In order to test the idea that secondary mesenchyme cells (SMCs) respond to the animal pole as a specific "target" site, as a postdoctoral fellow I performed several experiments. If the basic "rule" SMCs use is to continue their transient attachment behavior until they strike this target, then we would expect several things. First, allowing them to touch this region earlier than they would normally should cause them to make stable attachments earlier than usual. Second, delaying their contact with this region should delay the dramatic change in their behavior. Fortunately, we can test these ideas by changing the shape of the embryo. In each case, the results support the conclusion that physical contact with the animal pole target region induces the change in their behavior. The images below show pictures of actual embryos on which these experiments have been performed.


We can allow SMCs to touch the animal pole region earlier than they would normally by denting down the animal pole ectoderm. When this is done, in some cases the SMCs rapidly cease their exploratory behavior an make stable attachments to the animal pole, resulting in an abnormally short archenteron (C), whose attachment persists even after the dent is removed (D).

We can also perform the converse experiment to prevent SMCs from touching the animal pole region by sucking embryos into capillary tubes to elongate them along the animal-vegetal axis, as shown here. When this is done, the exploratory behavior of the SMCs continues for abnormally long periods of time. Eventually, in some cases the SMCs detach from the tip of the archenteron and those that migrate near the animal pole aggregate there (arrow). This provides further evidence that contact with this region results in the change of behavior of SMCs.