Permeability of the midgut basal lamina in the mosquito, Culex tarsalis Coquillett (Insecta, Diptera).
The permeability of the basal lamina of the midgut of the mosquito, Culex tarsalis, was determined for engorged and unfed mosquito midguts both pre- and postfixed. A range of materials, with diameters from less than 2nm (lanthanum) through 90 nm, (polystyrene spheres), was examined. The permeability of the basal lamina was constant under all experimental conditions; 5--8 nm particles (colloidal thorium) were the largest to consistently permeate. The discussion is centered on the question of how a virus particle, often 10 times the diameter of the established permeability limits of the basal lamina, can traverse this structure. Possible explanations are: 1. The basal lamina is a dynamic, plastic structure that easily distorts under physical and/or biochemical stresses, 2. The virion may possess enzymatic activity that locally alters the structure of the basal lamina, or 3. The extracellular surfaces of the midgut epithelium or the basal lamina itself may possess enzymatic properties that alter the size or possibly the structure of the virion as it passes through.