Emeritus Professor of Tokyo Medical and Dental University, Tokyo, Japan.
Department of Anatomy, Jeonbuk National University Medical School, Geunji-ro 20, Deokjin-gu, Jeonju, Jeonbuk, 54907, Republic of Korea.
Surg Radiol Anat. 2021 Sep;43(9):1503-1517. doi: 10.1007/s00276-021-02759-w. Epub 2021 May 31.
The longissimus (LO) and iliocostalis (IC) of adults consist of myofibers extending from the superolateral to the inferomedial side of the back and, because of the same course, they are fused in the thoracolumbar region. The LO also has a medial attachment to the long myofibers of the transversospinalis (TS) showing a course from the superomedial to the inferolateral side. However, there is apparently no information regarding when and how these similar longitudinal muscles differentiate from a cluster of dorsomedial myotome cells.
We examined sagittal and horizontal sections of the trunks of 39 human embryos and fetuses (18-330 mm crown-rump length).
At 6-7 weeks gestational age (GA), the surface aponeurosis appeared prior to and independent of the thoracolumbar fascia. At 6-9 weeks GA, the LO myofibers had a postero-inferior course, from the transverse process to the initial aponeurosis, whereas the TS myofibers had a postero-superior course, from a lateral extension of the intertransverse ligament to the aponeurosis. However, the IC consisted of supracostal longitudinal myofibers and was distant from the LO until 12 weeks GA. Because of the lack of ligamentous attachments and ribs, myofibers of the TS, LO, and IC took a similar inferior course in the lumbar region. When the early TS was represented by the transverso-aponeurotic muscle, consequently, the LO corresponded to the aponeuro-transversal muscle and was independent from the IC.
The classical model of TS and LO development does not recognize the essential role of the aponeurosis identified here.
成人的最长肌（LO）和髂肋肌（IC）由从背部的超外侧延伸到下内侧的肌纤维组成，由于同样的原因，它们在胸腰段融合。LO 还与横突间棘肌（TS）的长肌纤维有内侧附着，其走向从超内侧到下外侧。然而，显然没有关于这些相似的纵向肌肉何时以及如何从一群背内侧肌节细胞分化出来的信息。
我们检查了 39 个人类胚胎和胎儿（18-330 毫米头臀长）的躯干矢状和水平切片。
在 6-7 周妊娠龄（GA）时，表面腱膜出现于胸腰筋膜之前且独立于其。在 6-9 周 GA 时，LO 肌纤维呈后下走向，从横突到最初的腱膜，而 TS 肌纤维呈后上走向，从横突间韧带的外侧延伸到腱膜。然而，IC 由肋上的纵向肌纤维组成，并且在 12 周 GA 之前与 LO 分离。由于缺乏韧带附着和肋骨，TS、LO 和 IC 的肌纤维在腰椎区域采取相似的下走行。当早期的 TS 由横突腱膜肌表示时，因此，LO 对应于腱膜横突肌，并且与 IC 无关。
TS 和 LO 发育的经典模型没有认识到这里确定的腱膜的重要作用。
