The essence of the keratinization process
Unlike non-keratinized epithelium, here the surface cells accumulate keratin and transform into cornified scales.
The differentiation process (also known as the keratinization process) includes the following stages:
- Keratin filaments accumulate in the cell cytoplasm, forming bundles called tonofibrils.
- Cells synthesize special proteins (e.g., keratolinin) that thicken and strengthen the cell membrane from the inside.
- Keratin filaments gradually displace the nucleus and all organelles.
- The living cell turns into a post-cellular structure — a cornified scale — which is ultimately shed.
Border between the epidermis and dermis
Looking at a histological specimen (e.g., of fingertip skin), one can see that the lower border of the epithelium is very uneven. The connective tissue underlying the epithelium (dermis) projects into it in the form of papillae.
Such a tortuous border (and the basement membrane mirroring it) is necessary to increase the contact area between the layers, making their connection stronger.
Layers of stratified squamous keratinized epithelium
The epidermis is conventionally divided into layers depending on the stage of keratinocyte differentiation. In "thick" skin (on the palms and soles), 5 layers are distinguished (from bottom to top):
- Basal layer (Stratum basale). The deepest layer, consisting of a single row of cells lying on the basement membrane. The nuclei are oval and perpendicular to the membrane. Stem cells are located here, making it a germinative layer. There is still little keratin.
- Spinosum layer / Spinous layer (Stratum spinosum). Consists of 5–10 rows of polygonal cells firmly connected by desmosomes (hence the "spiny" appearance). Tonofibrils are located around the nucleus.
- Granular layer (Stratum granulosum). Contains 3–4 rows of flattened cells. This is the darkest-stained layer in the preparation due to basophilic keratohyalin granules (keratin on a filaggrin matrix).
- Lucid layer / Clear layer (Stratum lucidum). Present only in thick skin. These are 3–4 rows of flat cells that have lost their nuclei and organelles but have an envelope made of keratolinin. The cells refract light strongly, merging into a continuous eosinophilic (oxyphilic) band.
- Cornified layer / Horny layer (Stratum corneum). The thickest and most superficial layer. It represents multiple layers of dead cornified scales (corneocytes) filled with cross-linked keratin and covered with keratolinin.
Briefly: answers to topical questions
What other cell types, besides keratinocytes, are present in the epidermis?
In addition to keratinocytes, dendritic and neuroectodermal accessory cells are present in the epidermis. The following cell populations are distinguished:
- Melanocytes — pigment cells of neural origin (from the neural crest) located in the basal layer and synthesizing melanin.
- Intraepidermal macrophages (Langerhans cells) — phagocytic antigen-presenting cells of bone marrow origin involved in the immune response and containing specific Birbeck granules.
- Tactile cells (Merkel cells) — specialized cells performing a receptor function.
- Greenstein cells — are also part of the heterogeneous cell population of the epidermis.
How long does a complete cycle of epidermal renewal normally take?
The average time for complete epidermal renewal in humans is about 40 days. The process begins with the division of basal layer cells: keratinocytes move toward the surface, undergo successive changes, turn into cornified scales, and desquamate; new generations replace the shed cells. The rate of renewal depends on localization; on the soles, the process is faster.
Layers of the epidermis from bottom to top: Basal, Spinous, Granular, Lucid, Cornified. (The lucid layer is present only in thick skin!)
Class and exam questions
What is the main difference between thick and thin skin?
In thick skin (palms, soles), the epidermis consists of 5 layers — the stratum lucidum is present. In thin skin, the stratum lucidum is absent, and all other layers are significantly thinner.
What is a cornified scale?
This is a post-cellular structure, the result of keratinocyte cornification. It lacks a nucleus and organelles, is filled inside with keratin tonofibrils, and is covered on the outside with an envelope made of the protein keratolinin.
Why are dermal papillae needed?
Dermal papillae make the border between connective tissue and epithelium tortuous. This increases the contact area and securely fastens the skin layers together.
What else to study on this topic
- Structural features and functions of desmosomes in the spinous layer
- Stem cells of the basal layer and cellular renewal of the epidermis
- Chemical composition of keratohyalin granules (role of filaggrin)
- Molecular mechanisms of corneocyte cornified envelope formation
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