Tanycytes and Their Pivotal Role in Seasonal Physiological Adaptations

Tanycytes are glial cells whose cell soma are embedded in the ependymal layer surrounding the ventral region of the third ventricle in the hypothalamus. They send projections into the surrounding hypothalamus, with processes terminating in the neuropil of the hypothalamus, or extended to contact portal blood vessels in the median eminence or the surrounding pars tuberalis. Photoperiodic regulation of gene expression occurs in tanycytes through local signals emanating in the pars tuberalis. Of considerable importance across vertebrate taxa is photoperiodic regulation of type II (Dio2) and type III (Dio3) deiodinase enzyme gene expression in tanycytes as these regulate local thyroid hormone availability. This chapter explores the evidence for photoperiodic regulation of thyroid hormone and retinoic acid signalling by tanycytes, and considers this in relation to their other key functions, including their potential as a stem cell niche in the adult brain, their role as part of the blood brain barrier in transporting nutrients and hormones into the brain, and their role in regulating neuroendocrine secretion in the median eminence by virtue of their anatomical proximity to neuronal terminals.