We can see how such a morphogen acts by considering the development of the anterior-posterior axis in the fruit fly embryo. In the fly egg case, the oocyte, or fertilized egg, is accompanied by "nurse cells" at what will become the head end of the fly. This is called the anterior end; the tail end is posterior. Nurse cells create messenger RNA for a protein called bicoid, which mRNA messenger RNA they transport to the oocyte. Because these mRNAs originate in the anterior end, their concentration is highest there, and is lower towards the posterior end. Once the oocyte begins to divide, the mRNA is translated, and the bicoid protein is synthesized. Anterior cells have more of it than posterior cells, and the difference in concentration sets each cell group down its own developmental pathway, with anterior cells developing head structures, and posterior cells tail structures. Note that, in keeping with the "American plan," the fate of each cell is determined not by its ancestry, but by the environment it is in.
Retinoic acid is used as an acne treatment for humans, and must be avoided during pregnancy.
bicoid egg case bicoid egg case
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