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Ultrastructure of tracheal hemidesmosomes in mice. In a normal mice(a) there are well-defined, organized hemidesmosomes with darkened areas in the lamina densa abutting the hemidesmosome (arrows). In contrast, hemidesmosomes in Lamc2 -/- tracheas (b) are less organized, the intracellular component is more diffuse, and the lamina densa directly below the hemidesmosomal areas lacks the electron density seen in the littermate control (arrows). From Nguyen et al., 2006.[1]

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 when visualized by electron microscopy. While desmosomes link two cells together, hemidesmosomes attach one cell to the extracellular matrix. Rather than using cadherins, hemidesmosomes use integrin cell adhesion proteins. Hemidesmosomes are asymmetrical and are found in epithelial cells connecting the basal face to other cells.

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 structural 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.

An example configuration of a hemidesmosome might consist of cytosolic keratin, non-covalently bonded to a cytosolic plectin plaque, which is bonded to a single-pass transmembrane adhesion molecule such as the α6β4 integrin. The integrin might then attach to one of many multi-adhesive proteins such as laminin, resident within the extracellular matrix, thereby forming one of many potential adhesions between cell and matrix.

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 dense 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.

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