Pyramidalis muscle formation during human embryonic and early fetal periods.

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.