Orthogonal Arrays

Numerous aggregates of intramembranous orthogonal arrays of particles (OAPs) were detected initially in freeze-fracture images of astrocytic membranes adjacent to blood vessels and to the fluid surfaces of the brain (Dermietzel, 1973, 1974; Landis and Reese, 1974). The accumulation of OAPs in restricted domains, particularly when apposed to vascular structures, has been interpreted as evidence for a role in metabolic exchange. However, it has not been possible thus far to establish parallels between cellular activities and the distribution of astrocytic particle assemblies (Landis and Reese, 1981a).

The situation is no less uncertain in the case of Müller cells, where the unusual aggregation of particles forming small linear, rectangular, or square patterns (Fig. 1.17) is a prominent feature (cf. Wolburg and Berg, 1988; Gotow and Hashimoto, 1989; Berg-von der Emde and Wolburg, 1989; Richter, Reichenbach and Reichelt, 1990). In the well vascularized retinas of some species (e.g., mouse), Müller cells make extensive contact with blood vessels, and the presence of OAPs in relatively high density at these sites may permit interactions with the vascular system. However, in species with largely avascular retinae (e.g., rabbit, guinea pig), OAPs are found predominantly at the end feet of Müller cells, where they abut the vitreous body (Wolburg and Berg, 1987; Gotow and Hashimoto, 1989). While there is no opportunity for metabolic interaction with the retinal vasculature at the vitreous interface, it has been postulated that the OAPs form ionic channels for release (into the vitreous) of the potassium accumulated by Müller cells in the course of neuronal activity (see Chapter 5). Thus far there is no biochemical or physiological evidence to support the contention that OAPs constitute ion-selective channels. Although both OAPs and K+ channels are seen predominantly in the endfoot region of some species, the association is too tenuous to link the two structures (Wolburg et al., 1992); potassium channels are distributed at many loci along the Müller cell membrane that lack OAPs.

The possibility that OAPs serve as transmembrane ion channels or represent enzymes of the Müller cell transport system cannot be ruled out, but as yet there is no convincing evidence to associate the arrays with any aspect of Müller cell function. From the appearance of deep-etch freeze-fracture images, Gotow and Hashimoto (1988) have suggested that OAPs provide physical stability against membrane bending at the Müller cell-vitreal interface, but it is not clear why this region is more subject to

Figure 1.17. Freeze-fracture replicas showing orthogonal arrays of particles (OAP) among the multiple intramembranous particles at the vitreal surface of Müller cell membranes. The particle arrays (circled) are seen in P-face images of two Müller cell endfeet from a peripheral part of the rabbit retina (A), and a Müller cell endfoot in the goldfish retina (B) (Wolburg and Berg, 1987; Berg-von der Emde and Wolburg, 1989). ([A] Copyright 1987 Elsevier Science, reprinted with permission. [B] Copyright 1989 Wiley-Liss, a subsidiary of John Wiley & Sons, Inc., reprinted with permission.)

Figure 1.17. Freeze-fracture replicas showing orthogonal arrays of particles (OAP) among the multiple intramembranous particles at the vitreal surface of Müller cell membranes. The particle arrays (circled) are seen in P-face images of two Müller cell endfeet from a peripheral part of the rabbit retina (A), and a Müller cell endfoot in the goldfish retina (B) (Wolburg and Berg, 1987; Berg-von der Emde and Wolburg, 1989). ([A] Copyright 1987 Elsevier Science, reprinted with permission. [B] Copyright 1989 Wiley-Liss, a subsidiary of John Wiley & Sons, Inc., reprinted with permission.)

Was this article helpful?

0 0

Post a comment