Myosin Heavy Chain Transitions in Dog Skeletal Muscle Fibres during Post‐natal Development

Myosin heavy chain (MHC) isoform expression was established in the masseter, axial and limb skeletal muscles in 18 young dogs from late prenatal to 6 months of age. Results obtained by immunohistochemical procedure using 11 monoclonal antibodies, SDS‐PAGE electrophoresis, Western‐blotting and enzyme‐histochemical reactions were compared to those found in skeletal muscles of adult dogs. In the skeletal muscles of both fetuses in the last stages of development and neonatal dogs, the developmental isoforms MHC‐emb and MHC‐neo were prevalent. During the first 10 days after birth, the level of embryonic isoform expression declined, as did that of the neonatal isoform during the second and third weeks. Correspondingly, the expression of MHC‐IIa, and later, of MHC‐I increased. By 6 weeks of age the presence of MHC‐IIx became obvious. This transitional pattern was similar in all muscles except in the rhomboideus, where secondary slow fibres were present before birth. In the other muscles, only one primary, centrally located, slow fibre was present in a fascicle and the secondary slow fibres evolved from developmentally fast fibres between the second and third weeks of age. Our results indicate that timing of muscle maturation depends on its activity right after birth. The fastest developing muscles were the masseter muscle and the diaphragm. They were followed by others, postural and locomotory muscles. A pronounced swap from developmental to adult isoforms was noted between the 4 th and 6 th  week of age. At this age puppies are weaned and begin playing intensively and investigating their surroundings. The results of our study indicate that dog skeletal muscles exhibit an immature morphology at birth and develop relatively slowly compared to those of other domestic animals.