An Investigation of the Genesis of Lumbosacral Myeloschisis in the Chick Embryo

85 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.The genesis of lumbosacral myeloschisis in chick embryos was investigated by a study of serial cross-sections of embryos in which lumbosacral myeloschisis had been induced by the removal of the vitelline membrane over the blastoderm at the 13-, 14-, 15- or 16-somite stages. Defective embryos were examined histologically at 3-hour intervals from the time of treatment to 30 hours of reincubation and thereafter at 12-hour intervals until 66 hours of reincubation had elapsed. Another group of embryos was reincubated after treatment until a total of 120 hours of incubation was reached.The removal of the vitelline membrane proved to be effective in producing myeloschisis and Stage 11 embryos (13-15 somites) exhibited the highest incidence of lumbosacral myeloschisis (38.6%) from this method of induction. Thus the overlap zone of the chick embryo, a region which terminates at the posterior end of the lumbosacral level of the neural tube, could be analyzed for modifications of primary and secondary neurulation.Prior to the time of disappearance of the posterior neuropore (0-12 hours after treatment), Period I embryos exhibit a bulging of neural plate cells into the neural groove and a loosening of cellular contacts. Damage to the neuroepithelium results in continuity between the neural groove dorsally and the cavities formed ventrally within the medullary cord component of tail bud origin. Thus cells which do not normally participate in the mechanism of elevation and fusion of the neural folds come to line the ventral part of the neural groove in the lumbosacral region.During Period II, 15-30 hours after treatment, embryos exhibit exposed neural plates whose folds had never completed elevation nor fused to form a neural tube. In several of these embryos, a T-configuration defect, consisting of a closed ventral tube (the stem of the T) lying under the horizontal cap of neural tissue forming the crossbar of the T, is observed. The histological characteristics of neural plate ectoderm suggest that the crossbar of the T is derived from cells of neural plate origin. Similarly, there is evidence that cavitation and fusion of cavities within the medullary cord tissue form the canal of the ventral tube constituting the stem of the T. The T-configuration demonstrates not only the existence of neural plate and medullary components of the overlap zone but also their capacity to develop independently under experimental conditions.Asymmetrical outgrowths of one wall of the neural tube anterior to the myeloschisis or localized degeneration of dorsal regions of the neural tube suggest a susceptibility of the future alar half of the neural tube to injury after removal of the vitelline membrane.By Period III, 42-80 hours after treatment, the typical characteristics of myeloschisis, overgrowth of neural tissue and variable flaring of the neural folds are well established. Localized regions of damage to the neuroepithelium, where terminal bars are lacking persist as masses of loosely-packed cells protruding into the groove.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD