Formation and Ultrastructure of the Peritrophic Membrane in Larval Midge Chironomus tentans (Diptera : Chironomidae)

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The ultrastructural features of the cardia from fourth instar largae of the midge Chironomus tentans are consistent with it being the source of the peritrophic membrane. The group of anterior epithelial cells display short basal membrane infoldings, abundant rough endoplasmic reticulum, Golgi complexes, mitochondria, free ribosomes, vesicles and electron-dense secretory granules. The secretory material is secreted into the narrow luminal cleft between the evenly spaced microvilli and the apposing non-secretory esophageal valve. The valve and apices of secretory cells act as a press to mold the secretory product into the definitive peritrophic membrane. The single 255-488 nm thick uniform peritrophic membrane is devoid of holes and is composed of three layers in the posterior cardia, two layers in the rectum but only one 110 nm thick discernable layer in the midgut. since the inner loosely arranged third layer is absent in the midgut and the rectum, it is postulated that it is incorporated into the electron-dense second layer. The second or middle layer which represents the whole peritrophic membrane in the midgut displays unique vertical striations that enclose 6.5 nm diameter channels. It is suggested that these newly described open-ended channels facilitate passage of water, salts, digestive enzymes and digested food material in either direction.
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