可口革囊星虫的精子发生及精子结构

Spermatogenesis and sperm morphology of Phascolosoma esculenta

为了探究可口革囊星虫(Phascolosoma esculenta)精子发生过程及结构上的特殊性,用显微及亚显微技术研究了可口革囊星虫的精子发生和精子结构。可口革囊星虫的精巢位于收吻肌基部,为一曲折的带状组织。成熟精巢内可观察到精原细胞、精母细胞以及精细胞等各阶段的生精细胞。在精子形成早期,很多精细胞脱离精巢,以精细胞团的形式掉落到体腔中。精细胞团内的精细胞同步发育为精子后,脱离精子团进入肾管。成熟精子由头部和尾部组成。头部由钟形顶体与鼓形细胞核构成。顶体后段下包于精核的前端。顶体分内、中、外三层,外层有横隔;顶体下腔内有颗粒状物质不均匀分布,中央有一束丝状纤维组成的顶体棒。核物质电子密度高,核内含空泡。无核前窝,具浅的核后窝。尾部分中段和末段,中段由6个(偶见5个或7个)线粒体围绕近、远端中心粒构成;末段细长鞭状,由轴丝及包绕轴丝的质膜组成,轴丝为典型的"9+2"结构。分析认为:可口革囊星虫精子发生过程以及超微结构上存在特殊的结构与机制:①精细胞团保证了精子形成的同步性;②顶体后段下包于精核的前端使精子头部小而灵巧,利于快速运动;③顶体的横隔使精子顶体的牢固性增强,确保受精时顶体反应的正常进行;④中段较多的线粒体使精子具有更强的环境适应性,有利于有效的受精。

The characteristics of spermatogenesis and sperm structure were investigated in Phascolosoma esculenta by means of light and electron microscopy. The testis is a narrow band of tissue situated at the base of the ventral retractor muscles. Cells at various stages of spermatogenesis （including spermatogonia, spermatocytes and spermatids） are readily identifiable in the mature testis. Early in spermiogenesis, a spermatid mass is shed into the coelom. Spermatozoa develop within the spermatid mass, and then enter the nephridium. Each mature sperm consists of a head and a tail. The head includes a conical acrosome and contains a drum-shaped nucleus. The rear part of the acrosome is wrapped around the anterior portion of the sperm nucleus. The acrosome comprises three layers （exterior, interim and interior layers）. The exterior layer contains radiate septa. An acrosomal rod consisting of a sheath of fibrils is found in the subacrosomal space. The nucleus is highly condensed, and a shallow post fossa is identifiable, but no anterior fossa was found. The tail of the mature gamete comprises a middle piece and an end piece. 5 - 7 mitochondria surround the proximal and distral centrioles in the middle piece. The slim and ＂whip-like＂ end piece consists of a central axoneme, having the typical ＂9 ＋ 2＂ structure, and a plasma membrane. Our findings suggest that spermatogenesis andsperm structure of P. esculenta are characterized by （1） formation of a spermatid mass, facilitating synchronization of spermiogenesis ; （2） wrapping of the rear part of the acrosome around the anterior sperm nucleus ; this renders the sperm head more compact and may assist motility ; （3） septa in the exterior layer of the acrosome may strengthen it and guarantee that the acrosome reaction occurs normally and （4） increased mitochondrial loading in the middle piece which may assist fertilizing capacity .