The different types are present in varying proportions dependent on the tissue type. Collagen is of several types is found in the human body. Collagen type I is the most abundant and is found in bone, tendons and dermis. Many cell types can produce collagen from the amino acids glycine, proline and hydroxyproline in a multistep process involving many posttranslational modifications. The resultant thin fibrils spontaneously associate to form fibers and very stable bundles of fibers.
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.