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

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hours could provide evidence for one or the other mechanîsrn.
Use of a non¡.placental embryo for the experìmental nodel avoids
problems associated with maternal health and diet during gestatìon,
the sltes of lmplantation, and functlon of the placenta. ln the chÌck
enbryo, the close series of morphological stages al lows comparîsons of
lndlvidual embryos. Regia,ral subdi.¡lsion facil itates analysis of a
Particular developmental process ln embryos of different Stages oÉ through
different regions of the same embryo.
ln the case of early neurogenesis, the chíck is an excel lent experi;
mental model for human malformations. Hughes and Freeman (1974) compared
the development of the caudal region of the spinal cord in rat, mouse,
oppossum, plg, chick, and human embryos. 0f these embryos only the chick
and man show development of the caudal region of the neural tube by cavi t-
at¡on of tai l-bud material, after closure of the neural plâte.
ln amphibia, repti les, bìrds, and mammals the neural plate forms as
an ectodermal thickening overlyíng the cho rda -mesode rm, "nO
fold, to form
a tube. At caudal levels, however, development of the cord ís not so uníform.
l'lost work on neurulation has used amphibia, though studies on anurans
(with a bilaminar neural plate) are not str¡ctly comparable to other groups
(schroeder, t 97o) .
With the advent of electron microscopy, investigations of lgglglg1lq
,
have revealed. the existence of intracellulêr structures involved in neural
closure. Previous work attempted to explain neurulatlon în terms of extr¡ns¡c
forces, or by changes secondary to cell division and migrat.ion.
Hls (187q) suggested that the change ln shape of neuroeplthel ial cells,
from cuboidal to columnar, might be due to cell compression after a period