Which germ layer forms the circulatory system and muscles?

• A. Neural crest
• B. Mesoderm
• C. Endoderm
• D. Ectoderm

Answer: (B)

Germ layer origins determine which tissues arise from each layer during early development. The circulatory system and muscles come from the mesoderm, one of the three primary germ layers. The mesoderm differentiates into several tissue groups: somites give rise to skeletal muscle and dermis; the lateral plate and other mesodermal regions form the heart and blood vessels, as well as other components of the circulatory system. In contrast, the endoderm becomes the lining of the gut and its associated organs, and the ectoderm forms the skin and nervous system (with neural crest cells from ectoderm contributing to some structures but not the core muscle and circulatory tissues). So, the tissue origins for both muscles and the circulatory system are mesoderm.