Aspects of fine structure and function of the subcommissural organ of the embryonic chick.

The fine structure of the subcommissural organ (SCO) of the embryonic chick has been studied. No evidence of secretory release was obtained. The SCO seems not to be involved in such previously suggested roles as water regulation or response to varying light cycles. The introduction of electron dense markers into the 3rd ventricle demonstrated rapid and massive uptake into coated pits and coated vesicles of the ependymal cells and its subsequent incorporation into terminal bodies. It is suggested that the uptake of macromolecules from the cerebrospinal fluid may be an important and hitherto unsuspected function of the SCO.