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

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.