The tegmental proliferation region in the sea lamprey

The diencephalic/midbrain tegmental domain of the developing lamprey was characterized by the special features of the ventricular zone and distribution of some postmitotic neuronal populations, using proliferating cell nuclear antigen (PCNA) and HNK-1 immunocytochemistry. In late prolarvae and early larvae, the tegmental ventricular zone differentiated a striking arrangement of thin longitudinal crests with strongly PCNA-immunoreactive cells protruding toward the ventricle and separated by shallow valleys whose cells were faintly or moderately PCNA-immunoreactive. The tegmental ventricular zone was composed of a large caudal region located ventral to the pretectum and the midbrain tectum, and of a smaller rostral wedge-shaped region that extended dorsally between the dorsal and the ventral thalamus, in the last one, ventricular crests coursing in the zona limitans intrathalamica. In other diencephalic and midbrain regions, the ventricular zone lacked this crest-like organization and exhibited different smooth-surfaced domains rather homogeneous in thickness and appearance. The distribution of HNK-1-immunoreactive cell populations in the tegmental domain appears to be closely related with the pattern of proliferation crests. Together, our results reveal a shared basic organization in the tegmental domains of the diencephalon and midbrain of developing lamprey, indicating early appearance of the domain in vertebrate phylogeny.