Epithelial Tissue and Stem Cells

Comprehensive vocabulary flashcards covering the histology of epithelial tissues, cell polarity, membrane specializations, intercellular junctions, stem cell biology, and glandular classification.

Epithelial Tissue

An avascular tissue composed of tightly packed cells that exists in two primary forms: linings (covering external and internal body surfaces) and glands (specialized secretory structures).

Endothelium

The specialized simple squamous epithelium that lines the lumen of blood and lymphatic vessels, functioning primarily in transport control.

Mesothelium

The simple squamous epithelium that lines major closed body cavities, specifically the peritoneum, pleura, and pericardium.

Simple Squamous Epithelium

A single layer of flattened, plate-like cells specialized for passive diffusion, filtration, and transport control; found in pulmonary alveoli, Bowman's capsule, mesothelium, and endothelium.

Simple Cuboidal Epithelium

A single layer of cells that are as tall as they are wide, specialized for resorption and secretion; found in renal tubules, excretory ducts of glands, lens epithelium, and ovary surface covering.

Simple Columnar Epithelium

A single layer of tall, closely packed prismatic cells with nuclei aligned near the base; functions in secretion, resorption, and protection across the alimentary tract, bile ducts, and female reproductive tract.

Pseudostratified Epithelium

A variant of simple columnar epithelium containing cells of varying heights and nuclei at multiple levels, where every cell remains anchored directly to the basal lamina; lines the respiratory tract, ductus epididymis, and taste buds.

Stratified Squamous Keratinized Epithelium

A multilayered protective epithelium whose uppermost superficial cells die, lose their nuclei and organelles, and transform into tough, flat keratin-filled scales; forms the epidermis.

Stratified Squamous Nonkeratinized Epithelium

A multilayered protective epithelium whose surface cells remain nucleated and viable; covers moist, abrasion-prone mucosal surfaces such as the oral cavity, esophagus, vagina, cornea, and anal canal.

Urothelium (Transitional Epithelium)

A stratified epithelium lining the urinary tract (renal calyces, renal pelvis, ureters, and urinary bladder) featuring superficial dome-shaped umbrella cells that can flatten under distension.

Uroplakin

Stiff integral membrane protein plaques linked to actin filaments on the apical plasma membrane of umbrella cells in the urothelium, providing an impermeable osmotic barrier.

Self-Renewal

The hallmark capability of stem cells to undergo numerous mitotic divisions while maintaining an undifferentiated, unaged phenotype.

Asymmetric Division

A stem cell division mode yielding two distinct fates: one daughter cell that remains an undifferentiated stem cell and a sibling cell that enters a committed differentiation pathway.

Totipotent Stem Cells

Stem cells exhibiting the greatest developmental potency, able to form all cell types of the embryonic body as well as extraembryonic tissues (placenta); exemplified by the zygote.

Pluripotent Stem Cells

Stem cells capable of generating specialized cell types from all three embryonic germ layers, but incapable of generating extraembryonic tissues (placenta) or a complete organism; exemplified by inner cell mass cells of the blastocyst.

Multipotent Stem Cells

Stem cells restricted to giving rise to several specialized cell types belonging to a single germ layer or cell lineage (e.g., mesenchymal stem cells or hematopoietic stem cells).

Monopotent Stem Cells

Stem cells capable of differentiating into only a single mature cell lineage while retaining self-renewal capacity (e.g., adult cardiac stem cells).

Stem Cell Niche

A specialized physiological microenvironment containing differentiated support cells, extracellular matrix, blood vessels, and nerve endings that controls stem cell maintenance, self-renewal, and differentiation.

Induced Pluripotent Stem Cells (iPSCs)

Differentiated adult somatic cells artificially reprogrammed into an embryonic-like pluripotent state via transfection with defined sets of transcription factors (such as Oct3/4, Sox2, Klf4, and c-Myc).

Microvilli

Non-motile apical cytoplasmic projections supported by an internal bundle of parallel actin microfilaments bundled by fimbrin, fascin, and villin; significantly increases surface area for absorption.

Stereocilia

Exceptionally long and thick specialized non-motile microvilli with actin cores found restrictedly in the ductus epididymis, ductus deferens, and hair cells of the inner ear.

