Permissive effects of thyroid hormones on XXXX anterior pituitary mitotic activity.

@entity1 @entity1375 @entity8789 @entity1955 @entity1218 @entity35 @entity6145 @entity1954
The anterior pituitary is active mitotically and apoptotically under basal conditions and in response to a variety of physiological and pathophysiological stimuli. @entity1375 in @entity1 is associated with a modest but very occasionally dramatic increase in overall pituitary size. The mechanisms underlying this reversible phenomenon remain obscure. In the present study we have examined young adult @entity35 anterior pituitary following surgical thyroidectomy and subsequent @entity6145 treatment and withdrawal using an extremely accurate system for quantifying directly identified mitotic and apoptotic events. Despite the expected increase in the number and/or proportion of immunohistochemically identifiable thyrotrophs three weeks after thyroidectomy, mitotic and apoptotic activity remained unchanged, as did pituitary wet weight, in comparison with sham-operated and intact controls. In contrast, mitotic but not apoptotic activity was enhanced by treatment of thyroidectomized animals with @entity8789 ( @entity1954 (T3) and @entity1955 (T4) 1.8 microg and 3.6 microg/100 g body weight per day respectively), and once again declined to levels seen in intact animals within 72 h of subsequent @entity6145 withdrawal. @entity6145 -induced enhancement of mitotic activity was also seen in intact @entity35 treated with similar doses of thyroid hormones for 7 days and in thyroidectomized @entity35 treated for a similar period with very low dose @entity6145 replacement at a level that had no effect on raised hypothalamic @entity1218 - or pituitary TSHbeta-transcript prevalence (0.018 microg T3 plus 0.036 microg T4/100 g body weight per day). Thus changes in mitotic and apoptotic activity are unlikely to be the principle mechanism for the apparent increase in thyrotrophs up to 4 weeks after thyroidectomy. In contrast, the data indicate that thyroid hormones have a permissive effect on anterior pituitary mitotic activity in thyroidectomized male @entity35 . @entity6145 -induced enhancement of mitotic activity in intact @entity35 further suggests that in euthyroid @entity35 , ambient @entity6145 levels are a limiting factor for anterior pituitary mitotic activity. In summary, this time course study of young, male @entity35 has shown for the first time that thyroidectomy, @entity6145 replacement and subsequent withdrawal has no significant effect on anterior pituitary apoptotic activity. Secondly, it has shown that the anterior pituitary mitotic response to thyroidectomy is blocked by complete @entity6145 deprivation, but can be restored by very low level @entity6145 replacement, and thirdly that in intact animals @entity6145 levels significantly limit anterior pituitary mitotic activity.
