The Properties and Functions of Glial Cell Types of the Hypothalamic Median Eminence

Abstract

The median eminence (ME) is part of the neuroendocrine system (NES) that functions as a crucial interface between the hypothalamus and pituitary gland. The ME contains many non-neuronal cell types, including oligodendrocytes, oligodendrocyte precursor cells (OPCs), tanycytes, astrocytes, pericytes, microglia and other immune cells, which may be involved in the regulation of NES function. For example, in mice, ablation of tanycytes (a special class of ependymal glia with stem cell-like functions) results in weight gain, feeding, insulin insensitivity and increased visceral adipose, consistent with the demonstrated ability of these cells to sense and transport both glucose and leptin, and to differentiate into neurons that control feeding and metabolism in the hypothalamus. To give a further example, OPCs in the ME of mice have been shown to rapidly respond to dietary signals, in turn controlling composition of the extracellular matrix in the ME, derived from oligodendrocyte-lineage cells, which may contribute to the previously described role of these cells in actively maintaining leptin-receptor-expressing dendrites in the ME. In this review, we explore and discuss recent advances such as these, that have developed our understanding of how the various cell types of the ME contribute to its function in the NES as the interface between the hypothalamus and pituitary gland. We also highlight avenues of future research which promise to uncover additional functions of the ME and the glia, stem and progenitor cells it contains.

Keywords: NG2 glia; astrocytes; hypothalamus; median eminence (ME); microglia; oligodendrocyte precursor cells (OPCs); pituitary gland; tanycytes.

Figure 1

Location, structure and cell types of the murine median eminence (ME). Locations of the hypothalamus (red) and pituitary (blue) are outlined, in the ventral (A), sagittal (B) and coronal view (C). Red-boxed area on the left represents the red-highlighted area in the coronal plane (C), and shows the structure of the ME and its constituent cell types. OL, oligodendrocyte; OPC, oligodendrocyte precursor cell. Neurohormone-producing neurons project from the hypothalamus and abrogate in the ventral portion of the ME which contains fenestrated capillaries. Secreted hormones are then carried through the portal vasculature to the anterior pituitary gland. The 3^rd ventricle at the ME is lined with tanycytes. β1-tanycytes project laterally and are thought to isolate the ME milieu from the rest of the hypothalamus. β2-tanycytes project ventrally and junctions between their cell bodies prevent diffusion of substances from the ME to the CSF and vice versa. Myelinating oligodendrocytes are predominantly found in the dorsal section of the ME, which contains axons that project to the posterior pituitary. OPCs, microglia, and astrocytes are found throughout the ME body.