A scanning and transmission electron microscopic observation of the fourth ventricular floor in the mouse.

The floor of the fourth ventricle in the mouse was studied by scanning and transmission electron microscopy. 1. The fourth ventricular floor is covered by a simple layer of ependymal cells, the cytoplasm of which is characterized by numerous filaments and scanty ribosomes. 2. The ventricular surface of the ependyma is studded with numerous cilia and abundant microvilli. In the medial part of the ventricle the cilia grow densely and tend to beat posteriorly, whereas in the lateral part they are sparse and most of them beat posteriorly or posteromedially. In the area near the lateral aperture they beat posterolaterally to laterally. Only a few cilia are observable in the median sulcus.