AIPMT-NEET Biology Aspirants, learn about Connective tissue, its features and types in this post, important for AIPMT-NEET Biology Study material/ Notes.

One of the four basic types of body tissues in the human body is- Connective tissue. Connective tissue protects and supports the body. Helps the body to fight against infections, stores energy reserves as fat and help to bind the organs together, makes transport system within the body.

• It is the most abundant and widely distributed tissue in the body.
• It has 2 basic elements – (A) Connective tissue cells and (B) Extracellular matrix.

Cells and fibers of the Connective Tissue

(A) CELLS OF CONNECTIVE TISSUE

Cells of the connective tissue (let’s write connective tissue as C.T. for simplicity) originate from the mesenchymal cells (embryonic cells). Following types of cells are present in the connective tissue: Fibroblasts, Macrophages, Plasma cells, Mast cells, Adipocytes, Leukocytes.

(1) Fibroblasts: these are large, flat, moving cells with branching processes. Most numerous in no. and they are present in all types of C.T. These cells secrete fibers and ground substance of extracellular matrix.

(2) Macrophages: these are irregular shaped, large cells, developed from monocytes (monocytes are types of White blood cells). These cells engulf bacteria, cell debris and other foreign substances through the process of phagocytosis. Fixed macrophages are found in lungs and spleen, while wandering macrophages are found at the site of infection.

(3) Plasma cells: these are small cells developed from B-lymphocytes (type of White blood cell). These cells produce antibodies and help in neutralizing foreign substances. Mostly found in G.I. tract and respiratory tract, salivary glands, lymph nodes, spleen, red bone marrow.

(4) Mast cells: these cells produce histamine (helps to dilate small blood vessels for inflammatory response). Also able to ingest and kill bacteria. Found in association with blood vessels of C.T.

(5) Adipocytes: these cells help to store fats (triglycerides), so also called as fat cells or adipose cells. Found deep to the skin and around heart and kidneys.

(6) Leukocytes: leukocytes are white blood cells, which migrates from blood to C.T. when required. E.g. Neutrophiles migrate at the site of microbe infection and perform phagocytosis. Eosinophiles migrate at the site of parasitic infection and allergic response.

NOTE: Cells whose names ends with –blast like fibroblasts, chondroblasts in cartilage and osteoblasts in bones are undifferentiated, immature cells which secrete extracellular matrix and retain the capacity of cell division. Once the cells transform into mature cells their names become chondrocytes, osteocytes, fibrocytes, i.e. ending with –cyte. These mature cells have less capacity of cell division and help in maintenance of cell. Cell names ending with -clast like osteoclasts, help in breakdown of extracellular matrix.

(B) EXTRACELLULAR MATRIX (ECM) OF CONNECTIVE TISSUE

ECM of C.T. has 2 components : (a)Ground substance and (b) Fibers.

(a)Ground substance: It is the material present between cells and fibers of C.T.

• It may be fluid, semifluid, or calcified.
• It is made up of water and organic molecules like hyaluronic acid, chondroitin sulphate, glucosamine, called as glycosaminoglycans (GAGs).
• It supports cells and fibers, binds them together, and provides a medium for exchange of substances between blood and cells.
• Hyaluronic acid is viscous, slippery substance that binds cell together and make ground substance of C.T. jellylike. It helps to maintain the shape of the eyeballs. Enzyme hyaluronidase breaks hyaluronic acid making ground substance more liquid. Enzyme found in W.B.Cs, sperm and bacteria.
• Ground substance also has some adhesion proteins. Most important is Fibronectin.
• Adhesion proteins help in binding of components of ground substance to one another and to the surface of the cell. Fibronectin binds collagen fibers and ground substance together by binding to both.

(b)Fibers: fibers help to provide support and strength to the C.T. ECM has 3 types of fibers: Collagen fibers, Elastic fibers and Reticular fibers.

• Collagen fibers: these fibers are very strong and provide flexibility to the tissue also. Collagen fibers contain collagen protein (most abundant protein in the body, 25% of the total protein). This fiber provides great tensile strength to the tissue as these fibers occurs in the form of parallel bundles. Found in most types of C.T. like bones, cartilage, tendons (attach muscle to bone), ligaments (attach bone to bone).
• Elastic fibers: these fibers are smaller in diameter than collagen fibers. It forms a fibrous network in C.T. by branching and joining again. It contains elastin protein with fibrillin glycoprotein providing these fibers strength and stability. These fibers has great elasticity, i.e. can be stretched upto 150% of original length and then regain its original shape without breaking. Found in skin, blood vessels, and lung tissue.
• Reticular fibers: these fibers are much thinner than collagen fibers. It forms branching network in C.T. Made up of collagen protein and glycoproteins. Provides strength to the wall of blood vessels, smooth muscles, nerves etc.