Biology: Connective Tissue

Connective Tissue

• Connective tissue is composed of cells, extracellular matrix (ECM), and fibers, which provide structural support to the body. • The ECM consists of protein fibers (collagen and elastic fibers) and ground substance (glycoproteins and proteoglycans).

Mesenchyme and Mesoderm

• Mesenchyme is an embryonic tissue that develops from the mesoderm, one of the three primary germ layers. • Mesenchymal cells are undifferentiated, have large nuclei, and fine chromatin. • Mesenchyme gives rise to connective tissue, bone, cartilage, blood, vascular endothelium, and muscle.

Fibroblasts

• Fibroblasts are the key cells in connective tissue, responsible for producing and maintaining the ECM. • Fibroblasts originate from mesenchymal cells and produce collagen, elastin, and glycoproteins. • Quiescent fibroblasts have fewer processes, less rough endoplasmic reticulum (RER), and a darker, more heterochromatic nucleus.

Adipocytes

• Adipocytes are specialized connective tissue cells that store lipid in the form of neutral fats. • Adipose connective tissue provides cushioning and insulation to organs.

Macrophages and the Mononuclear Phagocyte System

• Macrophages are phagocytic cells that remove protein fibers, apoptotic cells, and tissue debris. • Macrophages originate from monocytes in the blood and mature in connective tissue. • Activated macrophages play a crucial role in inflammation and tissue repair.

Mast Cells

• Mast cells are oval or irregularly shaped cells with basophilic secretory granules. • Mast cells release bioactive substances, such as heparin, histamine, and cytokines, which contribute to localized inflammatory responses.

Plasma Cells

• Plasma cells are lymphocyte-derived, antibody-producing cells with basophilic cytoplasm rich in RER. • Plasma cells are present in most connective tissues, with an average lifespan of 10-20 days.

Leukocytes

• Leukocytes, such as macrophages, plasma cells, and other white blood cells, migrate from blood into connective tissue during inflammation.

Fibers

• The fibrous components of connective tissue include collagen, reticular, and elastic fibers. • Collagen fibers are formed from proteins secreted by fibroblasts and are essential for strength and resistance to shearing and tearing forces.

Collagen Synthesis

• Collagen synthesis occurs in many cell types, but is a specialty of fibroblasts. • Procollagen α chains are produced in the RER, aligned, and stabilized by disulfide bonds, and then cleaved to form a rodlike procollagen molecule. • Procollagen processing involves multiple posttranslational modification steps, including hydroxylation, glycosylation, and proteolytic cleavage.### Connective Tissue

• Connective tissue is composed of cells, fibers, and ground substance (a complex mixture of macromolecules)
• Provides support, structure, and mechanical strength to the body
• Has a diverse range of functions, including:
  • Supporting and connecting other tissues
  • Providing mechanical strength and resilience
  • Regulating movement and locomotion
  • Maintaining tissue fluid balance and pressure
  • Facilitating cell signaling and communication

Collagen Fibers

• Type I collagen fibers are the most abundant and are found in skin, tendons, and bones
• They are irregularly arranged in loose connective tissue, but are regularly arranged in dense connective tissue
• Type I collagen fibers are composed of:
  • Collagen subunits that are staggered by 67 nm, resulting in a characteristic transverse striation pattern
  • Small gaps (lacunar regions) between the ends of adjacent subunits
• Type II collagen fibers are found in cartilage and do not form fibers or bundles
• Type III collagen fibers are found in reticular tissue and form a delicate network that supports various types of cells

Elastic Fibers

• Elastic fibers are thinner than collagen fibers and are found in tissues that are subject to regular stretching or bending
• They are composed of:
  • Fibrillin (350 kDa) that forms a network of microfibrils
  • Elastin (60 kDa) that is deposited onto the microfibrils, giving the fibers their rubber-like properties
• Elastic fibers are found in:
  • Tendons and ligaments
  • Arteries and veins
  • Lungs and other organs that are subject to regular stretching

Reticular Fibers

• Reticular fibers are composed of type III collagen and are found in:
  • Delicate connective tissue of many organs
  • Immune system tissues
  • Reticular lamina of basement membranes
• They are characterized by:
  • A thin diameter (0.5-2 μm)
  • A network-like arrangement
  • A high content of sugar chains bound to type III collagen α chains

Ground Substance

• Ground substance is a highly hydrated, transparent, and complex mixture of macromolecules
• It is composed of:
  • Glycosaminoglycans (GAGs)
  • Proteoglycans
  • Multiadhesive glycoproteins
• Ground substance:
  • Fills the space between cells and fibers
  • Allows for diffusion of small molecules
  • Acts as a lubricant and barrier to the penetration of invaders

Glycosaminoglycans (GAGs)

• GAGs are long polymers of repeating disaccharide units
• They are negatively charged and have a high water-binding capacity
• The largest and most ubiquitous GAG is hyaluronan (also known as hyaluronic acid)
• Hyaluronan is synthesized directly into the ECM and forms a viscous, pericellular network

Proteoglycans

• Proteoglycans consist of a core protein to which are covalently attached various numbers and combinations of sulfated GAGs
• They are synthesized in the Golgi apparatus and secreted from cells
• Proteoglycans:
  • Are found in the ECM
  • Bind to various molecules, including growth factors and signaling proteins
  • Play a role in cell signaling and communication

Multiadhesive Glycoproteins

• Multiadhesive glycoproteins are large molecules with branched oligosaccharide chains
• They allow for adhesion of cells to their substrate
• Examples include:
  • Laminin (found in basal and external laminae)
  • Fibronectin (found in connective tissue)

Types of Connective Tissue

• Loose connective tissue:
  • Characterized by a delicate consistency
  • Has a moderate amount of ground substance
  • Is flexible and not very resistant to stress
• Dense connective tissue:
  • Has a clear predominance of bundled type I collagen fibers
  • Has a tough, three-dimensional collagen network
  • Is resistant to stress from all directions
• Reticular tissue:
  • Characterized by abundant type III collagen fibers
  • Has a delicate network that supports various types of cells
• Mucoid tissue:
  • Has a gelatinous consistency
  • Has a sparse amount of collagen fibers
  • Is found in the fetal umbilical cord and other tissues