Mandibular movements during elevation and fusion of palatal shelves evaluated from the course of Meckel's cartilage.

The purpose of the present study was to investigate whether the course of Meckel's cartilage could reveal the mandibular movements during the elevation and fusion of the soft tissue palatal shelves. Histological sections, cut serially in the horizontal plane from 64 human mandibles, 16-104 mm CRL, were analyzed. The course of the anterior and medial part of Meckel's cartilage changed markedly during the three palatal stages, i.e., before, during, and after palate formation. The medial part changed during these stages from a straight course through a curled, S-shaped course to a crochet-hook-shaped course. The anterior part of Meckel's cartilage developed from a separation in the symphysis menti region to a fusion and later to a separation again. It is suggested that these changes in the course of Meckel's cartilage are due to different muscle activities. It is supposed that during the palatal developmental stages the activity of the geniohyoid and genioglossus muscles caused the mandibular retraction and the widening of the angulation between the bilateral hemimandibles. The S-shape of Meckel's cartilage is a result of these movements. Later mandibular proclination and narrowing of the bilateral hemimandibles resulted in an anterior mechanical fusion of Meckel's cartilage due to the activity of the mylohyoid muscle. This stage is followed by a retraction and re-widening of the angulation between the bilateral bony components, which disrupts the fusion of Meckel's cartilage in the symphysis menti region. Thus, the course of Meckel's cartilage revealed the mandibular movements in the sagittal and transverse planes during palate formation.