Connective Tissue Biology Quiz

Connective Tissue Overview

• Connective tissue provides structural and metabolic support to other tissues
• It's a vascularized tissue, meaning it has blood and lymphatic vessels for nutrient delivery and waste removal.
• Plays a role in defense, protection, and repair of the body.
• Composed of cells and extracellular matrix (ECM).

Extracellular Matrix (ECM)

• A complex of macromolecules secreted by cells into the space between them.
• Present in large amounts in connective tissue with a limited number of cells scattered within it.
• Composed of two main components: ground substance and fibers.

ECM: Ground Substance

• Amorphous, gelatinous material
• Primarily composed of water and three other components:
  • Glycosaminoglycans (GAGs)
  • Proteoglycans
  • Structural glycoproteins

Glycosaminoglycans (GAGs)

• Long chains of repeating disaccharides
• Mostly sulfated, except for hyaluronic acid, which is the largest GAG and unsulfated.
• Negatively charged, attracting positive ions like Na+ and drawing in large amounts of water, making the ECM resistant to compression.

Proteoglycans

• Formed when GAGs bind to a protein core.
• Vary in size and type.
• Hyaluronic acid can bind numerous proteoglycans to form an aggregate.

Structural Glycoproteins

• Proteins bound to branched polysaccharides.
• Have binding sites for various ECM components
• Anchor epithelia to the matrix & connect matrix elements (e.g., collagen & proteoglycans).

ECM: Fibers

• Provide tensile strength and elasticity.
• Two main types in connective tissue:
  • Collagen
  • Elastin

Collagen Fibers

• Non-elastic, hard, and firm proteins.
• Most abundant proteins in the body.
• Highly resistant to traction.
• At least 35 different types, distinguished by amino acid sequence.

Collagen Biosynthesis

• Fibroblasts synthesize procollagen, a precursor molecule.
• Procollagen is released into the ECM, where it becomes activated tropocollagen.
• Tropocollagen molecules spontaneously assemble in a head-tail arrangement to form collagen microfibrils, then fibrils, then fibers.
• Vitamin C is crucial for proper collagen formation. Deficiency can lead to weakened tissues and bleeding disorders (scurvy).

Collagen Types

• Type I: Most abundant, supports skin, tendons, ligaments, and bones.
• Type II: Found in cartilage.
• Type III: Forms reticular fibers, supporting delicate organs and tissues.
• Type IV: Forms part of the basement membrane, supporting epithelial cells, muscles and Schwann cells.
• Others: at least 35 types, with various functions supporting tissue structure, such as binding other components or stabilizing the matrix.

Elastic Fibers

• Made of elastin protein and fibrillin.
• Thin, long, and branched.
• Provide elasticity to tissues.

Elastin Biosynthesis

• Fibroblasts and smooth muscle cells synthesize elastin.
• Tropoelastin molecules polymerize, binding to fibrillin microfibrils.
• Combining these creates elastic fibers.

Basement Membrane

• Non-cellular layer between epithelium and connective tissue.
• Similar structure surrounds skeletal, smooth muscle cells, adipocytes, and Schwann cells.
• Composed of basal lamina and reticular lamina.

Basal Lamina

• Synthesized by the epithelium.
• Comprised of structural glycoproteins and integrin fractions attaching to the epithelium; type IV collagen.
• Acts as a filter between epithelium and connective tissue.

Reticular Lamina

• Synthesized by the connective tissue.
• Consists primarily of reticular fibers (type III collagen) anchoring the basal lamina to the connective tissue.

Connective Tissue Cells: Permanent/Resident

• Fibroblasts (support and maintain the ECM).
• Adipocytes (store fat).
• Pericytes (support the endothelial cells lining blood vessels).
• Mast cells (release inflammatory mediators).
• Macrophages (phagocytosis, waste removal, defense).

Connective Tissue Cells: Transient/Wandering

• Leukocytes (part of the immune system).
• Plasma cells (secrete antibodies).

Origin of Connective Tissue Cells

• Many connective tissue cells derive from mesenchymal stem cells.
• Mesenchymal stem cells differentiate into various cells as needed.
• Cells also originate from the bone marrow and circulate in the blood stream to perform specific functions within connective tissues.

Mesenchymal Stem Cells

• Undifferentiated, pluripotent cells
• Irregular shape with cytoplasmic projections.
• Found primarily in embryonic connective tissue.
• Pericytes in adults have a similar function to Mesenchymal stem cells.

Fibroblasts

• Main cells of connective tissue, numerous and widely distributed.
• Elongated cells with ovoid nuclei.
• Generate the ECM.
• When inactive, are called fibrocytes.
• Specialized forms exist, such as myofibroblasts and reticular cells (specialized in generating reticular fibers).

Reticular Cells

• Fibroblasts specializing in reticular fiber generation.
• Extend many projections creating supportive networks in the cellular organs.

Myofibroblasts

• Fibroblasts with contractile properties.
• Resemble fibroblasts but have more contractile capabilities.
• Crucial in wound healing.
• Present in seminiferous tubules.

Pericytes

• Also derive from mesenchymal stem cells.
• Surround endothelial cells of capillaries.
• Can differentiate into other cell types (fibroblasts, endothelial cells, smooth muscle).

Mast Cells

• Largest permanent CT cells with oval shape and central nucleus.
• Contain granules (with heparin (anticoagulant), histamine (inflammation), and other molecules).
• Role in immune system, releasing contents when stimulated by antigens.

Macrophages

• Some are permanent, others are transient cells.
• Large, irregular cells derived from monocytes that circulate in the blood and then evolve into macrophages in tissues.
• Phagocytic (engulfing and destroying waste, pathogens, etc), and crucial in both removing waste and fighting infections.

Leukocytes

• Migratory cells of the connective tissue originating in bone marrow.
• Several types, each with different roles in inflammation and immunity (some granulocytes, monocytes, and lymphocytes).
• Critical in the body's immune activity.

Plasma Cells

• Derived from B lymphocytes (types of white blood cells).
• Rounded with an eccentric nucleus.
• Produce and secrete antibodies, helping the body defend against specific pathogens.

Classification of Connective Tissue

• Embryonic: mesenchymal CT (found in embryos), mucoid (umbilical cord, embryonic tissues)
• Adult: Loose, Dense, Elastic and Reticular tissues (each with different fiber arrangements, cell densities, and functions).

Loose Connective Tissue

• Abundant ground substance and tissue fluid.
• Few fibers, scattered randomly.
• Easily vascularized and innervated.
• Fills space between organs and other tissues under epithelium.

Dense Connective Tissue

• Abundant fibers, few cells.
• Collagen fibers bundled, offering high resistance to stress.
• Irregular dense connective tissue (disorganized bundles), found in the dermis, nerve sheaths, and muscles.
• Regular dense connective tissue (parallel bundles), found in tendons and ligaments.

Elastic Connective Tissue

• Thick bundles of elastic fibers arranged parallel and joined by surrounding loose connective tissue.
• Found in blood vessels and certain ligaments.

Reticular Connective Tissue

• Reticular cells and reticular fibers form a network to support tissues and organs with a high cellularity (like the spleen and lymph nodes).