Chapter 2: Integrating Cells Into Tissues

Epithelia are characterized by continuous layers of cells (predominantly cellular) that function as selective barriers, protecting underlying tissues, lining surfaces, and performing secretion or sensory roles. Epithelial classification is based on cell layering, distinguishing between unilaminar (simple) types—like squamous (for rapid diffusion), cuboidal, and columnar—and multilaminar (stratified) types, such as stratified squamous epithelia (keratinizing or non-keratinizing, specialized for abrasion) and the unique urothelium (transitional epithelium) found in the urinary tract that stretches greatly without losing integrity. Glands are specialized epithelial aggregates, divided into exocrine glands, which secrete onto surfaces via ducts, and endocrine glands, which are ductless and release hormones directly into the circulatory system. Glandular secretion mechanisms include merocrine (exocytosis, most common), apocrine (loss of apical cytoplasm), and holocrine (entire cell disintegration). All epithelia rest upon the basal lamina, a specialized layer of extracellular matrix (ECM) composed primarily of laminin and type IV collagen, which serves vital roles as a selective filter, anchor, and regulator of cell polarity and division. In contrast, connective and supporting tissues consist largely of abundant ECM secreted by widely dispersed cells, providing structural support, defense, and trophic functions. Resident cells include fibroblasts (matrix synthesizers), adipocytes (fat cells for energy storage and thermal generation, like brown fat, which also act as an endocrine organ secreting adipokines), and mesenchymal progenitor cells. The defensive, migrant cell population includes macrophages, lymphocytes (B and T cells), mast cells (which release inflammatory mediators like histamine and heparin), and granulocytes. The connective tissue ECM features insoluble protein fibers, mainly robust collagens (e.g., Types I, II, III, and IV), and elastic fibrils rich in elastin. The interfibrillar ground substance consists of adhesive glycoproteins (like fibronectin and laminin) and highly hydrated, polyanionic molecules like proteoglycans and hyaluronan, which resist compressive forces. Finally, connective tissues are classified based on fiber orientation and cell type predominance into irregular types (loose, dense irregular, adipose) and regular types (tendons, ligaments), and form compound structures like mucosae (epithelium, lamina propria, muscularis mucosae) and serosae.