Becker's World of the Cell - Chapter 15: Beyond the Cell: Cell Adhesions, Cell Junctions, and Extracellular Structures

Vocabulary practice flashcards covering cell-cell junctions, adhesive molecules, extracellular matrix components, and integrin-mediated signaling from Becker's World of the Cell, Chapter 15.

Epithelium

Sheets of polarized animal cells with discrete functional domains at opposite ends (apical and basal regions).

Connective Tissue

A loosely organized animal tissue in which cells are attached to each other, embedded in a rigid extracellular scaffold, or both.

Homophilic Interactions

Cell-cell adhesion events where identical receptor molecules on the surfaces of both interacting cells bind to each other.

Heterophilic Interactions

Cell-cell adhesion events where a cell-surface receptor on one cell binds to a different type of receptor on an adjacent cell.

Adherens Junction

A cadherin-mediated adhesive junction that forms a continuous zone of attachment with an intercellular space of $20\text{–}25\,\text{nm}$ and connects to intracellular actin microfilaments.

Cadherins

A family of transmembrane adhesion glycoproteins characterized by repeating extracellular domains, a transmembrane segment, and variable cytosolic tails that associate with the cytoskeleton.

E-Cadherin

An epithelial cadherin containing five repeats in its extracellular domain that dimerizes and "zips together" with cadherins on an adjacent cell in a homophilic fashion.

$\beta$-Catenin

A cytosolic linker protein that binds directly to the cytosolic tail of cadherin, links to $\alpha$-catenin, and can act as a gene transcription factor and metastasis regulator when freed.

$\alpha$-Catenin

A cytosolic protein that binds to $\beta$-catenin and recruits actin microfilaments to the adherens junction complex.

p120 Catenin

A cytosolic linker protein that binds to the cadherin cytoplasmic domain, stabilizing cadherin at the cell surface and regulating the activity of the small GTPase Rho.

Epithelial-Mesenchymal Transition (EMT)

The breakdown of an organized epithelial layer into migratory, loosely organized mesenchymal cells, driven by alterations in cadherin expression during development and cancer metastasis.

Desmosome

A button-like adhesive junction characterized by a $25\text{–}35\,\text{nm}$ intermembrane space that anchors desmosomal cadherins (desmocollins and desmogleins) to intermediate filaments (tonofilaments).

Tonofilaments

Intermediate filament bundles that loop into the intracellular dense plaques of desmosomes and hemidesmosomes to confer structural integrity against mechanical stress.

Selectins

Carbohydrate-binding cell surface glycoproteins (including L-selectin, P-selectin, and E-selectin) that mediate the transient rolling of leukocytes along endothelial cells during inflammation.

Tight Junction

A specialized junction that seals adjacent epithelial cells together along ridges with no intercellular space, blocking extracellular diffusion and restricting the lateral movement of membrane components.

Claudins

Tight junction transmembrane proteins with four membrane-spanning domains whose extracellular loops interlock to establish a seal and form ion-selective pores for paracellular transport.

Paracellular Transport

The selective, regulated passage of ions and small molecules through the intercellular space between adjacent cells across an epithelial barrier.

Gap Junction

A communicating junction featuring an intercellular space of $2\text{–}3\,\text{nm}$ that allows the direct chemical and electrical exchange of ions and small molecules between adjacent cells.

Connexon

A hollow cylindrical protein assembly in a gap junction composed of six connexin subunits arranged in a ring, forming a hydrophilic channel approximately $3\,\text{nm}$ wide.

Edema

The accumulation of excess fluid within the extracellular compartment, occurring as a hallmark of the inflammatory response when capillaries become leaky.

Extracellular Matrix (ECM)

An organized network of extracellular macromolecules—consisting of structural proteins, proteoglycans, and adhesive glycoproteins—that provides structural and biochemical support to animal tissues.

Collagen

The most abundant structural protein family in the animal ECM, characterized by high tensile strength and assembled into rigid triple-helical fibers composed of three intertwined $\alpha$ chains.

Procollagen

A precursor form of collagen synthesized in the lumen of the endoplasmic reticulum, consisting of a triple helix flanked by nonhelical terminal extensions.

Procollagen Peptidase

An extracellular enzyme that cleaves the nonhelical propeptides from procollagen following secretion, allowing spontaneous self-assembly into collagen fibrils.

Elastins

Stretchable ECM structural proteins rich in glycine and proline that are covalently crosslinked via lysine residues to provide reversible elasticity and recoil to tissues.

Proteoglycans

Gel-forming ECM glycoproteins consisting of numerous glycosaminoglycan (GAG) polysaccharide chains covalently attached to a central core protein.

Glycosaminoglycans (GAGs)

Large, unbranched carbohydrates composed of repeating disaccharide units that retain water and ions; examples include chondroitin sulfate, keratan sulfate, and hyaluronate.

Hyaluronate

A long, nonsulfated GAG that functions both as a backbone for cartilage proteoglycan complexes and as an unattached, lubricating molecule in synovial joint cavities.

Fibronectins

Dimeric adhesive glycoproteins linked near their C-termini by two disulfide bonds that contain binding domains for collagen, heparin, fibrin, and cell-surface integrins.

RGD Sequence

A specific tripeptide motif (Arg-Gly-Asp) on fibronectin and other ECM molecules that serves as the primary recognition site for integrin receptors.

Plasma Fibronectin

The soluble form of fibronectin present in blood that aids in hemostasis by binding to fibrin and anchoring platelets to the forming blood clot.

Basal Lamina

A thin, specialized sheet of ECM underlying epithelial cells that provides structural support, acts as a permeability barrier, and contains type IV collagen, proteoglycans, and laminin.

Matrix Metalloproteinases (MMPs)

A family of metal-dependent proteolytic enzymes that degrade ECM components locally, facilitating cell migration, leukocyte infiltration, and cancer cell invasion.

Laminin

A cross-shaped heterotrimeric glycoprotein found principally in the basal lamina, composed of $\alpha$, $\beta$, and $\gamma$ chains stabilized by disulfide bonds.

Integrins

Transmembrane heterodimeric cell-surface receptors comprising an $\alpha$ and a $\beta$ subunit that bridge extracellular ligands (such as fibronectin and laminin) to the cytoskeleton.

Focal Adhesion

A localized cell-ECM attachment site with a $20\text{–}25\,\text{nm}$ space where clustered integrins link extracellular matrices to actin microfilament bundles via talin, vinculin, and $\alpha$-actinin.

Hemidesmosome

A localized cell-matrix junction with a $25\text{–}35\,\text{nm}$ space that anchors intermediate filaments of epithelial cells to the underlying basal lamina via integrins, BPAG2, and plakin family proteins (BPAG1, plectin).

Anoikis

A specialized form of programmed cell death triggered in normal anchorage-dependent cells when they lose attachment to their extracellular matrix substratum.

Rho GTPases

A family of monomeric small G proteins (including Rho, Rac, and Cdc42) that act as molecular switches regulating actin remodeling, such as the formation of stress fibers, lamellipodia, and filopodia.