Connective Tissue Characteristics and Functions

Connective Tissue General Characteristics

• Connective tissue is derived from mesenchyme.
• Composed of cells, fibers (collagen, elastic, reticular) and ground substance.
• Cells are widely separated by a large amount of extracellular substance (matrix).
• Matrix can be firm, hard, or fluid, depending on the type of connective tissue.
• Often penetrated by blood vessels, lymphatics, and nerves.

Connective Tissue Functions

• Support and connection of cells, tissues, and organs.
• Cells provide healing of injured tissues.
• Resist stress, as seen in cartilage, bone, tendons, ligaments, and organ capsules.
• Medium for exchange, as blood, lymph, and connective tissue proper deliver nutrients, waste products, and signaling molecules.
• Immunity, as plasma, mast cells, and white blood cells (WBCs) leave the bloodstream and enter connective tissue to protect the body.
• Lipid storage and metabolism, performed by adipose cells and adipose tissue.

Connective Tissue Cells

• Fixed Cells: Stable and long-lived cells.
  • Fibroblasts
  • Fat Cells
  • Undifferentiated mesenchymal cells (UMCs)
  • Fixed Macrophages
• Wandering Free Cells: Transient and migrating cells from the blood to the connective tissue.
  • Plasma Cells
  • Mast Cells
  • Leukocytes

Undifferentiated Mesenchymal Cells (UMCs)

• LM: Small, irregular, branched, star-shaped cells with pale basophilic cytoplasm. Nucleus is large, oval, and centrally located with visible nucleoli.
• EM: Few organelles, many free ribosomes.
• Function:
  • Multipotent stem cell that can differentiate into other connective tissue cells.
  • In bone marrow they give rise to blood cells.

Fibroblast

• Origin: Undifferentiated mesenchymal cells, pericytes.
• Site: All types of connective tissue.
• LM: Flat, branched cells with thin processes; oval nucleus with a prominent nucleolus. Basophilic cytoplasm.
• EM: Euchromatic nucleus with a prominent nucleolus, RER, ribosomes, Golgi, mitochondria.
• Function:
  • Formation of connective tissue fibers.
  • Formation of ground substance.
  • Healing and repair of wounds.

Fibrocytes

• LM/EM: Smaller, spindle-shaped with few processes, elongated, dark nucleus. Involved in wound closure.

Myofibroblasts

• LM/EM: Similar to fibroblasts and smooth muscle cells. Contains actin and myosin.
• Function: Responsible for wound closure.

Fat Cells - Unilocular Adipocyte (White Fat)

• Site: White adipose tissue.
• LM: Rounded or oval, large cells with a peripheral, flattened nucleus. Cytoplasm is pale and reduced to a thin rim around a large fat droplet. In H&E stains, fat dissolves, creating large vacuoles (signet-ring appearance). Stains orange with Sudan III.
• EM: Fat droplets appear as electron-dense inclusions, with few cell organelles surrounding the nucleus.
• Function: Storage of fat, support organs, and heat insulation.

Fat Cells - Multilocular Adipocyte (Brown Fat)

• Site: Brown adipose connective tissue.
• LM: Small rounded cell with a rounded nucleus and many small fat droplets. Contains many mitochondria, giving it a brown appearance.
• Function: Heat generation.

Macrophages

• Origin: Blood monocytes.
• Site: Scattered in connective tissue or fixed to collagen fibers.
• LM: Large, branched cell with an irregular cell boundary (pseudopodia) and a dark, kidney-shaped nucleus. Cytoplasm is pale basophilic and vacuolated.
• EM: Pseudopodia, lysosomes, RER, Golgi.
• Function:
  • Phagocytic.
  • Antigen-presenting cells (APCs).

Mononuclear Phagocytic System

• Includes macrophages in different organs with specific names:
  • Von Kupffer cells in the liver.
  • Dust cells in the lung.
  • Langerhans cells in the skin.
  • Monocytes in the blood.
  • Microglia in the brain.
  • Osteoclasts in the bone.
  • Macrophages in the spleen, thymus, lymph node, bone marrow, and connective tissue.
• These cells share the same structure and function.

Reticular Cells

• Origin: Undifferentiated mesenchymal cells (UMCs).
• Site: Reticular stroma of parenchymatous organs (most common cell in reticular connective tissue).
• LM: Shape: Small, branched cell with many processes (reticular fibers are attached to their processes). Stains: Hx & E → Pale basophilic. Special stain → Silver stain (brown). Nucleus: Central, oval, and pale.
• Function:
  • Multipotent stem cell, supportive function.
  • Antigen-presenting cells (APCs).
  • Phagocytic function.