Neurulation in the rabbit embryo.

Among a broad range of factors and mechanisms involved in the complex process of neurulation a relationship between the curvature of the craniocaudal body axis and rate of neural tube closure has been proposed, but more examples and models are needed to further substantiate the existence of this relationship. This is particularly true for mammals, where marked differences in embryonic body curvature between species exist. The rabbit embryo has virtually no curvature during the main phase of neurulation and is therefore a suitable model, but neurulation is hardly documented in this species. In the present study, therefore, neural tube closure in the rabbit embryo is presented in detail by morphological and morphometrical parameters, as well as from scanning electron microscopic investigations. At the stages of 6-8 somites, the flat neural plate transforms into a V-shaped neural groove, beginning at the rhombo-cervical level. Between the stages of 8 and 9 somites, multiple closure sites occur simultaneously at three levels: at the incipient pros-mesencephalic transition, at the incipient mes-rhombencephalic transition, and at the level of the first pairs of somites. This results in four transient neuropores. The anterior and rhombencephalic neuropores close between the stages of 9-11 somites. The mesencephalic neuropore is very briefly present. The posterior neuropore is the largest and remains longest. Its tapered (cranial) portion closes fast within somite stages 9-10. Subsequently its wide (caudal) portion closes up to a narrow slit, but further closure slows down till full closure is achieved at the 22-somite stage. In comparing rabbit neurulation with that of chick and mouse, the sequence of multiple site closure resembles that of the mouse embryo, but other important aspects of neurulation resemble those of the chick embryo. In contrast to mouse and chick, no time lag between closure at the three closure sites in the rabbit was seen.