The sulcus medianus organ in the fourth cerebral ventricle of the rabbit: a previously undescribed circumventricular organ.

The sulcus medianus of the fourth ventricle in the rabbit contains a previously undescribed circumventricular organ. The cells in this region are predominantly nonciliated. The apical surface of each cell has characteristic microvillous projections with pinocytotic vesicles. The cell body is elongated, with the nucleus situated in the body or basal process; the cytoplasm of the cell is pale and contains microtubules and filaments. The general morphology of the cells is similar to tanycytes found in other circumventricular organs. Supraependymal cells were present on the non-ciliated ependyma in all of the animals examined, sometimes in large numbers.