Developmental Stage-Dependent Expression of Troponin T Isoforms in Chicken Embryonic Breast Muscles Grafted on Chorio-Allantoic Membrane

To examine whether the expression pattern of muscle-specific protein in embryonic muscle tissues at the different developmental stages depends on the types of myogenic cells, chicken breast muscle (pectoralis major) tissues of 10-day, 14-day, and 18-day old embryos (E10, E14, and E18, respectively) were grafted on chorio-allantoic membrane (CAM) of 9-day-old chicken embryos and cultured for 12 days at the longest, since the chorio-allantoic grafting is useful in pursuing muscle-cell lineage during muscle differentiation. The muscle fiber formation and expression of troponin T (TnT) isoforms in grafts were investigated by histological and immunohistochemical methods with anti-fast-muscle-type and anti-slow-muscle-type TnT (rabbit sera). In grafts of E10 breast muscle, most muscle fibers continued to develop without degeneration and TnT isoform expression of the fibers changed from the concomitant expression of fast-muscle-type (Ftype) and slow-muscle-type (S-type) to the expression of F-type only. In grafts of E14 and E18 breast muscles, the muscle fibers first degenerated with pyknotic nuclei and hyaline cytoplasm, and then new muscle fibers expressing F-type TnT isoforms were formed by the fusion of basophilic cells. The new muscle fibers in grafts of E14 muscles were different from those of E18 ones in that the former also expressed S-type TnT isoforms. In this paper, developmental stage-dependent TnT isoform expression of the embryonic breast muscles grafted on CAM is discussed in connection with cell-type difference of grafted muscle cells.