Secretory activity in the floor plate neuroepithelium of the developing human spinal cord: morphological evidence

The developing spinal cord at the cervical and thoracic levels in 14 human embryos ranging from Carnegie stages 14 to 20 were examined with the electron microscope. The floor plate-forming cells contained numerous cytoplasmic organelles, such as rough endoplasmic reticulum (ER), Golgi apparatus, and well-developed junctional complexes between the adjacent cells. Microvilli and cilia were numerous at the apical surface of neuroepithelial cells in the floor plate, but few were found in the lateral walls. Periodic acid-Schiff-positive substances were predominantly present in the neuroepithelial cells of the floor plate. In all specimens examined, multivesicular structures were observed in the floor plate neuroepithelium, but not in other regions of the spinal cord. The number of multivesicular structures appeared to increase with embryonic age. These structures contained numerous small and translucent vesicles within an electron-dense matrix; most vesicles were 40-70 nm in diameter. It appeared that the envelope of the multivesicular structures was first formed by the fusion of smooth ER-like cisterns, followed by invagination of the envelope by the vesicular contents. Presumably, the mature multivesicular structures were subsequently translocated to peduncular processes and their contents released into the central canal lumen in an exocytotic manner. This morphological evidence suggests that the floor plate cells of the spinal cord may have secretory activity during embryonic development.