Neurulation in the pig embryo

Neurulation is based on a multitude of factors and processes generated both inside and outside the neural plate. Although there are models for a gener al neurulation mechanism, specific sets of factors and processes have been shown to be involved in neurulation depending on developmental time and ros tro-caudal location at which neurulation occurred in the species under inve stigation. To find a common thread amongst these apparently divergent modes of neurulation another representative mammalian species, the pig, was stud ied here by scanning electron microscopy. The data are compared to a series of descriptions in other species. Furthermore. the relation of axial curva ture and neural tube closure rate is investigated. In the pig embryo of 7 s omites, the first apposition of the neural folds occurs at somite levels 5- 7. This corresponds to closure site 1 in the mouse embryo. At the next stag e the rostral and caudal parts of the rhombencephalic folds appose, leaving an opening in between. Therefore, at this stage four neuropores can be dis tinguished, of which the anterior and posterior ones will remain open longe st. The two rhombencephalic closure sites have no counterpart in the mouse, but do have some resemblance to those of the rabbit. The anterior neuropor e closes in three phases: (1) the dorsal folds slowly align and then close instantaneously, the slow progression being likely due to a counteracting e ffect of the mesencephalic flexure; (2) the dorso-lateral folds close in a zipper-like fashion in caudo-rostral direction; (3) the final round apertur e is likely to close by circumferential growth. At the stage of 22 somites the anterior neuropore is completely closed. In contrast to the two de novo closure sites for the anterior neuropore in the mouse embryo, none of thes e were detected in the pig embryo. The posterior neuropore closes initially very fast in the semitic region, but this process almost stops thereafter. We suggest that the somites force the neural folds to elevate precociously . Between the stages of 8-20 somites the width of the posterior neuropore d oes not change, while the rate of closure gradually increases; this increas e may be due to a catch-up of intrinsic neurulation processes and to the re duction of axial curvature. At the stage of 20-22 somites the posterior neu ropore suddenly reduces in size but thereafter a small neuropore remains fo r 5 somite stages. The closure of the posterior neuropore is completed at t he stage of 28 somites.