Abascal et al.: Testicular development in Thunnus thynnus 



409 



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Balearic Islands 



Figure 1 



Differences in gonad index (I G ) 



liver index 1 1, ), 



bluefin tuna [Thunnus thynnus) from Barbate and 

 male tuna from the Balearic Islands. 



(r 2 =0.21; P<0.0001) between I G and I L , which suggests a 

 slight growth of the liver with sexual maturation. 



Histology 



The testes of Thunnus thynnus are paired, elongate 

 organs that appear attached to the dorsal body wall by 

 a mesentery. The fat body, which is closely associated 

 with the gonad, consists of a variable amount of adipose 

 tissue. The testis is composed of a dense array of lobules 

 converging on the main sperm duct (vas deferens) and 

 terminating blindly beneath the tunica albuginea at the 

 periphery ( Fig. 3, A and B). Two distinct zones can be 

 distinguished in cross sections of the testes (Fig. 3A). At 

 the outer region, the seminiferous lobules have a thick 

 wall formed by the germinal epithelium, where germ cells 

 develop in association with Sertoli cells; the lumina of 

 the lobules are filled with spermatozoa that have been 

 released after completion of the spermiogenetic process 

 (Fig. 3, B and C). As a result of the release of mature 

 sperm from spermatocysts into the lobule lumina, the 

 germinal epithelium becomes discontinuous (Fig. 3B). 

 The transition from the outer to the central region of the 

 testis is marked by an abrupt change in the configuration 

 of the testicular lobules, which lose the germinal epithe- 

 lium and become ducts where lobule function has shifted 

 from sperm production to sperm storage (Fig. 3C). Thus, 

 the only sex cells that are found in the central part of 

 the testis are mature spermatozoa, which fill the swollen 

 lumina of the lobules. In this zone the testis ducts con- 

 stitute an intricate network of channels that convey the 

 spermatozoa produced in the proliferative region to the 

 main sperm duct (Fig. 3, A and D), which is thick walled 

 and located in the center of the testis (Fig. 3D). 



The gametes develop in groups of isogenic cells called 

 germinal cysts or spermatocysts, where the process of 

 differentiation is synchronous (Fig. 4). Primary sper- 

 matogonia are large, single cells (Fig. 4A) that are dis- 

 tributed all along the germinal epithelium, as is char- 

 acteristic of the teleost unrestricted testicular type. 

 Spermatogonia B resulting from successive mitoses of 

 spermatogonia A are found in small groups, whereas 

 spermatocytes and spermatids are grouped within larger 

 spermatocysts (Figs. 3, B and C, 4). The cysts contain- 

 ing late spermatids and spermatozoa, prior to spermia- 

 tion, display a particular alveolar appearance due to 

 the orientation of the spermatid heads facing the lobule 

 walls and the bundles of fiagella directed toward the 

 seminiferous lobule lumen (Fig. 4, A, C, and D). 



Active spermatogenesis was observed to occur both in 

 migrant bluefin tuna from the Strait of Gibraltar (Fig. 4, 

 A and B) and spawning fish from the Mediterranean 

 (Fig. 4, C and D). In both cases, all stages of the male 



