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

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This revlew of the histologlcal appearance of every sectloned embryo,
after al lowing for some cracking durÌ.ng processing, reveals a series of related
events during neurulation. The flattened neural plate, protonotochord,
and fused somltic mesoderm lying adjacent to Hensenrs node show
progresslve changes as they are fol lowed cran.ially.
Formatlon of the notochord ís accompanied by development of unsegmented
mesoderm and elevation of the neural folds. Cranial to this,
development of protosomites is accompanied by ínversion of the neuraì
folds. As the somites develop, with încreasing differentiat¡on ¡nto
centrêl and perlpheral regions, they show progressive expansîon which
ls accompanied by further inversion and then closure cif the neural folds.
After fusion of the neural folds the closed tube is inîtlally in
contact with the overlyîng newly-fused ectoderm. At stage l2 neural crest
cells begin to infiltrate between neural tube and ectoderm, beginning with
the point of inltial closure at the hindbrarin. By Stage 1!, mesenchyme
cells from locally-dispersing somites then. migrate between neural tube
and ectoderm, also beginning at the hìndbrain. ln embryos with myelodysplasia
a local reduction in neural volume allows the adjacent somites
to fuse across the midl ine, dorsal to neural tissue.
ln experimental and control embryos until Stage l! the notochord îs
separated from developing brain but lies in close contact *¡tn tÁ" developing
cord, except for the upper part of establ ished myeloschisis tesions
in most cases of myeloschisis.
}Jhlle fused and unsegmented somít¡c mesoderm are separated from neural
tlssue durlng early neurulation, the protosom¡tes are in contact wlth