The Questions of Developmental Biology

In frogs and chicks, secondary neurulation is usually seen in
the neural tube of the lumbar (abdominal) and tail vertebrae. In
both cases, it can be seen as a continuation of gastrulation. In
the frog, instead of involuting into the embryo, the cells of the
dorsal blastopore lip keep growing ventrally (Figure 12.9A, B).
The growing region at the tip of the lip is called the
chordoneural hinge (Pasteels 1937), and it contains precursors for both the posteriormost
portion of the neural plate and the posterior portion of the notochord. The growth of this region
converts the roughly spherical gastrula, 1.2 mm in diameter, into a linear tadpole some 9 mm
long. The tip of the tail is the direct descendant of the dorsal blastopore lip, and the cells lining
the blastopore form the neurenteric canal.
The proximal part of the neurenteric canal fuses with the anus, while the distal portion
becomes the ependymal canal (i.e., the lumen of the neural tube) (Figure 12.9C; Gont et al.
1993).
Differentiation of the Neural Tube
The differentiation of the neural tube into the various regions of the central nervous
system occurs simultaneously in three different ways. On the gross anatomical level, the neural
tube and its lumen bulge and constrict to form the chambers of the brain and the spinal cord. At
the tissue level, the cell populations within the wall of the neural tube rearrange themselves to
form the different functional regions of the brain and the spinal cord. Finally, on the cellular
level, the neuroepithelial cells themselves differentiate into the numerous types of nerve cells
(neurons) and supportive cells (glia) present in the body.
The early development of most vertebrate brains is similar, but because the human brain
may be the most organized piece of matter in the solar system and is arguably the most interesting
organ in the animal kingdom, we will concentrate on the development that is supposed to make
Homo sapient.