Keratinocytes forces

The boundary between the epidermis and dermis is a basement membrane (see Figure 3.9) it can be described by four planes proceeding from the basal epidermal side to the dermal side (a) the border of the basal ker-atinocyte (b) the lamina lucida, an electron lucent layer that lies beneath the epidermis (c) the lamina densa, an electron dense layer also known as the basal lamina and (d) the reticular lamina or subepidermal zone consisting of connective tissue immediately below the epidermis. The mechanical continuity at the epidermal-dermal junction, as well as between keratinocytes, is key to normal transfer of internal and external mechanical forces between the epidermis and dermis. 

Thus, external forces applied to the epidermis and/or internal forces present in dermis are transmitted through the cornified layer and lead to stretching of keratinocyte-keratinocyte cell junctions and the epidermal-dermal junction. These forces appear to be transmitted between epithelial cells via cadherins and may result in activation of secondary messengers which alter gene expression and protein synthesis. 

In Fig. 2.16 it shows the skin composition in layers, the stratum comeum, granular, spinosum, and basal. The epidermis uppermost layer, the stratum comeum, has keratinocytes tightly linked by des-mosomes in a hydrophobic cellular matrix. This sfructure CTeates an anatomical barrier against irritants, alleigens, and shear force. In addition to the physical barrier, there are mediators of resistance such as leucocytes, cytokines, complanent cascade, and host defense peptides [AMPs]. 

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