NUCLEAR TOPOGRAPHY

Part of “CHAPTER 8 – MORPHOLOGY OF THE ENDOCRINE BRAIN, HYPOTHALAMUS, AND NEUROHYPOPHYSIS“

HYPOTHALAMIC ZONES

Because the hypothalamus is so rich in perikaryal groups and so poor in landmarks (e.g., myelinated fiber bundles), the prudent approach is to consider easily identified hypothalamic zones as geographic points with respect to the more than two dozen classically identified hypothalamic nuclei, as detailed by Nauta and Haymaker.1 Zones in both longitudinal and coronal planes are illustrated in Figure 8-2.

FIGURE 8-2. Diagram showing anatomic relationship of hypothalamic nuclei, by zones. Here, the hypothalamus is considered as a hexagon, bordered anteriorly by the preoptic area and posteriorly by the midbrain. The third ventricle (wide black line) forms its midline. Most hypothalamic nuclei can be found within three longitudinal zones, designated as midline, medial, and lateral in relation to the third ventricle and fornix. These zones can be further subdivided into three anteroposterior regions or levels (supraoptic, tuberal, and mammillary), allowing identification of hypothalamic nuclei by position, an approach that is much simpler than distinguishing the nuclei by the classic cytoarchitectonic method. The three levels, shown in the coronal plane in Figure 8-3, depict the essential hypothalamic nuclei.

Longitudinal Plane.

The longitudinal zones are based on the phylogenetically primitive organization of lower vertebrates, in which the hypothalamus is characterized by a relatively cell-rich medial zone. A somewhat acellular lateral zone is separated from the medial zone by a prominent fiber bundle, the fornix. The medial zone can be subdivided further into a midline zone that is immediately adjacent to the third ventricle and contains a relatively homogeneous nuclear mass called the periventricular stratum. The more laterally placed portions of the medial hypothalamic zone contain reasonably differentiated cell clusters, including the medial preoptic, anterior hypothalamic, ventromedial, dorsomedial, paraventricular, posterior, and premammillary nuclei.

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