BIOL 330 - Connective Tissue

Connective tissue (definition)

Tissue specialized to support, connect, and maintain water balance for diffusion of metabolites between blood and cells

Extracellular dominance of connective tissue

Connective tissues consist primarily of extracellular material rather than cells

Stroma

Supportive framework of connective tissue within organs that supports functional tissue

Parenchyma

Functional cells of an organ responsible for its specific activity

Extracellular matrix (ECM)

Combination of fibers and ground substance forming the structural environment around cells

Ground substance

Nonfibrous, hydrated component of ECM rich in GAGs and water that fills space between fibers and cells

Mesenchyme

Embryonic connective tissue composed of undifferentiated cells in a gel-like matrix

Fibroblast

Major connective tissue cell that synthesizes and secretes ECM components; elongated with oval nucleus

Fibrocyte

Less active fibroblast with reduced synthetic activity

Adipocyte

Large fat-storing cell specialized for triglyceride storage; dominant in adipose tissue

Adipose tissue

Specialized connective tissue composed mainly of adipocytes

Macrophage

Phagocytic cell derived from monocytes that removes debris and presents antigens

Monocyte

Blood precursor cell that differentiates into macrophages in connective tissue

Macrophage functions

ECM turnover, phagocytosis of debris, and antigen presentation

Mast cell

Cell derived from blood precursors containing granules that release vasoactive substances during inflammation

Plasma cell

Antibody-producing cell derived from B lymphocytes

Leukocytes (in CT)

Wandering immune cells providing surveillance and stimulating repair

Collagen

Most abundant connective tissue protein forming strong fibers; ~20 types exist

Collagen synthesis

Process involving fibroblasts where procollagen is formed, modified, secreted, and assembled into fibrils

Hydroxylation (collagen)

Posttranslational modification of proline and lysine in RER required for collagen stability

Procollagen

Triple-helical precursor of collagen formed in RER

Collagen molecule

Formed after cleavage of procollagen ends; aggregates into fibrils

Collagen fibril

Small structural unit formed by collagen molecules with crossbanding pattern

Crossbanding

Regular banding pattern seen in collagen fibrils due to orderly assembly

Collagen bundle

Large structure formed by grouping of collagen fibrils

Matrix metalloproteinases (MMPs)

Enzymes (like collagenase) that degrade collagen, produced mainly by macrophages

Type III collagen

Forms reticular fibers, especially in immune and lymphoid tissues

Reticular fibers

Delicate network fibers that stain with silver and support immune cells

Elastic fibers

Fibers composed of elastin and fibrillin providing elasticity and recoil

Elastic lamellae

Sheet-like arrangements of elastic fibers

Elastin

Protein responsible for stretch and recoil properties of tissues

Fibrillin

Glycoprotein forming microfibrils that support elastin

Ground substance (definition)

Hydrated, gel-like ECM component often more abundant than fibers

Glycosaminoglycans (GAGs)

Long hydrated polysaccharides that form the backbone of ground substance

Hyaluronic acid (hyaluronan)

Very long GAG polymer composed of glucosamine-glucuronate units

Sulfated GAGs

Shorter GAG chains (e.g., chondroitin sulfate, keratan sulfate) attached to proteins

Proteoglycan

Core protein with attached GAG chains forming large hydrated complexes

Proteoglycan aggregates

Large complexes formed when proteoglycans attach to hyaluronic acid via linker proteins

Proteoglycan function

Bind water and growth factors, regulating ECM structure and cell activity

Multiadhesive glycoproteins

ECM proteins that link cells to ECM and aid in migration

Fibronectin

Glycoprotein binding collagen and integrins for cell adhesion

Laminin

Glycoprotein in basal lamina linking cells to ECM

Integrins

Cell membrane proteins that connect ECM to cytoskeleton

Connective tissue classification

Divided into loose and dense types based on collagen vs ground substance

Loose connective tissue

Tissue with more ground substance than collagen; surrounds vessels and epithelia

Areolar tissue

Common loose connective tissue with balanced fibers and gel-like matrix

Dense irregular connective tissue

Tissue with randomly arranged collagen fibers providing multidirectional strength

Dense regular connective tissue

Tissue with parallel collagen bundles providing strong unidirectional tension (tendons, ligaments)

Reticular tissue

Network of type III collagen supporting lymphoid organs

Mucoid (mucous) connective tissue

Embryonic gel-like tissue similar to mesenchyme found in umbilical cord

Connective tissue

A primary tissue type derived from mesenchyme composed of cells, fibers, and ground substance that supports, binds, and connects tissues.

