part of part of brain regional part of brain principal anterior division of supraoptic nucleus principal anterior division of supraoptic nucleus One of two divisions of the supraoptic nucleus observed in mammals, formed by the ascension of the optic chiasm to split the nucleus in two. The principal anterior division consists of a dense cluster of large cells that stain darkly in Nissl preparations adjacent to the optic chiasm, extending as far rostrally to the organum vasculosum of the lamina terminalis and caudally into the posterior hypothalamus. UBERON:0024078 NLXANAT:20090310