Fig. 1From: Thyroid and endostyle development in cyclostomes provides new insights into the evolutionary history of vertebratesTransition of the endostyle to the thyroid in lampreys and classical evolutionary hypothesis of the vertebrate thyroid gland. A Larval lampreys (lateral view) possess an exocrine endostyle (en), which transforms into thyroid follicles (thy) in the adult (ventral view). p, pharynx; ht, heart; or, orifice; bb, branchial basket; va, ventral aorta; hbm, hypobranchial muscle; tm, tongue muscle. Bars, 100 μm. B Classical evolutionary hypothesis of the relationship between the endostyle and the thyroid based on the previous studies. Although an endostyle-like thyroid primordium was observed in developing hagfish (Eptatretus stoutii) embryos in 1906, it has not been verified ever since [15]. The primitive endostyle has been retained only in modern lampreys but was independently lost in the gnathostome and possibly in hagfish.Back to article page