Histological investigation on the process of the fusion of the human mandibular symphysis

The process of the fusion of the mandibular symphysis was investigated both roentgenologically and histologically, using 34human fetuses from 12 to 49cm long.The mandibular symphysis was found roentgenologically as the radioparent shadow, which was conical in shape in fetuses from12to27cm long, band-like or ampullar in fetuses from over 30cm long and narrow-banded in one fetus of 49cm long.The mandibular symphysis histologically was divided into four layers: 1) connective tissue layer, 2) transitional layer, 3) osteopoietic layer, and4) bone trabecular layer.The connective tissue layer in fetuses from12to17cm long was broader in width but lower in height than in elder fetuses and became narrower and higher with increase in the body length.The transitional layer was composed mainly of the fibroblast-like cells.Such structure was not at all, or, at presence, very poorly developed in the immature fetuses as from 12 to 16cm long, but it became fairly apparent in the fetuses over17cm and quite distinct in the fetuses over 23cm.The osteopoietic layer was composed of the osteoblast, the hyaline homogenization and the deposition of the calcium in the bone matrix.As mentioned above, this layer also was indistinct in the immature fetuses as 12 to 16cm long, but became distinct in more mature fetuses as 20cm long.There were seen some differences in width and arrangement of the bone trabeculae, size of the osteocyte and calcium deposition in the matrix between the proximal and distal portion of the bone trabecular layer: the proximal portion showed an incomplete development and the distal portion a complete development.The bone trabecular layer in the fetuses from 12 to 16cm long showed completly, but in fetuses from 17to18cm long it was mixed completly and incompletly and in the fetuses over19cm long occupied mainly by incompletly.The cape-like bone trabeculae in the proximal portion were found in 14fetuses out of 34ones, predominantly from17to29cm long.The cartilaginous tissue was found in9relatively immature fetuses out of 34 ones examined, appearing mainly in the connective tissue layer.From the above mentioned data it is very likely that the fusion of the mandibular symphysis is due mainly to the collagenous ossification.