The ontogenesis of cranial neuromeres in the rat embryo. I. A scanning electron microscope and kinetic study.

The development of the cranial neuromeres, rostral to the level of the first somite, in rat embryos during neurulation is described. Examination of scanning electron micrographs of 4-, 8-, 12- and 16-somite-stage embryos has shown that the neuromeres develop in a strictly ordered pattern but the sequence is not ordered in either a rostrocaudal or caudorostral direction. The two mesomeres are shown to develop by subdivision of a previously formed mesomere, whilst the rhombomeres develop independently of each other. At the completion of cranial neurulation two mesomeres and nine rhombomeres (including the preotic sulcus) but no prosomeres are identifiable. A direct correlation of the position of peaks in mitotic index along the embryonic axis with the position of sulci on the surface of the neural epithelium, was not possible. However, it is suggested that the mitotic index peak is the initial means of identifying a neuromere, and once the neuromere is expressed morphologically as a surface sulcus the mitotic index returns to its basal level.