Ezure H
Department of Anatomy, Nippon Dental University, Tokyo, Japan.
Ann Anat. 1996 Jan;178(1):15-23. doi: 10.1016/S0940-9602(96)80004-5.
The development of the motor endplate (MEP) and the structure of the masseter muscle in the human fetus were examined by light and electron microscopy. At 12 weeks of gestation, the masseter muscle was composed mostly of irregularly-arranged myotubes. The number of muscle fibers increased while that of the myotubes decreases during development. After 28 weeks of gestation, the masseter muscle was composed only of muscle fibers, and these fibers rapidly increased in size in comparison with those prior to 28 weeks of gestation. The MEP appeared during the first 12 weeks of gestation, and were of an undeveloped simple type, with only one axon branch. After 20 weeks of gestation, MEPs were classified as complex or simple types in terms of the branching of the axons. The complex type of MEP was found in muscle fibers of large diameter, while the simple type was found in muscle fibers of small diameter. Schwann-like cells appeared at the surface of the MEP. After 28 weeks of gestation, only the complex type was found in the masseter muscle. These results suggest that, the development of the MEP is closely related to the development of muscle, and that the timing of the development of the masticatory muscles differs from that of other skeletal muscles, such as those in the trunk and limb.
通过光学显微镜和电子显微镜检查了人类胎儿咬肌运动终板（MEP）的发育情况以及咬肌的结构。妊娠12周时，咬肌主要由排列不规则的肌管组成。在发育过程中，肌纤维数量增加，而肌管数量减少。妊娠28周后，咬肌仅由肌纤维组成，与妊娠28周前相比，这些纤维迅速增大。MEP在妊娠的前12周出现，呈未发育的简单类型，只有一个轴突分支。妊娠20周后，根据轴突分支情况，MEP分为复杂型或简单型。复杂型MEP见于直径较大的肌纤维，而简单型见于直径较小的肌纤维。雪旺样细胞出现在MEP表面。妊娠28周后，咬肌中仅发现复杂型MEP。这些结果表明，MEP的发育与肌肉发育密切相关，咀嚼肌的发育时间与躯干和四肢等其他骨骼肌不同。
