What is a head chord?

In zoology, a head chord or acron is the anterior-most part of the notochord, a defining characteristic of chordates. It is made of mesodermal cells and extends forward from the anterior end of the notochord proper, which is derived from the endoderm. The dorsal part of the acron is invested by the prechordal plate and the rostral part of the neural tube, while laterally, it is invested by the parachordal cartilages.

During embryonic development, the head chord is involved in the formation of head structures, including the pituitary gland and the adenohypophysis. It is also thought to play a role in the development of the heart and the pharyngeal arches.

The head chord is present in all chordates, including vertebrates and tunicates, but it is reduced or absent in some groups, such as lampreys and hagfish.