![]() In the process, the three principal axes of the body are established: anteroposterior, from head to tail dorsoventral, from back to belly and mediolateral, from the midline outward to the left or to the right. (B) The asymmetric distribution of molecules inside the egg, and how this changes following fertilization so as to define a dorsoventral as well as an (more.)įertilization initiates a complex series of movements that will eventually tuck the vegetal cells and cells from the equatorial (middle) region of the animal-vegetal axis into the interior. (A) Side view of an egg photographed just before fertilization. As a result, the cells that inherit vegetal cytoplasm will produce signals to organize the behavior of adjacent cells and are committed to form the gut-the innermost tissue of the body the cells that inherit animal cytoplasm will form the outer tissues. ![]() Near the vegetal pole, for example, there is an accumulation of mRNAs coding for the gene regulatory protein VegT (a DNA-binding protein of the T-box family) and for signal proteins of the TGFβ superfamily, as well as some ready-made protein components of the Wnt signaling pathway ( Figure 21-66B). The animal and vegetal hemispheres contain different selections of mRNA molecules and other cell components, which become allocated to separate cells as the egg cell divides after fertilization. The light-colored lower end of the egg is called the vegetal pole the dark-colored upper end is called the animal pole. The Xenopus egg is a large cell, just over a millimeter in diameter ( Figure 21-66A). The Polarity of the Amphibian Embryo Depends on the Polarity of the Egg
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