Dynamic expression of thyrotropin-releasing hormone in the mouse definitive endoderm.

Thyrotropin-releasing hormone (TRH) is a well-characterized regulator of the hypothalamic-pituitary-thyroid endocrine axis. Here, we describe the expression of Trh during early embryonic development in the mouse. We find Trh to be highly expressed during postimplantation stages in the mouse embryo, with expression first observed in the epiblast at embryonic day (E) 6.5. During gastrulation, Trh is expressed in the newly formed definitive endoderm cells, and at embryonic day (E) 7.75, marks the entire definitive endoderm. Subsequently, Trh mRNA levels rapidly decrease such that, by E9.0, expression in the definitive endoderm is no longer detected, after which neural expression predominates. Thus, Trh is expressed dynamically and specifically in the developing mouse definitive endoderm from E7.0 to E8.5. Trh is unique among definitive endoderm markers as it transiently marks the entire definitive endoderm population and is not expressed in the extraembryonic endoderm. Trh will be a valuable tool to study definitive endoderm formation in the mouse embryo.