|EMBRYOLOGY HOME||EMBRYOLOGY TABLE OF CONTENTS||OBL HOME||OBL REFERENCES|
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.
|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).|
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 (
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
|Frog Neural Folds|
|Frog 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.
|Pig spinal cord|
|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).|