Histology - Connective Tissue (Part 1)

Histology - Connective Tissue (Part 1)

• Connective tissue (CT) originates from the mesoderm.
• CT is composed of widely separated cells with a significant amount of extracellular matrix.
• CT is penetrated by blood vessels, nerves, and lymphatic vessels.
• CT connects, supports, and protects other tissues and organs.

Objectives

• List the characteristics of connective tissue (CT).
• Recognize the histological structure of CT.
• Classify the types of CT cells.
• Describe the histological structure and functions of CT cells.
• List the components of the ground substance of CT

Connective Tissue Structure

• Cells: Blood vessels, ground substance, protein fibers (elastic, collagen, reticular).
• Extracellular Matrix: Fibers, ground substance.
• Fibers: Protein fibers (elastic, collagen, reticular).
• Ground Substance: Glycoproteins, glycosaminoglycans, proteoglycans.

Types of Connective Tissue (CT) According to Matrix Consistency

• Soft: CT proper
• Rubbery: Cartilage
• Hard: Bone
• Fluid: Blood

Types of CT Cells

• Fixed (Resident or Permanent Cells):
  • Originate locally within the CT.
  • Stable and long-lived cells.
    • Undifferentiated mesenchymal cells
    • Pericytes
    • Fibroblasts & fibrocytes
    • Adipocytes
• Wandering (Transient or Temporary Cells):
  • Originate from stem cells in bone marrow.
  • Circulate in blood then move into connective tissue to perform their functions.
    • Motile and short-lived cells.
    • Macrophages
    • Mast cells
    • Plasma cells
    • Leucocytes

Undifferentiated Mesenchymal Cells (UMCs)

• Origin: The mother cells of all types of CT.
• Sites:
  • Present mainly in embryos as stem cells.
  • In adults, some cells remain after birth around blood capillaries in the form of pericytes.
• Structure:
  • Shape: Small stellate-shaped cells with cytoplasmic processes (branched).
  • Nucleus: Large, pale, with prominent nucleolus.
  • Cytoplasm: Basophilic.
  • (EM) Nucleus: Euchromatic
  • (EM) Cytoplasm: Contains many free ribosomes.
• Function:
  • They can differentiate into different cells of all types of connective tissue (CT proper, cartilage, and bone).
  • Give rise to blood elements.
  • Some cells remain after birth around blood capillaries (pericytes).

Pericytes

• Origin: UMCs.
• Sites: Present around blood capillaries.
• Structure:
  • LM Shape: Branched cells with long cytoplasmic processes.
  • Cytoplasm: Basophilic.
  • Nucleus: Central and oval.
• EM Cytoplasm: contains network of actin and myosin
• Function:
  • In case of tissue injury, pericytes can differentiate into fibroblasts, endothelial cells, and smooth muscle cells.
  • They have contractile ability to regulate blood flow through capillaries.

Fibroblasts

• Origin: UMCs and pericytes.
• Sites: Most common type of CT cells.
• Structure:
  • LM Shape: Flat branched cells with cytoplasmic processes.
  • Nucleus: Large, pale, with prominent nucleolus.
  • Cytoplasm: Deep basophilic.
• EM
  • Well-developed rough endoplasmic reticulum (rER).
  • Well-developed Golgi apparatus.
  • Mitochondria.
  • Euchromatic nucleus.
  • No secretory granules in the cytoplasm.
• Function:
  • Synthesis of CT matrix (fibers and ground substance).
  • Synthesis of growth factors that enhance cell growth.
  • Healing of wounds.

Fibrocytes

• Smaller and spindle-shaped with fewer processes.
• Smaller and darker nucleus.
• Pale basophilic cytoplasm (less rER).
• Maintenance of CT matrix.

Adipocytes (Fat Cells)

• Origin: UMCs.

• Types: Unilocular and multilocular fat cells.

• Unilocular fat cells

  • Large oval cells with peripheral nucleus.
  • Cytoplasm with large fat droplet (by H&E, the fat droplet dissolves).
  • Thin rim of cytoplasm.
  • Signet ring appearance.

• Multilocular fat cells

  • Small rounded cells with central nucleus.
  • Cytoplasm contains many fat droplets of various sizes.
  • Numerous mitochondria.

• Special Stain:

  • Unilocular fat cells: Fat stains orange with Sudan III or black with Sudan black B and osmic acid.
  • Multilocular fat cells: Function: Heat generation (Brown fat). Energy storage (White fat)

Macrophages

• Origin: Blood monocytes.
• Structure:
  • LM Shape: Large irregular cells.
  • Nucleus: Eccentric and kidney-shaped.
  • EM Contains many lysosomes
  • Has many cell processes called pseudopodia.
• Function: Phagocytosis of foreign bodies, microorganisms, and dead cells, act as antigen-presenting cells(to T-lymphocytes), secrete growth factors important for tissue repair.

Plasma Cells

• Origin: B-lymphocytes.
• Site: Abundant in lymphoid tissue.
• Structure: Oval shape, eccentric nucleus (cart-wheel), heterochromatin alternating with euchromatin, deep basophilic cytoplasm, well-developed Golgi apparatus.
• EM: Well-developed rER, well-developed Golgi apparatus, mitochondria, no secretory granules in cytoplasm.
• Function: Synthesis and secretion of antibodies (humoral immunity).

Mast Cells

• Origin: Hematopoietic stem cells.
• Sites: Along blood vessels and mucosa of the GIT and respiratory tract.
• Structure: Oval or rounded shape, central nucleus, cytoplasm containing basophilic granules.
• Special stain: Granules are metachromatically stained by toluidine blue (appear purple or red).
• EM: RER, Golgi apparatus, mitochondria, electron-dense secretory granules.
• Function: Release chemical mediators (e.g. heparin, histamine, leukotrienes) resulting in an allergic reaction, carry IgE surface receptors. Binding of antigens to IgE leads to degranulation.

Ground Substance of CT Matrix

• Highly hydrated material.
• Composed of 3 macromolecules:
  • Glycosaminoglycans (GAGs): Polysaccharides (e.g., hyaluronic acid, chondroitin sulfate)
  • Proteoglycans: GAGs attached to a core protein
  • Multiadhesive glycoproteins: Large protein molecules (e.g., fibronectin) with branched oligosaccharide chains.
• Water in the ground substance is called interstitial fluid.
• Edema is excessive interstitial fluid accumulation in CT, often due to inflammation.

Pigment Cells

• Origin: UMCs
• Long branching processes
• Melanin storing cells
• Site: Iris of the eye, dermis of skin