Histochemical, ultrastructural and hormonal studies on the pars distalis of the echidna (Tachyglossus aculeatus).

There were no consistent significant differences between the concentrations of luteinizing hormone (LH) and adrenocorticotrophin (ACTH) in the rostral compared with the caudal zone of the echidna pars distalis. This suggests that LH is secreted by cells containing S-type granules (probably corresponding to secretory vesicles 200-300 nm diameter) which are distributed throughout the gland. Some of the cells containing vesicles 100-200 nm diameter, seen in small numbers in both zones of the gland, may be responsible for the secretion of ACTH. The concentration of pituitary LH is in the range of that found in eutherian mammals, but the concentration of ACTH is lower than that reported for other vertebrates, and this may be linked causally with the remarkably low rate of corticosteroid secretion in the echidna. The absence of significantly increased levels of pituitary LH and ACTH in a chronically orchidectomized and adrenalectomized animal adds to other evidence which suggests that mechanisms involving a negative feedback of steroid hormones on the hypothalamo-hypophysial axis may not be fully developed in the echidna.