Abstract
Ph.D.
Heavy metals like copper are toxicants commonly found in mining, industrial, agricultural and waste effluents in South Africa¡¯s riverine systems. In high concentrations these metals could have a negative effect not only on river systems, but also on fish populations. It is therefore important to determine the effect of these heavy metals on the fish at an early stage, before the fish show any sign of being exposed to the pollutants and before any bodily functions are affected. It is known that polluted water, especially heavy metal pollution like copper, can cause physiological and biochemical alterations in fish.
Histopathology is a mechanism, which can provide an indication of fish health by determining early injury to cells and can therefore be considered an important tool to determine the effect of pollutants like copper on fish tissue. At present very little is known about the effect of toxicants on the histology of fish tissue of freshwater fishes in South Africa. This is important in gonads, where many variables not only in different fish species, but also in the number of developing germ cells in the testes exist. The large number of fish species and the variation in their anatomy and reproductive patterns make knowledge of the anatomy of all fish species difficult.
In order to perform a histopathological analysis, it is important to be familiar with the normal histology of the fish organs that are being investigated. It was therefore necessary to describe the histology of selected organs in South Africa¡¯s freshwater fish species. The Mozambique tilapia, O. mossambicus was chosen as test organism and copper was selected as toxicant. The normal morphology and histology of the testes of O. mossambicus were described in order to serve as a baseline study and to provide histological parameters for further toxicological work on the gonadal histopathology of fish.
Fish were exposed to sub-lethal concentrations of copper (CuCl2) for a short-term (96 hours) in experimental flow-through systems under controlled laboratory conditions (23 ¡¾ 1¨¬C). An external investigation of each of the 30 control and 40 copper exposed fish was performed.
Standard length, body mass and gonadal mass were recorded and the maturity of each fish was obtained histologically. A light and electron microscopic examination was performed in order to describe the histology and histopathological changes. The histopathological changes were obtained by comparing the histology and ultrastructure of the testis of the control group with the information obtained for the testes that were exposed to copper.
Fish obtained from the control group that were run simultaneously with every exposure group were used to describe the normal histology of the testes of O. mossambicus. The aim was to describe the normal morphology and histology of the testes of O. mossambicus so that the information obtained during this investigation could serve as a baseline study when dealing with toxicity studies. The different germ cells of the spermatogenesis process were also described with a view to assess not only the reproductive pattern, but also to provide histological parameters which might form a basis for any toxicological work on the gonadal histopathology of fish.
Identification of reproductive biomarkers requires an understanding of all the principal cellular components of the testes and their normal range of variation. The cellular components that were described are the Sertoli and Leydig cells as well as the primary and secondary spermatogonia, primary and secondary spermatocytes, spermatids and spermatozoa. The information on the GSI, gonad reproductive stages as well as the histological examination provided the baseline which was needed for comparison of the testes of fish that were exposed to copper.
The cellular and sub-cellular responses at light and electron microscopic level were examined to determine early cell changes and damage in the testes which were caused by different sublethal exposures of copper. There were no differences in the gross structure of the testes of the control group and the group that were exposed to copper. The testes of the fish that were exposed to 0.16, 0.40 and 2.0 mg Cu/§¤ all revealed disorganisation of the lobules, disintegration of the spermatogonia as well as disintegration of the interstitial tissue.
The exposure of fish to sublethal concentrations of copper induced histopathological changes in the testes of all three groups of fish that were exposed to copper. The results also showed that the intensity of the histopathological changes increased with an increase in the concentration of copper.
As a biomarker of exposure to toxicants, histopathology represents a useful tool to asses the degree of pollution. The results from this investigation showed a decrease in the GSI after being exposed to copper, when compared with the control groups, which were not exposed to copper. The information on the effect of copper on the GSI, gonad reproductive stages as well as the histological examination also provides information on gonadal health and maturational stage.
This information however provides structural, rather than functional information regarding gonadal health. The results showed that the GSI and gonadal staging should be used together with the gonadal histopathology in order to provide a baseline for comparison when dealing with other toxicological studies.
The information obtained during this investigation showed that fish histology and histopathology could be used as a biomarker to provide information on the effect of heavy metals on fish health. It is however suggested that the information obtained during this investigation should be extended to field investigation in order to be validated as a reliable histopathological biomarker.
Prof. J.H.J. van Vuren
Prof. G.J. Steyn