Keratinocytes: The Basis of the Epidermis
The dominant cell population (at least 85%), named for the accumulation of a specific protein — keratin (in the form of tonofilaments).
As they move toward the surface, cells undergo a pathway of terminal differentiation. Depending on the stage, they are called:
- Basal keratinocytes.
- Spiny keratinocytes.
- Granular keratinocytes.
- Keratinocytes of the stratum lucidum.
- Cornified scales (corneocytes).
Functions:
- Barrier formation: provided by hydrophobic cornified scales, lipids, and durable 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: UV Protection
These cells are of neural 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 tyrosinase and DOPA oxidase enzymes). Through long processes, melanosomes are transferred to keratinocytes and dermal macrophages.
Interesting fact: The number of melanocytes in Caucasian and Negroid populations is the same. Differences in skin color are due to the size of the melanosomes and the concentration of pigment within them.
Immune Surveillance: Langerhans and Greinstein 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 include:
- 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.
Greinstein cells are a poorly understood population of dendritic cells. It is believed that they present antigens to T-suppressor cells, suppressing excessive reactions and maintaining immune balance in the skin.
Merkel Touch Cells
Located in the basal layer (especially densely in the 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 approach 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.
Briefly: answers to topical questions
What layers are distinguished in the epidermis of thick skin?
Five consecutive layers are distinguished in the epidermis of thick skin.
These include:
- Basal layer (stratum basale) — the deepest.
- Spinous layer (stratum spinosum).
- Granular layer (stratum granulosum).
- Lucid layer (stratum lucidum) — a distinctive feature of thick skin.
- Horny layer — the outer thick layer consisting of 15–20 rows of keratinized cells.
The first two layers are similar to non-keratinizing epithelium, while all layers except the basal one are multilayered.
From which embryonic sources do various cell populations of the epidermis develop?
Various cell populations of the epidermis develop from several independent embryonic sources.
Sources of cell origin:
- Keratinocytes — form from the cutaneous ectoderm through proliferation and differentiation of an initially single-layer sheet.
- Melanocytes — are of neural origin, develop from the neural crest (neuroectoderm), and migrate into the epidermis.
- Langerhans cells — are of bone marrow origin, developing from blood monocytes.
- Merkel cells — are presumed to be of neural origin.
What is included in the composition of the epidermal proliferative unit (EPU)?
The epidermal proliferative unit includes a Langerhans cell and its surrounding keratinocytes.
According to the EPU concept, this structure is formed by:
- Langerhans cell (intraepidermal macrophage) — the central element, whose vertical processes set the direction for correct layer-by-layer organization.
- Group of keratinocytes — cells of varying degrees of maturity that unite and organize around this macrophage.
Upon migration of Langerhans cells, the columnar structure of the epidermis may be disrupted due to the disintegration of this unit.
What is the mechanism of melanin transfer from melanocytes to keratinocytes?
The mechanism of melanin transfer from melanocytes to keratinocytes is associated with melanosomes.
The process includes the following stages:
- Melanin accumulates in membrane-bound organelles — melanosomes.
- Melanosomes emerge from the processes of melanocytes into the intercellular space.
- Neighboring epidermal cells — keratinocytes — phagocytose these melanosomes.
Melanosomes can also be captured by dermal macrophages — melanophages.
To remember the specialized cells of the epidermis, imagine their "professions": Melanocyte is a painter (colors the skin with melanin), Langerhans is a security guard (immunity and phagocytosis), Merkel is a communicator (touch and impulse transmission).
Class and exam questions
Why do people have different skin colors despite having the same number of melanocytes?
Skin color is determined not by the number of melanocytes, but by the activity of their organelles — melanosomes. In people with dark skin, melanosomes are larger and contain more melanin.
Why is the absence of desmosomes beneficial for intraepidermal macrophages?
The absence of strong intercellular contacts (desmosomes) provides Langerhans cells with the mobility necessary for antigen capture and subsequent migration to lymph nodes.
How does massage promote skin healing?
Tactile stimulation activates Merkel cells. In response, they release hormone-like substances (such as vasoactive intestinal peptide), which improve blood circulation and accelerate epithelial regeneration.
What else to study on this topic
- Morphological changes of keratinocytes during terminal differentiation
- Structure of the epidermal proliferative unit (EPU)
- Biochemical cascade of melanin synthesis
- Interaction of Grüneberg and Langerhans cells with T-lymphocyte populations
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