The development of skeletal muscles in premature infants. I. Fibre size and histochemical differentiation.

Biopsies from various skeletal muscles were taken post mortem from infants with conceptional ages between 25 and 40 weeks. Cryostat sections were analysed quantitatively. The growth of the mean fibre diameter follows an exponential curve, which is mainly due to an initial predominance of type II-fibres. Before the 34th week of gestation only isolated large "Wohlfart"-fibres show type I staining characteristics. A great number of smaller type I-fibres appear suddenly around the 34th week. Their percentage of the total fibre population increases linearly to 40% at term. The development of the "tonic" type I-fibres correlates with certain aspects of gross motor development. The importance of normal data in the diagnosis of certain congenital myopathies is emphasized.