Evolution, development and involution of the thymus

We studied the appearance of the thymus primordia of main representatives of the vertebrate evolution, observed these anlages in different places of the branchial region and followed their gradual reduction in phyletic line. On the assumption that the thymus potency has been encoded in evolution, we studied the human thymus ontogeny and described the entodermal thymus primordia not only in the 3rd but also in the 2nd and 4th pouch. We describe a very important contribution of the ectodermal canaliculus, projecting from the 3rd cleft, as well as the participation of the hypobranchial neural placode to the definitive thymus formation. Even though the entodermal thymus primordium in the 2nd pouch has reached an advanced stage, the thymus cannot complete its development because the ento-ectodermal relation is missing. In addition to this dysgenesis, we describe thymic involution in a later fetal period in connection with some malformations and confirm the close relation between the central immune organ and the neuro-endocrine system.