Connective Tissue Overview and Cells

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

Fixed Cells:
- Stable population, long-lived cells
- Produced in connective tissue and remain within it
- Examples: mesenchymal cells, pericytes, fibroblasts, fibrocytes, and adipocytes.
Free Cells:
- Changeable population, motile cells
- Belong to the immune system
- Enter the connective tissue from blood
- Short-lived and wander through the connective tissue
- Examples: macrophages and blood leukocytes

Embryonic stem cells capable of differentiating into various connective tissue cells
In adults, these cells remain undifferentiated in certain areas (e.g., pericytes, bone marrow) and act as stem cells
Small, irregular, branched cells
Pale cytoplasm, centrally located oval, pale nucleus with nucleoli
Few organelles (many ribosomes)

Most common type in most connective tissue
Derived from mesenchymal cells and pericytes
Flat, elongated cells with processes
Major function: formation of connective tissue fibers and matrix; crucial for tissue repair after injury

Old, inactive fibroblasts
Smaller size than fibroblasts with fewer processes
Less basophilic cytoplasm, with less rough endoplasmic reticulum (rER), and a darker nucleus
Maintain and renew connective tissue fibers and matrix

Unilocular (White) Adipocytes:
- Large, oval-shaped cells with a peripheral, flattened nucleus
- Cytoplasm contains a large fat globule
- Appear as signet rings in H&E stain; Sudan III stain (orange)
- Function: Fat storage, heat insulation, and body contouring
Multilocular (Brown) Adipocytes:
- Smaller, rounded cells with a central, rounded nucleus
- Contain many small fat droplets and numerous mitochondria
- Contain cytochrome pigment, giving them a brown color
- Function: Heat generation, less common than white adipocytes

Derived from monocytes
Attached to connective tissue fibers (mainly collagen), and are scattered throughout
Large, branched cells with pseudopodia, allowing for variable shapes
Small, dark, oval or kidney-shaped nucleus with vacuolated cytoplasm
Essential role in phagocytosis and immune responses

Phagocytosis, including removing microbes, cell debris, and other foreign material through pseudopodia, lysosome fusion.
Antigen presentation, playing a role in immune responses through phagocytosis.
Wound healing

Originate from mesenchymal cells
Branched cells with extensive processes, found in the stroma of organs
Silver stain can enhance visibility of the cell and their processes
Reticular cells, along with the reticular fibers produced by them, form a supportive network, maintaining the structure of organs like the liver, lymph nodes, and spleen

Originate from B-lymphocytes
Oval shape with eccentric (offset) nuclei appearing like a cartwheel or clockface
Extensive rough endoplasmic reticulum for rapid protein synthesis, creating a deep basophilic cytoplasm
Sites: Abundant in lymphoid tissues; rarely found in blood
Function: Antibody formation and secretion

Originate from mesenchymal cells in the bone marrow
Oval cells with an eccentric nucleus, located in connective tissue
Cytoplasm contains basophilic granules; their content is revealed with toluidine blue
EM: Well-developed Golgi apparatus, electron-dense granules, extensive rough endoplasmic reticulum (rER), and mitochondria
Function: Secrete histamine, heparin, and other factors involved in inflammation and allergic reactions

Same as histiocytes but freely move in connective tissue instead of being fixed to tissue fibers.

Various types of blood leukocytes migrate from blood vessels to connective tissue to perform immune functions
Ex: Lymphocytes (T-cells, B-cells, natural killer (NK) cells), monocytes, neutrophils, basophils, eosinophils

White Collagen Fibers: Parallel bundles formed by non-branching fibers, appearing pink in H&E stain. They are strong, flexible but not elastic, responsible for tensile strength and support in tissues
Yellow Elastic Fibers: Thin, branching fibers that run singly, appearing pink in H&E or brown with Orcein stain. They are stretchable, providing elasticity
Reticular Fibers: Very thin branching fibers that anastomose, forming a network; not visible with H&E and require silver stain. They are a supportive framework in some organs (stroma)

Loose (Areolar) C.T.: Contains all cell types and fibers, accommodating fluids or gases. Found throughout the body except the brain, usually under epithelium, in dermis, glands, organs. It binds tissues together.
Reticular C.T.: Dense network of reticular fibers, prevalent in lymphoid organs. Forms the framework (stroma)
Adipose (Fat) C.T.: Composed of adipocytes. Two types exist: white (unilocular) for insulation, fat storage, and body contouring, and brown (multilocular) for heat generation.
Mucoid connective tissue: Found in umbilical cord or Wharton's jelly, Vitreous humor of eye to protect nearby structures from pressure.
Dense C.T.:
- Regular White Fibrous C.T.: Parallel collagen fibers, resistant to tensile stress in one direction; found in tendons and cornea
- Irregular White Fibrous C.T.: Irregularly arranged collagen fibers with fibroblasts, resistant to stress in many directions. Common in dermis, periosteum, perichondrium.
- Yellow Elastic C.T.: Primarily elastic fibers to provide stretch and recoil; found in large arteries, ligaments, and vocal cords