Assessment of satellite cell number and activity status in human skeletal muscle biopsies.

The primary aim of our study was to validate the assessment of myonuclear and satellite cell number in biopsies from human skeletal muscle. We found that 25 type I and 25 type II fibers are sufficient to estimate the mean number of myonuclei per fiber. In contrast, the assessment of satellite cells improved when more fibers were included. Second, we report that small differences in counting satellite cells using CD56 and Pax7 antibodies can be attributed to the different staining profiles. Third, we provide support for the use of Ki67 in evaluating the proportion of active satellite cells. We observed very few (up to 1.3%) active satellite cells in healthy adult skeletal muscle at rest, but they increased significantly (up to 7-fold) following muscle activity. This study provides valuable tools to assess the behavior of satellite cells, both in pathological conditions and in response to physiological stimuli.

Effects of combined lower body endurance and upper body resistance training on the satellite cell pool in elderly subjects.

To distinguish the respective potential of endurance and resistance training to increase the satellite cell pool, we investigated the effects of 14 weeks of concurrent lower body endurance and upper body resistance training (3 sessions/week) on vastus lateralis (VLat) and deltoid (Del) muscles of 10 active elderly men. NCAM+ satellite cells and myonuclear number were assessed in VLat and Del. After 14 weeks of training the NCAM+ satellite cell pool increased similarly (+38%) in both muscles, mainly in type II muscle fibers (P < 0.05). There was no significant change in myonuclear number or myonuclear domain in either muscle. Combining resistance training in the upper limbs with endurance training in the lower limbs is an efficient strategy to enhance the satellite cell pool in upper and lower body muscles in elderly subjects. Our results provide a practical reference for the determination of optimal exercise protocols to improve muscle function and regeneration in the elderly.

The number of satellite cells in slow and fast fibres from human vastus lateralis muscle.

The aim of this investigation was to study the distribution of satellite cells in slow (type I fibres) and fast (type II fibres) fibres from human vastus lateralis muscle. This muscle is characterised by a mixed fibre type composition and is considered as the site of choice for biopsies in research work and for clinical diagnosis. Biopsy samples were obtained from five healthy young volunteers and a total of 1,747 type I fibres and 1,760 type II fibres were assessed. Satellite cells and fibre type composition were studied on serial muscle cross-sections stained with specific monoclonal antibodies. From a total of 218 satellite cells, 116 satellite cells were found in contact with type I fibres (53.6+/-8% of the satellite cells associated to type I fibres) and 102 satellite cells in contact with type II fibres (46.4+/-8% of the satellite cells associated to type II fibres). There was no significant difference (P=0.4) between the percentages of satellite cells in contact with type I and with type II fibres. Additionally, there was no relationship between the mean number of satellite cells per fibre and the mean cross-sectional area of muscle fibres. In conclusion, our results show that there is no fibre type-specific distribution of satellite cells in a human skeletal muscle with mixed fibre type composition.

The behaviour of satellite cells in response to exercise: what have we learned from human studies?

Understanding the complex role played by satellite cells in the adaptive response to exercise in human skeletal muscle has just begun. The development of reliable markers for the identification of satellite cell status (quiescence/activation/proliferation) is an important step towards the understanding of satellite cell behaviour in exercised human muscles. It is hypothesised currently that exercise in humans can induce (1) the activation of satellite cells without proliferation, (2) proliferation and withdrawal from differentiation, (3) proliferation and differentiation to provide myonuclei and (4) proliferation and differentiation to generate new muscle fibres or to repair segmental fibre injuries. In humans, the satellite cell pool can increase as early as 4 days following a single bout of exercise and is maintained at higher level following several weeks of training. Cessation of training is associated with a gradual reduction of the previously enhanced satellite cell pool. In the elderly, training counteracts the normal decline in satellite cell number seen with ageing. When the transcriptional activity of existing myonuclei reaches its maximum, daughter cells generated by satellite cell proliferation are involved in protein synthesis by enhancing the number of nuclear domains. Clearly, delineating the events and the mechanisms behind the activation of satellite cells both under physiological and pathological conditions in human skeletal muscles remains an important challenge.

The effects of heavy resistance training and detraining on satellite cells in human skeletal muscles.

The aim of this study was to investigate the modulation of satellite cell content and myonuclear number following 30 and 90 days of resistance training and 3, 10, 30, 60 and 90 days of detraining. Muscle biopsies were obtained from the vastus lateralis of 15 young men (mean age: 24 years; range: 20-32 years). Satellite cells and myonuclei were studied on muscle cross-sections stained with a monoclonal antibody against CD56 and counterstained with Mayer's haematoxylin. Cell cycle markers CyclinD1 and p21 mRNA levels were determined by Northern blotting. Satellite cell content increased by 19% (P= 0.02) at 30 days and by 31% (P= 0.0003) at 90 days of training. Compared to pre-training values, the number of satellite cells remained significantly elevated at 3, 10 and 60 days but not at 90 days of detraining. The two cell cycle markers CyclinD1 and p21 mRNA significantly increased at 30 days of training. At 90 days of training, p21 was still elevated whereas CyclinD1 returned to pre-training values. In the detraining period, p21 and CyclinD1 levels were similar to the pre-training values. There were no significant alterations in the number of myonuclei following the training and the detraining periods. The fibre area controlled by each myonucleus gradually increased throughout the training period and returned to pre-training values during detraining. In conclusion, these results demonstrate the high plasticity of satellite cells in response to training and detraining stimuli and clearly show that moderate changes in the size of skeletal muscle fibres can be achieved without the addition of new myonuclei.

Satellite cells and myonuclei in young and elderly women and men.

The overall aim of this study was to assess the effects of aging on the satellite cell population. Muscle biopsies were taken from the tibialis anterior muscle of healthy, moderately active young (age range, 20-32 years; n = 31) and elderly (age range, 70-83 years; n = 27) women and men with comparable physical activity pattern. Satellite cells and myonuclei were visualized using a monoclonal antibody against neural cell adhesion molecule and counterstained with Mayer's hematoxylin. An average of 211 (range, 192-241) muscle fibers were examined for each individual. Compared with the young women and men, the elderly subjects had a significantly lower (P < 0.011) number of satellite cells per muscle fiber but a significantly higher (P < 0.004) number of myonuclei per muscle fiber. The number of satellite cells relative to the total number of nuclei [satellite cells/(myonuclei + satellite cells)] was significantly lower in the elderly than in the young women and men. These results imply that a reduction in the satellite cell population occurs as a result of increasing age in healthy men and women.

Effects of endurance training on satellite cell frequency in skeletal muscle of old men.

We investigated the effects of endurance training on satellite cells, which are a major component of the regenerative capacity of muscles. Muscle biopsies were obtained from the vastus lateralis of 11 men aged between 70 and 80 years who trained for 14 weeks (work load corresponding to 65-95% of peak oxygen consumption, VO(2) peak). Satellite cells were identified by immunohistochemistry. There was a significant increase in satellite cell number. Additionally, VO(2) peak, citrate synthase activity, and the area of type IIA fibers were significantly increased. Fiber type distribution and the myonuclear number were not significantly affected. The enhancement of satellite cell frequency and fiber area indicate that endurance training is an efficient strategy to improve muscle function in the elderly.