Healthcare and Medicine Reference
In-Depth Information
Area opaca
Margin
Area
pellucid
Koller's sickle
Thickening
area of
blastoderm
Posterior
S1
Area opaca
Area
pellucid
Prosencephalon
S4
Thickening
area of
blastoderm
Mesencephalon
S7
Anterior
Hensen's
node
Cervical
Spinal Cord
S1
Area
pellucid
Area
opaca
S4
S7
S18
Primitive
groove
Thoracic
Spinal Cord
S24
Head
process
S28
Hensen's
node
Lumosacral
Spinal Cord
FIGURE 1.11 Development of the chick embryo. The blastoderm (area opaca) sits on top of the large yolk
and is the result of a large number of cleavage divisions. At the start of gastrulation, cells move posteriorly
(arrows) and migrate under the area opaca. The embryo begins to elongate in the anterior-posterior axis, and
the region where the cells migrate underneath the area opaca is now called the primitive groove and then
primitive streak. Details of the cellular movements are shown in the cutaway view. The cells migrate into the
blastocoel to form the mesoderm. At the anterior end of the primitive streak an enlargement of the streak is
called Hensen's node .
now underlies the dorsal ectoderm. A large number of
experimental studies in the early part of the twentieth
century revealed that these new tissue arrangements
were of critical importance to the development of a
normal animal. By culturing small pieces of embryos
in isolation, it was possible to determine the time at
which each part of the embryo acquired its character
or fate (Figure 1.13). When the dorsal ectoderm was
cultured in isolation prior to gastrulation, the cells dif-
ferentiated into epidermis, while when roughly the
same piece of tissue was isolated from gastrulating
embryos, the piece of ectoderm now differentiated into
neural tissue, including recognizable parts of the brain,
spinal cord, and even eyes. These results led Hans
Spemann, a leading embryologist of the time, to spec-
ulate that the ectoderm became fated to generate
neural tissue as a result of the tissue rearrangements
that occur at gastrulation (Hamburger et al., 1969). One
possible source of this “induction” of the neural tissue
was the involuting mesoderm, known at the time as
the archenteron roof. As noted above, the involuting
tissue is led into the interior of the embryo by the
dorsal lip of the blastopore. To test the idea that the
involuting mesoderm induces the overlying ectoderm
to become neural tissue, Spemann and Hilde Mangold
carried out the following experiment (Figure 1.14). The
dorsal lip of the blastopore was dissected from one
embryo and transplanted to the interior of another
embryo, and the latter embryo was allowed to develop
into a tadpole. Spemann and Mangold found that an
entire second body axis, including a brain, spinal cord,
and eyes developed from the ventral side of the
Search Pocayo ::




Custom Search