Proliferating cell nuclear antigen (PCNA) immunoreactivity and development of the pineal complex and habenula of the sea lamprey.

The development of the pineal complex and the habenula of the sea lamprey was studied with proliferating cell nuclear antigen (PCNA) immunocytochemistry. The pineal organ and the habenula primordia appeared in late embryos, and neuron differentiation began in prolarvae, as indicated by the presence of PCNA-negative cells. The parapineal primordium could not be distinguished until early prolarval stages, and cell differentiation was delayed to the larval period. Although the number of cycling (PCNA-immunoreactive) cells gradually decreased during the larval period in the three organs studied, their patterns of differentiation were different. We conclude that the unusual developmental pattern observed is related with the complex life cycle of lampreys.