Comparative analysis of satellite cell properties in heavy- and lightweight strains of turkey

The growth of muscle during postnatal development results partly from the proliferation of satellite cells and their fusion with muscle fibres. We analysed the properties of satellite cells in a heavyweight (HW) turkey strain characterized by high body weight and a fast growth rate, and in a lightweight farm strain (LW) characterized by low body weight and a slow growth rate. Satellite cell activation was then examined in stretched-overloaded anterior latissimus dorsi (ALD) muscle by weighting one wing in young turkeys from both strains. As early as day 1 of stretching for HW and day 2 for LW, small embryonic-like fibres expressing ventricular cardiac myosin heavy chain (MHC) isoform were observed. Following four days of stretching, the number of nascent fibres had increased in both strains but was significantly greater in HW than LW ALD muscle. The proliferation and differentiation capacities of satellite cells from HW and LW strains were investigated in culture. As judged by in vitro measurements of 3H-thymidine incorporation and DNA content, satellite cells of HW turkey exhibited a greater proliferative capability than those of LW turkey. No differences in the temporal appearance of muscle markers (desmin, MHC isoforms) were noted in vitro between the two strains. These data confirm our in vivo observations indicating that selection based on growth rate does not modify muscle fibre maturation. Our in vivo and in vitro observations suggest that variations in the postnatal muscle growth pattern between HW and LW strains may be related to a difference in the capacity of their satellite cells to proliferate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Desmin-lacZ transgene expression and regeneration within skeletal muscle transplants

The purpose of this study was to investigate the initiation and time course of the regeneration process in fragments of skeletal muscle transplants as a function of muscle tissue age at implantation. The appearance of desmin occurs at the very beginning of myogenesis. The transgenic desminnls lacZ mice used in the study bear a transgene in which the 1 kb DNA 5′ regulatory sequence of the desmin gene is linked to a reporter gene coding for Escherichia coliβ-galactosidase. The desmin lacZ transgene labels muscle cells in which the desmin synthesis programme has commenced. We implanted pectoralis muscle fragments from fetal transgenic embryos and mature and old transgenic mice into mature non-transgenic mice. Early events of myogenesis occurring during regeneration started sooner in transplants from 4-month-old (day 3 post-implantation) muscle than in those from 24-month-old (day 5-6 post-implantation) muscle, and they lasted longer in those from young (day 17 post-implantation) than in those from old (day 14 post-implantation) muscle fragments. In adult muscle, transgene activation proceeded from the periphery toward the centre of the transplant. In transplants from fetal 18-day-old pectoralis, myotubes with transgene activity were observed from day 1 to day 19. Desmin immunoreactivity, which appeared about one day after transgene activation, was followed by myosin expression. In adult transplants, the continuity of laminin labelling was disrupted around degenerative fibres, illustrating alteration of the extracellular matrix. Our data suggest that satellite cells from old muscle tissue have lower proliferative capacity and/or less access to trophic substances released by the host (damaged fibres, vascularization) than those from fetal or young adult muscle This revised version was published online in July 2006 with corrections to the Cover Date.     

Desmin-lacZ transgene, a marker of regenerating skeletal muscle

Transgenic C57 mice bearing a transgene of the desmin gene linked to the lacZ reporter gene which encoded for the enzyme β-galactosidase were used. In the muscle cell, a blue nuclear product appearing in the presence of the X-gal substrate for the enzyme provided evidence of the expression of the desmin gene. However, no transgene expression was observed 2 weeks postnatal in skeletal muscles, even though endogenous desmin was present. In order to investigate the regulatory mechanisms of the desmin gene during regeneration, adult pectoralis fragments (without expression of the desmin transgene) from transgenic mice were implanted into the tibialis anterior of 4 day or 6 week old Swiss mice. Adult pectoralis transplants re-expressed the transgene from day 4 to 10 after implantation. In addition, lesions were performed in adult transgenic pectoralis and transgenic expression in injured muscles was observed 2 days later. This new transgenic mouse is a powerful tool for the study of the various steps of skeletal muscle regeneration.     

Influence of spinal cord stimulation on the innervation pattern of muscle fibers in vivo.

In chick embryo, chronic stimulation of the brachial spinal cord at a fast rhythm from days 7 to 18 of development induced an increase in AChE activity sites and ACh receptor (AChR) clusters in slow anterior latissimus dorsi (ALD) muscle. Most AChR clusters and AChE spots were contacted by nerve endings. A previous study showed that such spinal cord stimulation causes changes in ALD muscle properties, especially the appearance of a high proportion of fast type II fibers (Fournier Le Ray et al., 1989). Analysis of the synaptic pattern in different fiber types of experimental ALD muscle indicated a decrease in the distance between successive AChE spots in slow type III fibers compared to controls, whereas the intersynaptic distance in fast type II fibers was very similar to that in the rare fast fibers developing in control ALD. Fast fibers of experimental muscles exhibited less AChR than did slow fibers. The increased number of neuromuscular junctions in ALD muscle after spinal cord stimulation appeared to be preferentially located in slow fibers. Electron microscopy showed no change in the number of axons in ALD nerve after spinal cord stimulation. The activity imposed on brachial motoneurons apparently caused terminal sprouting of ALD nerve in target muscle, thus accounting for the increase in neuromuscular contacts in ALD muscle fibers. Differences in the distribution of nerve contacts indicate that the type of muscle fiber innervated may play a critical role in the synaptic pattern during chick embryogenesis.