Micro-level adaptation of individual organs and tissue of sterlet (Acipenser ruthenus Linnaeus, 1758) under heat stress conditions

Abstract

Purpose. To evaluate the variability of the histological parameters of individual organs and tissues of sterlet, to establish the level of effect of heat stress on the physiological status of fish and the degree of their adaptation. To determine the perspective of using histological monitoring data in fish farming practice. Methodology. The primary materials were obtained in aquarium conditions of the Aquatic Bioresources Laboratory of the Kherson State Agrarian and Economic University. The laboratory processing of histological samples was performed according to generally accepted methods and manuals. Histological studies were carried out using the author’s equipment and original methods, specially intended for the diagnostics of fish tissues. The obtained results were processed by methods of variational statistics using a package of appropriate MS Excel applications, with emphasis on the standard error. Findings. Microanatomical studies of sterlet showed that under conditions of heat stress, the most significant changes occur in adipose tissue, somatic and cardiac muscle tissue, thyroid gland and renal tissue, which can conditionally be considered as bioindicative zones. Under the potential influence of the thermal factor, the response of sterlet is expressed in a decrease in the lipid content in the accompanying muscles of adipose tissue. The effect is visualized in the fragmentation and subsequent shredding of lipid vacuoles, which leads to variability in adipocyte shape from stellate to spindle-shaped. The obtained results indicate a decrease in the excretion of thyroid hormones into the bloodstream by the thyroid gland and is expressed in a gradual increase in the diameter of the thyroid follicles (by 16.0 µm and 26.5 µm) and a corresponding decrease in the height of the follicular cells (by 1.1 µm and 2.0 µm), which is statistically significant. Intensive development of “additional” follicles inside existing ones indicates the prevention of thyroid intoxication of the body, which indicates the development of an adaptive reaction of the body. Comparative studies of the ventricular myocardium of fish allowed establishing certain morphological differences in the form of moderate interstitial edema of the tissue and intracellular edema of cardiomyocytes. In the final phase of the experiment, karyopyknosis was locally observed in the ventricular cardiomyocytes of 50% of stressed fish. Somatic muscle tissue is generally resistant to stress, but the rate of its development at elevated temperatures slows down. Characteristic signs of the histological picture of muscle stress are approximately the same ratio of stromal and parenchymal components, the central orientation of the nuclei within a single muscle fiber and insufficient filling of the fibers with the fibrillar component. Antidiuretic hormone is forcedly released in fish under conditions of heat stress, which leads to a decrease in the volume of the primary filtrate. Evidence is a narrow gap in the urinary space between the visceral and parietal layers of the renal corpuscle. Originality. New data are presented and existing data on the histological structure of individual organs and tissues of sterlet under stress loads are significantly expanded. For the first time, a detailed histological analysis of the structure of muscle, adipose tissue, as well as the kidney and thyroid gland of sterlet under heat stress conditions was carried out. The comparative analysis, current and updated information on compensatory changes in tissues demonstrate their adaptive potential, which was developed during the evolution of the species. Practical Value. The information obtained is of value in morphophysiological studies as well as in fish farming practice in connection with the modern problem of a decrease in the life status of fish under stress. The use of the obtained data allows a qualitative assessment of the physiological state and can be used in the process of developing corrective measures in order to consolidate hereditary resistance, which is ultimately aimed at preventing diseases of valuable aquaculture objects. Keywords: sterlet, heat stress, adaptation, muscle tissue, thyroid gland, myocardium, adipose tissue, renal tissue.