Connective Tissues Overview

Overview

• Connective tissues (CT) have abundant extracellular components, are highly vascularized, and innervated.
• Divided into two major groups: fibrous connective tissues (loose, dense, reticular) and special connective tissues (elastic, adipose, cartilage, bone, blood).
• Main functions include connecting, supporting, aiding movement, providing nutrition, insulation, defense, thermoregulation, water metabolism, repair, regeneration, and storage.

Characteristics of Fibrous CT

• Composed of fibroblasts (cells) and extracellular fibers (collagen and elastin).
• Most common tissue type that forms the structural framework of organs.
• Characterized by:
  • High extracellular matrix content.
  • Diverse cell populations (resident and transient).
  • Low cellular density relative to epithelial tissues.
  • Rich vascularization and innervation.

Extracellular Matrix

• Stroma consists of fibrous proteins such as collagen and elastin, which provide structural integrity and resist stretching.

Collagen

• Most abundant protein in the body, unbranched and acidophilic.
• Acts as a structural glycoprotein.
• Numerous types, including type III for reticular fibers.

Reticular Fibers

• Type of loose connective tissue made from type III collagen.
• Found in lymphoid organs and bone marrow; not visible with H&E staining, requires silver staining.

Elastin

• Secreted by fibroblasts and located alongside collagen in fibrous CT.
• Provides elasticity due to its stretchable fibers composed of microfibrils and globular elastin protein; lacks banding pattern and is thin and branched.

Ground Substance

• Aqueous medium facilitating interactions among cells and fibrous components.
• Gel-like, amorphous, and undetectable with H&E staining.
• Contains:
  • Water (interstitial fluid).
  • GAGs (glycosaminoglycans) that are high molecular weight polymers made of repeating disaccharides.
    • Types include hyaluronic acid, chondroitin sulfates, dermatan sulfate, heparin sulfate, and keratan sulfate.
  • Proteoglycans formed from GAGs which combine to create large protein complexes, found in cartilage.

Heterogeneous Cell Populations

Resident Cells

• Mesenchymal cells: immature stem cells important for CT growth and repair.
• Fibroblasts: primary cells that secrete fibers and ground substance.
• Reticular cells: specialized fibroblasts in lymphatic tissues secreting type III collagen.
• Macrophages: phagocytic monocytes that remove debris and present antigens, can form giant cells like chondroclasts and osteoclasts.
• Mast cells: large, granulated cells near blood vessels that mediate inflammation and sensitivity.
• Pericytes: multipotent cells adjacent to small vessels, potential stem cells.
• Adipocytes: fat cells with own basement membrane, classified as brown or white based on function.

Transient Cells

• White blood cells (e.g., monocytes, lymphocytes, plasma cells, eosinophils) that migrate into and out of connective tissues.
• Pigment cells: melanocytes that produce melanin; melanophages lack the ability to synthesize melanin.

Explore the fundamental characteristics of connective tissues, focusing on their extracellular matrix and various cell types. This quiz covers the differences between fibrous and specialized connective tissues, their functions, and their structural components. Test your knowledge on how connective tissues contribute to the body’s overall function and support.