Satellite cells as the source of nuclei in muscles of growing rats

Abstract

The source of the new nuclei appearing during the growth of muscle fibers was examined in the tibialis anterior muscle of young Sherman rats (14-17 days of age) using radioautography at various intervals after a single injection of a small, non-toxic dose of 3H-thymidine ( 2 pCi/g body weight). Two techniques were employed: (1) labeled nuclei were detected in 1 $L thick radioautographs examined in the light microscope, and identified by simultaneous electron microscope examination of an adjacent section. The nuclei were then classified either as "true" muscle nuclei (within the plasmalemma of the fibers) or as belonging to "satellite cells" (which are mononucleated cells with scanty cytoplasm wedged between plasmalemma and basement membrane). (2) Muscle fibers freed by collagenase digestion were radioautographed one hour after 3H-thymidine injection in order to determine the total number of labeled nuclei (true muscle nuclei plus those of satellite cells) per unit length of fiber.Certain nuclei within the basement membrane of muscle fibers are labeled one hour after 3H-thymidine and, therefore, synthesize DNA. The electron microscope demonstrates that these nuclei invariably belong to satellite cells, never to true muscle nuclei. Furthermore, the total number of labeled nuclei per unit length of fiber doubles between 1 and 24 hours; and, therefore, the labeled satellite cell nuclei undergo mitosis.Following mitosis, half of the daughters of satellite cells are incorporated into the fibers to become true muscle nuclei. The remaining half divides again later; and half of their daughter cells are incorporated. Thus, satellite cells in young rats divide repeatedly and function as a source of true muscle nuclei.The early stages of muscle development are now well understood. At the start there is proli€eration of mononucleated cells, the myoblasts; groups of these fuse to form a multinucleated myotube, which then acquires myofibrils to become a muscle fiber. It is also known that the number of fibers in a muscle is fixed at birth or soon after (MacCallum, 1898;Morpurgo, 1898; Enesco and Puddy, '64), and that the extensive muscle growth which occurs during development in young animals is due to hypertrophy of the fibers (Enesco and Puddy, '64; MOSS, '68).However, it is not clear how this hypertrophy takes place. In vitro studies have shown that, once myoblasts have been incorporated into myotubes to become muscle fibers, the nuclei lose the ability to divide (Bintliff and Walker, '60; Stockdale and Holtzer, '61). Consequently, it is generally assumed that the nuclei of muscle fibers, unlike those of other tissues, do not divide and that their number does not increase during growth. On the other hand, in vivo studies of muscles of growing animals do not support this belief: DNA determinations have demonstrated that there is a considerable increase in the number of nuclei during muscle growth in rats (Enesco and Puddy, '64) and chickens (Moss, '68). Furthermore, radioautography with 3H-thymidine has shown that...

Temporal expression of regulatory and structural muscle proteins during myogenesis of satellite cells on isolated adult rat fibers

“…Indeed, our assays of a reactivity with an antibody against myogenin cannot indicate whether myogenin in the cells is in its active form, whether other important molecules are present to participate in the myogenic activity of myogenin, and whether the myogenin-expressing cells developed the final block that will make the cells terminally differentiated (see reviews by Olson (1992Olson ( , 1993 and Weintraub (1993) regarding the role of phosphorylation and other regulatory proteins in modulating the function of the myogenic regulatory factors). The notion that only 50% of the satellite cell progeny eventually terminally differentiated is in agreement with the studies of Moss and Leblond (1971) who, analyzing [ 3 H]thymidine incorporation in growing rat muscle, suggested that following each division of satellite cells only 50% of the progeny can fuse with the fiber while the second cell can continue to proliferate. In the present study the satellite cell progeny which are not bound for terminal differentiation (expression of DEVmyosin) do not continue to proliferate either.…”

“…Individual myofibers become encased by a basement membrane during late embryogenesis and it is at that stage that the distinction of satellite cells by their morphology and location is first possible (reviewed in Grounds and Yablonka-Reuveni, 1993). In vivo studies analyzing the incorporation of [ 3 H]thymidine into satellite cell nuclei and, subsequently, into fiber nuclei, indicated that at least some of the satellite cells are proliferative in young animals, contributing additional nuclei to the fibers (Moss and Leblond, 1971). In the adult, satellite cells are mitotically quiescent but can reinitiate proliferative activity in response to injury, and their progeny eventually fuse into preexisting or new myofibers (Snow, 1977;Schultz et al, 1978Schultz et al, , 1985Carlson and Faulkner, 1983).…”

Testosterone mediates satellite cell activation in denervated rat levator ani muscle

“…LA of Group B rats can be ascribed to the expansion of their satellite cell pool (Table 3). Autoradiographic studies by Moss and Leblond (1971) showed mitotic activity within this pool to be the mechanism of postnatal increase in myonuclei. The above changes indicate the very wide morphological range across which the LA responds to manipulations of its trophic inputs.…”

Caspase-3 Regulation of Diaphragm Myonuclear Domain during Mechanical Ventilation–induced Atrophy

“…The ability of skeletal muscle to hypertrophy seems to be tightly coupled to the addition of myonuclei via satellite and myogenic cell fusion with existing myofibers (21). For example, the myonuclear domain of muscle fibers is maintained during muscle fiber hypertrophy due to postnatal growth (22)(23)(24), synergistic muscle ablation (25,26), stretch (27), or exercise (28,29). Muscle disuse atrophy due to a variety of experimental models results in the concurrent loss of myonuclei (14,21,(30)(31)(32)(33)(34).…”