Accessory thoracic muscles in human foetuses

Background: Typically, the anterior thoracic wall musculature is composed of the pectoralis major and pectoralis minor. Embryologically, these 2 muscles originate from a common pectoral muscle mass; therefore, disruption of the normal development and differentiation could give rise to an aberrant or accessory muscle. The main aim of this study is to demonstrate and classify the accessory muscles of the pectoralis region in human foetuses.

Material and methods: Fifty spontaneously aborted human foetuses (25 male and 25 female, 100 sides) aged 18-38 weeks of gestation at death, and fixed in 10% formalin solution were examined. Following parental approval, the foetuses were donated to the Medical University anatomy program. The pectoralis major and minor muscle's morphology, the possible occurrence of accessory muscles of the pectoral region and its morphology, their origins, and insertions, as well as the morphometric details, were assessed.

Results: The pectoralis major and minor were bilaterally found in all foetuses (100 cases). The accessory muscles of pectoral region were found in 16 cases (16%), and 4 types were differentiated. The pectoralis quartus muscle was the most common type of accessory muscle found in this study and occurred in 8 cases. The axillary arch muscle was observed in 3 cases. The chondrocoracoideus muscle was observed in 3 cases. The sternalis muscle occurred in 2 cases, and one of them was bifurcated.

Conclusions: The thoracic region is characterised by a large number of morphological variations, which are observed not only in the adult population, but also among human foetuses. The pectoralis quartus was the most frequent variation in this study. Accessory structures like the sternalis muscle, chondrocoracoideus muscle, pectoralis quartus muscle, or axillary arch muscle may have clinical implications, and knowledge about them is very useful for clinicians, especially plastic surgeons, thoracic surgeons, and orthopaedic surgeons.