Fibroblasts and myofibroblasts

The fibroblasts are the principal and most common cells in connective tissue.

Fibroblasts are responsible for the synthesis of collagenous, elastic and reticular fibers and the complex carbohydrates of the ground substance.

The fibroblasts have two stages of activity:

-active and

- quiescent

The quiescent cell is fibrocyte.

The active cell is fibroblast.

The active cell (fibroblast) has an abundant and irregularly branched cytoplasm. The cytoplasm contains rough endoplasmic reticulum, which synthesis procolagen and elastin, and Golgi complex, which synthesis GAG (glycosaminoglycan) of the intercellular ground substance.

The fibrocyte has an acidophilic cytoplasm with a small amount of rough endoplasmic reticulum.

Where active production of extracellular matrix material is in progress during active growth or in wound, the cytoplasm of the fibroblast is more extensive and displays a slight basophilia as a result of increased amounts of rough endoplasmic reticulum associated with protein synthesis. The myofibroblast displays properties of both fibroblast and smooth muscle cell. These cells have the morphologic characteristics of fibroblasts but contain increased amounts of actin microfilaments and myosin. Their activity is responsible for wound clossure after tissue injury, a process called wound contraction.

Macrophages

Macrophages are phagocytic cells derived from connective tissue macrophages, also known as tissue histiocytes; contain a large Golgi, rough and smooth endoplasmic reticulum, mitochondria, secretory vesicles and lysosomes.

The lysosomes of the macrophage, along with the surface cytoplasmic projections, are the structures most indicative of the specialized phagocytic capability of the cell.

Although the main function of the macrophage is phagocytosis, the macrophage also plays a role in immune reactions by presenting lymphocytes with concentrated antigens derived from phagocytosed foreign cells or proteins.

Mast cells

Mast cells are oval to round connective tissue cells, whose cytoplasm is filled with basophilic granules.

The secretions of mast cell granules can result in immediate hypersensivity reactions, allergy and anaphylaxis.

Mast cell granules include four substances:

-histamine

-slow reacting substance of anaphylaxis (SRS-A)

-eosinophil chemotactic factor of anaphylaxis (ECF-A)

-heparin

In the immune response, histamine and SRS-A increase the permeability of small blood vessels thereby causing edema in the surrounding tissue.

Heparin is an anticoagulant.

The ECF-A stimulates eosinophils to migrate to the sites where mast cells have released their agents, the eosinophils counteract the effects of the histamine and SRS-A.

Adipose cells

The adipose cell is a connective tissue cell specialized to store neutral fat. When they accumulate in very large numbers, they are called adipose tissue.

Plasma cells

Plasma cells are antibody-producing cells derived from B lymphocytes. Their average life is 10-20 days. Plasma cells are large, ovoid cells. The cytoplasm displays strong basophilia due to the presence of an extensive rough endoplasmic reticulum.

Pericytes

The pericytes is a cell that serves as an undifferentiated mesenchymal cell. Pericytes, also called adventitial cells and perivascular cells, are found around capillaries and venules.

Cartilage

Cartilage is a form of connective tissue composed of cells called chondrocytes and a highly specialized extracellular matrix, composed of fibers and ground substance.

Cartilage is an avascular tissue that consists of chondrocytes and extensive matrix that is produced and maintained by the chondrocytes. Chondrocytes are located in matrix cavities called lacunae.

80% of cartilage consists of water, 10-15% of organic substances and 5-8% of nonorganic salts.

Three different kinds of cartilage of cartilage are distinguished on the basis of characteristics of matrix:

-hyaline cartilage, characterized by a homogeneous amorphous matrix

-elastic cartilage whose matrix contains elastic fibers and elastic lamellae

-fibro cartilage, whose matrix contains large bundles of type I collagen

The perichondrium is a sheath of dense connective tissue that surrounds cartilage in most places, forming an interface between the cartilage and the tissue supported by the cartilage.