The median ventricular formation. A distinct structure at the mesencephalic apex.

The prenatal ontogenesis of the median ventricular formation (MVF)--a cell group at the seam between both sides of the mesencephalic roof--was analyzed ultrastructurally, autoradiographically and for the expression of intracytoplasmatic structures, i.e. glial filament antigen. As compared with other regions of the mesencephalic roof it was found that from embryonal day 12 onwards DNA synthesis of ventricular cells in the dorsal midline is significantly reduced. This reduction is more pronounced at later developmental stages. On the other hand, the MVF gains drastically in width during ontogenesis. It was shown that this increase may be caused by an immigration of postmitotic neighbouring ventricular cells. The characteristic morphological feature of MVF cells is their extension from the ventricular lining to the pial basement membrane. Their dorsal processes are joined to a thin fibre bundle and predominantly display microtubules as well as filaments and glycogen within their electrolucent cytoplasm. They also contain intracellular structures that react with antibodies against glial filaments as revealed by an enzyme-coupled immunolabelling. The perikarya of MVF cells, on the other hand, are almost all situated at the same level within the ventral third of the mesencephalic roof, thus bulging concentrically at the lateral sides of the MVF. Characteristically, a subfraction of MVF cells exhibits vast amounts of rough ER. The nature and function of the MVF cells is discussed in the light of the concept of guidance of preneurons by radial glia (Sidman and Rakic 1973).