Ossification of the human fetal basicranium.

Previous investigations of prenatal development of the human cranium have not identified the sequence of its ossification. The purpose of the present study was to elucidate the pattern of skeletal maturity of the cranial bones in the midsagittal region anterior to the foramen magnum. This study is based upon a radiographic and histochemical investigation of midsagittal tissue blocks of the cranial bases of 73 human fetuses derived from the first half of the prenatal period. A marked regularity in the ossification pattern of the bones in the midsagittal part of the human cranium was observed. Ossification starts in the frontal bone. The sequence in which the next bones ossify is occipital bone, basisphenoid bone, presphenoid bone, and ethmoid bone. The material was divided into 7 maturity stages devised for this analysis. The stages were related to general fetal size (crown-rump length) and to general fetal maturation (composite number of ossified bones in hand and foot). Skeletal development of the median part of the human cranium is not strictly correlated with the size or the stage of general maturation of the fetuses. Knowledge of normal skeletal development is necessary for understanding anomalies of development.