1. Morphology and differentiation
The morphology of keratinocytes changes during their life cycle, that lasts about 1 month in humans. The changes are progressive from the basal layer, where keratinocytes are born, until the superficial layer, where they die and detach from the epidermis. The morphological differences of keratinocytes lead to the features of the epidermal layers. The layers or strata (stratum in singular) are the stratum basale, stratum spinosum, stratum granulosum and stratum corneum. Excepting the stratum basale, which is relatively constant, the layers may change in thickness depending on the body region (Figure 1). Thus, the summed thickness of the epidermis, i.e., the thickness of the strata together, may range from 50 µm in body regions with little mechanical stress to 1 mm in those regions like the palms and the soles, where friction is higher.
Epidermis
Figure 1. Epidermis showing different thickness. A) Thick epidermis from the sole. B) Thick epidermis from the palm. C) and E) Thin epidermis. D) Moderately thick epidermis. All the images are from rat skin. The different layers of the epidermis are only distinguished in the thick epidermis, because the proliferation rate of keratinocytes is higher. However, the same keratinocyte life cycle happens in the thin skin. The black lines indicate the basal, spinosum, and granulosum strata, but not the corneum stratum.
2. Stratum basale
The stratum basale, or germinative layer, (Figures 1 and 2) is a one thick cell layer made up of keratinocytes, and other very scarce cells, found in the deeper part of the epidermis. Stratum basale keratinocytes are attached to the basal lamina by hemidesmosomes. These connections helps to maintain the integrity of the epithelium and controls the proliferation and differentiation of keratinocytes.