Advanced Cell Ch. 20 Extracellular matrix and connective tissues: Plant cells The supportive matrix is called a cell wall Cell walls are tough but not rigid Can be filled with water to distend the cells and create turgor pressure Two types of cell wall: Primary cell walls: Primarily composed of pectin Not as strong and sturdy as secondary walls Allow growth; usually found on the outside of plant Secondary cell walls: Primarily composed of cellulose Formed by thickening of primary wall or deposition of new layers with a different composition Cellulose: Polysaccharide Form long fibers along the lines of stress Cellulose microfilaments are interwoven with other polysaccharides and structural proteins It is synthesized on the outer surface of the cell by enzyme complexes embedded in the plasma membrane Complexes transport sugar monomers across the membrane and add them to growing chains Each chain assembles to form cellulose microfibril Complexes move through the membranes trailing cellulose behind them Complexes follow microtubules under the plasma membrane. Plant cells can become specialized: Waxy, waterproof leaves Lignin: network of polymers embedded in the cellulose cell wall Hard, thick, woody walls Animal cells Connective tissues Extracellular matrix is extensive It carries the bulk of the tensile force Depends heavily on Collagen A protein with many varieties Chief proteins in bone, tendon, and skin Has a long, stiff triple-helical structure Made of three collagen polypeptide chains These collagen molecules in turn assemble into polymers called collagen fibrils Thin cables of collagen triple-helices Pack together further to form collagen fibers Connective tissue cells: In skin, tendon and other connective tissues they are called fibroblasts In bone they are called osteoblasts In extracellular matrix To prevent formation in the cell, a precursor form of collagen, called procollagen, is made and transported out of the cell Procollagen proteinases cut off extensions and assembly of collage occurs Organization of collagen: In alternating layers to resist tensile stress in multiple directions In one direction to resist tensile stress in one direction Fibroblasts control this organization Deposit collagen in an oriented fashion Then they rearrange it Integrins couple the matrix to the cytoskeleton inside the cell Cells do not attach well to bare collage So fibronectin, another protein, binds to collagen and another part forms a connection site for a cell Cell uses a receptor protein called an integrin to attach to the fibronectin Internal part of integrin binds to an actin filament through a set of adaptor molecules Integrins undergo conformational changes as the cell moves Integrins on white blood cells help them crawl out of blood vessels at sites of infection Deficiency of this integrin is leucocyte adhesion deficiency Suffer from repeated bacterial infections Glycosaminoglycans (GAGs): macromolecules in the extracellular matrix that prevent compression They are negatively charged polysaccharide chains made of repeating disaccharide units Highly hydrophilic Form hydrophilic gels because their negative charges attract cations and water Causes swelling Swelling with stiff collagen can create a tough and durable framework, like cartilage. GAGs are usually covalently linked to core proteins to form proteoglycans These are extremely diverse in size, shape and chemistry Typically many GAG chains are attached to a single core protein Core protein could be linked at the other end to more GAGs Structure of this looks like a bottle brush In compact connective tissues like bone and tendon: GAGs proportion is small, matrix consists almost entirely of collagen In eye: Almost entirely GAGs and water Proteoglycans form gels that can filter out different sized and charged molecules Epithelial tissues Stratified: many cells thick Epidermis Simple epithelium: only one cell layer thick Lining of the gut Columnar: tall column shaped cells; one layer Cuboidal: square and squat; one layer Squamous: flattened cells Epethilial cells have two faces: Apical surface is free and exposed to the air or to a watery fluid Basal surface is attached to a sheet of connective tissue called the basal lamina Lamina consists of a thin, tough sheet of extracellular matrix, composed of many special types of collagen Also protein called laminin Provided adhesive sites for integrin molecules in the basal plasma membranes of epithelial cells The two faces are chemically different is polarized cell Junctions: Tight Junctions: sealing function seal neighboring cells together so that water-soluble molecules cannot easily leak between them formed by proteins called claudins and occludins they are arranged in strands along the lines of the junction to create the seal help maintain cell polarity Adherens Junctions: bind one cell to another a cadherin molecule is tethered inside the cell, via linker proteins, to actin filaments adherens form a continuous adhesion belt around each of the interacting cells near the apical end bundles of actin filaments are connected from cell to cell across the membrane allows movement and vesicle formation important in embryonic development of the neuronal tube; gives rise to central nervous system and lens vesicle=lens of eye Desmosomes: bind one cell to another Different set of cadherin molecules connects to keratin filaments Intermediate filaments found in epithelial Keratin criss-cross the cytoplasm in bundles Spot-welded with desmosome junctions Hemidesmosomes: bind cells to the basal lamina Integrins connect to the basal lamina Inside the cell, integrin tails are linked to keratin fibers Gap Junctions: Transmembrane proteins called connexons are aligned to form narrow, water-filled channels across the two plasma membranse Allow cytosolic inorganic ions and small molecules to pass from cell to cell Can be opened and closed in response to extracellular or intracellular signals i.e. dopamine in retina neurons in response to increase in light intensity Plasmodesmata: in plants only Like gap junctions Differ in that the channels of plasmodesmata are lined with plasma membrane Tissue Maintenance and Renewal: Basic Cell requirements: All tissues need mechanical strength Need for oxygen, nutrients, and waste disposal Maintenance of the different tissue types that are in co-existence Three factors contribute to this: Cell communication: Cells continually monitor the environment for signals from other cells Selective cell adhesion: Cells adhere selectively due to different cadherin proteins, etc. Homophillic binding to cells of the same type Can bind to different cells, but only if the same adhesion protein Cell memory: gene expression of past divisions is kept. Fibroblast always produces fibroblasts Different tissues are renewed at different rates: Cell turnover varies from cell to cell. Ionizing radiation blocks cell division Halts renewal Cancers originate through violation of the controls mandating cell renewal Stem Cells: Most of the specialized cells that need frequent renewal cannot renew themselves They are terminally differentiated At the end of their developmental pathway Instead they are generated from a stock of proliferating precursor cells These derive from a much smaller number of self-renewing stem cells Both reside with the differentiated cells Stem cells can divide with no limit Must choose between making more stem cells or making differentiated cells Precursor cells are already slightly differentiated Lining of small intestine: Absorptive and secretory cells are arranged in a single layer covering the villi Continuous with the epithelium lining the crypts Descend into the underlying connective tissue Stem cells are near the bottom of the crypts Give rise mostly to proliferating precursor cells Move upward in the plane of the epithelial sheet As they move upward they terminally differentiate Cells die when they reach the tip of the villi Cancer: