[Comparative study of masticatory muscles in the suncus and the mouse]

The external forms of head of the adult suncus and mouse resemble each other, but those of their newborns differ. In the newborn suncus, the upper view of external form of the head is narrow in width and long antero-posteriorly as is the cranial bone, but the adult form is almost triangular in shape based on the posterior portion of the head. In contrast, the external forms of the newborn and the adult are similar in the mouse. The postnatal changes of external form seem to be related to the development of the masticatory muscles in the suncus. Therefore, the present report comparatively studied the development of the masticatory muscles in suncus and mouse. To examine the developmental change of the volume of muscles on two animals, serial sections from fetus through adult were prepared by a commonly used staining method. Muscle volume was determined by the number of voxcels obtained from an image processor. Endplates stained by AchE staining and the course of muscle fibers of the masseter and temporal muscle (especially the MT1-temporal muscle in suncus) were also studied in the two animals. Ten measuring points on each craniofacial bone related with muscle growth were selected and the lengths between each point were measured to confirm the development of the musculoskeletal system in suncus. In suncus, the suncus masticatory muscles, both MT1-temporal and masseter muscle, were shown to have a multipinnate structure. This type of structure increases the physiological cross-sectional area to increase the masticatory force. The direction of these muscles, as a whole, run anteroposteriorly in this animal. Comparing the growth patterns between newborn and adult, the volumes of temporal and masseter muscle increase in the suncus more than those of in the mouse. The diameters of the temporal and masseter muscle fibers increase greatly after birth in the suncus in comparison with those of the mouse, though the fetus and newborn of both animals have approximately the same diameters. The differences in external forms of the head between newborn and adult in the suncus in comparison with those in the mouse may be caused by the greater increase in the volumes of temporal and masseter muscle, resulting in a remarkable increase in the forces of muscular contractions in the suncus. Postnatal dry weight of each masticatory muscle was measured in the suncus. The weights of the MT1-temporal and masseter muscle increased more than those of other muscles, especially from 14 days through 28 days. The results obtained here thus may be closely correlated to the growth of the diameters of muscle fibers. The whole parietal bone was not covered with the MT1-temporal muscle at 7 days after birth, but was covered at 14 days. Sagittal and nuchal crests were not observed at the former stage but was clearly evident at the latter in the suncus. In the suncus, the length and width of the MT1-temporal muscle became larger in the second week after birth than in the later weeks. In the suncus, during from 7 days through 14 days after birth the growth rate of the portion between Etf and Dwp was enhanced compared to that of other portions. In this term the length of the MT1-temporal muscle also grew noticeably. The distance between Etf and Dwp corresponds to the length of MT1-temporal muscle at the ventral portion. The growth between Etf and Dwp precedes that of muscle volume in the suncus. In conclusion, the developmental change of the cranial bone between Etf and Dwp precedes that of the masticatory muscles in the suncus. The formation of the well-developed sagittal and nuchal crests and the expansion of muscle attachment may well have led to the increase of muscle volume. As a result, the difference of external forms of head between newborn and adult in the suncus has been shown clearly to be due to the developmental changes of the volume of each masticatory muscle from 14 days through adult after birth, while such a difference is not seen in mouse.