Fetal development of the human trapezius and sternocleidomastoid muscles

Human trapezius muscle development during the early fetal period

This study aimed to observe human trapezius muscle (TpzM) development during early fetal period and apply diffusion tensor imaging (DTI) analysis to describe the muscle architecture that leads to physiological functions. Human embryonic and early fetal specimens were selected for this study. TpzM was first detected at Carnegie stage 20. The position of the TpzM changed with the formation of the scapula, clavicle, and vertebrae, which are its insertions and origins. DTI revealed the fiber orientation from each vertebral level to dissect each muscle. Fiber orientation in the ventral view gradually changed from the cervical to thoracic vertebrae, except for the middle part at which the insertions changed, which was almost similar in all early fetal specimens. The TpzM volume increased from C1 to C7 in the upper part, reached local maxima at C6 and C7 in the middle, and then decreased. These muscles can be categorized into three parts according to their insertions and presented with the features of each part. The fiber orientation and distribution of the three parts at the vertebral level were almost constant during the early fetal period. The border between the upper and middle parts was mainly located around the C6 and C7 vertebral levels, whereas the middle and lower parts were between the Th1 and Th2 vertebral levels. A three-dimensional change in the fiber orientation in the upper part of the TpzM according to the vertebral level was noticeable. Our data will help to elucidate the developmental processes of TpzM.

Accessory Head of Sternocleidomastoid Muscle in Indian Cadavers: A Report of Three Cases

During dissection sessions for undergraduate students, the unilateral accessory clavicular head of the sternocleidomastoid muscle was observed in three cadavers. These accessory heads extended from the middle third of the clavicle and joined the sternocleidomastoid muscle in the middle third. The variations in the sternocleidomastoid muscle may be attributed to abnormal mesodermal splitting or fusion failure during the development of the post-sixth branchial arch. Anomalies of the sternocleidomastoid may be misdiagnosed as cervical dystonia, fibromatosis colli, or muscular spasm. In rare cases, an accessory head could result in torticollis in adults. These anomalies warrant particular attention during interventional procedures conducted by anesthesiologists. The internal jugular vein is accessed at the lesser supraclavicular fossa for cannulation during central venous access and temporary hemodialysis. Variations in its anatomy can pose challenges during these procedures. Moreover, the clavicular head may be utilized for muscle flaps in the upper neck and occipital regions.