Ultrastructural Studies On Spermatogenesis And Oogenesis In Meretrix Sp.

Meretrix sp. belongs to Genus Meretrix(Mollusca, Class Bivalvia, Subclass Heterodonta,Order Veneroida, Family Veneridae). It is found in recent years during researching the GuangxiClam. The shell of this specie is almost plain white except the trailing edge, so it is usuallycalled white shell clam. It is mainly distributed in the south China sea. Lin Zhihua et al.considered it might be a new specie after comparing it to Liaoning Clam, Shandong Clam,Jiangsu Clam and Guangxi Clam using molecular biological technique. Transmission electronmicroscopy was used to study the morphological structures changes of nucleus and cellorganelle during spermatogenesis of Meretrix sp.. The ultrastructure of spermatozoamorphology of Meretrix sp. was analyzed, and compared with other species of Genus Meretrix.Meanwhile, vitellogenesis of Meretrix sp. was also studied. Through this research, we not onlyenrich the developmental biology of Meretrix sp., but also provide more base date for artificialgrow seedings, cultivation and species identification. The main results are as follow:1SpermatogenesisSpermatogenesis of Meretrix sp. is developed through the following five main stages, thatis, the spermatogonium stage, primary spermatocyte stage, second spermatocyte stage,sprmatid stage and mature sperm stage. The chromatin distribute in small blocky patterns in thenucleus of spermatogonium, and there are few organelles in the cytoplasm. During primaryspermatocyte stage and second spermatocyte stage, the chromatin condense and form a plaqueshape. All kinds of cell organelles undergo from scratch, from few to many. Since the primaryspermatocyte stage, proacrosomic granules can be clearly observed. Aftersecond meiotic division, second spermatocyte turns into sprmatid. While the sprmatiddeveloping into a mature sperm, the chromatin get together and fill up the nucleus, the nucleusshape also changes and finally transform into the shape like a bullet. During this stage, themitochondria mix together and distribute at the base of the nucleus, and eventually fuse to forma plum blossom of five, other cell organelles disappear.2Ultrastructure of mature spermThe sperm of Meretrix sp. is prototype and divided into three parts: head, mid-piece andend-piece. The head comprises a reversing-v-shaped acrosome and a bullet-like nucleus. Theelectron density of subacrosomal space is low and there are a few of nuclear vesicles in thenucleus.The nucleus has posterior nuclear fossa and no anterior nuclear fossa. The acrosome liesin front of the nucleus and its horizontal axis is not perpendicular to the vertical axis of thenucleus. Five plum blossom-shaped mitochondria enclosing a centrosome constitute themid-piece. The end-piece is composed of flagellum which has “9+2”arrangement.3Oogenesis Oogenesis of Meretrix sp. is developed through the following five main stages, that is, theoogonium stage, early vitellogenesis stage, middle vitellogenesis stage and end vitellogenesisstage. At the early oogonium stage, mitochondria are the major cell organelle and they aremostly underdeveloped. At the later oogoninum stage, golgi, endoplasmic reticulum andlysosome increase gradually. At early vitellogenesis stage, all cell organelle are highlydeveloped. There are large number of mitochondria distributing in the cytoplasm, which havedeveloped cristae. Rough endoplasmic reticula are circular lamella and mostly lie around thenucleus.The golgi are membrane capsule-shaped, their ends inflate and go into the cytoplasmto participate in the vitellogenesis. At the middle stage, mitochondria, rough endoplasmicreticula, golgi, ribosome and lysosome are all involved in vitellogenesis. At the end ofvitellogenesis, the oocyte is full of yolk and lipid droplets, only few cell organelle such asmitochondria and golgi could be observed.