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

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C¡osure of the anterior neuropore was completed by Stâge lJ in the
control emhryos, apart fiom one Stage l/ embryo with an open anterior
neuropore (regarded as an open braln defect by thls Stage). Several
experimental embryos after Stage 12, however, showed open anter¡or neuropores
(regarded as an open brain defects)at the Stages lmmediately following
Stage 12, provlding evidence of non-closure rather than closure and
reopenlng of the braln. These defects were not seen ln large enough numbers
to allow detai led hîstologlcal study. The appearance of open braÌn defects
was very similar at 3 days and at 12 days, and closely resembles the welli
preserved.human exencephal lc embryo l l lusrrated by Hunter (,|934-35).
Closure of the rhombold sinus occurred at Stage 15-f6 in both experlmental
and control groups. A trlangular rhombold slnus, however, was
seen only in experlnental embryos of Stages 11-16. Open cord defects
first appeared at Stage 13, and were seen at all Stages after this, agaîn
suggesting that they arose by non-cloSure rather.than by reopening of the
closed neural tube.
0n comparlng the drawings of whole embryos with their subsequent
histologlcal appearânce, It became apparent that a tr¡angular rhomboid
slnus is the precursor of myeloschisis. The fact that the development of
rryeloschlsis can be predícted from the shape of the rhomboid sînus before
the perlod of normal closure is strong evidence that myeloschîsís arîses
by non-cl osure.
Skeletal stalning of l1-12 day embryos revealed an increasing
sever¡ty of axial defects from cervlcal to caudal level's. Spina bifîda
occulta was seen mainly ln the cervical r.egion. Spina blflda manifesta
occurred (wlth open cord defects) from the lower thoracic to the upper