Fourth ventricular floor in human embryos: scanning electron microscopic observations.

The ultrastructural surface features of the normal fourth ventricular floor of seven human embryos ranging from Carnegie stage 14 to stage 19 (crown-rump length: 7.6-16.2 mm) were examined by using scanning electron microscopy (SEM). Low-power SEM views showed the median sulcus, sulcus limitans, and neuromeres, transient structures characteristic of the earlier embryonic period. High-power SEM observation revealed supraependymal cells (SE cells) and supraependymal fibers (SE fibers) which exhibited a characteristic localization, as well as generalized surface-membrane modifications such as microvilli and cilia. SE cells could be classified into two major groups. The type 1 SE cells seem to possess neuronal functions, as deduced from morphological similarities to their counterparts in adults and the specialized distribution closely related to neuromeres. The type 2 SE cell morphologically resembled the phagocytic SE cell described in related literature. SE fibers ran a course either rostrocaudally in the median sulcus or mediolaterally on the neuromeres, most frequently near the interneuromeric cleft; they made contact with type 1 SE cells and ependymal surface modifications and then penetrated the ependymal layer.