Reticular fibers are thinner and made mostly of type III collagen. They are abundant in the framework of blood forming organs such as the spleen and bone marrow. Fibroblasts are the most common cells in connective tissue and are responsible for making the fibers and ground substance. They have an irregularly branched cytoplasm, large ovoid nucleus, and well developed endoplasmic reticulum and Golgi apparatus. In adults, fibroblasts rarely undergo division within connective tissues.
Macrophages are resident phagocytic cells in connective tissues.
They derive from monocytes that migrate into the connective tissue. Macrophages can proliferate within the tissue when needed. They are antigen presenting cells and are active in cell-mediated resistance to bacterial , viral , and parasitic infections as well as scavengers of aged erythrocytes . Mast cells are abundant in the dermis and in the digestive and respiratory tracts where they are the storage sites for chemical mediators of the inflammatory response . Secretory granules within mast cells contain histamine, chemotactic factors, and glycosaminoglycans like the anticoagulant heparin.