Growth in perspective

Muscle-specific Drp1 overexpression impairs skeletal muscle growth via translational attenuation

Mitochondrial fission and fusion are essential processes in the maintenance of the skeletal muscle function. The contribution of these processes to muscle development has not been properly investigated in vivo because of the early lethality of the models generated so far. To define the role of mitochondrial fission in muscle development and repair, we have generated a transgenic mouse line that overexpresses the fission-inducing protein Drp1 specifically in skeletal muscle. These mice displayed a drastic impairment in postnatal muscle growth, with reorganisation of the mitochondrial network and reduction of mtDNA quantity, without the deficiency of mitochondrial bioenergetics. Importantly we found that Drp1 overexpression activates the stress-induced PKR/eIF2α/Fgf21 pathway thus leading to an attenuated protein synthesis and downregulation of the growth hormone pathway. These results reveal for the first time how mitochondrial network dynamics influence muscle growth and shed light on aspects of muscle physiology relevant in human muscle pathologies.

Asymptotic weight and maturing rate in mice selected for body conformation

Growth patterns of four lines of mice selected for body conformation were analyzed with the logistic function, in order to provide baseline information about the relationship between asymptotic weight and maturing rate of body weight. Two lines were divergently selected favoring the phenotypic correlation between body weight and tail length (agonistic selection: CBi+, high body weight and long tail; CBi-, low body weight and short tail), whereas the other two lines were generated by a disruptive selection performed against the correlation between the aforementioned traits (antagonistic selection: CBi/C, high body weight and short tail; CBi/L, low body weight and long tail). The logistic parameters A (asymptotic weight) and k (maturing rate) behaved in CBi/C and CBi- mice and in CBi+ females as expected in terms of the negative genetic relationship between mature size and earliness of maturing. An altered growth pattern was found in CBi/L mice and in CBi+ males, because in the former genotype, selected for low body weight, the time taken to mature increased, whereas in the latter, selected for high body weight, there was a non-significant increase in the same trait. In accordance with the selective criterion, different sources of genetic variation for body weight could be exploited: one inversely associated with earliness of maturing (agonistic selection), and the other independent of maturing rate (antagonistic selection), showing that genetic variation of A is partly independent of k.

Sexual dimorphism and ontogenetic allometry of soft tissues in Rattus norvegicus

Most studies of sexual dimorphism in mammals focus on overall body size. However, relatively little is known about the differences in growth trajectories that produce dimorphism in organ and muscle size. We weighed six organs and four muscles in Rattus norvegicus to determine what heterochronic and allometric scaling differences exist between the sexes. This cross-sectional growth study included 113 males and 109 females with ages ranging from birth to 200 days of age. All muscle and organ weights were ultimately greater in males than in females, because males grew for a longer period of time, had a greater maximum rate of growth, and spent more time near the maximum rate. No ontogenetic scaling differences existed between the sexes in organ weight except for lungs and gonads. During growth, organ weights were negatively allometric to body weight. No scaling differences relative to body weight existed between the sexes for muscles; however, there was variation in the allometric relations among muscles relative to body weight. Sexual dimorphism in muscles and organs appears to be a size difference resulting from differences in the duration and rates of growth.

Morphometric skeletal traits, femoral measurements, and bone mineral deposition in mice with agonistic selection for body conformation

Morphometric skeletal traits, femoral histomorphometry, and bone mineral deposition were investigated in two lines of mice (CBi+ and CBi-) divergently selected for body conformation (CBi+: high body weight, long tail; CBi-: low body weight, short tail) and in the unselected control line CBi. Linear morphometric measurements, absolute and relative skeletal weights, absolute and relative femoral weights, and total biomass sustained per unit of total or tail-less skeletal weight were increased in CBi+ mice in comparison with controls. This greater biomass implies a greater mechanical demand that is satisfied by a heavier skeleton. Looking specifically to the femur, CBi+ mice had heavier bones, both absolute and relative, with a greater diameter and a greater cortical thickness, resulting in a greater cortex/diameter ratio than controls. Although morphometric measurement and absolute skeletal weight were lower in CBi- than in CBi mice, the relative skeleton weight and the biomass sustained per unit of skeletal weight were not modified in the downward selection line when compared with controls. Therefore, CBi- mice did not exhibit a greater mechanical demand as CBi+ mice did. These results led us to consider at least three main aspects: bone length growth; cortical thickness/bone diameter ratio; and bone calcification. The long bones appeared to have a genetically determined predisposition to achieve a given length, which, however, could be modified by artificial selection. Cortical thickness would be directly related to the biomass sustained. This variable increased in CBi+ mice, a genotype that supports a greater biomass than controls, and did not change in CBi- mice, which sustained the same biomass as CBi. The pattern of mineral deposition did not accompany the functional demand because it was higher in CBi- than in CBi+; however, as artificial selection separately affected bone material quality and bone architectural design, these genotypes could express architectural modifications that override any change in bone material quality.

The effects of muscular dystrophy on craniofacial growth in mice: a study of heterochrony and ontogenetic allometry

Mechanical loading of muscles on bones at their sites of attachment can regulate skeletal morphology. The present study examined the effects of muscle degeneration on craniofacial growth, using two strains of muscular dystrophic mice, Mus musculus, differing in pathological severity. We collected radiographic and weight data longitudinally and digitized radiographs to obtain distances between anatomical landmarks in different functional regions of the skull. We then quantified heterochronic and allometric differences among genotypes and between sexes. Because growth is nonlinear with respect to time, we first used the Gompertz model to obtain heterochronic growth parameters, which were then tested with ANOVA. Ontogenetic allometric analyses examined the scaling relationships between various measurements with linear regressions. For most measurements the severely dystrophic mice are significantly smaller in final size than both the control and the mildly dystrophic mice, which are statistically indistinguishable. Measures of total growth and the neurocranium exhibit more differences among groups in heterochronic parameters of early ontogeny because growth in these regions is controlled primarily by brain expansion that ceases early in development. In contrast, the face and mandible exhibit more differences in later growth parameters possibly because of the increased influence of muscles on these regions as growth progresses. The severely dystrophic mice have flatter, more elongate skulls and mandibles than those of the other two genotypes, concurrent with an absence of muscular forces to stimulate growth in a superior-inferior direction.