A bi-lobed organ surrounded by a connective tissue capsule. It is located in the upper anterior portion of the chest, behind the sternum. It is composed predominantly of lymphocytes and fewer epithelial cells. Connective tissue septa separate the lobes into lobules. The lobules contain an outer portion (cortical zone) which is rich in lymphocytes and an inner portion (medullary zone) which is rich in epithelial cells. It is an organ essential for the development of the immune system. Its function is the maturation of the progenitor lymphoid cells to thymocytes and subsequently to mature T-cells. It reaches its greatest weight at puberty and subsequently begins to involute.