Connective tissues contain a variety of cells embedded in a matrix of fibers and ground substance, which consists of water, proteins, carbohydrates, and lipids. The cells of connective tissues carry out diverse tasks. Fibroblasts produce the proteins, through secretion, from which the fibers of connective tissues are made. White blood cells enter connective tissue to fight infection. Macrophages, differentiated from circulating white blood cells called monocytes, help during inflammation: Irregularly shaped, these cells can engulf particles such as bacteria or dead red blood cells and move by amoeboid motion. Fat cells are highly specialized for the storage of fat: Connective tissue that has a high concentration of fat is called adipose tissue. Mast cells secrete two substances: heparin which prevents blood clotting and histamine which increases the permeability of capillary walls.
There are three types of fibers found in connective tissues. The most common type of fibers are the collagenous fibers, composed of many strands of the protein collagen (which occurs in several forms) bound together. Collagenous fibers impart strength to the tissues in which they are located. They are flexible but resistant to stretching. Elastic fibers, on the other hand, are easily stretched, returning to their original shape when the stretching ceases. Elastic fibers, made of the protein elastin, are significantly thinner than collagenous fibers. Reticular fibers arise at the points where connective tissues joins with other tissues. Reticular fibers, branched and interwoven in structure, help hold the connective and non-connective
