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THE DORSAL, HOLLOW NERVE CORD

THE FORMATION OF THE NERVOUS SYSTEM

     The primitive streak of the epiblast marks the point where the central nervous system begins to form. A neural plate forms two neural folds which will fuse to form the neural tube.
EMBRYO
      Although the organizing region of amphibians forms a circular blastopore compared to the linear primitive streak of amniotes, the two regions are similar.  In amniotes, Henson’s node at the anterior end of the primitive streak is homologous to the dorsal blastopore lip of amphibians, as evidenced by shared gene expression such as goosecoid, Xnot/Cnot, noggin, and nodal/XNr in addition to physical and functional similarities (Arendt, 1999).
EMBRYOS

     Once the primitive streak forms, if it possible to identify the cranial/caudal and dorsal/ventral axes of the embryo.  By the fourth week the primitive streak has disappeared although it does occasionally persist later in development in some fetuses.  In 1/35,000 children the tissue which persists from the primitive streak forms a saccrococcygeal teratoma (Moore, p.68).

      The nervous system forms as neurectoderm differentiates to neural tissue.  The neural plate forms two neural folds which, upon fusing, become the neural tube.

Frog Neural Plate
NEURAL PLATE
Frog Neural Folds
Frog Neural Folds Frog Neural Folds Frog Neural Folds
Frog Neural Tube
Neural Tube Neural Tube Neural Tube

HOLLOW NERVE CORD AND SPINAL CORD

     One of the four basic characteristics of chordates is the presence of a hollow, dorsal nerve tube.  In adults, nervous tissue surrounds the central canal of the spinal cord and the ventricles of the brain through which cerebrospinal fluid passes.  The small relative size of these spaces in adult nervous systems is unlike the large hollow center of the embryonic nerve cord.

Frog
frog
chick brain
chick brain
pig brain
pig brain
Pig spinal cord
Pig spinal cord Pig spinal cord
20 mm pig brain showing lateral ventricles and fourth ventricle
20 mm pig brain showing lateral ventricles and fourth ventricle 20 mm pig brain showing lateral ventricles and fourth ventricle
20 mm pig brain showing lateral ventricles and fourth ventricle 20 mm pig brain showing lateral ventricles and fourth ventricle
Chimps and humans have the shortest spinal cords (relatively) in the order primates (MacLarnon, 1996).  Human embryos begin with coccygeal ganglia which degenerate although a coccygeal medullary vestige is retained until later in fetal life (Ariens, p. 221).