GENERAL AND COMPARATIVE ENDOCRINOLOGY 29, 433-440 (1976) Compensatory Hypertrophy of the Remaining Ovary and the Effects of Methallibure in the Unilaterally Ovariectomized Tilapia aurea’ STEPHEN DADZIE AND MOHAMED HYDER Gecaga Institute of Tropical Comparative Endocrinology. Department of Zoology, University of Nairobi, Nairobi, Kenya Accepted October 16, 1975 In Tilapia aurea unilateral ovariectomy resulted in a compensatory hypertrophy of the remaining ovary. The compensatory hypertrophy is manifested by: (a) an increase in the number of dividing oogonia; and (b) hypertrophy and hyperplasia of the plasma membrane of the oocyte indicating a physiological stimulation. Methallibure treatment of unilaterally ovariectomized T. aurea resulted in a blockage of the compensatory hypertrophy of the remaining ovary 15 days after the commencement of the treatment, suggesting the in- volvement of gonadotrophins in the changes observed. These results could be interpreted either as an increase in gonadotrophin secretion or an increased utilization of an un- changed amount of gonadotrophin. Unlike total gonadectomy, unilateral ovariectomy has been conducted on only a few teleostean species and the available in- formation is based on isolated experiments conducted on small numbers of animals. Earlier reports include those of Bock (1928) and Craig-Bennett (1931). In all cases, the authors reported some degree of compen- satory hypertrophy of the remaining ovary. More recently, the compensatory hyper- trophy response of the ovary after unilat- eral ovariectomy was studied at various phases of the annual reproductive cycle of the Indian catfish Heteropneutes fossilis by Goswami and Sundararaj (1968a). The au- thors observed that the remaining ovary had the ability to hypertrophy and compen- sate for the loss of its contralateral coun- terpart in all periods of the reproductive cycle except the postspawning period. They stated, however, that if the experi- ment was performed nearer to the spawning season full compensatory hypertrophy was i A report based on this paper was presented at the Seventh Symposium on Comparative Endocrinology. achieved more quickly than in those in which the operation took place in an out- of-season period. The present investigations have been concerned not only with an account of gen- eral weight increases and qualitative studies of the cell types of the Tilapia ovary but also with an analytical description of the functional mechanism of the ovary con- tributing towards the manifestations of compensatory hypertrophy . An attempt has also been made to elucidate the endo- crine regulation of the reproductive cycle under these conditions of unilateral ovari- ectomy. The involvement of gonadotrophins in hemiovariectomy-induced compensatory hypertrophy in fish was suggested by Gos- wami and Sundararaj (1968b) who inhibited this response with oestradiol bensoate (EB). These authors suggested that EB might act by modifying the rate of synthesis and release of pituitary gonadotrophins. In the present study another antigonado- trophic agent, methallibure, has been used in an attempt to explain the degree of in- 433 Copyright @ 1976 by Academic Press, Inc. All rights of reproduction in any form reserved