Lec41n - Lecture 41 Developmental Mechanisms Part 1 6th Ed...

Lecture 41 Developmental Mechanisms, Part 1
6 th Ed.; 402-406; Fig. 21.12; 419-420; 1015-1017
7 th Ed.; 411-414; 420; 425-427; 1005-1006
8 th Ed.; 367-368, 372, 1038-1041 + scanned C. elegans development figure
Introduction . Development is one of the most actively studied areas of biology today, and we don’t
yet have a complete picture of how it works. I’ll describe 2 classic experiments, done years ago,
that each identified an important principle of development, without showing exactly it worked. The
2 principles are Induction and Cytoplasmic Determinants . For each of these, I’ll also describe an
example from later work, when specific molecules could be identified, that provides a more detailed
example of how these principles actually play out in animal development.
Induction is the ability of some cells to affect the development of other cells. Here’s an example of
how we know that induction is important in development.
Spemann’s organizer; an important inducer in development . At the end of gastrulation, the opening
of the archenteron is called the blastopore. Because the basic body plan is already set up in the
gastrula, we already know that one edge or “lip” of the blastopore will become part of the back of
the animal. For this reason, this region, which forms at gastrulation, is called the “dorsal lip of the
blastopore”. (“Dorsal” means the side of a vertebrate organism closest to the backbone. “Ventral”
is the opposite; the side closest to the belly. We can define a dorsal-ventral axis, meaning a line
from the back to the front of the organism). A group of cells here, together called “Spemann’s
organizer”, plays a crucial role in development. This was shown in a key experiment by Spemann.
(Follow along in Campbell, Fig. 47.22 (6 th )/47.25 (7 th )/47.24 (8 th )). He took a gastrula – the
“Donor” gastrula- and cut out the Organizer cells. He then transplanted these cells into another
gastrula – the “Recipient” gastrula. (The 2 gastrulas were different colors, so he could tell them
apart). It’s important that Spemann didn’t transplant the Organizer cells from the Donor gastrula
into the same place in the Recipient. Instead, he inserted the cells into the other side of the
Recipient gastrula – on the opposite side from the Recipient’s own Organizer. He let development
proceed. The result was formation of 2 embryos, joined belly-to-belly. One embryo developed
normally from the Recipient gastrula, just as if no transplantation had occurred. The second embryo
was the abnormal one. It contained cells from BOTH the Donor AND the Recipient gastrulas. This
result showed that the Organizer cells could direct formation of an entire embryo. The fact that the
second embryo contained cells from the Recipient gastrula (as well as the Donor gastrula) was
important. This showed that the Donor Organizer wasn’t JUST developing into the second embryo.