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

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6.1.3 l:legral Cþ;ure and Neglq I qe!e!:ts
By examlnlng and drawing such large numbers of embryos, normal and
abnormal neural closure could be followed ín detall. The anteríor neuro,
pore, normally closed by Stage 12 (Hami lton, 1965), hras still open in some
experìmental and control ernbryos at Stailes 13 - ZO. Such a contînuous
series of embryos showing an open anterior neuropore at Stâges immediately
after normal closure suggests that the establ ished open brain defects seen
În groups 3OE, l+28, € 42C ar¡se by non-closure (Table 26; Fi9s. tg €. 20)..
During normal development, the rhomboid sinus assumes an oval shape
and closes by Stage 15 .(Hami lton, 1965), though fÌnal closure cannot be
fully confirmed until Stage 16 in whole embryos. ln sonie embryos with
ên open rhomboid sínus, the neural folds formed an inverted triangular
outl íne rather than the normal oval shape (F¡gs. 24 ê 29. ln groups lgE,
t8C, 30E 6 JOC open neural defects were present just craniaì to the
rhomboîd sinus, sometimes showing contînuity with the neu.al fold, of a
triangular rhomboid sinus. This suggests that establ ished open cord
defects aríse by non-closure of the rhomboid sinus, whose neural folds
form a trìangular rather than an oval contour during non-closure
(Flss. 19 È 2A.
Ar
.
slightly later Stages (30E, 3OC, \zE, 42c) the additíon- of more
somític mesodern in the caudal region resulted in open cord defects beíng
located in the somite regîon. lJhen these lesíons were examîned careful ly
some formed a regular open area while others showed an irregular contour
(F¡s . 20).
The histological appearances of regular and irregular
open defects were later found to be quite d¡stinct (see Section 6.3 ).