Abstract
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.
3. The cilium is about 10μm long and is the thickest at the proximal portion except the most proximal stiff looking part (diameter 300nm), slightly tapering near the tip. It ends in a rounded and occasionally bent tip. Its ultrastructure in sections is that of a kinocilium.
4. Adjacent cells contact each other forming zonulae occludentes and zonulae adherentes near the surface, and interdigitations in the deeper portion. Usually the surfaces of the cells are separated by about 40nm.
5. Besides possible lymphocytes and other wandering cells which are seen occasionally, several multiform supraependymal cells and neuron-like cells are observed on the ventricular surface. They are oval or polygonal cells with bipolar or multipolar processes which presumably coincide with the unmyelinated nerve fiber-like structures seen by transmission electron microscopy. Some of these cells are supposed to be supraependymal cells of unknown nature but some may be included in nerve cells. A few spider form astrocyte-like cells are also recognized.
6. Occasionally protrusions of ependymal cells and that of possible nerve cells underneath the ependymal layer are observable on the ventricular surface. The latter contains numerous mitochondria but its continuation with a nerve cell is not verified. Sometimes small granules of various sizes are found among microvilli by scanning electron microscopy and they are assumed to be substances secreted through the ependyma.
