Classification of epithelia
When describing any epithelium, all its characteristics must be stated sequentially. The classification is based on several features.
- Number of layers: simple (all cells contact the basement membrane) and stratified (some cells lack contact with the basement membrane).
- Number of rows (for simple epithelia): single-row and multi-row (pseudostratified). In pseudostratified epithelium, all cells maintain a connection with the basement membrane via a thin strip of cytoplasm, whereas in stratified epithelium, such a connection is absent.
- Cell shape: squamous, cuboidal, columnar (cylindrical is a synonym).
- Keratinization (for stratified epithelia): keratinizing or non-keratinizing. Transitional epithelium (urothelium), which lines the urinary tract and is capable of stretching, is distinguished separately.
- Specific structures: cilia (excretion of mucus in the upper respiratory tract), microvilli (increase of absorption surface area and membrane digestion in the small intestine).
Epidermis and epithelium: delimitation of concepts
The skin consists of three main layers: the epidermis, dermis, and hypodermis (subcutaneous adipose tissue). The epidermis includes an epithelium, but epithelial tissue is found far beyond just the skin surface: simple squamous epithelium covers the renal corpuscle capsule, forms the vascular endothelium and peritoneal mesothelium, and lines the excretory ducts of salivary glands.
In addition to proper epithelial cells, the epidermis contains cells of non-epithelial origin (neural and hematopoietic), which is why the concepts of "epidermis" and "epithelium" are not synonymous.
Thick and thin skin
Two types of skin are distinguished. Thick skin covers the palms and soles; it is characterized by a prominent stratum corneum, the presence of the stratum lucidum, and the absence of skin derivatives such as hair follicles and sebaceous glands. Thin skin covers the rest of the body: its stratum corneum is significantly thinner, the stratum lucidum is absent, but hair follicles and sebaceous glands are present.
Layers of stratified squamous keratinized epithelium
The epithelium of the epidermis typically consists of five layers. The epithelial cells are called keratinocytes because they contain a large amount of keratin, the main intermediate filament of epithelial tissue.
- Stratum basale (basal layer) — a single-layered row of cells lying on the basement membrane and forming papillary projections. The papillary layer of the dermis, made of loose fibrous connective tissue with blood vessels that provide diffuse nutrition to the epithelium, projects into it. The stratum basale is a cambial layer: the renewal of the entire epidermis occurs through cell division. The cells are basophilic due to an active synthetic apparatus and a large amount of RNA.
- Stratum spinosum (spinous layer) — polygonal cells tightly bound together by desmosomes. It provides the barrier function of the skin. Disruption of intercellular connections leads to the formation of intraepidermal cavities — blisters.
- Stratum granulosum (granular layer) — 3–4 rows of flattened spindle-shaped cells. It contains keratohyalin granules, the protein keratolininin (which forms the dense plasmalemma of overlying cells), the protein filaggrin (an amorphous matrix for keratin tonofilaments), as well as keratinosomes — granules containing precursors of the stratum corneum cementing substance.
- Stratum lucidum (clear layer) — present only in thick skin. Due to the identical refractive index of the cell contents and the intercellular cementing substance, cell boundaries are indistinguishable under a light microscope, and the layer appears as a uniform oxyphilic strip.
- Stratum corneum (horny layer) — the most superficial layer, represented by cornified scales without nuclei and organelles. It is filled with keratin tonofilaments, surrounded by a dense envelope of keratolininin; the scales are glued together by a cementing substance that forms the hydrolipid mantle. The layer gradually desquamates, and complete renewal of the epidermis takes about 28 days.
Cells of non-epithelial origin
The basal and spinous layers contain cells that are not epitheliocytes:
- Melanocytes synthesize the pigment melanin, which protects tissues from ultraviolet radiation.
- Langerhans cells — intraepidermal macrophages that provide immune defense and engulf damaged cells.
- Merkel cells are associated with nerve endings and function as mechanoreceptors.
Key points
- Epithelia are classified by the number of layers, rows, cell shape, presence of keratinization, and specific structures.
- The epidermis is a broader concept that includes the epithelium and cells of non-epithelial origin.
- Thick skin is characterized by prominent stratum corneum and stratum lucidum layers and the absence of hair and sebaceous glands.
- Stratified squamous keratinizing epithelium consists of the basal, spinous, granular, clear, and horny layers.
- The basal layer is cambial; the spinous layer provides strength via desmosomes.
- Keratohyalin, keratolininin, filaggrin, and keratinosomes accumulate in the granular layer.
- Melanocytes, Langerhans cells, and Merkel cells perform protective, immune, and receptor functions.
- Complete renewal of the epidermis takes approximately 28 days.