Abstract: Ultrastructure changes of oocytes during oogenesis and vitellogenesis in sea cucumber Holothuria scabra were studied by scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM). The results show that oocyte of H. Scabra belongs to isolecithal egg and its oogenesis includes following three stages: oogonium, pre-vitellogenic oocyte and vitellogenic oocyte, and that the stage of vitellogenic oocyte includes early vitellogenic oocyte, mid vitellogenic oocyte and late vitellogenic oocyte. Besides a large and oval nucleus, oogonium contains fewer mitochondrias in this stage. The nuclear of pre-vitellogenic oocyte contains a vacuolated nucleoli, also known as a germinal vesicle. At the early stage of vitellogenesis, various well-developed organelles emerge in cytoplasm. At the stage of mid vitellogenic oocyte, active endogenous yolk formation greatly occurs and many kinds of organelles directly participate in this process. At the stage of late vitellogenic oocyte, the oocytes are the biggest and contain fewer numbers of organelles, and the germinal vesicle of eccentric position is near the basal protuberance. Golgi apparatus, mitochondrias and ribsome are the dominant organelles, which take part in the formation of endogenous yolk, while active microphagocytosis of plasma membrane play the main role in forming exogenous yolk.

Key words: Holothuria scabra, oogenesis, vitellogenesis, ultrastructure