Differentiation of Myosin in Chick Embryos*

Differentiation of myosin in chick embryos was studied by the fluorescent antibody technique: 1) Fluorescent antibodies against either the heavy chains of myosin from adult chicken breast muscle (fast white) or cardiac muscle stained both the breast muscle and cardiac muscle of early embryos. Staining of these muscles with the antibody against myosin from the other kind of muscle decreased as the embryos developed and no staining was observed after hatching. However, staining with antibody against myosin from the same kind of muscle did not decrease during development. 2) Essentially similar results to those described in 1) were obtained with troponin. 3) Fluorescent antibody against the heavy chain of myosin from adult anterior latissimus dorsi muscle (slow red) also stained, both the breast muscle and cardiac muscle of early embryos. Staining gradually decreased during embryonic development. After hatching cardiac muscle did not stain while breast muscle showed small, clearly limited spots of stain. 4) Double staining with two of the three anti-myosin antibodies mentioned above revealed that single glycerinated myofibrils from 10 day chick embryos stained with all three antibodies. 5) The process of differentiation of embryonic myosins is discussed on the basis of these results.