DEVELOPMENTAL BIOLOGY CLASS NOTES
Classes 7 and 8
Establishment of body axes in the early Xenopus embryo: the importance of the “dorsal” side, inductive
events, and the establishment of mesoderm
Note: these notes are for the entire week—these class periods are interwoven, and complicated, so I put
the two days together.
Chp 10: Xenopus: 291-295, 302-312
Learning goals: Be able to:
Explain how the D-V axis is established in Xenopus, including the timing of this event and the
Explain how the organizer was shown to be capable of inducing the dorsal axis.
Compare the role and origin of the Niewkoop center to the role and origin of the organizer
Design experiments that prove the role of mesoderm-inducing factors.
Propose outcomes of experiments that prevent normal mesodermal patterning.
Relate the concept of morphogens to the determination of mesoderm and the role of the organizer in
Our goal in this unit is to understand the logic of how animal body plans can arise from a one-cell embryo. Not
long ago, this process seemed miraculous. At present, particularly in Drosophila and amphibians, we have a
pretty good idea of how it works, and we have identified many of the molecules that are involved. In
Drosophila, most of what we know has come from a genetic approach; in Xenopus, from experimental
embryology. Most of the same mechanisms appear to be used in all embryos in similar ways.
We’ll focus on
the Xenopus embryo for this section, and return to Drosophila when we discuss developmental biology from the
perspective of genetics in Unit 3.
Establishment of embryonic polarities in amphibians
In most organisms (except mammals), axis determination relies upon maternally contributed factors,
deposited directly into the oocyte.
As we’ve already discussed, cleavage then partitions these factors into
different cells, sometimes giving adjacent cells different capabilities.
In addition to the cytoplasmic
determinants, signaling is a major component of cell fate determination as well.
Thus, the establishment of the
axes (we’ll consider mostly D-V and A-P) often begins with segregation of a transcription factor, and continues
In amphibia, the animal-vegetal axis is the first to be established. A TGF-b ligand, Vg-1, is localized to
the vegetal half of the embryo at the formation of the oocyte, along with numerous additional factors (including
another factor called VegT, a transcription factor).
The animal half seems to have very few maternal mRNAs.
The cleavages are displaced towards the animal pole due to the high concentration of yolk at the vegetal pole.
Sperm entry specifies the dorsal side via cortical rotation.
The mature amphibian egg is radially symmetric
around the animal/vegetal axis. Early experiments showed that the sperm can enter the egg anywhere in the
animal hemisphere, and that the future dorsal side where the blastopore forms is almost always on the meridian