MCH development enlightens a switch in the prosencephalic basic plan.

<p>The medial neurogenic column (mnc) characterized in the embryo by the longitudinal expression of Nkx2.2, extends from the anterior to posterior pole of the neural plate (a). Specific neuronal phenotypes will be produced at specific stages of the mnc; serotoninergic neurons (dark purple dots) in the presumptive hindbrain, and MCH phenotype (blue dots) in the presumptive prosencephalon. After neural tube closure (b), first MCH neuroblasts (blue) are effectively produced. They send their axons through the <i>tpoc</i> that run parallel to the Nkx2.2 expression domain (dark green arrow show the direction of the tpoc). At the same stage, serotoninergic neurons (purple) differentiate and send their axons in the medial longitudinal fasciculus (<i>mlf</i>). Later (c), as neurogenesis begins in the telencephalon, axons from late produced MCH neurons (red) extend toward this structure. At the same stage, mesencephalic dopaminergic neurons (DA - light green area and arrow) differentiate, and their projections gather in a dense bundle in the mfb with MCH axons. In the adult animal (d), the mfb is the main longitudinal tract of the ventral diencephalon, connecting the rostral brainstem with the telencephalon, and defining the anteroposterior axis of the adult forebrain. The thin black arrows point the presumptive (a, b) or the anatomical position (c, d) of the pituitary which is at the rostral pole of the neural plate, but ventral surface of the adult brain.