Cell Biology Terms Starting With B

Basal lamina is a thin, specialized sheet of extracellular matrix that underlies epithelial cells, muscle fibers, and blood vessels, anchoring them to surrounding connective tissue and regulating molecular exchange between tissue compartments.

The basal lamina is typically 50 to 100 nanometers thick and contains four major components: type IV collagen, laminin, nidogen, and the heparan sulfate proteoglycan perlecan, which self-assemble into a mesh without requiring a cellular template. Laminin binds integrin receptors on the overlying cell surface, transmitting mechanical signals between the matrix and the cytoskeleton. In kidney glomeruli, the basal lamina between capillary endothelial cells and podocytes filters blood by size and charge, normally excluding proteins larger than about 70 kilodaltons.

During tissue repair and embryonic development, the basal lamina also provides directional cues that guide migrating cells along defined paths.

Beta-catenin is a protein that links the cell-adhesion molecule E-cadherin to the actin cytoskeleton at cell-cell contacts and, when released from that complex, travels to the nucleus to activate genes controlling cell growth and differentiation.

Normally, a destruction complex containing the proteins APC, Axin, and GSK-3 beta continuously phosphorylates beta-catenin, marking it for proteasomal degradation within minutes of synthesis. When a Wnt ligand binds its Frizzled receptor, this destruction complex is inactivated, beta-catenin accumulates in the cytoplasm, and the stabilized protein translocates to the nucleus. There it binds TCF/LEF transcription factors and drives expression of proliferation genes including MYC and CCND1.

During vertebrate embryogenesis, this pathway patterns the dorsal-ventral body axis and maintains intestinal stem cell identity throughout adult life.

Bile canaliculus is a narrow intercellular channel formed between adjacent hepatocytes that collects bile secreted by those cells and carries it toward larger bile ducts.

Each bile canaliculus forms at the apical membranes of two neighboring hepatocytes and measures approximately 1 micrometer in diameter. Microvilli projecting from those apical surfaces increase the membrane area available for bile salt secretion into the channel. Tight junctions composed of claudin and occludin proteins seal the canalicular space from the blood sinusoid, preventing bile from leaking into the circulation.

Bile flows through the canalicular network toward the canals of Hering and then into interlobular bile ducts at the portal triad of each liver lobule.

Brush border is a dense array of microvilli on the apical surface of absorptive epithelial cells that dramatically expands the membrane area available for nutrient uptake and digestion.

Each microvillus in the brush border measures roughly 1 micrometer long and 0.1 micrometers wide, and a single human intestinal enterocyte carries approximately 1,000 to 3,000 of them, increasing functional surface area up to 25-fold compared to a flat membrane. Actin filaments bundled by villin and fimbrin proteins form the structural core of each microvillus and anchor it to the terminal web beneath the apical membrane. Digestive enzymes including lactase, sucrase, and aminopeptidase are embedded directly in the brush border membrane, coupling final hydrolysis of nutrients to their immediate transport into the cell.

Celiac disease and rotavirus infection both damage the brush border, flattening microvilli and reducing absorptive capacity, which leads to malabsorption and diarrhea.

Budding is an asexual reproductive process in which a new cell or organism develops as a small outgrowth on the parent, then separates to live independently.

In the baker’s yeast Saccharomyces cerevisiae, a bud emerges at a genetically determined site on the mother cell surface, grows by localized insertion of new membrane and cell wall material, and reaches roughly 30 percent of the mother cell’s volume before cytokinesis severs the connection. The bud site is marked by a ring of septin proteins that recruit the machinery for polarized growth. Mitochondria, ribosomes, and other organelles are actively transported into the bud before separation, so the daughter cell is fully equipped to survive independently.

Multicellular animals including hydra (Hydra vulgaris) also reproduce by budding, producing tentacled outgrowths that detach as complete juvenile organisms.