The vertebrate diencephalic MCH system: A versatile neuronal population in an evolving brain

Neurons synthesizing melanin-concentrating hormone (MCH) are described in the posterior hypothalamus of all vertebrates investigated so far. However, their anatomy is very different according to species: they are small and periventricular in lampreys, cartilaginous fishes or anurans, large and neuroendocrine in bony fishes, or distributed over large regions of the lateral hypothalamus in many mammals. An analysis of their comparative anatomy alongside recent data about the development of the forebrain, suggests that although very different, MCH neurons of the caudal hypothalamus are homologous. We further hypothesize that their divergent anatomy is linked to divergence in the forebrain – in particular telencephalic evolution.

► MCH neurons are observed in the posterior hypothalamus of all vertebrates.
► The anatomical organization of MCH systems is clade/species specific.
► The differentiation of the MCH phenotype is SHH dependent in the embryo.
► SHH is a key player of forebrain morphogenesis.
► Divergent anatomy of the MCH systems corresponds to divergent prosencephalic evolutionary patterns.