Chapter 5: Epithelial Tissue Structure & Function

Epithelium is an avascular tissue that covers the exterior surfaces of the body, lines its internal cavities and tubes, and forms glands, functioning primarily as a selective barrier between the external environment and the underlying connective tissue. Epithelial cells are characterized by their close adherence to one another through specialized cell junctions and by exhibiting functional and morphologic polarity, defining distinct apical, lateral, and basal domains. Epithelia are classified traditionally based on the number of cell layers (simple, stratified, or pseudostratified) and the shape of the surface cells (squamous, cuboidal, or columnar), with notable specific types including endothelium, mesothelium, and transitional epithelium (urothelium). The apical domain often displays specialized modifications for specific functions, such as microvilli (actin-cored projections increasing surface area for absorption, visible as striated or brush borders) and stereocilia (unusually long microvilli found in the epididymis and inner ear sensory cells). The apical surface may also feature cilia, extensions containing a microtubule-based axoneme. Motile cilia possess a 9 plus 2 axonemal arrangement, utilizing dynein motor proteins to propel fluid or mucus (e.g., trachea), while primary cilia are generally immotile with a 9 plus 0 arrangement, acting as crucial chemosensors and mechanosensors (e.g., in kidney tubules). The lateral domain maintains strong adhesion via junctional complexes, often visible in light microscopy as terminal bars. These complexes include occluding junctions (zonula occludens or tight junctions) which form the most apical seal, separating the apical and lateral domains and preventing paracellular transport using proteins like claudins, occludins, and tricellulin; anchoring junctions provide mechanical stability, exemplified by the zonula adherens (linking adjacent actin cytoskeletons via cadherins) and the macula adherens (desmosomes, spot-like junctions linking intermediate filaments via desmogleins and desmocollins); and communicating junctions (gap junctions) allow direct molecular exchange via connexons. The basal domain is secured to the underlying connective tissue by the basement membrane, which includes the epithelial product, the basal lamina (containing type IV collagen and laminins), and the connective tissue layer, the reticular lamina (type III collagen). This interface is stabilized by anchoring fibrils (type VII collagen) and cell-to-extracellular matrix junctions like dynamic focal adhesions (integrin-based, linked to actin) and stable hemidesmosomes (integrin-based, linked to intermediate filaments). Glands are organized as either exocrine (secreting via ducts using merocrine, apocrine, or holocrine mechanisms) or ductless endocrine tissues. Epithelial dynamics involve cell renewal via stem cells, and the reversible epithelial-mesenchymal transition (EMT), a process where epithelial cells lose polarity and adhesion to become migratory mesenchymal cells, relevant in development, fibrosis, and tumor metastasis.