Basal membrane

Multi-layered extracellular structure which each epithelial tissue has on its basal side facing the connective tissue and by which the epithelial and connective tissues are linked together. The actual basement membrane and its individual layers are only visible by electron microscopy:

Adjacent to the plasma membrane of the keratinocytes is the 20-40 nm thick lamina lucida, which consists of laminin and fibronectin, among other substances, and is ultrastructurally transparent.

This is followed by the 30-50 nm thick, electron-dense lamina densa (basal lamina), which consists predominantly of type IV collagen and amorphous protein-polysaccharide complexes.

The electron microscopically transparent sublamina densa consists of bundles of elastic microfibrils, anchoring fibrils and glycoproteins.

Below the basement membrane lies another layer of reticular fibers (anchoring fibrils) and amorphous glycoproteins that is thicker than the other layers combined.

Above the basement membrane is the cell membrane of the basal keratinocytes with the hemidesmosomes: important proteins of this zone are plectin and bullous pemphigoid antigen 1 (BPAG1), components of the hemidesmosomes, which on the one hand are anchored to keratin filaments of the intracellular cytoskeleton of the basal keratinocytes. On the other hand, they are connected to the extracellular anchor filaments of the lamina lucida by the transmembrane proteins alpha-6-beta-4 integrin (see integrins below) and BPAG2.

All layers together represent the so-called basement membrane zone (BMZ or dermo-epidermal junction zone) visible under the light microscope.