[Quantification of the diameter of muscular fibres in the course of the development of the quadriceps].

Muscle biopsies were taken from the medium portion of the vastus lateralis of a hundred fetuses (aged between 10 and 40 weeks), children, adolescents and adults without any neuro-muscular disease. Diameters of the muscle fibers were measured with a Leitz ASM 68K on frozen sections which had been stained for myofibrillar ATPase. The results have been expressed as average diameters and histograms. The results demonstrated a gradual increase in diameter of the fibers during fetal development. At the birth, the average diameter of the type I and II fibers is 10-15 microns. Growth continues during postnatal development up until puberty, when the fibers reach their final diameter of 60-65 microns. Analysis of the histograms shows that before 18 weeks of gestation, there is a large dispersion in the diameter of the fibers. The first type I fibers are larger than the others and correspond to Wohlfart type b fibers. This is followed by the appearance of a second population of smaller type I fibers. These fibers gradually increase in diameter and one observes a homogenous population of type I fibers at the birth.