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

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reach the floor-plate; asymmetry of the neural canal; a pear-shaped external
contoür of the neural tube; an hour-glass external shape of the neural
tube; accessory canals în the tail-bud nraterial; and finally a solid tailbud,
deep to a foldi.ng neural plate. Using these features, rather than
sirnply the presence of accessory canals, the overlap zone was detectable
ln control embryos of Stage tl-16. After ttr¡s, complete fusion obscured
the extent of overlap in normal enbryos, but the zone was still recognisable
in myeloschisis lesions because of the contínued separation of the
tv'ro sources of neural materlal
The overlap zone could only be analysed quantltatively between
Stages lJ and 16, when it was promînent ín control embryos. There was
little dîfference in the extent of overlap between embryos wlth myeloschisis
and the experímental and control embryos wi th no neural defects,
showlng that myeloschisis does not arlse through changes ln the extent
of the zone.
The two embryos with myelodysplasía at Stage 16, however, showed
a very short overlap zone due to a greâtly reduced contribut¡on by the
neural plate. The lower boundary of the zone (marking the lowest extent
of the neural plate material) lay at almost the same level as the upper
boundary (markíng the h¡ghest extent of the taìl-bud material).
'
For the analysis of volúmetric changes associated with open neural
defects, measurement of the ratìo of neural tube to notochordal areas
allowed comparison of: embryos of different sizes; reglons of different
lengths; sections cut in different planes; poor I y-p rocessed and wellprocessed
material ¡ and groups of embryos with different reglonal
boundarles. Statistical analysis revealed greât constancy in notochordal
areas, wlth no evîdence of excesslve ¡eural tîssue in myeloschisls, but