The Process of Keratinization
Unlike non-keratinized epithelium, superficial cells here accumulate keratin and transform into cornified scales.
The differentiation process (keratinization) includes the following stages:
- Keratin filaments accumulate in the cell cytoplasm, forming bundles known as tonofibrils.
- Cells synthesize specialized proteins (such as involucrin / keratolin), which thicken and reinforce the cell membrane from the inside.
- Keratin filaments gradually displace the nucleus and all organelles.
- The living cell transforms into a post-cellular structure — a corneocyte (cornified scale) — which is eventually shed.
Epidermal-Dermal Junction
When examining a histological specimen (e.g., from fingertip skin), the lower boundary of the epithelium appears highly irregular. The underlying connective tissue (dermis) projects into it as papillae.
This convoluted boundary (and the basal lamina mirroring it) increases the contact surface area between the layers, providing a stronger mechanical attachment.
Layers of Keratinized Stratified Squamous Epithelium
The epidermis is typically divided into layers based on the stage of keratinocyte differentiation. In "thick" skin (palms and soles), 5 layers are identified (from deep to superficial):
- Stratum basale (Stratum basale). The deepest layer, consisting of a single row of cells resting on the basal lamina. Nuclei are oval and oriented perpendicularly to the membrane. It contains stem cells, making this the germinative layer (stratum germinativum). Keratin content is initially low.
- Stratum spinosum (Stratum spinosum). Consists of 5–10 rows of polyhedral cells firmly connected by desmosomes (giving them a "spiny" appearance). Tonofibrils are arranged around the nucleus.
- Stratum granulosum (Stratum granulosum). Contains 3–4 rows of flattened cells. This is the most intensely stained layer in histological preparations due to basophilic keratohyalin granules (keratin embedded in a filaggrin matrix).
- Stratum lucidum (Stratum lucidum). Present only in thick skin. It consists of 3–4 rows of flat cells that have lost their nuclei and organelles but possess an envelope made of cross-linked proteins. The cells strongly refract light, forming a continuous eosinophilic band.
- Stratum corneum (Stratum corneum). The thickest and most superficial layer. It consists of multiple layers of dead cornified scales (corneocytes) filled with cross-linked keratin and coated with a protective protein envelope.