Morphogenesis of Human Scalene Muscles Between Weeks 6 and 13 of Development: Anatomical Aspects and...

The scalene muscles represent a crucial muscle group in cervical anatomy, with significant clinical and functional implications. Despite their importance, studies on their embryonic and fetal development remain scarce. This study examined the morphogenesis of the scalene muscles bilaterally in 33 developing human specimens [66 sides from 12 embryos (Weeks 6-8) and 21 fetuses (Weeks 9-13)] using serial histological sections and conventional light microscopy. The scalene blastema appeared during Week 6 of development, comprising myoblasts, mesenchymal cells, and collagen fibers, and was surrounded by fibroblast-like cells and collagen fibers. The ventral rami of spinal nerves forming the brachial plexus, along with the subclavian artery, divided this blastema into two components: a medial portion (scalenus anterior muscle primordium), pierced by the phrenic nerve, and a lateral portion (scalenus medius muscle primordium), pierced by the dorsal scapular nerve and the branches forming the long thoracic nerve. The scalene triangle, through which these neurovascular elements pass, formed between the scalenus anterior and medius primordia. The scalenus minimus muscle was identified in 7 of 66 sides (10.6%). Our findings suggest that the scalene blastema originates from both hypaxial myotomes and sclerotomes of the cervical somites. The developmental relationship between the scalene blastema and adjacent neurovascular structures may help explain anatomical variations in this region with clinical significance. Furthermore, the insertion of the scalenus anterior muscle into the parietal pleura via the suprapleural membrane primordium provides new insight into its functional role in respiratory mechanics.