Transient morphologies at muscle attachments to the future medial margin of the developing scapula: a histological study using human embryos and early fetuses.

BACKGROUND: The medial margin of the scapula and its related muscles [the serratus anterior (SA), rhomboideus, levator scapulae (LS)] have been among the major interests of embryologists because of their common somitic origin. A systematic and synchronous mechanism appeared to operate during the formation of muscle attachments. MATERIALS AND METHODS: Sagittal and horizontal histological sections from 24 human embryos and fetuses at approximately 5-7 weeks of gestation were examined. RESULTS: Instead of the upper half of the medial margin, the mesenchymal or cartilaginous scapula had a superoposterior bar that received the SA, LS and rhomboideus. The LS extended almost horizontally because of the high position of the scapula and was adjacent to the rhomboideus. The SA surrounded the superoposterior cartilage bar of the scapula in 10 of the 24 specimens examined. The extended attachment of the SA interfered with the scapular insertions of the rhomboideus and LS. Irrespective of whether the SA extended or not, in 15 of 24 specimens, the rhomboideus extended beyond the superoposterior bar toward the inferior angle of the scapula. This inferiorly extended rhomboideus was used by the SA as an attachment site. The latissimus dorsi appeared to attach to the scapula, but it did not extend posteriorly beyond the SA. CONCLUSIONS: Interference with the insertions of other muscles (= competition), as well as provision of a tentative attachment (= cooperation), appeared to occur among muscles sharing the narrow medial part of the scapula.Redundancy in attachment morphology might be resolved by later posterolateral migration of the scapula and growth of the supraspinatus fossa.