324 
Fishery Bulletin 95(2), 1997 
Table 2 
Definition of oocyte stages of Atka mackerel based on major histological characteristics. 
Oocyte 
stage 
Mean oocyte 
size (pm) 
(range) 
Major histological characteristics 
1 
Early perinucleus 
55 (30-80) 
Small oocyte with hematoxylin-positive cytoplasm. Nucleoli on the outer 
margin of nucleus. 
2 
Late perinucleus 
147 (117-176 ) 
Oocyte becoming larger, cytoplasm lighter, nucleoli still present. 
3 
Cortical alveoli 
230 (216-255 ) 
Cortical alveoli present as a ring on the outer margin of the cytoplasm. 
Cortical alveoli appear as white droplets since they do not stain in H&E. 
The zona radiata can be seen developing as a thin, pink layer. Cytoplasm 
in center of oocyte appears granular. 
4 
Oil droplet stage 
490 (313-628) 
Oil droplets appear first on inner margin of cytoplasm and then start to 
fill out the inner half of the oocyte. Zona radiata thickens, nucleoli are 
still present in nucleus, granulosa cells in tight circle around zona 
radiata. 
5 
Yolk globule stage 
677 (549-843 ) 
Eosin-positive yolk droplets appear between the inner layer of oil 
vesicles and the outer layer of cortical alveoli, giving the oocyte a three- 
layered appearance. Vacuoles appear in oil droplet or cortical alveoli 
layer. Cytoplasm around nucleus granular, staining eosin-positive. With 
further development, yolk droplet zone and oil droplet zone may fuse 
together. Zona radiata thickens and oocyte increases in size. 
6 
Migratory nucleus 
944 (686-1,294 ) 
Yolk platelets form by the fusion of smaller yolk droplets. Oil droplet and 
yolk platelet zone have fused, with cortical alveoli still on the margin of 
the oocyte. Nucleus in the center loses its shape (nuclear membrane gets 
dissolved) and migrates towards the micropyle. 
7 
Early hydration 
1,277 (999-1529 ) 
Zona radiata thickens to almost twice the thickness characterizing 
migratory nucleus stage. Oocyte increases rapidly in size. Yolk fuses to 
uniform, pink mass (H&E stain) in center of oocyte, with still some large 
yolk platelets surrounding it. The margin of the cytoplasm does not 
stain, nucleus is no longer visible. 
8 
Late hydration 
1,932 (1,646-2,195 ) 
Yolk fused to one mass in the center of oocyte, surrounded by 
nonstaining area. Oocyte still within follicle. 
9 
Ovulation 
Same as in stage 8 
Oocyte same as in stage 8, but oocyte no longer inside follicle and 
usually found within lumen of ovary. 
Histological preparation 
After storage for several months in formalin, the 
ovary pairs were reweighed and sections from the 
middle of one ovary were taken and processed for 
histological examination. The tissue samples were 
embedded in Paraplast and sectioned with a micro- 
tome to a thickness of 5 pm. All samples were rou- 
tinely stained with hematoxylin and eosin (H&E). 
Selected samples were stained with Periodic Acid 
Schiff reagent (PAS) to identify carbohydrate com- 
plexes in cortical alveoli while other samples were 
sectioned frozen and stained with Sudan black in 
order to demonstrate the presence of oil droplets 
(Galigher and Kozloff, 1971). Oocytes in each ovary 
were subsequently classified into histological oocyte 
stages (Table 2). Postovulatory follicles and atretic 
oocytes were also recorded and classified according 
to the categories defined by Hunter and Macewicz 
(1985). 
Mean oocyte stage per month 
Each ovary was classified to the most advanced oo- 
cyte stage present using the histological criteria sum- 
marized in Table 2. Mean oocyte stage per month 
was determined by summing the individual speci- 
men’s oocyte stages (most advanced) by month and 
dividing the sum by the number of specimens col- 
lected in that month as follows: 
