Rapid synthesis of photoreceptor membrane and assembly of new microvilli in a crab at dusk

Abstract

Large areas of photoreceptor membrane are synthesized in the retinula cells of the crab Leptograpsus variegatus at dusk. Initially, new membrane differentiates from rough endoplasmic reticulum (ER) as large tubules of smooth ER. These tubules transform to concentric ellipsoids of closely apposed pairs of membranes ("doublet ER"), sometimes passing through an intervening crenate form. The new membrane is transported through bridges of cytoplasm that cross the palisade to the rhabdom region, from which the remains of the rhabdomeres that were built during the previous dusk have been dissolved. The degradation of the old microvilli of one rhabdomere is acomplished without affecting neighbouring rhabdomeres of other cells. New microvilli are assembled in situ from sheets of doublet ER, which are converted to tubules oriented in the same direction as the future microvilli. The cytoplasmic face of the ER remains the cytoplasmic face of the tubules, which become progressively narrower, partly by further longitudinal division, until the final diameter of the microvillus is reached. A central core is often seen in transverse sections of mature microvilli. It may be involved in the final consolidation, but rhabdomeric microvilli are not formed in the same manner as those of intestinal brush border cells. There is no evidence that new membrane passes through the Golgi compartment before incorporation into the rhabdom, as is the case for rod outer segment membrane in vertebrate photoreceptors.

Mosaic Eyes is a novel component of the Crumbs complex and negatively regulates photoreceptor apical size

“…It has been estimated that as much as 10% of the OS is shed and renewed daily in mammals (Young, 1967), and both processes are regulated by cyclic light (Besharse et al, 1977;Hollyfield and Rayborn, 1979;Stowe, 1980); experiments in the locust suggest these processes are controlled locally (Williams, 1982). To our knowledge, no molecules or molecular mechanisms have been identified that regulate the process of apical membrane renewal in photoreceptors, although many genes, including that encoding Rhodopsin, are required for the formation of the OS (Lem et al, 1999).…”

Ultrastructural Changes of Rhabdoms of the Eyes of Ocypode Species in Relation to Different Regimes of Light and Dark Adaptation

“…DeBruin and Crisp, 1957;Bryceson and McIntyre, 1983;Shelton et al, 1986) has been devoted to elucidate circadian effects on the responsiveness of distal, proximal, and reflecting pigments (Fig. 13), and considerable excitement was generated in the 70s and 80s after the discovery was made that photoreceptor turnover processes are diurnally modulated (Nässel and Waterman, 1979;Stowe, 1980;Toh and Waterman, 1982). Numerous intracellular changes, associated with rhabdom degradation (Hafner et al, 1982b) and up to 20 fold differences in rhabdom volume between day and night conditions were recorded in some species (Nässel and Waterman, 1979).…”

Chemical cues from fish heighten visual sensitivity in larval crabs through changes in photoreceptor structure and function

“…Chamberlain and Barlow (1987) similarly observed wider, shorter rhabdoms in Limulus at night when animals are known to be most visually sensitive. Furthermore, rhabdom size increased in adult H. sanguineus and several other adult crab species during the nocturnal phase of the diel cycle (Nässel and Waterman, 1979;Stowe, 1980;Toh and Waterman, 1982;Arikawa et al, 1987) and several studies have similarly reported greater rhabdom width or volume in more-sensitive dark-adapted eyes (Meyer-Rochow and …”