Bowers: Reproductive cycle of oocytes and embryos of Sebastes flavidus 



235 



Figure 1 (left page) 



(A) Oogonial nest (Stage-I oocyte) from Sebastes jlavidiis, contain- 

 ing several primary oocytes collected Dec. 1985, 400 x . (B) Section 

 of oocytes in ovary of S. flamdus collected May 1985, 250 x . Baso- 

 philic properties of cytoplasm in Stage-II cells and their large nucleoli 

 are shown. Distribution of vacuoles are seen in the larger Stage-Ill 

 cells. (C) Cross-section of an ovary collected Aug. 1985 showing ar- 

 rangement of Stage-IV oocytes in grape-like clusters on outer 

 margins of a lamellar branch, 63 x . (D) Typical Stage-IV (initial yolk 

 accumulation) oocyte showing the first indications of yolk, 250 x . 



The cellular composition of the mature follicle was 

 best observed in Stage-IV or Stage-V oocjdes (Fig. 2D). 

 A bilaminar vitelline membrane about 1^< in thickness 

 was next to the plasma membrane of the oocyte. Out- 

 side the vitelline membrane was a single inner epithelial 

 layer, the granulosa. Encapsulating the granulosa was 

 an intricate capillary network filled with erythrocytes. 

 The theca, a single epithelial layer consisting of squa- 

 mous cells with large nuclei, surrounded the profuse 

 capillary system. 



Stage VI: Migratory nucleus Cell diameters ranged 

 from 600 to ~750^. Lipid material had coalesced to 

 form a single, large vacuole, usually centrally located. 

 Nuclear material was ameboid in appearance and no 

 longer occupied a centralized position in the cell (Fig. 

 2C). Nucleoli were small, indistinct, or entirely absent. 



Stage VII: Ovulation/Fertilization Histological 

 evidence of ovulation was verified by observing the in- 

 tegrity of the surrounding follicle. Follicles appeared 

 either as irregularly shaped and shrunken away from 

 the oocyte or displayed a loss of continuity. Postovu- 

 latory follicles appeared throughout the sectioned ovary 

 (Fig." 2C). 



Because fertilization of the mature oocyte occurs 

 rapidly after ovulation, distinction between ovulated 

 oocytes and recently-fertilized oocytes was unneces- 

 sary. Therefore, fertilization was considered an event 

 rather than a stage of histological distinction, and is 

 included in Stage VII to maintain logical continuity of 

 the developmental process. Following fertilization, 

 however, the yolk material became a single homog- 

 eneous mass staining bright-purple in histological 

 preparations, appearing clear or translucent in unfixed 

 samples (Fig. 3 A, B). This distinguishes fertilized from 

 recently ovulated ova. 



Embryonic development 



A complete series of sequential embryonic developmen- 

 tal stages was not obtained from field collections due 

 to the sampling interval and rapid development of 



embryos. Embryos from field collections were, how- 

 ever, satisfactorily placed into one of three broad 

 categories: (1) early-celled, (2) embryonic body, or (3) 

 eyed-larvae (where retinal pigmentation was visible). 



Early-celled The early celled stage of embryonic 

 development observed from field collections of yellow- 

 tail rockfish ovaries corresponded to stage 9 of Op- 

 penheimer's classification for Fundulus heteroclitus 

 (Oppenheimer 1937). The early-celled stage was seen 

 as an undifferentiated mass of cells (blastodermal cap) 

 on top of a large yolk mass (Fig. 3A). This stage was 

 first collected in January, most frequently seen in 

 February, and last occurred in March. 



Embryos in a more advanced state (i.e., flattening 

 or expansion of the blastula) occasionally occurred 

 within an ovary primarily containing early-celled 

 embryos. This suggests rapid cellular divisions and 

 growth. 



Embryonic body The appearance of an embryonic 

 body was first seen in an ovary collected in February 

 and last seen in March. This embryonic stage closely 

 corresponds to Oppenheimer's stages 14 or 15 (Op- 

 penheimer 1937). At the beginning of this stage, an un- 

 differentiated mass of cells (taking on the appearance 

 of tissue rather than individual cells) was located in a 

 high ridge lying over the yolk mass. The oil globule was 

 evident at the opposite pole (Fig. 3B). With further 

 development, embryos displayed optic vesicles orig- 

 inating from lateral buds, distinguishing the cephalic 

 region (Fig. 3C). By the end of this growth phase, 

 somites along the trunk were visible along with 

 lengthening of the tail. The head had fiuther developed 

 to include lens formation (Fig. 3D). 



Figure 2 (overleaf, left page) 



(A) Cross-section of ovary from Sebastes flavidus with clutch of 

 oocytes in late Stage V, 63 x . (B) Cross-section through nucleus of 

 a Stage-V oocyte showing distribution of nucleoli and lampbrush 

 chromosomes in the nucleoplasm, 400 x. (C) Section through two 

 nearly-mature oocytes in Stage VI (migratory nucleus). 63 x . (D) 

 Tangential section of Stage-V oocyte showing components of the 

 follicle outside of vitelline membrane, 400 x. 



Figure 3 (overleaf, right page) 



(A) Early-celled embryos from Sebastes flavidus ovary collected Feb. 

 1986, 63 X . (B) First appearance of embryonic body, showing cellular 

 differentiation. Whole embryo, formalin-fixed, 40 x. (C) Section 

 through developing embryo (embryonic body, late stage) showing 

 optic vesicle formation originating from lateral expansions in cephalic 

 region, 63 x . (D) Embryonic body stage further developed than in 

 Fig. 3C, with better definition of brain, retina, and lens formation, 

 63x. 



