Connective Tissue

Connective tissue is one of the four types of tissues found in animal bodies. They support, separate and connect different types of tissues in the body. Connective tissue consists of varying amounts of an extracellular substance which separates its cells. This tissue comprises of only a small fraction of cells. The extracellular substance is comprised of fibers fixed in the ground substance which contains tissue fluid. There are three types of fibers in the connective tissue.

Extracellular Substance

Collagen Fibers

It is the most abundant type of fibers found in the connective tissue. Their main function is to give support and strength to the tissue. These fibers are 1 to 10 µm thick. The fibers are composed of smaller fibrils of 0.2 to 0.5 µm. Fibrils are further made up of microfibrils.

Microfibrils are assemblies of tropocollagen which is a triple helix of collagen molecules. The organization of tropocollagen is very regular. Coarse collagen fibers are made up of type I tropocollagen. These fibers can stretch by only 15-20 %.

Reticular Fibers

These fibers are composed of a fine network of fibers instead of bundles. Reticular fibers consist of collagen but the main type of tropocollagen is type III in reticular fibers, which is different from the coarse collagen fibers.

Elastic Fibers

There are no substructures in the elastic fibers. It is revealed by electron microscopy that elastic fibers comprise of individual microfibrils. These microfibrils are embedded in an amorphous matrix. The matrix is made up of 90 percent of fiber and is composed of elastin protein. Neither the microfibrils nor the elastin is made of collagen. If tensile forces are applied, elastic fibers can stretch up to 150 percent.

Ground Substance

Ground substance is an amorphous gel-like structure found in connective tissue, which surrounds the cells. Cells are supported by an extracellular matrix, in a tissue. Ground substance does not include fibers (elastin and collagen). The composition of the ground substance varies depending on the tissue. The ground substance usually comprises of water, proteoglycans, glycosaminoglycans (mostly hyaluronan), and glycoproteins.

Connective Tissue Cells

The cells of the connective tissue are divided into two types based on their ability to move within the tissue: fixed cells (fibrocytes and fat cells) and wandering cells (macrophages, plasma cells,  lymphocytes etc).

Fibrocytes

These are the most common type of cell in the connective tissue, considered as the true connective tissue. The cytoplasm of the fibrocyte (inactive) does not contain many organelles but if they are activated, for example by damage to the surrounding tissue then, the situation is changed. In that case, it becomes a fibroblast and it contains many organelles essential for the synthesis of proteins required for repair.

Reticular Cells

These are larger than fibrocytes and form a network of reticular fibers, for example in lymphoid organs. Nuclei of reticular cells are large.

Adipocytes

Adipocytes or fat cells are fixed cells in the loose connective tissue. Their main function is to store lipids. in a well-fed adipocyte, the cytoplasm is found in the form of a small rim around the large lipid droplet and the nucleus is found in slightly thick part of the cytoplasm. But, a starving adipocyte, it would resemble a fibrocyte and contains many small lipid droplets. Adipocyte mobilization is controlled by nervous system or hormones. They secrete

Adipocyte mobilization is controlled by nervous system or hormones. They secrete leptin protein, which gives feedback to the brain centers about the bodily fat reserves. They are very long-lived and their number is controlled by the number of lipoblasts produced during foetal and early postnatal development.

Macrophages

Macrophages are formed from monocytes (precursor cells). Monocytes are originated in the bone marrow. From bone marrow, they are released into the blood stream. From blood stream, they enter the connective tissue, because they are highly mobile. Upon reaching connective tissue, the differentiate into macrophages. The appearance of the macrophages may change depending on the type of phagocytic activity.

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