human mandible: a detailed study on the prenatal and postnatal developmental changes in the sites of articulations between its two halves

ABSTRACT

Thirty-two human mandibles, aging from nine weeks in utero [I.U.] till one year postnatally [P.N.] have been used in the present study. For gross anatomical structure, axial and coronal scan planes were taken, and a three- dimensional computerized tomography [C.T.] reconstruction was carried out. The bodies of the mandibles were separated from the two rami and fixed in 10% formalin. The specimens were processed for light microscopic study. At the ninth week I.U., the symphysis menti was only formed of mesenchymal tissue bounded on either side by labial bony trabeculae, Meckells cartilage and oral bony trabeculae. Rostrally, the bilateral rods of Meckells cartilage approached each other but were separated by a rim of mesenchymal tissue. The three-dimensional C.T. scan revealed a triangular defect between the two hemimandibles. By the twelfth week LU., two secondary cartilaginous structures, completely separated from MeckelIs cartilage, were observed in the caudal portion of the midline mesenchyme. By the twenty- eighth week I.U., signs of endochondral ossification appeared in the secondary cartilages and ended by the formation of a mental ossicle at the fortieth week I.U. The mental ossicle appeared conical in shape, and showed consistent gradual growth reaching its maximum by the age of first month P.N. Finally, complete fusion of the mental ossicle with the hemimandibles had occurred by the age of five months P.N., while the hemimandibles were still separated by a rim of mesenchymal tissue rostral to the mental ossicle. Fusion of the hemimandibles in that region proceeded from an inward to an outward direction. Thus, by the end of the first year P.N., the mandible became a single bone. The study revealed that the single mental ossicle is not an integral part of the symphyseal region, but is rather a contributor in the construction of the symphysis menti. Moreover, the study showed that the symphysis menti is not simply a midline one as thought long ago, and confirmed the existence of two different sites of articulations between the hemimandibles. The first one was located in the midline, between the rostral portions of the hemimandibles and simulated a fibrous joint. The second one was observed to lie on either side of the midline, between the mental ossicle and the caudal portion of each hemimandible, and simulated in structure a primary cartilaginous joint. Both articulations ended by synostosis, by the end of the first year P.N