Biology Reference
In-Depth Information
Figure 18.1 Diagrams show how the morphogenic movements of convergence
(narrowing) and extension (lengthening) of the presumptive notochordal (No), somitic
(So), and posterior neural (PN) tissues contribute to gastrulation and body axis elongation.
As the mesodermal tissues involute, they converge and constrict the blastopore (black
arrows, A). After involution they continue to converge in the mediolateral direction and
extend in the anterior-posterior direction, and thereby elongate the body axis (B, arrows,
No, So). Meanwhile, the posterior neural tissue (PN) converges and extends in a congruent
manner on the outside (A, B, arrows). These movements are produced by internally
generated forces as shown by the fact that explants excised from the early gastrula (A,
dashed lines), and sandwiched with their inner surfaces together (C), will also show
convergence and extension of both the mesodermal and neural components (C-E).
Forebrain: FB
body axis during gastrulation and neurulation of many species of vertebrates,
and occurs during many other morphogenic events in both vertebrates and
invertebrates (Keller, 2002). One example of these movements occurs during
gastrulation of the amphibian Xenopus laevis. As the presumptive notochordal
and the somitic tissues involute, or turn inside the blastopore, they converge in
their mediolateral dimension and extend in their anterior-posterior aspect,
which elongates the body axis and at the same time constricts the
circumference of the closing blastopore (Figure 18.1A-B). Simultaneously,
the overlying prospective hindbrain and spinal cord tissue of the neural plate
also converges and extends congruently with the underlying mesoderm but on
the outside (Figure 18.1A-B). These movements elongate and narrow the
neural plate and also aid the mesoderm in constricting the blastoporal lip
(Keller et al., 2000) (Figure 18.1A-B). Convergence and extension also occurs
