Hes1 regulates formations of the hypophyseal pars tuberalis and the hypothalamus

The hypophyseal pars tuberalis surrounds the median eminence and infundibular stalk of the hypothalamus as thin layers of cells. The pars tuberalis expresses MT1 melatonin receptor and participates in mediating the photoperiodic secretion of pituitary hormones. Both the rostral tip of Rathke's pouch (pars tuberalis primordium) and the pars tuberalis expressed alpha GSU mRNA, and were immunoreactive for LH, chromogranin A, and TSH beta in mice. Hes genes control progenitor cell differentiation in many embryonic tissues and play a crucial role for neurulation in the central nervous system. We investigated the Hes1 function in outgrowth and differentiation of the pars tuberalis by using the markers for the pars tuberalis. In homozygous Hes1 null mutant embryos, the rostral tip was formed in the basal-ventral part of Rathke's pouch at embryonic day (E)11.5 as well as in wild-type embryos. In contrast to the wild-type, the rostral tip of null mutants could not extend rostrally with age; it remained in the low extremity of Rathke's pouch during E12.5-E13.5 and disappeared at E14.5, resulting in lack of the pars tuberalis. Development of the ventral diencephalon was impaired in the null mutants at early stages. Rathke's pouch, therefore, could not link with the nervous tissue and failed to receive inductive signals from the diencephalon. In a very few mutant mice in which the ventral diencephalon was partially sustained, some pars tuberalis cells were distributed around the hypoplastic infundibulum. Thus, Hes1 is required for development of the pars tuberalis and its growth is dependent on the ventral diencephalon.