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 hypothalamic−pituitary system .................................................... 228 Background ................................................................................................ 228 Glycoprotein hormones (GPHs) .............................................................. 232 Identication of hagsh GTH .................................................................. 234 Feedback regulation of hagsh GTH functions by sex steroid hormones .................................................................................................... 235 Gonads and gonadal sex steroids ................................................................ 238 Gonads and germ cells ............................................................................. 238 Plasma levels of sex steroid hormones in the hagsh gonads ............ 238 Expression of steroidogenic enzymes in the hagsh gonads ............. 240 Localization of steroidogenic cells in the hagsh gonads ................... 241 Functional corpus luteum in the hagsh ............................................... 243 Hypothalamic factors regulating the gonadotropic function of hagsh ............................................................................................................244