The amino acid proline is considered to be one of the non-essential amino acids as it is synthesized naturally via biochemical pathways in the body and is not required in the diet - it is synthesized from other amino acids in the liver. There is not need for proline from dietary sources. The amino acid proline serves as a precursor for a derivative called hydroxyl-proline. This form of proline is considered to be a major component of the connective tissues of the body – namely the fibrous protein collagen. The structural nature of proline differs from all other amino acids as it possess a secondary amino group and has a pyrrole ring - a structure seen in hemoglobin and the cytochromes. It is correctly termed an imino acid due to its peculiar structure. The structural peculiarity of proline caused it to impart a rigid protein structure on all proteins that contain proline. The texture of human skin is improved by proline as it aids in the collagen synthesis and also reduces the loss of collagen through the normal process of aging. The healing of cartilage and strengthening of joints, various tendons and heart muscles is also achieved by their high proline content. The health of the connective tissues in the body is promoted and maintained by proline working in tandem with the vitamin C. Meat sources are the primary source of proline and most other amino acids found in the body. The ability of a person to learn is promoted by proline. The restoration of tissues and wound recovery following injury is also actively aided by proline.