Epithelial Structure and Function

A comprehensive set of vocabulary flashcards detailing epithelial cell polarity, plasma membrane domains, junctional complexes, transport pathways, surface specializations, and glandular classification.

Epithelial Polarity

A condition where a cell is polarized if its functions are preferentially directed towards one end, evidenced by apical specializations, supranuclear position of the Golgi complex, and apical secretory product accumulation.

Apical Plasma Membrane

The cell membrane domain responsible for regulation of nutrient and water uptake, regulated secretion, and protection; it contains glycolipids, cholesterol, $H/K\text{ ATPase}$, and anion channels.

Lateral Plasma Membrane

The cell membrane domain involved in cell contact, adhesion, and communication, characterized by cell adhesion molecules and the junctional complex.

Basal-Lateral Membrane

The cell membrane domain that manages signal reception and transduction, generation of ion gradients, and constitutive secretion; it contains receptors for hormones and binding sites for basal lamina constituents.

Cell adhesion molecules (CAM)

A family of glycoproteins in the cell membrane that maintains the cohesion and arrangement of cells within an epithelium.

Cadherins

Specific glycoproteins responsible for the mutual recognition and cohesion of similar cells.

Zona Occludens

Also known as a tight junction, this belt-like membrane specialization encircles columnar cells to serve as a barrier between apical and basolateral domains and enable selective permeability.

Transcellular Pathway

A transport method involving the active uptake of substances at the free surface by pinocytosis, followed by vesicle transport across the cell and release at the base.

Paracellular Pathway

A transport method where molecules passively diffuse through the intercellular spaces of the epithelium.

Zonula Adherens

A junctional site below the zonula occludens that encircles the apical portion of cells, stabilizing the epithelium by connecting the terminal webs of adjoining cells.

Desmosomes

Also called macula adherens, these small, round junctional specializations on the sides of adjoining cells link cytoskeletons together to contribute to epithelial stability.

Hemidesmosomes

Junctions found in stratified squamous epithelium that enable the tissue to withstand shearing stresses and blows without separating from the basal lamina.

Gap Junction

Also known as nexus, this junctional complex is primarily concerned with cell-to-cell communication and is abundant in muscle to ensure simultaneous contraction.

Connexons

Structures that project into the intercellular space and link end-to-end to form channels for the free passage of ions, amino acids, and cyclic AMP less than $2\text{ nm}$ in diameter.

Basal Lamina

The extracellular layer found beneath all epithelia, consisting of the lamina lucida and lamina densa.

Lamina lucida

A transparent layer adjacent to the epithelium composed of large proteoglycan, laminin, and atypical collagen (type IV).

Lamina densa

A meshwork of fine filaments composed of collagen (type VII) with anchoring fibers that loop around type I collagen in the underlying matrix.

Brush border

A specialized apical region for absorption exhibiting fine vertical striations made of closely spaced microvilli.

Microvilli

Slender cell processes $1-2\text{ }\text{μm}$ long and $80-90\text{ nm}$ in diameter that amplify the surface area for absorption.

Glycocalyx

A furry coat over the brush border formed by very fine branching filaments extending from the tips of microvilli.

Cilia

Motile cell processes $7-10\text{ }\text{μm}$ in length and $0.3\text{ }\text{μm}$ in diameter that beat in the same direction.

Epithelial Renewal

The process where epithelial cells with limited lifespans are continually lost by exfoliation or programmed cell death and replaced by mitotic division.

Absorptive Epithelia

Epithelial cells specialized for the uptake of ions and small molecules from the lumen and their movement to the extracellular fluid.

Secretory Epithelia

Epithelial cells specialized for the synthesis of a specific product and its release onto an internal or external surface.

Exocrine Glands

Glands that deliver secretions onto a body surface via a system of ducts, classified as simple (unbranched) or compound (highly branched).

Goblet Cells

Solitary secretory cells found among absorptive cells, featuring a narrow basal region and an expanded apical region for storing secretory products.

Mucous Glands

Glands that produce a viscous surface layer composed of mucopolysaccharide.

Serous Glands

Glands that produce a watery secretion often rich with enzymes.

Merocrine

A method of secretion where the product is released by the fusion of granule membranes with the cell membrane, such as in sweat glands.

Holocrine

A secretory process where the whole cell is exfoliated and the secretion consists of cell residue and synthesized products, such as in sebaceous glands.

Apocrine

A secretory process where the apical portion of the cell containing the product pinches off, such as in mammary glands.

Endocrine glands

Ductless glands that release protein or steroid hormones directly into the surrounding extracellular fluid and blood to reach distant target organs.