An Anatomical Description

SUMMARY. Hybridomas were prepared from mice immunized with myosin from the enlarged left ventricle of a 53-year-old female with an obstructive cardiomyopathy. The specificity of 15 monoclonal antibodies to myosin heavy chains was assessed by the reactivity of muscle extracts and of chymotryptic myosin fragments of different sizes with these antibodies, as determined by the immune replicate technique; some of the monoclonal antibodies cross-reacted only with the ventricular V3-type myosin from hypothyroid rats, whereas the other antibodies cross-reacted both with the latter and with the ventricular VI-type myosins from normal young rats. Immu-nological heterogeneity of the fibers from human atrial muscles and from human ventricular muscles was detected by some of the antimyosin antibodies by means of indirect immunofluores-cence. Histochemical fiber heterogeneity was also detected by adenosine triphosphatase staining of the same tissues. Because of the close correspondence observed between the immunological and histochemical responses of atrial fibers, it has been postulated that at least two distinct types of myosin exist in the human atrium, each myosin form being histochemically related to either a- or /3-like ventricular myosin heavy chains. In contrast, there was no direct correspondence between the two experimental approaches in human ventricles, and it is postulated that at leas