Motile Cilia

Elongated, hair-like apical surface organelles powered by an internal $9 \times 2 + 2$ axoneme anchored to a basal body, executing coordinated rhythmic beating to propel surface fluids.

Axoneme ($9 \times 2 + 2$)

The cytoskeletal microtubule arrangement of motile cilia and flagella, consisting of nine outer peripheral microtubule doublets encircling a central pair of single microtubules.

Nexin

An elastic linking protein connecting adjacent peripheral microtubule doublets within an axoneme, responsible for restoring ciliary shape during the passive backstroke.

Metachrony

The coordinated, sequential, wave-like beating movement executed by carpets of motile cilia that unidirectionally propels mucus, oocytes, or spermatozoa across an epithelium.

Primary Cilium

A solitary, non-motile cell sensory organelle possessing a $9 \times 2 + 0$ axoneme that acts as a mechanosensor and signaling hub regulating cell cycle progression and tissue differentiation.

Zonula Occludens (Tight Junction)

A continuous belt-like occluding junction formed by transmembrane claudins and occludins that seals intercellular clefts to control paracellular permeability and maintain cell membrane polarity.

Zonula Adherens (Adhesion Belt)

An anchoring junction that forms a continuous band around epithelial cells immediately beneath the tight junction, connecting transmembrane cadherins across cells to intracellular actin filament bundles.

Desmosome (Macula Adherens)

A button-like anchoring junction that anchors cytokeratin intermediate filaments to desmoplakin attachment plaques and links adjoining cells via transmembrane desmogleins.

Focal Contact

An anchoring cell-to-extracellular matrix junction mediated by integrins that attaches intracellular actin filaments to extracellular matrix ligands such as fibronectin, laminin, and collagen.

Hemidesmosome

A specialized anchoring junction on the basal surface of epithelial cells that rivets intermediate filaments to the underlying basal lamina via integrins, collagen XVII, and laminin 5.

Gap Junction (Nexus)

A communicating intercellular junction composed of arrays of transmembrane connexons allowing direct exchange of ions, secondary messengers, and small metabolites ($\le 1.5\,\text{nm}$) between cells.

Connexon

A hexameric transmembrane protein ring composed of six connexin subunits that forms a central hydrophilic pore spanning the plasma membrane.

Terminal Bar

An apical junctional complex present in certain simple epithelia comprising, in apical-to-basal order: a zonula occludens (tight junction), a zonula adherens (adhesion belt), and a desmosome.

Intercalated Disk

Specialized complex junctional structure connecting adjacent cardiac myocytes, containing modified adhesion belts, desmosomes for mechanical cohesion, and gap junctions for electrical syncytium.

Basal Striation

The light microscopic appearance created by deep basal plasma membrane infoldings packed with vertically aligned mitochondria, specialized for high-capacity active ion transport.

Basal Lamina

A non-cellular supportive extracellular matrix sheet produced by epithelial cells, comprising a lamina rara and lamina densa, and composed of laminin, collagen IV, entactin, and perlecan.

Basement Membrane

A compound structural layer visible beneath epithelia, formed by the epithelial-derived basal lamina combined with an underlying reticular layer synthesized by connective tissue fibroblasts.

Exocrine Gland

A multicellular epithelial gland that releases its secretory product through an epithelial duct system directly onto an external or internal apical epithelial surface.

Endocrine Gland

A ductless epithelial gland that releases secretory products (hormones) directly into the interstitial fluid and bloodstream for transport to distant target tissues.

Merocrine (Eccrine) Secretion

The most common mode of secretion wherein exocrine or endocrine products are released from membrane-bound vesicles via standard exocytosis with no loss of cellular cytoplasm.

Apocrine Secretion

A secretory mechanism in which the secretory material (typically lipid droplets) is released along with a portion of the apical cytoplasm enclosed by plasma membrane (e.g., mammary glands).

Holocrine Secretion

A secretory mechanism in which cells fill with lipid secretory material, undergo programmed necrosis, and disintegrate entirely to release the accumulated product (e.g., sebaceous glands).

Pemphigus

An autoimmune blistering skin disease in which autoantibodies attack desmosomal adhesion proteins (desmogleins), causing detachment of epidermal cells and widespread skin blister formation.