Quantitative (muscle fibre number and cross-sectional areas) and qualitative (myosin isoforms and metabolic enzyme activities) characteristics of two muscles, M. pectoralis major and M. anterior latissimus dorsi, were compared among male chickens of two lines during growth from hatching to adulthood. The lines were derived from a divergent selection based on growth rate. The two muscles were chosen on the basis of their histochemical profile. Pectoralis major muscle contains only fast contracting muscle fibres whereas anterior latissimus dorsi muscle is almost entirely made up with slow contracting fibres. At both ages, the two lines showed similar fibre type distributions. At hatching, fibre cross-sectional areas were equivalent in the two lines, but after the first week, animals from the fast growing line exhibited wider fibre areas, whatever the muscle, than animals from the slow growing line. The total number of fibres in a muscle was found greater in the fast growing line, irrespective of whether it was exactly determined (anterior latissimus dorsi muscle, + 20%) or only estimated (pectoralis major muscle). This number remains constant in the two lines throughout the growth. Myosin isoform profiles and metabolic enzyme activities were similar in the two lines, at both ages, and were in good agreement with the histochemical muscle fibre profiles.

The expression of the Na(+)/Ca(2+) exchanger was studied in differentiating muscle fibers in rats. NCX1 and NCX3 isoform (Na(+)/Ca(2+) exchanger isoform) expression was found to be developmentally regulated. NCX1 mRNA and protein levels peaked shortly after birth. Conversely, NCX3 isoform expression was very low in muscles of newborn rats but increased dramatically during the first 2 wk of postnatal life. Immunocytochemical analysis showed that NCX1 was uniformly distributed along the sarcolemmal membrane of undifferentiated rat muscle fibers but formed clusters in T-tubular membranes and sarcolemma of adult muscle. NCX3 appeared to be more uniformly distributed along the sarcolemma and inside myoplasm. In the adult, NCX1 was predominantly expressed in oxidative (type 1 and 2A) fibers of both slow- and fast-twitch muscles, whereas NCX3 was highly expressed in fast glycolytic (2B) fibers. NCX2 was expressed in rat brain but not in skeletal muscle. Developmental changes in NCX1 and NCX3 as well as the distribution of these isoforms at the cellular level and in different fiber types suggest that they may have different physiological roles.

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The staging of murine cardiomyocyte specification and determination was investigated in cultures of tissue explants from pre- and postgastrulated embryos and after transplantation of cardiac or cardiogenic tissues from mouse embryos into 2-day-old chick embryos in different locations. The development of transplanted and cultured cells in cardiomyocytes was evaluated by testing the expression of several cardiac transcription factor genes (Nkx 2.5, eHAND, dHAND, GATA-4), alpha-cardiac actin mRNA, and beta-myosin heavy chain protein. In vitro analyses showed that cells with the potential to form cardiac muscle were present prior to gastrulation in 6.5-day postconception (dpc) epiblasts, as indicated by the expression of Nkx 2.5, eHAND, dHAND, and GATA-4 cardiac transcription factors; desmin transgene; alpha-cardiac actin; and beta-myosin heavy chain. Conversely, epiblasts transplanted into the chicken somitic environment did not exhibit full cardiogenic cell differentiation. It was determined that chick host axial structures did not influence cardiogenesis in transplants. Mesoderm from late streak explants was capable of differentiating into the cardiac phenotype in the avian heterotopic environment, indicating that the specification of cardiac precursors (under way by 6.5 dpc) became irreversible at around the late streak stage in mouse embryo. Although in vitro analyses showed that interaction with endoderm is not required for the specification of murine cardiac cells, the presence of endoderm in explant cultures between mid- and late streak stages stimulated emerging mesodermal cells to adopt a myocardial pathway, whereas ectoderm had no influence on cardiomyogenesis.