Fetal development and growth of the human neck axial musculature.

Neck epaxial muscles, which are differentiated for suspending the head, occupy a large space posterior to the cervical lordosis. Limited information exists regarding developmental process that determines the muscle fiber direction and bony attachment of neck epaxial muscles. We examined histological sections of 28 human fetuses aged approximately 7-18 weeks (crown-rump length, 20-150 mm). In place of the underdeveloped lordosis, the transverse process of cervical vertebrae was shifted anteriorly at the cervicothoracic junction. The semispinalis and longissimus were distinguished by the direction of muscle fibers connecting between the surface aponeurosis and transverse process. The semispinalis capitis and splenius capitis had a bulky anterior margin without bony attachments. The obliquus capitis inferior continued to both the rectus capitis posterior major and the semispinalis cervicis, but the obliquus capitis superior was consistently independent. Muscle attachments to the scapula were quite different from the final morphology: 1) the levator and rhomboidei usually extended inferiorly along the developing scapula beyond the inferior angle and 2) the splenius capitis or semispinalis cervicis rarely issued an aberrant bundle attaching to the scapula. The scaleni, rhomboidei, levator scapulae, iliocostalis and longissimus were arranged in parallel from the anteromedial to the posterolateral planes and together formed a thick oblique muscle bundle originating from the cervical transverse process and running toward the upper thoracic vertebra and ribcage. The transient oblique muscle bundle seen in early fetuses seemed to provide the so-called intermediate axial muscle between the epaxial-hypaxial muscles: a concept postulated in recent molecular neurology and embryology.