Hypothalamic−pituitary−gonadal endocrine system in the hagfish

The hypothalamic−pituitary system, which is specic to vertebrates, is considered to be an evolutionary innovation and seminal event that emerged prior to or during the differentiation of the ancestral jawless vertebrates (agnathans) (Sower et al., 2009). Such an evolutionary innovation is one of the key elements, leading to physiological divergence, including reproduction, growth, metabolism, stress, and osmoregulation in subsequent Hagsh …