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

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'c¡osure of the neural folds were closely integrated with formation of the
notochordr and the developnent of nature somites from undifferentlated
mater¡al of the streak and node, and later the tail-bud.
ln the experlmental embryos the development of lnyeloschîsis was
preceded by everslon of the neural folds at the rhomboid sinus in
serial sections, producî.ng a trlangular shape on examining the whole embryo.
ln the'earl iest myeloschisis leslons the rhomboid sinus was s.tlll open.
Exanlnation of the serlal sect¡ons showed that myeloschlsis consisted of
an open defect of the lower part of the neural plate material, extending
caudally to involve the upper part of the ta¡l bud materlal. ln older
embryos an appêrently normal neural tube formed at a more caudal level.
Establ ished myeloschisis lesions on hlstology showed a flat surface
plaque (continuous with neural plate), overlyíng neural materîal containing
accessory canals (derived from the têíl-bud). The two sources of neural
material were clearly separated,.wíth dîfferent orientation of their constituent
cells. The majority of mitotîc figures were seen at the luminal
surface of the closed neural tube, and along the dorsal surface of the
exposed plaque. The everted neural plate showed smooth continulty
wi th adjacent .ectoderm.
Neural crest cells were seen at the margin of most myeloschîsis lesions,
and adjacent structures were wel I deveìoped. The notochord was uniformly
normal ln appeârênce, but wldely separated from neural tlssue at the
cranlal end of most myeloschisis lesions after Stage ,l6. The somîtes
appeared normal , and the impressíon of separation of somites from affected
areas of neural tube wâs not confirmed quantítatively.
I'leasurements of neural tlssue/notochord ratios provided no evidence
of local overgrowthrr of neural tlssue, which could therefore not be