Distribution of thyrotrophic activity in the hypophysis of necturus before and after thyroidectomy as measured by I¹³¹ bioassay in mice

Abstract

Hypophyses of non-thyroidectomized and thyroidectomized winter Necturi were divided into rostral and caudal portions which were frozen-dried, refrigerated, and injected into mice for bioassay according to the method of McKenzie ('58). Each mouse received the homogenized equivalent of two pituitary portions, either rostral or caudal. The per cent increase in blood which resulted was used as a gauge of thyrotrophic activity.In the non-thyroidectomized Necturus, nearly two-thirds of the thyroid stimulating activity was in the rostral portion which consisted of the rostral two-thirds of the anterior zone tissue. The remaining TS activity was in the caudal portion which consisted of the caudal third of the anterior zone, the transitional zone and basophilic bed area. These bioassay results coincide with the distribution of basophile cells described previously as thyrotrophs on the basis of histological observations. After thyroidectomy, caudal portions yielded 95% of the thyrotrophic activity. This observation is consistent, with some reservation, with the number and distribution of thyroidectomy cells that differentiate after thyroidectomy, taking into account the vacuolation and degranulation of these cells which occurs in rostrocaudal sequence.The extensive investigation of the localization of pituitary hormones leaves little doubt that these hormones or their precursors are represented by specific granules contained within particular cells and that regional assays of the gland reflect the localization of the cells themselves rather than any distribution of their released products throughout the hypophysis (Hymer and McShan, '63) and associated references). The ultrastructure of the mechanism of release of pituitary cell granules has been described by Farquhar ('61) in the rat and by Cardell ('63) in Triturus. These studies show that extruded granules dissolve in pericapillary spaces bordering the cells.The purpose of the present study was to assess regional concentrations of thyrotrophic activity in the hypophysis of Necturus before and after thyroidectomy, and to compare these regional concentrations of TSH as shown by bioassay with the regional distributions of cells identified as thyrotropic basophiles and thyroidectomy cells on the basis of histological changes (Aplington, '62). ANAT. REC., 160: 273-278 MATERIAL AND METHODSSpecimens of Necturus m. maculosus (Rahesque) were obtained during the winter months from E. G. Steinhilber and Co., Oshkosh, Wisconsin and placed in controlled temperature aquaria at 17°C. Nonthyroidectomized animals were sacrificed within two weeks after they were received, thyroidectomized animals two weeks after operation.The thyroid gland of Necturus and the procedure here used for its removal are described by Aplington ('62). The hypophysis was exposed intact by removing a rectangle of parasphenoid bone from the ventral calvarium after the head had been severed from the body above the lower jaw angle.All hypophyses were divided with iridectomy scissors into two portions, a rost...