Circumventricular Organs - Secretory Organs - Median Eminence

The median eminence (ME) is located in the inferior portion of the hypothalamus and is ventral to the third ventricle. While some publications do not list the ME as a CVO, when it is considered to be a circumventricular organ, it is classified as a secretory organ. The median eminence is rich in fenestrated capillaries, allowing for the passage of proteins and neurohormones. More specifically, the median eminence allows for the transport of neurohormones between the CSF and the peripheral blood supply. The major cell type that makes up the median eminence are specialized ependymal cells known as tanycytes. These contribute to the organ's ability to selectively allow macromolecules to pass from the central to the peripheral neuroendocrine systems.

Tanycytes line the floor of the third ventricle and can be characterized by a singular long projection that delves deep inside the hypothalamus. Tanycytes have been evolutionarily linked to radial glial cells of the central nervous system. The tanycytes of the median eminence are often found along the fenestrated peripheral capillaries. They are tightly packed on the capillaries, forming a seal between the third ventricle and the median eminence. This seal can be attributed to the tight junctions observed between tanycytes and functions to restrict the travel of molecules between the median eminence and the third ventricle. The median eminence is also closely linked to the transport of GnRH between the median eminence and the anterior pituitary. Neuronal projections of GnRH neurons actually end at the median eminence, allowing for its release into the portal blood system.