The induction of axial blisters in the chick embryo by trypan blue

The hemorrhage, blister formation, and rumplessness observed in the chick embryo following treatment with trypan blue may be due to (1) increased ventricular blood pressure or (2) to necrosis and edema in the caudal region of the embryo by inhibiting nutrient utilization. To test the role of increased ventricular blood pressure in the induction of caudal blisters, primitive streak to 7-somite chick embryos were cut in half, separating the upper presumptive heart region from the lower presumptive trunk and tail regions. Each half was then explanted on media containing trypan blue for 24 hours. In intact embryos treated with 0.04 mM trypan blue the frequency of blisters in the posterior region was 80.8%. The blisters usually appeared on both sides of the neural tube, below or in the region of the last few somites. In transected embryos treated with trypan blue, the frequency of blisters in the posterior halves which had beating hearts was 2.6%. However, the frequency of blisters in the posterior halves which were not connected to beating hearts was 47.8%. In some cases the blisters were found in posterior halves in which the rump was not well developed or present at all. Thus, we may conclude that: (1) direct connection between the heart and the rump is not necessary for the induction of caudal blisters. (2) The presence of a well-differentiated rump is also not necessary for blister formation. We suggest that trypan blue acts directly on organs or structures found in the caudal region of the chick embryos.