Fertility in mammals is controlled by hypothalamic neurons that secrete gonadotropin-releasing hormone (GnRH). These neurons differentiate in the olfactory placodes during embryogenesis and migrate from the nose to the hypothalamus before birth. Information regarding this process in humans is sparse. Here, we adapted new tissue-clearing and whole-mount immunohistochemical techniques to entire human embryos/fetuses to meticulously study this system during the first trimester of gestation in the largest series of human fetuses examined to date. Combining these cutting-edge techniques with conventional immunohistochemistry, we provide the first chronological and quantitative analysis of GnRH neuron origins, differentiation and migration, as well as a 3D atlas of their distribution in the fetal brain. We reveal not only that the number of GnRH-immunoreactive neurons in humans is significantly higher than previously thought, but that GnRH cells migrate into several extrahypothalamic brain regions in addition to the hypothalamus. Their presence in these areas raises the possibility that GnRH has non-reproductive roles, creating new avenues for research on GnRH functions in cognitive, behavioral and physiological processes.
Author contributions
F. Casoni was involved in the experimental design, research and preparation of the manuscript and figures. S.A.M. performed 3DISCO experiments and was involved in the preparation of the figures. M.B. performed the 3D imaging with LSM. F.L. performed the 3D reconstruction analysis. C.A. performed the in situ hybridization experiments. F.Collier, E.H., S.R., V.P. and A.C. were involved in the interpretation of results and preparation of the manuscript. P.G. designed the study and wrote the paper.
Funding
This work was supported by the Institut National de la Santé et de la Recherche Médicale (INSERM), France [grant number U1172]; Agence Nationale de la Recherche (ANR), France [ANR-14-CE12-0015-01 RoSes and GnRH to P.G.]; and the Fondation pour la Recherche Médicale [DEQ20130326524 to V.P. and DEQ20120323700 to A.C.].
Data availability
Movies are available at the Dryad digital repository (Casoni et al., 2016): http://dx.doi.org/10.5061/dryad.7v928.
