Origin and composition of the peritrophic membrane of the mosquito, Aedes aegypti

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An electron microscope study was made of the development of the peritrophic membrane (PM) of the mosquito, Aedes aegypti. Thin sections were stained with heavy metals, some of which were unusual, and both normal and extracted membranes were examined. The PM is secreted in a blind pouch formed by protrusion of the oesophagus into the anterior end of the midgut. The cuticle of the oesophagus tapers in thickness and at the end of the oesophagus consists of only epicuticle. There is no intergradation to midgut. At one particular cell boundary cuticle ceases, microvilli appear, and the appearance of the cytoplasm changes. The PM is formed from material secreted by a ring of midgut cells in the anterior half of the proventricular pouch. Cross-sections show about 40 cells; longitudinal sections show that the PM material comes from the anteriormost 8 to 10 of these cells; therefore, some 300 to 400 cells are involved. The PM is already present in larvae when they hatch from the egg. It becomes thicker and with more microfibrous layers as the larvae grow. The fully formed PM in an older larva is a differentiated structure consisting of a granular-appearing layer about 0·25 μm thick underlain by usually three layers of crossed microfibres separated by low density regions with very faint irregular fibres. The total thickness is about 1 μm. Extraction studies show that all of these parts contain chitin-protein units, and that accordingly chitin occurs in the PM in both fibrous and non-fibrous form. Reasons are detailed for concluding that the ring of midgut cells secrete a material that is or contains a chitin-protein component, that this material is of unknown physical state (viscous fluid?) but appears granular in stained thin sections, and that as this material moves posteriorly some physico-chemical phenomenon induces post-secretion aggregation to produce layers of microfibres within this secretion. The nature of the fibre-inducing and fibre-orienting mechanisms remains unknown but does not seem to involve a direct control by the secreting cells or their microvilli. The PM is not comparable in ulstrastructure to the cuticle of the larva. There is no equivalent of an epicuticle. Particles of a few dozen nanometers diameter do not penetrate into the PM from either side. The PM is affected by digestive enzymes and some other chemicals, and begins to show degradation in the more posterior parts of the gut. In contrast to existing literature, there is no evidence for the existence of a ‘mould’ or a ‘press’ and extrusion mechanism. There are a pair of cuticular collars at the end of the oesophageal valve but these do not form any mould or press—perhaps they restrict food from getting back into the proventricular pouch where the PM material is secreted, or aid in propelling the PM posteriorly. It is suggested that the only difference between the so-called type I and II of the PM's is the restriction of secretion to a ring of cells at the anterior end of the midgut in type II. There is no PM in the pupal stage, in the adult male, or in the unfed young adult female. However, within a short time after taking a blood meal a membrane, which we must call a PM, appears around the food bolus. The PM of the adult female is different from that of the larva: it is induced by feeding, it is thin, it shows no internal differentiation. This species, then, is an exception to Peters' generality that different stages of any one species have similar PM's.