A thymus candidate in lampreys

The lamprey's thymoid and vertebrate immunity The immune system of lampreys — jawless fish with roots in the early separation of the vertebrates into jawed and jawless lines — is of particular interest to evolutionary biologists. Much has been made of the differences between the 'alternative' immune system in the lamprey and that of today's jawed vertebrates, but the recent discovery that lampreys have lymphocytes resembling B and T cells, which are central to the adaptive immune response of the jawed vertebrates, puts more emphasis on the search for similarities. Histological surveys of lampreys have failed to reveal an organ equivalent to the mammalian thymus, the organ that generates the development of T lymphocytes. Now, gene expression analysis reveals previously unrecognized thymus-like structures — termed thymoids — at the tips of gill filaments of lamprey larvae. This suggests that the common ancestor of the jawed and jawless vertebrates may have had not only T- and B-like lymphocytes, but also anatomically distinct tissues for their development.