Histological study of functional ectopic pituitary transplants in a teleost fish ( Poecilia formosa )

Abstract

The histological study of the homotransplanted pituitary of Poecilia formosa, grafted for 7 weeks in fish that were hypophysectomized 3 weeks after the transplantation, revealed considerable variation in the size and cellular composition of the grafts. However, certain general conclusions were drawn from the study. Apart from one graft that was infiltrated with connective tissue, eta cells (secreting the teleostean prolactin-like hormone) are invariably present. These cells remain chromophilic with an active appearance, and they appear to ensure the survival in fresh water of the grafted-hypophysectomized fish and to assist in the maintenance of the normal appearance of the skin. Growth hormone (alpha) cells are found rarely in the grafts and in feeble numbers, despite evidence of a limited secretion of growth hormone. Corticotrophs (epsilon cells) are almost always to be found, though their appearance of activity varies from graft to graft; they are usually associated with the maintenance of a slightly stimulated interrenal, and with the maintenance of normal blood cell counts. Thyrotrophs (delta cells) are among the most active-looking cells in the grafts, and do not appear to be dependent on hypothalamic stimulation for the secretion of large amounts of TSH, which maintained normal thyroid histology and provoked hyperplasia of the thyroid in several cases. In contrast to the above cells, the gonadotrophs in the grafts are regressed and no longer able to stimulate the ovary; a hypothalamic factor appears to be essential for the maintenance of structure and function in these cells. The pars intermedia and neurohypophysis, intimately associated in teleosts, simultaneously undergo a marked involution accompanied by the disappearance of typical neurosecretory material. The identification of the various cell-types in the grafts is generally more difficult than in the normal pituitary, but is nevertheless possible, most easily in those grafts which retain a general structure comparable to that of the normal gland. These findings have been compared with those obtained in the higher vertebrates.