Poorly Myelinated Hypothalamus
This includes the preoptic region in front of the optic chiasm, the tuber cinereum, the lateral field (A1) lying dorsolaterally to thetuber cinereum, and the dorsocaudal area (B2) above the mamillary bodies. The poorly myelinated hypothalamus is the part of the brain that is richest in peptidergic neurons. The different neuropeptides can be demon-strated in diffusely scattered cells (luliberin, cholecystokinin, thyoliberin), in per-iventricular cell aggregations (soma-tostatin), and in the nuclear areas (neu-ropeptides in various compositions). Numerous fiber connections within the hy-pothalamus and some of the long projection tracts are peptidergic.
The preoptic region (C3) extends from the anterior commissure (C4) to the optic chiasm (C5) as a parvocellular field surrounding the most rostral excavation of the third ven-tricle, the preoptic recess. The region con-tains a large number of peptidergic neurons (containing primarily enkephalin). Two prominent magnocellular nuclei lie adja-cent to it, namely, the supraoptic nucleus(AC6) and the paraventricular nucleus (AC7). The supraoptic nucleus borders on the optic tract (A8). Its neurons contain cholecys-tokinin and dynorphin. The paraventricular nucleus lies close to the wall of the third ventricle, separated from the ependymal(the simple cell layer lining the walls of the ventricular system, p. 284) only by a layer of glial fibers, and extends as a narrow band obliquely upward into the region of the zona incerta. Its peptidergic neurons have been shown to contain corticoliberin, neu-rotensin, cholecystokinin, as well as other neuropeptides.
The principal nucleus of the tuber cinereum (A9) is the ventromedial nucleus (AC10), a round body occupying most of the tuber cinereum. It contains medium-sized neu-rons, among them many peptidergic (con-taining primarily neurotensin), and is sur-rounded by a delicate fiber capsule formed by the pallidohypothalamic fasciculus. The dorsomedial nucleus (AC11) is less distinctand contains small neurons. At the base of the infundibulum lies the parvocellular in-fundibular nucleus (AC12). Its cells encirclethe infundibular recess and reach directly into the ependyma. The neurons of this nu-cleus contain primarily endorphin and ACTH (adrenocorticotropic hormone).