Substructure in an Epithelial Basal Lamina (Basement Membrane)

Ultrastructural studies of the basal lamina (basement membrane) of most tissues describe a felt or meshwork of fibrillar material embedded in a gellike amorphous matrix (7, 22, 23, 36). In contrast, a regularly banded structure associated with the basal lamina or in connective tissue has been described in studies on a variety of tissues including Descemet's membrane (8, 12, 13, 14), the subcommissural organ of rat brain (20, 21, 34, 35), the trabecular meshwork of the eye (9-11, 29, 30, 32), Meissner's touch corpuscles (2), Schwann cell tumors of the VIIIth nerve (17, 18, 25), and in experimentally traumatized rat peripheral nerve (24). In all of these cases, it was inferred or directly stated that the banded structure was a form of collagen, probably the long-spacing type. Bertram and Bird (I) also noted banding in the basal lamina of Aedes aegypti midgut epithelium. Apparently, their observations were an example of longitudinal sectioning through this structure. The present work adds another dimension to these observations and provides a further demonstration of highly ordered substructure in a tissue noted for metabolic activity (1, 28). MATERIALS AND METHODS The midguts of Aedes aegypti mosquitoes infected with the avian malarial parasite, Plasmodium gallinaceum, were fixed in (i) 2.5 % glutaraldehyde in 0.1 N phosphate buffer or (ii) 2.5 % glutaraldehyde in 0.1 N sodium citrate. The tissue was postfixed in 1 % OsO4 in 0.1 N phosphate buffer or 1 % OsO4 in 0.1 N sodium citrate. Some tissue was fixed in phosphatebuffered OsO4 alone. The tissue was dehydrated in graded alcohols and embedded in Epon. Sections were cut on a Porter-Blum MT-2 ultramicrotome and stained with lead citrate alone or with alcoholic uranyl acetate followed by lead citrate according to Reynolds (26). Sections were viewed in a Siemens-Elmiskop I electron microscope.