Lamellar to Tubular Conformational Changes in the Endoplasmic Reticulum of the Retinal Pigment Epithelium of the Newt, Notophthalmus Viridescens

The retinal pigment epithelium (RPE) of the newt (Notophthalmus viridescens) was examined ultrastructurally under both in-vivo and in-vitro conditions. Five distinct conformations of smooth endoplasmic reticulum (SER), two lamellar and three tubular, were observed. The two lamellar conformations included myeloid bodies, which have previously been described (Yorke and Dickson 1984), and fenestrated SER. The latter appeared as layers of flattened or curved cisternae which were penetrated by fenestrations. Fenestrated SER became indistinguishable from the highly branched and convoluted random-tubular SER through the formation of an intermediate configuration ("tubular sheets"). The remaining tubular SER conformations appeared to arise from random-tubular SER through a progressive reduction in branching and a straightening of individual tubules. Fascicular SER was represented by the hexagonal organization of straight, unbranched tubules into bundles (fascicles). Spiral SER consisted of a similar hexagonal arrangement, but the unbranched tubules spiralled about one another. Neighbouring tubules in areas of spiral SER were also joined together by pairs of electron-dense bars. Although lamellar (especially myeloid bodies) and random-tubular configurations of the SER were common features in vivo, fascicular and spiral SER were primarily conformations encountered in vitro. Conditions favouring bilayer lipid phases also appear to facilitate the formation of both myeloid bodies and fascicular SER. These conditions included increased duration of incubation, low (less than 20 degrees C) incubation temperatures, and Ca2+-free incubations with EGTA. Random-tubular SEB was most prevalent in media supplemented with fetal calf serum and also after warmer (30 degrees C) incubation temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)