The term medial preoptic area refers to a subdivision of the preoptic area defined on the basis of Nissl stain. It is largely bounded medially by the preoptic periventricular nucleus and the anteroventral periventricular nucleus, laterally by the lateral preoptic area, dorsally by the anterior commissure and the thalamus, ventrally by the optic chiasm, rostrally by the nucleus of the diagonal band and caudally by the anterior hypothalamic area. It's internal structure varies considerably by species. In the human it is relatively undifferentiated ( Saper-1990 ). In the macaque ( Paxinos-2009a ), rat and mouse the area contains a well defined medial preoptic nucleus. In the rat ( Swanson-2004 ) and mouse ( Hof-2000; Dong-2004 ) the area contains four other nuclei as well: the anterodorsal preoptic nucleus, the anteroventral preoptic nucleus, the posterodorsal preoptic nucleus and the parastrial nucleus. The cell sparse portion of the area between nuclei is known as the medial preoptic area proper ( Swanson-2004 ).