Mesenchyme

Embryonic connective tissue of undifferentiated cells with large euchromatic nuclei and abundant ground substance rich in hyaluronan.

Extracellular matrix (ECM)

Non-cellular component of connective tissue consisting of fibers and ground substance that supports cells and regulates cell behavior.

Ground substance

Highly hydrated, transparent material filling spaces between cells and fibers; composed of GAGs, proteoglycans, and glycoproteins.

Fibers

Structural proteins in connective tissue including collagen, elastic, and reticular fibers.

Connective tissue functions

Maintains organ form, binds tissues, supports metabolism via diffusion, and stores growth/differentiation factors.

Mesoderm

Embryonic germ layer that gives rise to connective tissue and migrates to surround developing organs.

Fibroblast

Active connective tissue cell that produces fibers and ECM components; has large euchromatic nucleus and abundant RER/Golgi.

Fibrocyte

Inactive fibroblast with smaller, darker nucleus and reduced synthetic activity.

Collagen fibers

Strong fibers providing tensile strength to tissues.

Elastic fibers

Fibers composed of elastin that allow tissues to stretch and recoil.

Reticular fibers

Thin type III collagen fibers forming supportive networks in organs.

Glycosaminoglycans (GAGs)

Long polysaccharides that bind water and provide resistance to compression.

Proteoglycans

Core proteins with GAG chains that form hydrated gel-like ECM.

Multiadhesive glycoproteins

ECM molecules (fibronectin, laminin) that connect cells to ECM.

Fibronectin

Glycoprotein that binds cells to ECM and aids in adhesion and migration.

Laminin

Glycoprotein in basal lamina that anchors epithelial cells to connective tissue.

Fibroblast function

Produces collagen, elastic fibers, reticular fibers, GAGs, proteoglycans, and glycoproteins.

Wound healing

Process where fibroblasts proliferate and produce collagen to replace damaged tissue with scar tissue.

Myofibroblast

Modified fibroblast containing actin that contracts to close wounds.

Scar tissue

Dense collagen-rich tissue replacing normal tissue after injury.

Granulation tissue

Early wound healing tissue rich in fibroblasts and new blood vessels.

Keloid

Excessive scar tissue extending beyond wound boundaries due to persistent myofibroblasts.

Adipocyte

Fat-storing cell specialized for energy storage and heat production.

Macrophage

Phagocytic cell derived from monocytes that removes debris and pathogens.

Monocyte

Circulating precursor cell that differentiates into macrophages.

Phagocytosis

Process of engulfing and digesting particles or pathogens.

Pinocytosis

Uptake of fluid and small molecules by cells.

Lysosome

Organelle containing digestive enzymes for intracellular breakdown.

Mononuclear phagocyte system

Network of long-lived phagocytic cells involved in antigen processing and presentation.

Kupffer cells

Liver macrophages.

Microglia

Macrophages of the central nervous system.

Langerhans cells

Antigen-presenting macrophages in the skin.

Dendritic cells

Antigen-presenting cells in lymphoid organs.

Osteoclast

Bone-resorbing cell formed by fused macrophages.

Mast cell

Connective tissue cell with granules containing histamine and heparin involved in inflammation and allergy.

Heparin

Anticoagulant substance in mast cell granules.

Histamine

Mediator that increases vascular permeability and causes vasodilation.

Leukotrienes

Lipid mediators that promote inflammation and smooth muscle contraction.

Chemotactic factors

Substances that attract immune cells to sites of inflammation.

IgE

Antibody that binds mast cells and triggers allergic responses.

Mast cell degranulation

Release of histamine and mediators after antigen-IgE binding.

Immediate hypersensitivity

Rapid allergic reaction mediated by mast cells and IgE.

Edema

Swelling caused by fluid accumulation in tissues.

Hydrostatic pressure

Force pushing fluid out of capillaries.

Colloid osmotic pressure

Force pulling fluid into capillaries due to plasma proteins.

Transudate

Fluid with low protein content due to pressure imbalance.

Exudate

Protein-rich fluid due to inflammation and vascular permeability.

Mastocytosis

Disease with excessive mast cell proliferation.

Plasma cell

Antibody-producing cell derived from B lymphocytes.