Fig 5.- Type 2 deiodinase mRNA expression in the rat brain. A: euthyroid rat. D2 mRNA is distributed all over the brain with high concentration in walls of the lower third of the 3rd ventricle. In this region, D2 is expressed in specialized glial cells called tanycytes. In the rest of the brain D2 is expressed in the astrocytes (the inset shows D2 mRNA hybridization silver grains over a stained astrocyte). B: D2 expression in a hypothyroid rat. An increased expression is observed in most parts of the brain, but especially in the barrel cortex and some structures such as the ventromedial nucleus, serving as relay stations for somatosensory pathways. C and D: Dark field photomicropgraphs of the 3rd ventricle, infundibular region and median eminence showing high expression of D2 mRNA in this area (C), and its increased expression in iodine deficient rats. Upper panels are data from the author. Lower panels are adapted from Peeters et al, Am J Physiol Endocrinol Metab 281: E54-E61, 2001.