[Development of parietal bone based on ossification in the fetus]

Seventy-two parietal bones from human fetuses aged 15 to 29 weeks were studied. Three age groups were formed: I--15-19 weeks; II--19-24 weeks, III--25-29 weeks. The width and length of each parietal bone was determined. Microradiological studies were done with the Unipan 401 X-ray apparatus. Subsequently, pulverized bone for biochemical studies was prepared. Calcium, magnesium, zinc and iron content was determined using atomic spectrophotometry. Phosphates were measured with a colorimetric method and fluorine was assayed using an ion-selective electrode. The dimensions of fetal parietal bones changed almost linearly with age until the 20th week of life when a deceleration in the growth rate was noted (Tables 1, 2, 3, 4). This finding could be useful for estimation of fetal age when other methods are unavailable. The parietal bone is formed from one or two primary ossification centers lying in the same plane or one above the other (Figs. 1, 2, 3). Fusion of primary ossification centers was observed between 15 and 19 weeks of life (Fig. 3). Trabeculae radiate from the primary ossification center to the periphery of the parietal bone and take the form of a "coral reef" within the tuber (Figs. 2 and 5). Transverse ossification which was not coordinated with the growth of trabeculae from the primary ossification center was observed between week 15 and 19 of intrauterine life (Fig. 2). Fusion of peripheral trabeculae with those from the ossification center takes place at a later stage of osteogenesis (Fig. 4). Parietal bones of 29-week-old fetuses consisted of two layers of trabeculae and a distinct tuber, except for the sagittal and coronal sutures and the anterior fontanel, where a single layer of trabeculae was observed. Trabeculae were well developed and densely packed (Fig. 5). Analysis of the mineral composition of fetal parietal bones confirmed that the major elements are calcium and phosphorus and that their content increases with age (Figs. 6 and 7). Fluorine content differed widely and was unrelated to age (Fig. 9, Tab. 1). The importance of fluorine in the process of mineralization of parietal bones requires further study. Magnesium content increased with age (Fig. 8) and did not correlate with calcium or phosphorus. Apparently, these elements play different roles in the process of mineralization. The content of zinc and iron, two labile components of bone, decreased with age (Figs. 10 and 11). Some of these changes could have taken place during storage of the bones.