The median eminence is a region at the base of the hypothalamus where neurosecretory terminals release regulatory hormones near the first capillary bed of the hypophyseal portal system. It is an interface between hypothalamic neurons and the vascular route to the anterior pituitary. A GnRH terminal here is a site of release; the neuron's soma need not occupy the same location.[ref]
Vascular endothelial cells promote acute plasticity in ependymoglial cells of the neuroendocrine brain by Sandrine De Seranno and others. Introduction and Abstract: median-eminence neurosecretory interface
Access to the capillaries also depends on neighboring glial cells, including tanycytes. In a 2004 study, female-rat tissue containing the median eminence was incubated for 30 minutes with or without added arginine, a substrate for nitric oxide production. Immunoelectron microscopy identified GnRH-containing terminals. In treated preparations, the investigators observed structural changes bringing terminals into direct contact with the space beside the capillaries. Complementary cell-culture experiments implicated signals from endothelium in glial remodeling.[ref]
Vascular endothelial cells promote acute plasticity in ependymoglial cells of the neuroendocrine brain by Sandrine De Seranno and others. Abstract; Figure7:500micromolar L-arginine,30min and immunogold localization
The comparison is illuminating because the route is more than a fixed pipe connecting two glands. The arrangement of neurons, glial cells and blood vessels can alter access to the release surface. A microscopic structural observation and a measurement of circulating hormone nevertheless remain different kinds of evidence: proximity alone does not quantify how much GnRH enters the blood.
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