Studies on the hypothalamo-pituitary-interrenal axis of the elasmobranch fish, Dasyatis sabina




Klesch, William Lawrence, 1941-

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As a group, the elasmobranch fish have been in continuous existance since the early Devonian era, evolving for more than 300 million years, independently, from their intensely studied osteicthyean relatives, the teleosts. This independant evolution from the rest of the vertebrates places them in a unique situation. Through comparative biochemical and/or physiological studies it should be possible to obtain from them, evidence of either similarities or differences in function and interrelations of their endocrine systems with those known to exist in more recently evolved vertebrates. One of the more significant characteristics of this group is the utilization of urea to maintain their body fluids hyperosmotic to the environment. Urea is also used for similar functions in estivating lungfish, some anurans and is present in Latimeria. This significant physiological achievement, tolerating urea concentrations that would denature enzymes and affect oxygen transport in many vertebrates (Gordon, et al., 1961), warrants these animals unique and deserving of further investigation. Clearly it is important to study the structure and function of the endocrine organs in these animals and to establish the chemical identity of their hormones. [...] Of particular evolutionary and physiological interest is the elasmobranch pituitary's control of the interrenal gland and the possible intervention of the hypothalamus in a manner analogous to the situation seen in the higher vertebrates. The literature on elasmobranch interrenal physiolology and its control is both sparse and conflicting. [...] The identification of corticosteroids within the fishes has concerned the comparative biochemist and endocrinologist alike for many years. One obvious reason being the evolutionary significance in comparing the chemical and physiological function with those steroids of the more recently evolve vertebrates. [...] The present investigations were intended to repeat and extend these studies in order to gain a more complete understanding of the hypothalamo-pituitary-interrenal axis and its contribution to the physiology of an elasmobranch fish. The animal used for these investigations was the stingray, Dasyatis sabina, found in large numbers in Texas coastal and estuarine waters. This species has an encapsulated interrenal gland composed entirely of cortical tissue and a large pituitary gland with easily identifiable lobes, which together, lend themselves well to experimental surgery and culture. Thus the principal method of investigation for this study was an in vitro preparation, allowing the experimenter to study a given system free from the nervous and humoral influences that would interact with the system under investigation in the intact animal. The in vitro data was supplemented by in vivo experiments involving interrenalectomy and regional hypophysectomy and by various histological and anatomical examinations