A tubular network associated with the brush-border surface of the Aedes aegypti midgut: Implications for pathogen transmission by mosquitoes

The mosquito Aedes aegypti is capable of transmitting a variety of pathogen s to man and to other vertebrates. The midgut of this insect has been well- studied both as the tissue where the first contact occurs between ingested pathogens and the insect host, and as a model system for blood meal digesti on in blood-sucking insects. To understand better the nature of the midgut surface encountered by parasites or viruses, we used scanning electron micr oscopy to identify the most prominent structures and cell morphologies on t he luminal midgut surface. The luminal side of the midgut is a complex and layered set of structures. The microvilli that are found on most, but not a ll, cells are covered by a network of fine strands that we have termed the microvilli-associated network (MN), The MN strands are membranous, as shown by a membrane bilayer visible in cross sections of MN strands at high magn ification in transmission electron micrographs. The MN is found in blood-fe d as well as unfed mosquitoes and is not affected by chitinase treatment, s uggesting that it is not related to the chitinous peritrophic membrane that is formed only after blood feeding. The cells in the midgut epithelium hav e two distinct morphologies: the predominant cell type is densely covered w ith microvilli, while cells with fewer microvilli are found interspersed th roughout the midgut, We used lectins to probe for the presence of carbohydr ates on the midgut surface. A large number of lectins bind to the luminal m idgut surface, suggesting that a variety of sugar linkages are present on t he structures visualized by electron microscopy, Some of these lectins part ially block attachment of malaria ookinetes to the midgut surface in vitro, Thus, the mosquito midgut epithelium, like the lining of mammalian intesti nes, is complex, composed of a variety of cell types and extensively covere d with surface carbohydrate that may play a role in pathogen attachment.