Which organ is NOT derived from the endoderm?

The cardiovascular system is not derived from the endoderm. It is primarily formed from mesoderm, one of the three primary germ layers in embryonic development. The mesoderm gives rise to structures such as the heart and blood vessels.

In contrast, the thymus, pancreas, and thyroid gland originate from the endoderm. The endoderm is responsible for forming the inner lining of the gastrointestinal tract and associated organs, including glands. The thymus develops from the third pharyngeal pouch, the pancreas arises from the foregut endoderm, and the thyroid gland develops from the floor of the pharynx. Understanding these developmental origins helps clarify which organs are derived from each germ layer.