东方扁虾卵子发生的超微结构

根据卵细胞的形态、内部结构特征及卵母细胞与滤泡细胞之间的关系,东方扁虾的卵子发生可划分为卵原细胞、卵黄发生前卵母细胞、卵黄发生卵母细胞和成熟卵母细胞等四个时期。卵原细胞胞质稀少,胞器以滑面内质网为主。卵黄发生前卵母细胞核明显膨大,特称为生发泡;在靠近核外膜的胞质中可观察到核仁外排物。卵黄发生卵母细胞逐渐为滤泡细胞所包围;卵黄合成旺盛,胞质中因而形成并积累了越来越多的卵黄粒。东方扁虾卵母细胞的卵黄发生是二源的。游离型核糖体率先参与内源性卵黄合成形成无膜卵黄粒。粗面内质网是内源性卵黄形成的主要胞器。滑面内质网、线粒体和溶酶体以多种方式活跃地参与卵黄粒形成。卵周隙内的外源性物质有两个来源:滤泡细胞的合成产物和血淋巴携带、转运的卵黄蛋白前体物。这些外源性物质主要通过质膜的微吞饮作用和微绒毛的吸收作用这两种方式进入卵母细胞,进而形成外源性卵黄。内源性和外源性的卵黄物质共同参与成熟卵母细胞中富含髓样小体的卵黄粒的形成。卵壳的形成和微绒毛的回缩被认为是东方扁虾卵母细胞成熟的形态学标志。  

Ultrastructure of oogenesis in Thenus orientalis

Oogenesis and oocyte vitellogenesis in the flathead lobster Thenus orientalis were observed in detail by transmission electron microscopy.According to the shape and size of ootids,ultrastructural changes of nucleus and cytoplasm,vitelline accumulation and the relationship between oocytes and follicle cells,the oogenesis of T.orientalis could be divided into four stages,i.e.oogonia, previtellogenic oocytes,vitellogenic oocytes and mature oocytes.Besides a large and oval nucleus,the oogonia mainly contain vesicles of smooth endoplasmic reticula.The nucleus of previtellogenic oocyte had become markedly swollen,and was named specially as germinal vesicle.The nucleolar exudates appeared in the cytoplasm near out nuclear envelope of previtellogenic oocyte.The oocyte in vitellogenic stage was surrounded progressively by follicle cells and formed a follicle.More and more yolk granules were formed and accumulated in the cytoplasm of vitellogenic oocyte.In T.orientalis,vitellogenesis appeared to involve both endogenous and exogenous yolk formation.The free ribosomes were the first organelles involved in the endogenous yolk formation.And the dense proteinous particles synthesized by these free ribosomes could be accumulated and developed directly in ooplasm into yolk granules without membranes.The rough endoplasmic reticula were the main organelles involved in the endogenous yolk formation.The smooth endoplasmic reticula took part in the endogenous yolk formation by being transformed to rough endoplasmic reticula or fusing with rough endoplasmic reticula.By various ways,mitochondria and lysomes were involved actively in the formation of yolk granules.In addition,golgi vesicles were also involved in the yolk formation.There were two sources,the synthetic product of follicle cells and the precursor of lipovitellin carried by haemolymph,for exogenous materials accumulated in the perivitelline space.These exogenous materials were uptaken into oocytes by the micropinocytosis of plasmalemma and the absorption of microvilli,which were the two main ways of exogenous yolk formation.When oocyte became mature,ooplasm was nearly filled with yolk granules.Both endogenous and exogenous yolk materials were involved in the formation of yolk granules with myelin figures in mature oocytes of T.orientalis.The formation of chorion in perivitelline space and the retraction of microvilli from chorion and perivitelline space could be regarded as the morphological markers of mature oocyte in(T.orientalis.)