Orthogonal arrays of particles in plasma membranes of Müller cells in the guinea pig retina

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Abstract
Plasma membranes of guinea pig Müller cells were examined with a freeze‐fracture technique to see how orthogonal arrays are distributed in the avascular retina. Examination of the portion approximately intermediate between the optic disc and equator of the eyeball showed that all end‐feet of Müller cells were provided with arrays. Orthogonal arrays were concentrated on vitreal end‐foot membranes, i.e., membranes that were covered by the basal lamina and contacted the vitreous body, called vitreal membranes here. The arrays were rarely observed in the portions of end‐feet that did not contact the vitreous body, called lateral membranes. The distribution density of arrays in the vitreal membranes was 122.5 ± 45.3/μm2, which was over 10 times higher than that (9.6 ± 9.6/μm2) in the lateral membranes. The arrays became numerous and extended in shape at the periphery of the vitreal membrane, characteristically aligned in rows at the border where vitreal met lateral membrane, but never intruded into the domain of lateral membrane. Some arrays were composed of loosely attached particles and/or rod‐like profiles. Sometimes rod‐like profiles, 9–13 nm wide and 20–50 nm long, called linear structures here, were isolated, and sometimes they appeared in rows. Ordinary intramembrane particles (IMPs) were significantly smaller and less numerous in vitreal than in lateral membranes. IMPs larger than 9 nm in diameter were significantly fewer in the vitreal membranes, which suggests that they have been consumed to form the arrays. Although the distribution of orthogonal arrays is similar to that of K+ channels (Newman: J. Neurosci., 7:2423–2432, 1987), we consider the array an unlikely candidate for the ion channel, because its subunit particles do not protrude onto either the inner or outer surface of the membrane (Gotow and Hashimoto: J. Neurocytol., 17:399–413, 1988). Judging from their unique alignment in rows where the membrane is bent and vitreal and lateral membranes meet, the arrays may contribute to some membrane stability, resisting the physical tension at the interface with mesenchymal tissue.