Diagnostic ultrasound ( PDFDrive )

CHAPTER 35 The Fetal Spine 1217
A
B
C
FIG. 35.1 Cross Section of Trilaminar Embryonic Disc (Germ
Disc). (A) Cross section of midportion of embryonic disc 17 days
after conception. The notochordal process is a hollow tube (black circle)
that lies between the ectoderm (EC) (red) and endoderm (EN) (green)
and is lanked by the mesoderm plate (M) (blue). (B) Cross section of
midportion of embryo 21 days after conception. The medial portions
of the mesoderm plate (M) (blue) are organizing into somites (s). The
ectoderm (EC) (red) is folding at the midline into the neural fold, which
will soon become the neural tube. This folding is induced by the neighboring
notochord (solid black circle). Note that the notochord is now a solid
cord that has evolved from the hollow process of day 17. (C) Cross
section of midportion of embryo 28 days after conception. The
neural fold has evolved into a closed neural tube (hollow red ovoid
structure) that has separated from its ectoderm layer (EC) (red). The
somites have ruptured along the medial sides. Migrating cells from the
somites (the sclerotome) envelop the neural tube (red ovoid) and become
the vertebral arches. The sclerotome surrounding the notochord (black
circle) becomes the vertebral bodies and intervertebral discs. The
notochordal remnants differentiate later to become the nucleus pulposus
of the discs. The rest of the notochordal cells degenerate and disappear.
AC, Amniotic cavity; YS, yolk sac. (Illustrations by Karen Sauerbrei, RT,
BA.)
has evolved into a solid cord called the notochord, and the
paraxial mesoderm has developed multiple discrete bumps called
somites, of which there are 37 pairs when fully developed (Fig.
35.1B).
he notochord and the rest of the intraembryonic mesoderm
induce the development of the neural plate in the ectoderm
layer (amniotic cavity side of the germ disc), starting on conceptual
day 18. he neural plate grows in length and breadth until
conceptual day 21, when neurulation begins. Neurulation is the
process of folding of the neural plate into the neural tube,
probably induced by the adjacent notochord. he lateral edges
of the neural folds begin to fuse dorsally into a closed neural
tube in the occipitocervical region, leaving an opening at the
cranial end (cranial neuropore) and the caudal end (caudal
neuropore). he hollow center of the neural tube is called the
neural canal, which will become the central canal of the spinal
cord and ventricular system of the brain. By day 24 conceptual
age, the cranial neuropore closes, and by day 25 the caudal
neuropore closes (Table 35.1).
he cranial end of the neural tube becomes the brain, and
the caudal end becomes the spinal cord. In week 4 conceptual
age, ater the neural tube has formed, the adjacent 37 pairs of
somites in the intraembryonic mesoderm give rise to the vertebral
bodies and vertebral arches that will surround the spinal cord.
A group of cells, the sclerotome, migrates from the somites and
surrounds the adjacent neural tube and notochord. he ventral
portion of the sclerotome surrounds the notochord and forms
the rudiment of the vertebral body. he dorsal portion of the
sclerotome surrounds the neural tube and forms the precursors
to the vertebral arch (Fig. 35.1C). In the fetus, the notochordal