Layers of epidermis
Epidermis of the skin consists of four layers in thin skin and five layers in thick skin. The layers are called strata, and their names correspond to the layer's structure and functionality.
Figure 1. Cross section of the epidermis showing all five layers together with the cells.
Stratum basale is the bottom layer of the epidermis that connects it with the dermis. It consists of a single row of cuboidal or columnar newborn keratinocytes. Between them are stem cells, called basal cells, that undergo continuous cell division and produce new keratinocytes, pushing older cells up towards the surface of the skin.
Besides keratinocytes and stem cells, stratum basale also has melanocytes and Merkel cells scattered among the basal cells. Merkel cells are the link to the sensory neuron (nerve cell). They contact the flattened process of the neuron (called tactile disc). This connection detects touch sensations. The other type of cell, melanocyte, has long, slender projections that extend between the keratinocytes in stratum basale and stratum spinosum. These projections transfer melanin granules, a pigment produced by melanocytes, to the keratinocytes. The granules surround the cell's nucleus and shield the nuclear DNA from damage by UV light.
Second layer of the epidermis, stratum spinosum, consists of 8 to 10 layers of keratinocytes. They are slightly flattened and tightly joined by desmosomes. This connection provides strength and flexibility to the skin. In stratum spinosum, keratinocytes start to produce bundles of keratin in the form of intermediate filaments.
Scattered among keratinocytes in the stratum spinosum are Langerhans cells. These cells belong to the immune system thus they migrated to the epidermis from red bone marrow. Langerhans cells participate in an immune response against microbes that invade the skin. First, they recognize invading microbes, then help other immune cells destroy them.
Stratum granulosum consists of 3 to 5 layers of keratinocytes that slowly start to undergo apoptosis (programmed death). Their organelles start to degenerate as they move further away from the source of nutrition (blood vessels embedded in the dermis). In this layer, keratinocytes are rich in keratohyalin and lamellar granules. Keratohyalin molecules assemble keratin, and lamellar granules release a lipid-rich secretion that fills spaces between cells of the stratum granulosum, lucidum, and corneum. The secretion is water repellant, aiding water entry and loss regulation through the skin.
Stratum lucidum is built from 4 to 6 layers of flattened, clear, dead keratinocytes. At this point, the cells contain large amounts of keratin and have thickened plasma membrane. Their organs degenerated. Keratohyalin molecules present in keratinocytes in stratum granulosum are transformed into eleidin - transparent protein. Eleidin is converted into keratin in the final layer of the epidermis.
Stratum lucidum is only present in the thick skin.
The uppermost layer of the epidermis consists of 25 to 30 layers of flattened, dead keratinocytes that were transformed into corneocytes. Corneocytes are thin, flat, plasma membrane-enclosed packages of keratin that no longer contain a nucleus or any internal organelles. The cells are no longer connected by desmosomes, as they will eventually be shed off the skin surface. This thick, multiple layers of dead cells protects deeper layers of skin from injury and pathogenic invasion.