The embryonic origin of connective tissue mast cells.

To find out whether mast cells are derived from the neural crest or from mesoderm, explants of avian blastoderm, from which the neural crest was excluded, were grown on the chorio-allantoic membrane of chick hosts. The grafts were fixed in aldehyde fixatives and stained to demonstrate metachromatic connective tissue mast cells. In grafts of chick endoderm and mesoderm, mast cells were present despite the absence of neural crest derivatives. To ascertain whether these mast cells were perhaps of host origin, in a second experiment quail embryos were used as donors; explants included all three germ layers and were delimited so as to exclude neural crest cells. The proportion of the mast cells present that showed very large nucleoli such as are characteristic of quail cells was far in excess of that observed in mast cells of a young chick and much closer to that seen in these cells in a young quail. The same was true for grafts in which quail mesoderm had been combined with chick endoderm. Taken together, the results show that connective tissue mast cells are mesodermal rather than neural crest in origin.