Specialized ependyma in the posterior mesencephalon of the chicken: The fine structure of the subtrochlear organ

Summary A formation of specialized ependymal cells in the posterior mesencephalon of the domestic fowl, designated as the subtrochlear organ, was examined with light-,scanning-and transmission electron microscopy. This organ possessing the form of the letter V is located in the ventricular wall of the posterior mesencephalon. Its apex marks the median sulcus, while the arms of the V are directed rostrolaterally. Ependymal cells lining the subtrochlear organ usually project an extremely elongated process into the subependymal region and are classified into three types according to their surface features: (1) cells with a bulbshaped protrusion that projects into the ventricle, (2) single cilium-bearing cells, and (3) cells with a tuft of cilia. The first type of cell is restricted to the median portion of the subtrochlear organ; its bulb-shaped protrusion contains numerous ribosomes. The second type of cell predominates in the arm (rostrolateral) area; in its apical cytoplasm such ciliary structures as basal body are rarely seen. The third type of cell is usually assembled into several small islands on the arm area; it has many basal bodies and other ciliary structures in the apical cytoplasm.