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 locomo tor control (e.g. cats, rats) possess incompletely differentiated 'fas t-twitch' and 'slow-twitch' muscles at birth; full development proceed s slowly in the postnatal period and involves myogenic, hormonal, neur al and behavioural factors. The gradual emergence of specific motor pa tterns and the exercise of individual muscle groups is thought to play a major role in the final development of each muscle and the fibre ty pes which comprise them. In contrast, precocial species such as the sh eep are born with skeletal muscles, especially those of the limbs, whi ch are fully differentiated at birth. The relative importance of neura l and hormonal factors in allowing this functional specialization to o ccur in the presumed absence of significant load-bearing exercise in t he intrauterine environment is unclear. In this brief review, the chan ges 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 considere d.