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

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The defects in early human embryos, together with the f indi.ngs in
experimental dysraþhism, suppoFt,the assertiori that excencephaly, craniorachlschisIs,
and nyeloschisis arise by neural non-closure. Secondary
neural overgrowth and later degenerative châ.nges then produce the character¡stic
lesions of anencephaly and mye I omen i ngoce I e . Format¡on of the
caudal spinal cord fron thc ta¡l-bud wouid urpl"in sparing of the sacral
reglon, though complicated by relative shortening of the cord in the fetal
per iod.
Several forms of myelodysplasia can be explained as reductions în
the vo I ume
of neural plate material, tail-bud materîêl, or both.
Dipl'onryelía at the caudar rever. may arise by persistent cavîtation of taí r.-
bud material. This cannot apply to dlplomyel ia at hlgher levels, which
was not encountered in the windowed chíck embryos and so is not discussed
further.
Local overgrowth of neural tissue was suggested by patten (1952,
1953) as a posslble cause of non-crosure, because'¡t wês seen in embryos
w¡thout visíble externar defects, as well as in dysraþhism. Neural overgrowth
can fol low experimental incision of the roof of the closed avian
neural tube, at either the cord level (Fowler, 19531 or in the brain
(.lel ínek, 1960).
Harked folding of the ruminar surface of the chick neurar tr¡be has
been reported, w¡th and wíthout dysraphísm, after exposure to lead
chlorlde (Catlzone and Gray, l94l), tentanus tox¡n (Corl íss et al., 1966)
and several viruses (Hamburger and Haber, 19\7r Heath et ar"' r.956; rJir riamson
et al., 1956). However analysis of closed brain overgrowth produced by
influenza A vîrus revealed that the apparent folding and thíckening of