The
basal lamina is a layer of
extracellular matrix secreted by the epithelial cells, on which the
epithelium sits. It is also known as the
basement membrane This structure is visible only with the electron microscope, where it appears as an electron-dense layer, 20-100 nm thick (with exceptions such as the basal laminae that compose the 100-200 nanometre thick
glomerular basement membrane).