General Principles of Structure and Classification
The primary morphological criterion for any stratified epithelial tissue is that exclusively the bottom row of cells is attached to the basement membrane. All other layers lie above it and lack this contact.
Subgrouping of stratified epithelia is based on two key features:
- Presence or absence of keratinization (cornification).
- The ability of the entire cell sheet to change its thickness (e.g., during organ distension).
Tissue turnover relies on stem cells that permanently reside in the basal layer. Following mitotic division, some daughter cells enter the differentiation pathway. As they age, they are progressively pushed upward. Upon reaching the apical (superficial) zone, the old cells slough off. Although this renewal mechanism works regularly, it proceeds more slowly than in simple epithelia.
Non-Keratinized Stratified Squamous Epithelium
This tissue exhibits prominent stratification. Although the total number of cell rows may exceed ten, histologically the layer is divided into three main zones:
- Basal layer (stratum basale). Located directly on the basement membrane. Contains stem and differentiating cells. Nuclei are oval and oriented perpendicularly to the membrane (vertical orientation).
- Spinosum (prickle cell) layer (stratum spinosum). Multilayered. Cells acquire an irregular polygonal shape, and their nuclei become round. Cells form short cytoplasmic projections ("spines") that bind firmly to neighboring cells via intercellular junctions known as desmosomes.
- Squamous cell layer (superficial layer). Also multilayered. Cells here are flattened, and their narrow, rod-like nuclei are elongated parallel to the surface of the sheet. Over time, these cells desquamate.
Note: The shape and horizontal orientation of the nuclei in the superficial layer serve as a crucial diagnostic feature distinguishing this epithelium from transitional epithelium on histological slides.
Topography: This type of epithelium lines the oral cavity, esophagus, conjunctiva, and forms the corneal epithelium of the eye.
Corneal Characteristics and Stratified Cuboidal Epithelium
The anterior corneal epithelium has a unique histological feature. Directly beneath it lies the anterior limiting lamina (Bowman's layer) — a specialized, very thick basement membrane. Unlike basement membranes in most other organs, it is clearly visualized under light microscopy as a wide eosinophilic band.
Stratified cuboidal non-keratinized epithelium is also found in the body, though it is a relatively rare tissue type. Its main locations include:
- Excretory ducts of salivary, sebaceous, and sweat glands;
- Columnar zone of the anal canal;
- Coverings of growing ovarian follicles.