Connective Tissue Notes

Connective Tissue

• Connective tissue supports and connects tissues/cells to form organs.
• Interstitial fluid provides metabolic support via nutrient/waste diffusion.
• Main component: extracellular matrix (ECM).
• ECM includes protein fibers (collagen, elastic fibers) and ground substance.
• Ground substance: proteoglycans, glycosaminoglycans (GAGs), multiadhesive glycoproteins (laminin, fibronectin).
• Glycoproteins stabilize ECM by binding matrix components and cell membrane integrins.
• Water in ground substance facilitates nutrient/waste exchange.
• Connective tissue diversity arises from variations in cells, fibers, and ground substance.
• Origin: embryonic mesenchyme (from mesoderm).
• Mesenchyme: viscous ground substance, few collagen fibers, undifferentiated cells.
• Mesenchymal cells can differentiate into connective tissue, bone, cartilage, blood, endothelium, and muscle.
• Mesenchyme-like cells in adults may be stem cell sources for repair/regeneration.

Cells of Connective Tissue

• Key cells: Fibroblasts originate from mesenchymal cells and reside in connective tissue.
• Macrophages, plasma cells, and mast cells from hematopoietic stem cells circulate and enter connective tissue.
• Leukocytes are transient, functioning briefly before apoptosis.

Fibroblasts

• Most common cells, produce/maintain ECM components.
• Synthesize collagen, elastin, GAGs, proteoglycans, and glycoproteins.
• Secrete ECM components with modification outside the cell.
• Active fibroblasts: abundant cytoplasm, much RER, large euchromatic nucleus.
• Quiescent fibroblasts (fibrocytes): smaller, less RER, darker heterochromatic nucleus.
• Fibroblasts respond to growth factors influencing growth/differentiation.
• Adult fibroblasts rarely divide unless stimulated by growth factors during tissue repair.
• Myofibroblasts (wound healing): contractile function, enriched with actin.

Adipocytes

• Derived from mesenchymal cells and are specialized for lipid storage.
• Adipose tissue cushions/insulates organs.

Macrophages & the Mononuclear Phagocyte System

• Macrophages phagocytose protein fibers, apoptotic cells, debris, and particulate material.
• They're abundant at inflammation sites.
• Vary in size/shape based on activity.
• Function in ECM fiber turnover, innate immune defense, removing debris/ invaders.
• Macrophages are antigen-presenting cells for lymphocyte activation.
• Stimulation leads to activated macrophages with increased phagocytosis, digestion, metabolism, and enzyme activity.
• Activated macrophages secrete ECM breakdown enzymes, growth factors, and cytokines.
• Stimulated macrophages may fuse into multinuclear giant cells.
• Macrophages have irregular surfaces, Golgi complexes, and lysosomes.
• Originate from blood monocytes that enter connective tissue and differentiate.
• Mononuclear phagocyte system: macrophage-like cells in various organs.
• Macrophages become activated during inflammation/tissue repair, increasing in number.
• They secrete growth factors and present antigens for lymphocyte activation.

Mast Cells

• Oval/irregular cells filled with basophilic secretory granules.
• Granules display metachromasia.
• Mast cells release bioactive substances for inflammation, immunity, and repair.
• Releases Heparin (anticoagulant), Histamine (vascular permeability, muscle contraction), Serine proteases (inflammation mediators), Eosinophil and neutrophil chemotactic factors