DIVISIONS OF THE PITUITARY GLAND AND RELATIONSHIP TO THE HYPOTHALAMUS
The pituitary gland (hypophysis) is composed of the neurohypophysis (posterior pituitary lobe) and adenohypophysis (anterior pituitary lobe). The neurohypophysis consists of three parts: the median eminence of the tuber cinereum, infundibular stem, and infundibular process (neural lobe). The adenohypophysis is likewise divided into three parts: the pars tuberalis, pars intermedia, and pars distalis (glandularis). The infundibular stem, together with portions of the adenohypophysis that form a sheath around it, is designated as the hypophysial (pituitary) stalk. The extension of neurohypophysial tissue up the stalk and into the median eminence of the tuber cinereum constitutes approximately 15% of the neurohypophysis. A low stalk section may leave enough of the gland still in contact with its higher connections in the paraventricular and supraoptic nuclei to prevent the onset of diabetes insipidus. Atrophy and disappearance of cell bodies in the supraoptic and paraventricular nuclei follow damage to their axons in the supraopticohypophysial tract. If the tract is cut at the level of the diaphragma sellae, only 70% of these cells are affected; if the tract is severed above the median eminence, about 85% of the cells will atrophy. Thus, approximately 15% of the axons terminate between these levels.
The main nerve supply, both functionally and anatomically, of the neurohypophysis is the hypothalamo-hypophysial tract in the pituitary stalk. It consists of two main parts: the supraopticohypophysial tract, running in the anterior or ventral wall of the stalk, and the tuberohypophysial tract in the posterior, or dorsal, wall of the stalk. The tuberohypophysial tract originates in the central and posterior parts of the hypothalamus from the paraventricular nucleus and from scattered cells and nuclei in the tuberal region and mamillary bodies. The supraopticohypophysial tract arises from the supraoptic and paraventricular nuclei. On entering the median eminence, it occupies a very superﬁcial position, where it is liable to be affected by basal infections of the brain and granulomatous inﬂammatory processes. The tuberohypophysial tract in the dorsal region of the median eminence is smaller and consists of ﬁner ﬁbers. In the neural stalk, all the ﬁbers congre- gate into a dense bundle lying in a central position, leaving a peripheral zone in contact with the pars tuberalis, which is relatively free of nerve elements. The hypothalamohypophysial tract terminates mainly in the neurohypophysis.
The hypothalamus has ill-deﬁned boundaries. Anteroinferiorly, it is limited by the optic chiasm and optic tracts; passing posteriorly, it is bounded by the posterior perforated substance and the cerebral peduncles. On sagittal section, it can be seen to be separated from the thalamus by the hypothalamic sulcus on the wall of the third ventricle. Anteriorly, it merges with the pre- optic septal region, and posteriorly, it merges with the tegmental area of the midbrain. Its lateral relations are the subthalamus and the internal capsule.
A connective tissue trabecula separates the posterior and anterior lobes of the pituitary; it also extends out into the anterior pituitary lobe for a variable distance as a vascular bed for the large-lumened artery of the trabecula. The embryonic cleft, which marks the site of the Rathke pouch within the gland, may be contained, in part, in this trabecula. It is easier to see in newborns and tends to disappear in later life. Colloid-ﬁlled follicles in the adult gland mark the site of the pars inter- media at the junction between the pars distalis and the neurohypophysis. This boundary may be quite irregular because ﬁngerlike projections of adenohypophysial tissue are frequently found in the substance of the neurohypophysis.