Growth, maturation, and pituitary activity in tilapia species.
The endocrine control of reproduction and aspects of growth in mouthbrooding cichlids Tilanid. mossambica (Peters) and Tilapia aurea (Steindachner), have been investigated.
Comparative studies on the processes of development of the gonads have revealed no differences, on the broader aspects, in the two species. In the females, germ cells proliferate through mitotic divisions and become transformed into oocytes. Growth of the oocytes, as a result of protoplasmic, and then, trophic inclusions in the oocytes, results eventually in maturation. In the males, maturation is brought about as a result of a series of divisions of the germ cells, living rise, eventually, to ripe spermatozoa.
Using histological and cytochemical techniques, eight cell types have been identified in the pituitary gland of the two species; eta or prolactin cells and Î±1-, or ACTH cells in the pro-adenohypophysis; Î±- or STH cells, Î²1- cells, Î²2 – cells (or two types of gonadotrophs), and Î³- or chromophobic cells in the meso-adenohypophysis; and two acidophilic cell types, the function of which are not clear, in the meta-adenohypophysis.
Studies of the cyclical activities in the meso-adenohypophysis and gonads have revealed the presence of a correlation in development. During the reproductive cycle, there is a gradual increase in the quantity of the meso-adenohypophyseal basophils, reaching a peak at the prespawning period.
Maintenance of fish under low temperatures of 13Â° – 15Â° C and in continuous darkness results in an inhibition of the development of the pituitary and gonads, and in an abolition of somatic growth.
Total gonadectomy in Tilapia results in degranulation, vacuolization and depletion of the basophils. At the same time, hypertrophy, hyperplasia and granulation of the meso-adenohypophyseal acidophils (STH cells) are observed. Significant weight increases in the gonadeotomized fish are recorded. The operation also results in the abolition of the secondary sexual characters.
Administration of testosterone propionate, restored the Secondary sexual characters and spawning behaviour in gonadeotomized males. Evidence of masculinization was found in the TP – treated females. Oestradiol benzoate stimulated the growth in size of the genital papilla in fish of both sexes but nuptial coloration was not restored in the males. Dependence of the secondary sexual characters and spawning behaviour on sex steroids has been inferred.
Treatment of fish with a dithiocarbamoylhydrazine derivative (Methallibure) reveals an inhibition of the pituitary gonadotrophic function. A draclual depletion in the basophilic cells of the meso-adenohypophysis is observed, with concomitant regressive changes in the gonads. In the ovary, all yolky oocytes undergo atresia but oogonia and protoplasmic oocytes remain. In the testis, germ cells beyond the spermatogonial stage become necrotic. However, ripe sperm present in the testis at the time of the treatment, persist for some time. Methallibure also abolishes the secondary sexual characters and spawning behaviour, but the feeding habits and consequently, growth, arc not interrupted. ‘The compound also has a secondary, favourable effect on growth in female Tilapia.
Unilateral ovariectomj’- results in compensatory hypertrophy of the remaining ovary, but Methallibure suppresses the compensatory hypertrophy.
Replacement therapy experiments have revealed that the chorionic gonadotrophins (HOG and PMS) have stimulatory effects on the regressed gonads and the appearance of the secondary sexual characters, and spawning behaviour of Methalliburetreated Tilapia. On the other hand, the gonadotrophic hormones (FSH and LH), when used singly were less effective. However, the gonads, the secondary sexual characters, and spawning behaviour, were all stimulated when the two hormones were administered in combination. Evidence is presented on the regulation of maturation directly by the pituitary. The possibility of the elaboration of two gonadotrophic hormones in the pituitary gland of Tilapia is discussed. Extracts of carp pituitary suppressed gametogenesis and gonadal weights. In the testis, the treatment led to an abnormal development.
Testosterone propionate also suppressed gametogenesis and gonadal weights, but restored the secondary sexual characters and spawning behaviour. Evidence is presented on the control of the secondary sexual characters and spawning behaviour by the gonads (androgens) and its mediation through the pituitary.
The sites of steriod production in the gonads were investigated histochemically. 3B – hydroxysteriod dehydrogenase ac ivities were detected in the follicular cells of the ovary and the interstitial cells of Leydig in the testis.
A report on fish culture in Israel, with special emphasis on Tilapia is also presented, and discussed in the light of the experimental findings.