Abstract：
The pyramidalis muscle (PM) is a paired small triangular muscle of the anterior abdominal wall; however, its physiological significance is unclear. Recent studies have failed to detect this muscle during embryonic period. Hence, the present study aimed to determine the time when PM is emerging and reveal its features using high-resolution magnetic resonance imaging. Fourteen embryos between Carnegie stage (CS)18 and CS23 and 59 fetuses (crown-rump length: 39.5-185.0 mm) were selected for this study. The PM was first detected in one of the three samples at CS20. It was detected in five of the seven samples (71.4%) between CS21 and CS23. Forty-eight samples (81.4%) at early fetal period had PMs on both the right and left sides, and 3 (5.1%) had it only on the right side. Eight samples (13.6%) had no PMs. No side-differences or sexual dimorphisms were detected. The PM length was larger than the width in most samples, although the length/width ratio varied among the samples. The PM/rectus abdominis muscle length and PM/umbilicus-pubic symphysis length ratios were almost constant, irrespective of the crown-rump length. The PM was located ventrally inferior to the rectus abdominis and closer to the medial muscle groups of the lower limb than the rectus abdominis. The present study demonstrated that PM formation occurred in the late embryonic period, and that the frequency, side differences, sex dimorphism, and spatial position of the PM in the early fetal period were similar to those in adults.
摘要：
金字塔肌（PM）是前腹壁的一对小三角形肌肉；但是，其生理意义尚不清楚。最近的研究未能在胚胎时期检测到这种肌肉。因此，本研究旨在确定PM出现的时间，并使用高分辨率磁共振成像揭示其特征。本研究选择了卡内基阶段（CS）18和CS23之间的14个胚胎和59个胎儿（冠部长度：39.5-185.0mm）。首先在CS20在三个样品之一中检测到PM。在CS21和CS23之间的七个样品中的五个（71.4％）中检测到。四十八份样本(81.4%)在胎儿期早期有左右两侧的PMs,3名(5.1%)只在右侧。8个样本(13.6%)没有PMs。没有检测到侧差或性二态。大多数样品的PM长度大于宽度，尽管长宽比在样品之间变化。PM/腹直肌长度和PM/脐-耻骨联合长度比几乎恒定，无论皇冠-臀部的长度。PM位于腹直肌的腹侧下方，并且比腹直肌更靠近下肢的内侧肌群。本研究表明，PM的形成发生在胚胎晚期，频率，侧面差异,性别二态,胎儿早期PM的空间位置与成人相似。
