608 



Fishery Bulletin 103(4) 



could also be found in the center portions of lobes adja- 

 cent to the characteristic male precursor lobule struc- 

 tures. The primary duct was now well formed; however 

 there were still no definitive morphological characterstic 

 that would enable sex determination. 



Male differentiation 



The initial differentiation of males was evident in the 

 morphological features of the germ-cell tissue located 

 along the peripheral portions of each lobe. The germ 

 tissue began to form elongated bands perpendicular to 

 the edge of the lobe, whereas the somatic tissue began 

 to form fibrous bands originating along the edges of the 



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Size class (mm) 



Figure 7 



Maturity stage by size class for male and female striped mullet 

 {Mugil cephalus L. ) from South Carolina estuaries, October 1997 

 to December 2000. Males, n=1850; females, n=1250. 



primary duct (Fig. 12A). The primary duct was defined 

 structurally at this point. With continued increase in 

 fish length, lobes increased in size and vasculariza- 

 tion. The germ tissue continued to elongate medially 

 within the lobe in a corradiating pattern (Fig. 12B). 

 Somatic tissue continued to form bandlike structures 

 that would eventually become secondary ductwork, and 

 the germ-cell expanded to form lobules (Fig. 12C). As the 

 lobules became more developed, spermatogonia began 

 to line the lobules as part of the germinal epithelium 

 (Fig. 12D). Sertoli cells were not visible because of the 

 lack of resolution at this magnification (400x) level. 

 Mitotic proliferation of spermatogonia caused lobular 

 enlargement, a;though spermatogonia were very small at 

 this stage (2-3 f*m). The overall male aspects of 

 the physical structure of the lobes was clear at 

 this point (Fig. 12E). Melanomacrophages were 

 found in the lobes of some specimens (Fig. 12F). 

 The melanomacrophages were found only in 

 immature and differentiating males. 



Female differentiation 



The first sign of female sexual differentiation 

 was the organization of germ-cell tissue into 

 round nests of 8-10 cells each (Fig. 13A). The 

 germ-cell nests, which eventually gave rise to 

 oogonial nests, were first found along the lat- 

 eral periphery of the lobe and were infrequently 

 scattered within the gonad lobe. There was 

 evidence of early ovarian wall development, 

 which consisted of a single layer of cells form- 

 ing the outer layer of the lobe, separate from 

 the oogonial nests (Fig. 13B). Although some 

 ductwork was present, there was no evidence of 

 the formation of lamellae. Ductwork tended to 

 become reduced as the germ-cell nests became 

 more numerous. With continued development, 

 individual cells within the nests became more 

 visible and the ovary wall became more evi- 

 dent (Fig. 13B). Stalks or buds of tissue were 

 observed growing out of the base of the stroma 

 on the dorsolateral surface (Fig. 13C). As devel- 

 opment progressed, the ovarian wall attached 

 to these stalks or tissue buds appeared to grow 

 over the dorsal surface of each ovarian lobe. 

 This ovary-wall stalk bud was not necessarily 

 an indicator of female differentiation because 

 a small number of samples (0.6%) with definite 

 male structure also had indications of these 

 stalk buds. However, these tissue stalks were 

 present in 68% of the differentiating females. 

 The presence of both the ovary wall stalk 

 buds and the rounded germ-cell nests located 

 throughout the gonad lobe were diagnostic of 

 female differentiation. 



Primary growth oocytes increased in num- 

 ber and began to aggregate, forming distinct 

 lamellae (Fig. 13D). The ovary wall continued 

 to differentiate at this point but was only a few 



