Development

The thymus develops from branchial components on both sides, so contributing to its two lobes:

• endoderm: from ventral part of third and fourth pouches
• ectoderm: from third and fourth clefts

During fetal life, the rudimentary thymic tissue elongates caudally into the mediastinum from the equivalent of the future neck region. The thymic tissues detach from the branchial pouches and settle along a line in the median plane posterior to the mediastinum.

The branchial layers form the epithelia of the thymus which:

• orders intervening rows of lymphocytes into a net-like arrangement
• produces thymic corpuscles
• liberates local hormones such as thymosin that govern the differentiation of T-lymphocytes

In fetal life, the thymus receives lymphocytes from the liver, but when blood cell formation is assumed by the bone marrow, this is the source of lymphocytes.