Orthogonal arrays of intramembranous particles: a review with special reference to astrocytes

This review describes properties and occurrence of the so-called orthogonal arrays of intramembranous particles (OAPs). These peculiar components of membranes were described in astrocytes, intestinal cells, kidney collecting tubule cells, lens fiber cells, muscle cells and a number of other cell types in vertebrates. They are shown only by means of the freeze-fracture technique and are not understood functionally at present. They are suggested to play a role in ionic transport processes. They appear to represent proteins, but it is not known whether or not OAPs observed in different tissues are identical structures. In the central nervous system, they are confined to macroglial cell types such as astrocytes, retinal Müller cells, ependymal cells and tanycytes. Oligodendrocytes are devoid of them. Interrelations between OAPs and other membrane specializations such as tight junctions and caveolae are described. The most conspicuous property of astrocytic OAPs in situ is their unequal distribution. The resulting OAP-related polarity is lost in cultured astrocytes and therefore seems to require the intact microenvironment of the brain. The possible role of polarized astrocytes for the maintenance of the blood-brain barrier is discussed. In addition, the relationship between the capability of a nervous system to regenerate after injury and the occurrence of OAPs in astrocytes is considered.