Functional development of fetal limb muscles: a review of the roles of activity, nerves and hormones

Abstract

Animals that are immature at birth with respect to postural and locomotor control (e.g. cats, rats) possess incompletely differentiated 'fast-twitch' and 'slow-twitch' muscles at birth; full development proceeds slowly in the postnatal period and involves myogenic, hormonal, neural and behavioural factors. The gradual emergence of specific motor patterns and the exercise of individual muscle groups is thought to play a major role in the final development of each muscle and the fibre types which comprise them. In contrast, precocial species such as the sheep are born with skeletal muscles, especially those of the limbs, which are fully differentiated at birth. The relative importance of neural and hormonal factors in allowing this functional specialization to occur in the presumed absence of significant load-bearing exercise in the intrauterine environment is unclear. In this brief review, the changes which occur in contractile function and fibre type differentiation during the last one-third of gestation in fetal sheep are described, and some of the factors which influence this development are considered.

Precocial development of axial locomotor muscle in bottlenose dolphins (Tursiops truncatus)

“…2) (see also Dubowitz, 1965;Rubinstein and Kelly, 1978;White et al, 1978;Bechtel and Kline, 1987;Wigston and English, 1992;Umezu et al, 1992). These studies demonstrated that muscles of terrestrial, precocial locomotors were similar to those of adults both in mass (as a percentage of total body mass) (Grand, 1992) and in fiber-type profile, as demonstrated by the myosin ATPase assay (Dubowitz, 1965;Gutmann et al, 1974;White et al, 1978;Bechtel and Kline, 1987;Walker and Luff, 1995).…”

“…Neonatal muscles with a fiber-type profile dominated by slowtwitch fibers can have contraction times that are faster than in the adult (Kelly and Rubinstein, 1980;Reiser et al, 1988;Walker and Luff, 1995). Conversely, neonatal fast-twitch muscle contraction times are slower than in adult muscles (Walker and Luff, 1995).…”

“…tigated: 1) relative mass (e.g., Grand, 1992), 2) histochemistry, 3) immunocytochemistry, 4) biochemistry (e.g., Cobb et al, 1994), and 5) contractile properties (e.g., Reiser et al, 1988;Walker and Luff, 1995). This study examined three of these five characteristics of neonatal muscle.…”

Differentiation of original and regenerated skeletal muscle fibres in mdx dystrophic muscles

“…The force-generating potential of skeletal muscles is determined by the contractile and metabolic properties of muscle fibres. The differentiation of muscle fibres has been reported to be closely linked to motoneurone activity, mechanical factors and changes in the levels of some hormones (Buller et al 1960;Dhoot and Perry 1981;Vrbova et al 1985;Dhoot 1992;Sabry and Dhoot 1992;Walker and Luff 1995). The changes in the levels of metabolic enzymes also contribute to fibre type specialisation.…”

Contribution of Stretch to the Change of Activation Properties of Muscle Fibers in the Diaphragm at the Transition from Fetal to Neonatal Life

“…It is not known when this “hybrid” property emerges during development, or what effect the transition to air breathing, or the mechanics of regular forceful breathing might have on the activation properties of the diaphragm fibers. In fetal sheep, limb muscle fibers initially have a “fast-twitch” phenotype, whether characterized by the contraction speed of whole muscle groups (Walker and Luff, 1995), or by the Ca 2+ and Sr 2+ activation properties of single “skinned” muscle fibers (West et al, 1999). We hypothesized that, as for hind limb muscle fibers in fetal sheep, all diaphragm fibers would initially have a fast-twitch phenotype, and that slow-twitch characteristics would emerge either late in gestation or at birth.…”