Hemidesmosome

Hemidesmosomes (HD) are very small stud- or rivet-like structures on the inner basal surface of keratinocytes in the epidermis of skin. They are similar in form to desmosomes. The HD comprises two rivet-like plaques (the inner and outer plaques), together with the anchoring fibrils and anchoring filaments these are collectively termed the HD-stable adhesion complex or HD-anchoring filament complex. Together, the HD-anchoring filament complex forms a continuous strucutral link between the basal keratinocyte keratin intermediate filaments and the underlying basement membrane zone (BMZ) and dermal components. Over the past decade, these structures have been shown to comprise a variety of some 10 or more molecular components.

Electron microscopic analysis of the epidermal basement membrane zone (BMZ) reveals that it comprises a narrow and sometimes folded interface between the basal keratinocytes and the dermis. At high power, several complex structures are observed within the epidermal BMZ. The epidermal BMZ shows small (< 0.5 micrometers), regularly spaced electron dense structures which are the hemidesmosomes. Thin, extracellular, electron-dense lines, parallel to the plasma membrane, subjacent to the outer plaque are visible in one third of HDs and are termed sub-basal dense plates (SBDPs). Anchoring filaments traverse the lamina lucida space and appear to insert into the electron densa zone, the lamina densa. Beneath the lamina densa, loop-structured, cross-banded anchoring fibrils extend more than 300 nm beneath the basement membrane within the papillary dermis. The length of these loops may enable them to link or encircle dermal collagen fibers or other components such as those of the elastic microfibril network.

See also: desmosome, epidermolysis bullosa