Keratinocytes: The Foundation of the Epidermis
The dominant cell population (at least 85%), named for their accumulation of a specific protein—keratin (in the form of tonofilaments).
As they migrate toward the surface, cells undergo terminal differentiation. Depending on their stage, they are called:
- Basal keratinocytes.
- Spinous keratinocytes.
- Granular keratinocytes.
- Lucid keratinocytes (stratum lucidum).
- Cornified squames (corneocytes).
Functions:
- Barrier formation: provided by hydrophobic cornified squames, lipids, and strong desmosomes in the deep layers.
- Immune regulation: keratinocytes synthesize thymic hormone analogs (thymosin, thymopoietin) that stimulate T-lymphocytes.
- Metabolism: activation of vitamin D under the influence of ultraviolet radiation.
Melanocytes: Protection Against Ultraviolet Radiation
These cells are of neural crest origin and are localized strictly in the basal layer (making up 10–25% of its cells). Unlike keratinocytes, they lack desmosomes.
Their cytoplasm contains melanosomes—membrane-bound organelles where the pigment melanin is synthesized from the amino acid tyrosine (with the participation of the enzymes tyrosinase and DOPA oxidase). Through long processes, melanosomes are transferred to keratinocytes and dermal macrophages.
Fun fact: The number of melanocytes is identical in people of Caucasian and Negroid races. Differences in skin color are due to the size of the melanosomes and the concentration of pigment within them.
Immune Surveillance: Langerhans and Greenstein Cells
Langerhans cells (intraepidermal macrophages) develop from blood monocytes. They are located in the basal and spinous layers, do not form desmosomes, and can migrate to lymph nodes. Their tasks:
- Phagocytosis of foreign antigens and their presentation to lymphocytes.
- Secretion of interleukins (IL-1, IL-6) and antimicrobial factors.
- Structural role: their processes form the epidermal proliferative unit (EPU), which organizes keratinocyte growth.
Greenstein cells are a poorly understood population of dendritic cells. They are believed to present antigens to T-suppressor cells, suppressing excessive reactions and maintaining immune balance in the skin.
Merkel Tactile Cells
Located in the basal layer (most densely in fingertips and hair follicles). They function as mechanoreceptors reacting to light touch.
Unlike other accessory cells, Merkel cells are connected to neighboring keratinocytes by desmosomes. Sensory neuron terminals abut them. In addition to generating nerve impulses, they perform a neuroendocrine function: upon tactile stimulation, they paracrinely release peptides (enkephalin, vasoactive intestinal peptide). This regulates vascular tone and accelerates epithelial regeneration.