Connective Tissue Characteristics

General Characters of Connective Tissue

• Originates from mesoderm
• Supports, binds, and connects various tissues and organs
• Composed of connective tissue cells, fibers, and ground substance (matrix)
• Rich in blood vessels and nerves

Structure of Connective Tissue

• Consists of matrix, fibers, and cells

Connective Tissue Ground Substance

• Jelly-like substance formed of acid mucopolysaccharides
• Produced by fibroblast cells
• Present between connective tissue cells and fibers, binding them together
• Allows diffusion of tissue fluids, nutrients, and waste products between components and body tissues

Connective Tissue Fibers

White Collagen Fibers

• Appear white in fresh state
• Formed of bundles, each consisting of a group of fibers
• Each fiber formed of a group of fibrils, composed of tropocollagen molecules
• Appear pink in H&E stain
• Types:
  • Collagen type I: found in tendons of muscle and bone, formed by fibroblast cells and bone
  • Collagen type II: found in cartilage, formed by chondroblasts
  • Collagen type III: forms reticular fibers
  • Collagen type IV: present in basement membranes, formed by epithelial cells
  • Collagen type V: found in fetal membranes, blood vessels, and around muscles, formed by fibroblasts

Yellow Elastic Fibers

• Thin, branching, slender, and long fibers
• Appear yellow in great amounts
• Resistant to boiling and hydrolysis by acids or alkalies
• Formed of protein called elastin
• Formed by fibroblast and smooth muscle cells
• Stained yellow by Van Gieson stain and brown with orcein stain

Reticular Fibers

• Very fine, delicate fibers forming the stroma of glands like liver and salivary glands
• Not visible by H&E stain
• Appear black with silver stain and red with PAS
• Formed of collagen type III, proteoglycan, and glycoproteins
• Formed by fibroblasts and smooth muscle cells
• Function: forms reticular meshwork of stroma of organs

Connective Tissue Cells

Fixed Cells

• Long-lived cells like fibroblasts, histocytes, fat cells (adipocytes), mesenchymal cells, pericytes, and reticular cells

Free Cells

• Short-lived motile cells that enter connective tissue from blood, like free macrophages, plasma cells, mast cells, and leucocytes

Fixed Cells of Connective Tissue

Fibroblasts

• Irregular or fusiform in shape
• Basophilic cytoplasm rich in RER, Golgi complex, mitochondria, and secretory granules
• Pale oval nucleus
• Old, non-active fibroblasts are called fibrocytes, with acidophilic cytoplasm and dark nuclei
• Function: forms connective tissue fibers and ground substance; helps in wound healing

Reticular Cells

• Star-shaped cells with long processes and central rounded nuclei
• Functions: form reticular stroma of organs like spleen, lymph node, and thymus
• Can change into phagocytic cells when stimulated by antigens

Mesenchymal Cells

• Embryonic undifferentiated cells
• Pale basophilic cytoplasm with central oval nucleus and multiple cytoplasmic processes
• Function: can differentiate into other types of connective tissue cells

Pericytes

• Irregular cell membrane with multiple cell processes and oval central nuclei
• Present in the wall of blood capillaries between the basement membrane and endothelial lining
• Functions: can change into fibroblasts, smooth muscle cells, and endothelial cells; change diameter of capillaries

Histocytes (Fixed Macrophages)

• Irregular cell surface with protrusions and indentation and oval eccentric nucleus
• Cytoplasm rich in lysosomes and residual bodies
• Stained with vital stain like trypan blue
• Functions: phagocytosis of foreign bodies; antigen presentation; can fuse to form...