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Muscle biopsies were taken at the oesophageal opening and in the costal part of the diaphragm from 15 foetuses (aged between 11 and 40 weeks) and from four children aged 1 week, 5 months, 3 and 16 years. Subjects with multiple congenital abnormalities or neuromuscular diseases were not included in this study. Enzyme histochemistry (myosin ATPases) and immunohistochemistry using antibodies directed against vimentin, desmin, titin and different isoforms of the myosin heavy chains (MHCs) were carried out on serial frozen sections. The diameters of the different types of fibres were measured using a Leitz ASM 68K and the corresponding results were expressed either as mean values or as histograms. In both of the regions of the diaphragm which were examined (oesophageal opening and costal portion). The skeletal muscle fibres are formed from two successive generations in a manner very similar to that which we have previously described for the limb muscles. The chronology of the various events involved in the maturation of the diaphragm does however present certain distinctive features since the adult MHCs are expressed well before 30 weeks even though paradoxically large amounts of fetal MHC were still present at birth. These findings suggest the existence of a special regulatory mechanism in the diaphragm resulting either from its innervation by the phrenic nerve or from its specific functional constraints.

The ossification of human femoral bone was studied in 10 fetuses aged from 8 to 40 weeks, and in a 3 1/2 month-old child. After dissection and radiographic examination, each femur was sectioned in a sagittal plane. One part was processed for routine histology, the remaining one for scanning electron microscopy. By comparing the radiographic, light microscopic and scanning electron microscopic pictures, it is possible to precise the pattern of ossification and to define 3 main periods during human fetal development (pure cartilage, metaphyseal and diaphyseal ossification, constitution of a cortex and a medulla in the diaphysis).

Muscle biopsies were taken from the superficial portion of the M. masseter in 10 foetuses (aged between 12 and 38 weeks), in a child of 18 months and in an adult without any neuromuscular disease. Serial frozen sections were processed for immunocytochemistry using antibodies specific for the embryonic, foetal, slow and fast myosin heavy chains (MHCs). Diameter of the different types of fibers were measured with a Leitz ASM 68 K; the results have been expressed as average diameters and histograms. During foetal development, the masseter is formed from two successive generations of muscle fibers in a manner very similar to that which has been previously described for other skeletal muscles. After birth, a particular phenotype appears. This phenotype is characterised by the persistence of embryonic and foetal MHCs and by the presence of two distinct populations of fibers: small diameter fibers which coexpress embryonic, foetal and fast myosin isoforms but never express the slow MHC; large diameter fibers which express slow myosin either exclusively or in variable associations with the other isoforms.

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.

The adrenal glands from a series of 300 fetuses and infants which had died in the Hospital Saint-Vincent-de-Paul, Paris, were dissected, weighed and processed for routine histology. The development of the fetal cortex and its regression have been compared to the general development of the gland. Our results show that there is an important development of the fetal cortex up to birth, when the adrenal glands represent almost one third of the size of the kidneys and possess only very reduced permanent cortex and practically no medulla. The regression of the fetal cortex in the postnatal period is marked by a decrease in weight of the adrenal glands up to the 2nd or 3rd month, by the persistence of a few dispersed cells up to the fifth month and by the persistence of a few fibrous elements up to 2 years of age.

A scanning electron microscopic study of the development of the human quadriceps was performed in 30 fetuses ranging from 6 to 40 weeks gestation. The results clearly illustrate the hyperplastic phase of myogenesis, showing the differentiation, multiplication and fusion of the myoblasts and the subsequent formation of different generations of myotubes. The myoblasts decrease relatively in number within the developing quadriceps until the 19th week of gestation, and remain afterwards as quiescent satellite cells. The maturation of the myotubes to myofibers and the metabolic differentiation of the fibers are not accompanied by significant changes in their external form in terms of scanning electron microscopy. The hypertrophic phase of myogenesis and general fetal growth can be followed with scanning electron microscopy by the increase in size of the fibers or bundles of fibers, as well as by the differentiation of the connective tissue components of the muscle.

A scanning electron microscopic study of the coxo-femoral articular cartilage was performed in ten human fetuses aged from 15 to 40 weeks. An irregular surface was observed at the level of both femoral head and acetabulum. These surface irregularities are modified during development.