Histological characterization of a monoclonal antibody raised against the branchial arches of the chick embryo: reactivity with myogenic lineages and a few non-mesodermal derivatives.

Monoclonal antibody A7H2 has been selected from a library of monoclonals raised against the branchial arch cells of 3-day-old chicken embryos, and its histological distribution was examined at different embryonic stages and levels. The immunoreactivity appeared during neurulation and was almost general in extraembryonic areas. As the embryo formed, A7H2-fluorescence disappeared from most tissues, but was concentrated in the splanchnomesoderm-derived smooth muscle lineages and epithelial linings of the coelomic cavity. A strong and durable reactivity was also observed in the myogenic condensations constituting the axis of the branchial arches, whereas myotomal cells of the differentiating somites were labelled more weakly and for a shorter time. Interestingly, non-mesodermal fluorescence was restricted to the branchial arch epithelium and some ectodermal placodes, to the thinning-out zones of the neural tube destined to form the choroid plexus, and to the pharyngeal and cloacal extremities of the digestive tract.