Connective Tissue Overview Quiz

Connective Tissue Overview

• Originates from mesoderm
• Composed of widely separated cells with a large amount of ground substance (matrix)
• Contains blood vessels, nerves, and lymphatic vessels
• Connects, supports, and protects other tissues (e.g., epithelium and organs)

Connective Tissue Cells

• Fixed Cells: Stable population, long-lived cells produced in the connective tissue and remain there.

  • Undifferentiated mesenchymal cells: Stem cells in embryos that can differentiate into other connective tissue cells. In adults, they remain undifferentiated in bone marrow and act as stem cells; pericytes, and bone marrow cells. Small, irregular, branched cells with a pale basophilic cytoplasm, central, large oval pale nucleus and nucleoli. Few organelles (many ribosomes).
  • Function (in embryo): act as stem (mother) cells, differentiating into all types of CT cells, smooth muscle cells, and endothelial cells. Function (in adult): remain undifferentiated in certain areas to act as life-long source for some cells (e.g., in bone marrow -- blood cells, around blood vessels -- pericytes).
  • Pericytes: Present around blood capillaries, branched with an oval nucleus and pale cytoplasm. Give rise to fibroblasts and smooth muscles throughout life. Involved in vasoconstriction.
  • Function (injury): can differentiate into fibroblasts, smooth muscle cells, and endothelial cells.
  • Fibroblasts: Most common type in connective tissue proper. Originate from mesenchymal cells, pericytes. Flat, elongated cells with processes. Produce protein-forming cells. Functions: formation of connective tissue fibers; build connective tissue matrix; repair connective tissue after injury.
  • Fibrocytes: Old, inactive fibroblasts; smaller, fewer processes; less basophilic cytoplasm; less rER; dark nucleus. Continuous slow turnover of connective tissue fibers and matrix (maintenance of connective tissue).
  • Adipocytes (fat cells):
    • Unilocular (white): Large oval cells with a flattened nucleus to the periphery; cytoplasm is thin film around a large fat globule. H&E stain - signet ring appearance; Special stain - Sudan III (orange). Function: storage of fat, heat insulator.
    • Multilocular (brown): Small, rounded cells with a central rounded nucleus; many small fat droplets; many mitochondria which contain cytochrome pigment causing brown color. Less common; function: heat generation.
  • Fixed Macrophage (hisitoctye): Derived from monocytes; attached to connective tissue fibers, mainly collagen fibers; large, branched cells with pseudopodia (variable shape); small, dark, oval or kidney-shaped nucleus with vacuolated cytoplasm. Vital stain: trypan blue. Functions: phagocytosis, clean wounds, fuse with other forms to form giant cells to engulf large foreign bodies; antigen presenting to lymphocyte; destruction of old RBCs (liver, spleen).
  • Reticular cells: Present in the stroma of organs; small, branched cells with long processes. Silver stain can demonstrate them. Functions: form reticular fibers; act as supporting cells; form a reticular network with reticular fibers; act as phagocytic cells as needed.

• Free Cells: Changeable population, motile cells belonging to the immune system that enter the connective tissue from the blood; short-lived and wander through connective tissue.

  • Plasma cells: Originate from B-lymphocytes; differentiate to plasmablasts then plasma cells. Oval with eccentric nucleus (cart-wheel or clock-face); cytoplasm is deep basophilic with a visible Golgi image. Sites: abundant in lymphoid tissue, rarely found in blood. EM: well-developed Golgi apparatus; closely spaced cisternae of rER & mitochondria. Functions: formation and secretion of antibodies.
  • Mast cells: Originate from mesenchymal cells in bone marrow. Sites: around blood vessels and sub-epithelial connective tissue (respiratory and digestive systems). Oval cell, with eccentric nucleus; basophilic cytoplasm. Stain: metachromatic stain with toluidine blue. Function: secretion of histamine, heparin, and eosinophil chemotactic factor.
  • Free Macrophages: The same as fixed macrophages but they wander in connective tissues.
  • Blood Leukocytes: All types migrate from the bloodstream to the connective tissue to perform their defensive functions (e.g., lymphocytes, monocytes, basophils, neutrophils, eosinophils)

Connective Tissue Fibers

• White Collagen Fibers: Synthesized by fibroblasts, chondroblasts, and osteoblasts; wavy branching bundles of non-branching parallel fibers; stain pink in H&E; strong, flexible, not elastic; give tissues strength and resist stretching.
• Yellow Elastic Fibers: Synthesized by fibroblasts and chondroblasts; thin, long branching fibers running singly; stain pink in H&E, brown with Orcein; stretchable; give elasticity to tissues.
• Reticular Fibers: Synthesized by fibroblasts and reticular cells; very thin fibers branching and anastomosing to form a network; not seen by H&E; brown with silver stain (argyrophilic); delicate, flexible; form the stroma (background) that supports organs.

Types of Connective Tissue Proper

• Loose (Areolar) Connective Tissue: Contains all types of connective tissue cells and fibers, embedded in an abundant matrix; contains potential cavities (areolae); present throughout the body; functions: binds tissues together, surrounds organs, and supports organs and tissues.
• Reticular Connective Tissue: Reticular fibers form network; presence of reticular cells.
• Mucoid Connective Tissue: Large amount of jelly-like ground substance rich in mucus and hyaluronic acid; UMCs and fibroblasts that communicate via processes; fine collagen fibers; found in umbilical cord (Wharton's jelly), pulp of growing teeth, vitreous humor of the eye. Protects near-by structures from pressure.
• Adipose Connective Tissue:
  • White Adipose: Unilocular fat cells; each cell contains a large single fat droplet; fat is not pigmented; located under skin, mammary gland, around kidneys; functions: storage of fat, heat insulator, support of organs, forms body contours.
  • Brown Adipose: Multilocular fat cells; each cell contains many fat droplets; fat is pigmented due to high vascularity and cytochrome pigments; abundant in newborns; limited to supraclavicular, interscapular, axillary, thoracic, and paravertebral regions in adults; function: heat generation.
• Dense Connective Tissue:
  • Regular White Fibrous: Parallel collagen bundles with fibroblasts between; located in tendons and cornea; functions: withstand stretch in one direction.
  • Irregular White Fibrous: Irregularly arranged collagen bundles with fibroblasts between; located in periosteum, perichondrium, dermis of skin, capsule of organs; functions: withstand stretch in different directions.
  • Yellow Elastic: mainly elastic fibers; appears yellow in fresh state; located in the aorta, bronchi/bronchioles, and ligaments; function: recoil after stretch.