Ultrastructure and changes during metamorphosis of the lympho-hemopoietic tissue of the larval anadromous sea lamprey Petromyzon marinus.

The ultrastructure of the lympho-hemopoietic tissue of the typhlosole and opisthonephros of larval Petromyzon marinus was studied throughout the life cycle. In both locations, lympho-hemopoietic tissue houses a connective tissue stroma and is irrigated by a system of enlarged blood sinuses lined by a continuous endothelium. The typhlosolar and opisthonephric lympho-hemopoietic tissue consists of mature and developing blood cells belonging to erythroid, lymphoid, monocytic and granulocytic lineages. During metamorphosis there is a total degeneration of the larval opisthonephros and a full transformation of the digestive apparatus. As a consequence of these modifications the typhlosole and the opisthonephros lose their hemopoietic capacity. These changes are discussed in view of the role played in them by the so-called "hemopoietic inductive microenvironments", principally the stroma supporting apparatus and vascularization pattern.