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

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somite area (through asymmetry of the neural tube) but lts fusion to
the neural plåte materîa.l was so gradual that the upper limit of the
overlap zone was difficult to determîne (Fîgs. !1-54 änd 55-66).
t/here neurâl closure was st¡ll occurfing în the caudal region of
Stage 13 embryos, the exposed surface of the neural plate was wellpreserved,
with normal or¡entation of cells perpendicular to thÍs surface.
6.3.6 Staqe I3-16 Experîmental Embrvos. (Grouo I I t)
ln this group of experimental embryos two types of establ ished
defects were ev i den t.
The majoríty of Stage ll-1! embryos showed elevatlon or eversîon
of the neural folds in the protosom¡te, unsegmented mesoderm and caudal
êreas, constitutlng neural defects whích could be followed into Group lV
embryos. The defects consisted of unclosed neural folds, showîng marked
necrosis of the exposed surface and loss of cell orientation, lying dorsal
to more normal neural tissue derived from the ta.i l-bud. ln many cases
there was some distlnction between the two sources of neural tissue at
some part of the lesions,though no clear line of separation. As similar
lesions were present ln a more advanced foim.in many Group lV experimental
embryos, these were regarded as the f¡rst stage of estêbl ished myeloschisis.
ln experîmental embryos wìth and without myeloschísís the notochord
was in contact with both closed and open sections of the neural .tube over
the somite, protosom¡ìe, and unsegmented mesoderm areas, but not ¡n the
braîn or caudal areas. Areas of myeloschisis showed continuity with adjacent
ectoderm, implying that the neural folds at these sítas had never
c I osed. (r¡gs. 6S-Zo).
Somltic mesoderm was generally ín contact with neural tube at the
somite level , but separated from neural tube at the unsegmented mesoderm
and caudal areas. ln the protosomite area the degree of cóntact varied,