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

l8
ln some very early human embryos wlth cranioschisís an exposed
mass of well-preserved brain tissue is seen, forming an exencephaly
rather than an anencephaly (Hunter, 1934-,35; van der Zrrran,..195l). Thts
suggests that exencephaly gives rise to anencephaly by necrosis and
sloughing of the exposed mass of brain tîssue, followed by vascularization
of the open area.
ln spina bifida cystica a comparable process wâs demonstrated
by Cameron (1956) who found that the basic dêfect was probably an open
neural plaque. Secondary overgrowth by squamous ep¡thelium and scar
tissue with accumulation of fluid suggested an încorrect diagnosis of
ul cerat i ng meningocele or rrmyelomeningocele't.
Several early human embryos with establ ished spina bif¡da cystica
do indeed show an exposed neural plaque and open ependymal cana! wíth
no covering of epithel ial or vascular tissue (lngalls, 1932i patten,
1953; Lemire et al,, 1965).
2,3.2 Productíon of Experimental Dysraphis¡n in AnÍmal Molþls
The belief that external factors may influence embryonic development
is an ancîent concept common to many cul tures. ln the nineteenth
century congenital malformations were frequently produced in lower
animals, but the mammalian embryo was thought to be protected by its
uterine envlronment. During the present century, however, experimental
terêtology has produced many defects in mammalian embryos, and demonstrated
many malformation syndromes in man due to environmental êgents.
Agents whích have been reported to produce experîmental dysraphism
in mammalian embryos by maternal treatment are shown in Table l.