In this study, we have evaluated the ability of human satellite cells isolated from subjects aged from 5 days to 86 years to proliferate in culture. Cells were cultivated until they became senescent. The number of cell divisions was calculated by counting the number of cells plated in culture compared to the number of cells removed following proliferation. Telomere length, which is known to decrease during each round of cell division, has been used to analyze the in vitro replicative capacity and in vivo replicative history of human satellite cells at isolation. The rate of telomere shortening in myonuclei of these muscle biopsies was also examined. Our results show that both proliferative capacity and telomere length of satellite cells decreases with age during the first two decades but that the myonuclei of human skeletal muscle are remarkably stable because telomere length in these myonuclei remains constant from birth to 86 years. The lack of shortening of mean terminal restriction fragments (TRF) in vivo confirms that skeletal muscle is a stable tissue with little nuclear turnover and therefore an ideal target for cell-mediated gene therapy. Moreover, our results show that it is important to consider donor age as a limiting factor to obtain an optimal number of cells.

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To determine the postural role of longus colli (LC) and dorsal neck muscles, we have studied the relationship between their cross-sectional areas related to their force of contraction and the lordosis and the length of the cervical spine. This study was carried out in 36 healthy subjects. Muscle cross-sectional areas were measured by computerized tomography. The index of lordosis and the length of the cervical spine were measured on an X-ray profile. The cross-sectional area of LC was correlated to the lordosis index (R = -0.432, p < 0.02) whereas all the other parameters were not correlated. The authors conclude that LC counteracts the lordosis increment related to the weight of the head and to the contraction of the dorsal neck muscles. Postural functions of LC and postcervical muscles are complementary. They form a sleeve which encloses and stabilizes the cervical spine in all positions of the head.

In this study we have developed an in vitro cell culture system which displays the majority of the defects previously described for congenital myotonic dystrophy (CDM) muscle in vivo. Human satellite cells were isolated from the quadriceps muscles of three CDM fetuses with different clinical severity. By Southern blot analysis all three cultures were found to have approximately 2300 CTG repeats. This CTG expansion was found to progressively increase in size during the proliferative life span, confirming an instability of this triplet in skeletal muscle cells. The CDM myoblasts and myotubes also showed abnormal retention of mutant RNA in nuclear foci, as well as modifications in their myogenic program. The proliferative capacity of the CDM myoblasts was reduced and a delay in fusion, differentiation and maturation was observed in the CDM cultures compared with unaffected myoblast cultures. The clinical severity and delayed maturation observed in the CDM fetuses were closely reflected by the phenotypic modifications observed in vitro. Since the culture conditions were the same, this suggests that the defects we have described are intrinsic to the program expressed by the myoblasts in the absence of any trophic factors. Altogether, our results demonstrate that satellite cells are defective in CDM and are probably implicated in the delay in maturation and muscle atrophy that has been described previously in CDM fetuses.

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