Hypothalamic Organization

As its name implies, the hypothalamus is found below the thalamus in the diencephalon (Fig. 1). It is

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Figure 1 The disposition of the hypothalamus in relation to other regions of the central nervous system is illustrated in sagittal and ventral exposures of the human brain. In sagittal exposures of the brain the hypothalamus occupies a small region bounded by the anterior commissure (AC), optic chiasm (OC), and mammillary body. Ventral exposure of the brain reveals the three prominent landmarks that define the floor of the hypothalamus: the OC rostrally, the infundibular stalk (IS) arising from the tuber cinereum, and the paired spherical protuberances that constitute the MBs. III, third ventricle; IV, fourth ventricle.

Figure 1 The disposition of the hypothalamus in relation to other regions of the central nervous system is illustrated in sagittal and ventral exposures of the human brain. In sagittal exposures of the brain the hypothalamus occupies a small region bounded by the anterior commissure (AC), optic chiasm (OC), and mammillary body. Ventral exposure of the brain reveals the three prominent landmarks that define the floor of the hypothalamus: the OC rostrally, the infundibular stalk (IS) arising from the tuber cinereum, and the paired spherical protuberances that constitute the MBs. III, third ventricle; IV, fourth ventricle.

distinguished by distinct external landmarks on the ventral surface of the brain that define its full rostrocaudal extent. These include the optic chiasm rostrally, the intermediate tuber cinereum marked by the prominent infundibular stalk, and the caudally placed mammillary bodies. Classic literature used these external landmarks to define internal subdivisions of the hypothalamus. Thus, it is still common to find references to the chiasmatic, tuberal, and mam-millary subdivisions of hypothalamus. However, as our knowledge of the organization, connectivity, and function of hypothalamic cell groups has improved, the rationale for dividing the hypothalamus into three regions defined by external landmarks has become less compelling. Nevertheless, these landmarks remain useful designations for defining the general location of hypothalamic cell groups and are commonly found in the literature.

Coronal sections through the hypothalamus reveal prominent internal landmarks that have proven useful in defining the basic organization of hypothalamic cell and fiber systems. Among the most prominent is the third ventricle that separates much of the hypothalamus into identical halves. The fluid-filled reservoir is particularly prominent in the intermediate portion of the hypothalamus demarcated by the tuber cinereum. In coronal sections through this level, the lumen of the ventricle defines the dorsoventral extent of the hypothalamus. The floor of the third ventricle is formed by a thin bridge of tissue, commonly known as the median eminence, that is continuous with a stalk connecting the ventral hypothalamus to the pituitary gland. The median eminence is among the most important interfaces through which the hypothalamus exerts regulatory control over peripheral systems in that it is essential for regulation of hormone secretion from the pituitary gland. It is similar to another midline strip of tissue that contains the organum vasculosum of the lamina terminalis (OVLT) and that forms the rostral wall of the third ventricle in that it lacks a blood-brain barrier (BBB). The absence of the BBB at these sites provides the essential means for neurohumoral communication and will be considered in greater detail later.

The internal organization of the hypothalamus is characterized best in histological preparations of coronal sections. Classic Nissl preparations reveal three longitudinal zones that extend throughout the rostrocaudal extent of the hypothalamus. The location of each zone can be defined in relation to its proximity to the third ventricle and a prominent myelinated fiber bundle, the fornix, that enters the hypothalamus rostrally and then courses caudally to terminate in the mammillary bodies. The periventricular zone is a densely packed group of neurons immediately adjacent to the third ventricle. Occasionally, it contains well-demarcated cell groups, such as the suprachias-matic, arcuate, or paraventricular nuclei. However, in the majority of its extent it is composed of a thin, densely packed group of neurons immediately adjacent to the ependymal lining of the ventricle. The medial zone is found between the periventricular zone and a vertical plane passing through the fornix. Nuclear groups in this region (e.g., the ventromedial and dorsomedial nuclei) are among the most prominent and well-delineated cell groups in the hypothalamus. The lateral zone is found between the medial zone and the optic tract and internal capsule at the lateral extent of the diencephalon. Neurons in this region are more dispersed and do not form the distinct nuclear groups characteristic of the periventricular and medial zones. Nevertheless, immunohistochemical studies that have revealed distinct phenotypic parcellation of neurons in the lateral zone have contributed greatly to improving our understanding of the functional organization of this region.

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