Septum transversum is part of which germ layer?

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

Answer: (B)

Understanding where the diaphragm’s central tendon comes from hinges on germ layer origins. The septum transversum arises from mesoderm, the middle embryonic layer that forms muscles, bones, connective tissue, and serous membranes. This structure grows from the ventral body wall and gives rise to the central tendon of the diaphragm, helping separate the thoracic and abdominal cavities. Endoderm forms gut lining and associated organs; ectoderm forms skin and the nervous system; neural crest cells are a specialized ectoderm-derived population with other derivatives—not the septum transversum. Therefore, the septum transversum is mesodermal in origin.