Epithelial Tissue

Epithelium is one of the four basic tissue types and is derived from all three germ layers: ectoderm (skin), mesoderm (lining of cardiovascular system), and endoderm (GI tract). Epithelial cells are polar, possessing an apical surface facing a lumen or exterior, and a basal surface anchored to the basement membrane via hemidesmosomes. These cells are avascular, relying on diffusion from underlying vascularised connective tissue. Junctional complexes, including tight junctions (zonula occludens), adherens junctions, and desmosomes, maintain structural integrity and regulate paracellular transport. Simple epithelium (one layer) is optimised for diffusion and absorption, whereas stratified epithelium (multiple layers) provides protection against mechanical or chemical stress. Specialised types include pseudostratified ciliated columnar epithelium (respiratory tract) and transitional epithelium (urothelium), which allows for distension in the urinary system. Glandular epithelium, derived from invaginations of surface epithelium, is classified as endocrine (secreting into blood) or exocrine (secreting into ducts). Understanding these arrangements is fundamental for identifying normal organ structure and pathological changes such as metaplasia or carcinoma.

The structural signature of epithelium is determined by its cytoskeleton and intercellular junctions. Keratin filaments provide mechanical strength. The apical domain may feature specialisations: microvilli (actin-based, enhancing absorption), cilia (microtubule-based 9+2 arrangement, facilitating motility), or stereocilia (long microvilli). The lateral domain contains junctional complexes that regulate permeability and cell-cell communication. The basal domain is anchored to the basement membrane, a complex structure of Type IV collagen, laminin, and proteoglycans. This membrane acts as a selective filter and a scaffold for regeneration. Epithelial renewal occurs via mitotic division of stem cells located near the basement membrane. Morphological classification is essential: squamous cells are flat, cuboidal are square-like with central nuclei, and columnar are tall with basal nuclei. Stratification is named according to the surface layer (e.g., keratinised stratified squamous epithelium of the epidermis).

Epithelial histology is central to understanding oncology (carcinomas) and chronic disease. In Barrett's Oesophagus, the stratified squamous lining replaces with simple columnar cells (metaplasia) due to acid reflux, increasing adenocarcinoma risk. In Celiac disease, the simple columnar epithelium of the small intestine loses its villous structure (villous atrophy). Immotile cilia syndrome (Kartagener's) leads to chronic respiratory infections. Pemphigus vulgaris is an autoimmune condition targeting desmosomes, causing epithelial cell separation (acantholysis) and blistering. Knowledge of these transitions is high-yield for MLA pathology questions.

Histological examination via biopsy is the gold standard. H&E staining typically shows blue (basophilic) nuclei and pink (eosinophilic) cytoplasm. Immunohistochemistry (IHC) for cytokeratins verifies epithelial origin. Electron microscopy may be used to identify ultrastructural defects in cilia or basement membranes in specific genetic conditions.

Not directly applicable to this basic-science topic; see related clinical topics on Gastro-oesophageal reflux disease, Celiac disease, or specific malignancies.