McDonough et al. Sexual differentiation and gonad development in Mugil cephalus 



609 



cell layers thick. There was still a great deal of 

 stroma and somatic tissue left in the ovary, but 

 it began to form bands of fibrous tissue, result- 

 ing from the regression of stroma and somatic 

 tissue (where present) as the lamellae contin- 

 ued to develop. The primary duct was greatly 

 reduced. Oogonia began proliferating and dif- 

 ferentiating into primary growth oocytes as fol- 

 liculogenesis commenced. The ovary wall, now 

 becoming vascularized, began to separate from 

 the lamellae, opening a space that would become 

 the ovarian lumen (Fig. 13, D-F). The ovary 

 wall was made up of squamous cells on the in- 

 side layers and collagen and elastic tissue on 

 the outer layers. The stroma and somatic cells 

 continued to be reduced until they were primar- 

 ily fibrous tissue from which the lamellae were 

 suspended. Histological ovarian cross-sections 

 changed from the leaf or spade shape of the 

 undifferentiated gonad to a more rounded one. 

 Once ovarian differentiation was completed, the 

 individual lamellae were seen to have oocytes 

 within each and the stroma was reduced to sus- 

 pensory tissue for the lamellae (Fig. 13F). The 

 primary growth oocytes present in the lamellae 

 remained small (80 to 100 pm) and relatively 

 uniform in size. At the initiation of reproductive 

 development, the oocytes started to grow from 

 the arrested prophase of the first meiotic divi- 

 sion (Stenger, 1959; Kuo et al., 1974). 



Morphological features of atretic females 



Females undergoing atresia were captured in all 

 months except August-October, and 78% were 

 seen January-March of each year. The first sign 

 of alpha atresia was the breakdown of the most 

 advanced residual oocytes. Vacuoles (of various 

 sizes) began to appear (Fig. 14A), merging to 

 form large spaces within the decaying oocyte. 

 The overall diameter of oocytes decreased from 

 600 to 300-400 Jim; however oocytes retained 

 their overall shape during alpha atresia and 

 showed no signs of collapse (Fig. 14B). Beta 

 atresia was the shortest phase. The oocytes had shrunk 

 in size (<300 |um) but retained their previous overall 

 structure and shape. A distinct hollow matrix retaining 

 only the outer layers of the oocyte (follicle layers and the 

 vitelline membrane) was the defining characteristic for 

 beta atresia (Fig. 14C). The tissue retained this structure 

 while the oocyte continued to decrease (150-180 t/m). 

 During gamma atresia the oocytes collapsed (Fig. 14D) 

 or shrank. Some vacuoles remained in partially collapsed 

 oocytes, but they were fewer in number and smaller in 

 size (<150 f/m) (Fig. 14E). The areas in and around the 

 collapsed oocytes and ovarian lumen became more vas- 

 cularized during this stage, and this helped facilitate 

 rapid resorption of decaying cellular material (Fig. 14F). 

 Undeveloped oocytes became more visible and numerous. 

 Gamma atresia ended when only masses of broken-down 



i:o(i 



iiiiiii 



roo - 



(.00 



40(1 



Males 



r^i 



K\M Immature 

 I I Developing 

 ^^B Alretie 

 Resting 



1 



1 



1 2 



i I 1 i r 1 1 — 



4 5 6 7 8 9 10 



500 



300 



200 



100 



B 



Females 





-T 



iwm Immature 

 i-   i Developing 

  Atretic 

 I 1 Resting 



-,. 



I 



G 



H W M = _ 



4 5 6 7 



Age 



10 



Figure 8 



Maturity stage, by age class, for male and female striped mullet 

 {Mugil cephalus Ljfrom South Carolina estuaries, October 1997 

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



cellular material remained. Delta atresia was character- 

 ized by the presence and decay of nondescript cellular 

 material from the previous spawning (Fig. 14G). Delta 

 atresia was present in approximately 30% or more of 

 the ovary. There was also a decrease in the amount 

 of vascularization within the ovarian lamellae during 

 this stage because most of the old oocyte material had 

 been resorbed. The lamellae contained only undeveloped 

 oocytes and all the remaining material from the previous 

 spawn was concentrated medially in the lamellae. 



In the resting stage, no reproductive activity occurred 

 in the ovaries. Infrequently, resting ovaries showed 

 some minor evidence of the previous spawning. The 

 remaining undeveloped oocytes were previtellogenic 

 and varied widely in size (80-120 um). The ovary wall 

 was relatively thick, particularly in comparison to the 



