Oocyte development in the kuruma prawn Penaeus japonicus

Female kuruma prawns (Penaeus japonicus Bate) with undeveloped, early developing, developing, nearly ripe and ripe ovaries, were collected from Ise Bay, Japan, in 1984. Oocyte development of the kuruma prawn was classified into ten stages according to morphological characters, namely: (1) synapsis stage, (2) chromatin nucleolus stage, (3) early perinucleolus stage, (4) late perinucleolus stage, (5) oil globule Stage I, (6) oil globule Stage II, (7) yolkless stage, (8) yolk granule stage, (9) prematuration stage, and (10) maturation stage. The synapsis stage is a multiplication stage. The chromatin nucleolus stage, early and late perinucleolus stages are previtellogenesis and primary growth stages. Oil globule Stage I is an initial stage of primary vitellogenesis and secondary growth. Follicle cells on the oil globule Stage I oocytes expand rapidly and reach maximum size during oogenesis. Yolk granule stage oocytes are in the initial stages of secondary vitellogenesis. Strongly acidophilic yolk granules accumulate within basophilic vesicles of the cytoplasm. The yolk granules are first concentrated in the inner part of the cytoplasm, then gradually spread to the periphery. Cortical crypts, which are separated from the oocyte cytoplasm by the cytoplasmic membrane, are situated outside of oocyte cytoplasm. Germinal vesicle breakdown (GVBD) is initiated in the late phase of prematuration and continues until the late phase of maturation immediately prior to spawning. At the beginning of the maturation stage, the oocytes are ovulated, after which the nuclei further shrink and migrate out-wards. After ovulation, meiotic division of the ovarian oocyte progressed up to the metaphase of primary maturation division. Finally, the meiotic metaphase is visible just beneath the cytoplasmic membrane in the mature oocyte. Though ovulation is synchronous within the same ovary, GVBD is not completely synchronous. Ovulated mature oocytes have many club-shaped cortical crypts in the peripheral part of the cytoplasm and contain extensive accumulations of yolk granules dispersed throughout the cytoplasm. The apical end of the club-shaped cortical crypts and cytoplasmic membrane are coated by the vitellin envelope in the mature oocyte.