A new mechanism for transitory,local endocytosis in photoreceptors of a spider,Dinopis

Summary Intermediate segment outgrowths (ISOs) are transitory specialisations of the plasma membrane of intermediate segments of the posterior median photoreceptors of Dinopis. Local regions form outgrowths into the glial partitions separating the receptors and remain connected to their parent intermediate segments by narrow necks. ISOs, only a few m in diameter, are sites of intense endocytosis. Coated pits in their plasma membranes give rise to saccular internalisations. Unusual, slender, endocytotic tubules either pinch off coated vesicles, or become detached to yield a tubular detritus. Products of endocytosis are assembled to yield multivesicular and dense bodies, which are usually surrounded by smooth saccules derived from the endoplasmic reticulum of the intermediate segment. ISOs also contain arrays of tubules, thought to be stacks of haemocyanin molecules. There are usually at least 10 times the number of empty ISOs as full and active outgrowths. The number of active ISOs increases rapidly at dawn to peak at about 3 h after sunrise and then rapidly declines. The present sample suggests that the number of empty ISOs increases steadily throughout the day. Thus, ISOs turn over and are probably rapidly formed and short-lived. The contents of ISOs are observed to be evicted into the intermediate segments, where they presumably join the population of secondary lysosomes and are digested. The cyclical activity of ISOs is greatest after shedding of rhabdomeral membrane at dawn (Blest 1978). ISOs are thought to be concerned with the return to the receptors of a minor fraction of rhabdomeral material lost to extracellular space during shedding of the microvillar membrane.The authors thank Professor D.T. Anderson, F.R.S., for use of field facilities of the Crommelin Biological Field Station of Sydney University at Warrah, Pearl Beach, N.S.W., and Andrew and Sally Austin and Carolyn Lawson-Kerr for help in the field. We are indebted to Rod Whitty and the Electron Microscopy Unit for support during these studies, and to Gary Brown for preparing Figs. 1 and 16