Diagnostic ultrasound ( PDFDrive )

CHAPTER 49 The Pediatric Spinal Canal 1673
ectoderm diferentiate into neural crest cells. Primary neurulation
(closure of the neural tube) begins during days 22 and 23. he
lateral margins of the neural plate thicken to form the neural
folds, and the thinner midline of the neural plate forms the
neural groove. he neural folds bend dorsally, converge, and
fuse in the midline to form the neural tube. he process of tube
closure begins in the region of the craniocervical junction and
extends cephalad and caudad with several fusion sites, and the
FIG. 49.1 Cartilaginous Gaps in Vertebral Ring Allow Penetration
by Scanning Beam. Transverse radiograph through a thoracic vertebral
body specimen from an infant demonstrates the cartilaginous posterior
median intraneural synchondrosis (arrow) and the paired neurocentral
synchondroses (arrowheads). (Courtesy of Dr. Paul Kleinman, Children’s
Hospital, Boston.)
posterior neuropore closes last by day 27. Ater closure of the
neural tube, the overlying ectoderm separates from the neural
tissue and fuses in the midline, forming a continuous ectodermal
covering of the neural structures; this process is known as disjunction.
Ater the neural tube has closed and separates from the
supericial ectoderm, mesenchyme also migrates dorsal to the
neural tube and forms precursors of the neural arches, in addition
to meninges and paraspinous muscles. At the time of closure,
the lateral edges of the neural plate, which form the neural crest
cells, are extruded from the neural tube, move to the dorsum of
the neural tube, and eventually migrate laterally and give rise to
the sensory dorsal root ganglia, the autonomic nervous system,
and other structures 7,8 (Fig. 49.2).
he distal neural tube is formed by secondary neurulation.
Distal to the posterior neuropore, undiferentiated cells from
the primitive streak form the caudal cell mass (day 30). Microcysts
develop and coalesce to form a tubular structure that unites with
the more distal neural tube that was formed by primary neurulation.
At 38 days the process of retrogressive diferentiation occurs,
with decrease in size of the cell mass and central lumen of the
caudal neural tube. he segment formed by this process eventually
forms the ilum terminale and cauda equina and ascends to
form the conus medullaris and ventriculus terminalis. he
regression process continues into the postnatal period with
minimal migration, and the conus reaches the adult level of L1-2
by approximately 3 months of age. 9
he spinal column develops in parallel with the spinal cord
beginning at the future occipital region and sweeps caudally.
SONOGRAPHY OF NEONATAL SPINE
Neural crest
Surface
ectoderm
Notochord
Neural plate
Neural crest
Neural groove
Spiral
ganglion
Conus
medullaris
Ventriculus
terminalis
A
Neural tube
Filum
terminale
B
FIG. 49.2 Three Stages of Spinal Cord Development. (A) Neurulation (closure of neural tube) is the process of progression from neural
plate to neural groove to neural tube. (B) Canalization occurs when multiple microcysts form and coalesce in caudal cell mass (arrows), which
fuses to distal neural tube (arrowheads), forming the primitive spinal cord. (C) Retrogressive differentiation (programmed cell death) is the process
by which the caudal cell mass and neural tube regress in size to form the fetal conus medullaris, ventriculus terminalis, and ilum terminale.
(A with permission from Sadler T. Langman’s medical embryology. 5th ed. Baltimore: Lippincott; 1985; B and C with permission from Barkovich
AJ. The nervous system. Pediatric neuroimaging. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 2000. 8 )
C