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Lecture 41Devel Mechs Pt1 notes

Lecture 41Devel Mechs Pt1 notes - Lecture 41 Developmental...

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Lecture 41 Developmental Mechanisms, Part 1 6 th Ed.; 402-406; Fig. 21.12; 419-420; 1015-1017 7 th Ed.; 411-414; Fig. 21.14; 420; 425 (bottom)-427; 1005-1006 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 an example of how these principles act in animal development. Induction is a key principle of 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 . As we said last time, 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. 47.22/47.25). 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.
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