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

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.