Spatial distribution of “tissue‐specific” antigens in the developing human heart and skeletal muscle. I. An immunohistochemical analysis of creatine kinase isoenzyme expression patterns

Using monoclonal antibodies against the M and B subunit isoforms of creatine kinase (CK) we have investigated their distribution in developing human skeletal and cardiac muscle immunohistochemically. It is demonstrated that in skeletal muscle, a switch from CK‐B to CK‐M takes place around the week 8 of development, whereas in the developing heart, CK‐M is the predominant isoform from the earliest stage examined onward (i.e., 4½ weeks of development). In all hearts examined, local differences in concentration of the CK isoforms are observed. The CK‐M expression in the developing outflow tract (OFT) and conduction system is described in detail. Between the weeks 5 and 7 of development, the distal portion of the OFT is characterized by low CK‐M expression, whereas around the week 8–10 of development the myocardium around the developing semilunar valves in the OFT expresses a very high level of CK‐M. At all stages examined, a relatively low CK‐M level is observed in those regions in which the “slow” components of the conduction system do develop (e.g., the sinoatrial junction and atrioventricular junction), whereas a relatively high concentration of CK‐M is observed in those areas that are destined to become the “fast” components, i.e., the subendocardial myocardium of the ventricles. The high expression of CK‐M in the developing “fast components” of the conduction system contrasts with the relatively low expression of CK‐M in the force‐producing myocardium of the interventricular septum and free ventricular wall.