominantly responsible for restricting the entry of leucocytes into the CNS. Several features of the CNS endothelium may contribute to its unique barrier function the cells are interconnected by tight junctions, are relatively devoid of pinocy-totic vesicles and fenestrations, cany an unusually high net negative surface charge,24 and have a low or inappropriate expression of surface adhesion molecules that support leucocyte attachment.21 25"27 Of the small number of leucocytes present in the normal brain, the majority are lymphocytes.3 When the BBB is compromised serious pathological manifestations may ensue as illustrated by multiple sclerosis (MS) in which the features of encephalomyelitis are attributed to the CNS entry, perivascular sequestration, and local activation of sensitized lymphocytes and other blood mononuclear leucocytes. 

Figure9.12 (a) Cerebral capillary covered with astrocyte end-feet, (b) Cross section of a brain microvessel formed by endothelial cells, pericytes, and astrocyte end-feet (from Ref. [104]).

Fig. 2. Metabolic interactions between neurons and astrocytes. Glucose enters the brain through the astrocytic end feet that envelop brain capillaries. In the astrocytes some of the glucose is metabolized to lactate which is exported to the extracellular fluid and taken up by neurons. In neurons lactate is converted to pyruvate which is either decarboxyiated to acetyl-CoA or carboxykited to malate to enter the TCA cycle. Glutamate may therefore be formed in neurons from a-ketoglutarate or from glutamine, which is imported from astrocytes. The glutamate that is released is taken up by astrocytes and amidated to glutamine or metabolized via the TCA cycle. The malate thus formed may leave the TCA cycle and become decarboxyiated to pyruvate and lactate. For lack of space, astrocytic pyruvate carboxylation is indicated only by the reversible formation of lactate. Notice that the relative importance of the various pathways in vivo is a matter of debate (see text).

Matute C, Gutierrez-lgarza K, Ri o C, Miledi R (1994) Glutamate receptors in astrocytic end-feet. Neuroreport 5 1205-1208. 

There are two barrier systems in the normal adult brain the blood-brain barrier (BBB) and the blood cerebrospinal fluid barrier (BCSFB). The BBB is composed of three cell types, all of which play an important role in BBB integrity and regulation endothelial cells of the cerebral vasculature, astrocytes, and pericytes (Fig. 1). Pericytes of BBB localize to the capillary basement membrane, sandwiched next to endothelial vessel cells by astrocytic end feet, which form very extensive and intimate bundles around both pericytes and endothelial vessel cells (Fig. 1). Specific characteristics of the BBB are believed to be maintained and induced by the astrocytic processes [28 30]. The endothelial cells of the cerebral vasculature differ from their counterparts in peripheral systems in that they lack fenestrations [31 33], demonstrate little to no... 

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