“Tanycytes, lining the ventral wall of the third ventricle, have long been speculated to regulate essential aspects of hypothalamic function in energy homeostasis. To directly test this, we genetically ablated tanycytes of the arcuate nucleus and median eminence and observed the resulting effects. We found that tanycyte ablation increases fat mass, decreases the use of fat as an energy source, and reduces insulin sensitivity in male mice, without significantly affecting body weight.  These effects are enhanced under thermoneutral conditions. While tanycyte ablation disrupts the CSF-hypothalamus barrier, it does not significantly affect the blood-hypothalamus barrier, hypothalamic responses to leptin, or regulation of the hypothalamic-pituitary axis. This study clarifies the physiological role of tanycytes, and demonstrates its essential role in metabolic homeostasis.”