Secondary neurulation of human embryos: morphological changes and the expression of neuronal antigens

The morphological changes and expression patterns of neuronal antigens of human embryos, obtained from the therapeutic termination of pregnancy or from surgical procedures, were analyzed in order to characterize the secondary neurulation.

A total of 21 human embryos from Carnegie stages 12 to 23 and two fetuses in early stages were studied. The markers used for immunohistochemical study were neural cell adhesion molecule (N-CAM), neuronal nuclear antigen (NeuN), neurofilament-associated protein (3A10), synaptophysin, and glial fibrillary acidic protein (GFAP).

The formation of the caudal neural tube to the tip of the caudal portion of the embryo was finished at stage 17. The postcloacal gut had completely disappeared at stage 18, and multiple cavities of the caudal neural tube were clearly visible. The caudal portion of the neural tube showed findings suggestive of involution at stage 19. The expression patterns of neuronal antigens were as follows: N-CAM and NeuN showed immunoreactivity at the germinal layer of the spinal cord at stages 17 and 18. Neurofilament-associated protein (3A10) showed persistent immunoreactivity at the caudal cell mass and notochord during the observation period, along with the spinal cord, and the positive reactions were mainly located at the dorsal white matter at stage 17. Synaptophysin showed a weak positive reaction at the caudal cell mass and notochord at stages 13 and 14, evident by staining observed at the spinal cord at stages 15 and 16. There was no definite positive reaction for GFAP.

These characteristic patterns might be helpful for the understanding of human congenital anomalies involving secondary neurulation processes.