Universlty of Manitoba, ln Partîal Fulfiìlment - MSpace at the ...

ii¡
occurred in the cerV¡cal and upper thoracic regions¡spina bifida manifesta
(associated wiih opeá cord defects) from the lower thoracic to sacral
regÎons, whlle vertebral deletionswere almost confirmed to the caudal '
r.eg ion ,
Examînation of a closely-spaced series of embryos recovered wíthin
42 hours of windowlng revealed open braîn and cord defects. These
occurred at every Stage after the expected closure of the anterior
neuropore and rhomboid sinus, suggesting a process of non-closure.
Furthermore, lncípient non-closure of the spinal cord could be predicted ..
from the abnormal triangular shape of the rhomboíd sinus.
Serial sectioning of selected early.embryos revealed two types
of open cord defects. HyeloschisÌs arose by eversÌon of the neural
folds at the rhomboid sinus, and formed regular defects w¡th separation
between the neural plate and taÌl-bud sources of neural tissue. The
development of myeloschisis was associated wi th local separation of
the notochord from the open neural tissue, but not with trovergrowthr¡
of neural tíssue.
ln myelodysplasia neural plate material was absent from the êrea
of the open defect, and the spinal cord u¡as derived from tail-bud
material êlone. Myelodysplasia was characterized by a local reduction
in neural volume, and assocîated wîth cystic and hemorrhagic changes
in mesoderm and reductìon in the volume of adjacent somites.