Landmark discoveries in elucidating the origins of the hypothalamic-pituitary system from the perspective of a basal vertebrate, sea lamprey

•This review highlights our major discoveries and findings of the major components of the lamprey hypothalamic-pituitary axis.•The HP system is specific to vertebrates that emerged prior to or during the differentiation of the ancestral jawless vertebrates, the agnathans.•Lampreys have three hypothalamic gonadotropin-releasing hormones (GnRH) and three GnRH receptors regulating the pituitary-gonadal axis.•We propose that lampreys only have two pituitary glycoprotein hormones, lamprey GpH and thyrostimulin that in some unknown ways differentially regulate two glycoprotein hormone receptors (I and II)•Our working hypothesis suggests the existence of a primitive, overlapping yet functional HPG and HPT endocrine system in lampreys.