ULTRASTRUCTURAL OBSERVATION ON OOGENESIS,VITELLOGENESIS AND VITELLINE MEMBRANE FORMATION DURING YOLK-SYNTHESIZING STAGE IN PHASCOLOSOMA ESCULENTA

Electron microscopy technology was employed to investigate the ultrastructural changes during oogenensis, vitellogenesis and vitelline membrane formation in the sipunculan Phascolosoma esculenta. The results show that oogenesis in this species could be divided into three stages:the primary yolk synthesis stage, rapid yolk synthesis stage and maturation stage. Organelles including mitochondria, endoplasmic reticulum and Golgi apparatus can evolve into yolk granules via pinocytosis. The plasma membrane of oocyte is covered by vitelline membrane which is formed by the oocyte itself and thickens as the oocyte grows. The vitelline membrane is permeable, so external nutrients can penetrate through it and be utilized during oocyte development and vitellogenesis. The vitelline membrane of the growth mature oocyte is 7—9μm thick, consisting of the fiber layer, the dense layer, granular protuberance layer and jelly coat. This tough and flexible structure could help the oocyte to maintain the specific shape.