The cytological changes of sperm and oocyte nuclei during fertilization in Phascolosoma esculenta (Sipuncula: Phascolosomatidea)

Abstract In this study, we investigated fertilization cytology in Phascolosoma esculenta. The sperm binds to the oocyte envelope in a random position at the point of initial contact. An acrosomal reaction then occurs and the acrosomal filament penetrates the oocyte envelope. The apex of the acrosomal filament contacts with the plasma membrane of the oocyte at the point where the fertilization cone is formed. The fertilization cone then retracts, towing the acrosomal filament back and drawing the sperm into the oocyte. Subsequently, the pores on the oocyte envelope are blocked and filled with fibrin and colloid. After the oocyte is activated by sperm, the first and second meiosis processes start sequentially, followed by the release of the first and second polar bodies. After the second polar body is extruded, the haploid female nucleus rapidly reorganizes to form a female pronucleus. In comparison, the sperm nucleus begins to decondense and dilate after the first polar body is released. The male pronucleus is not formed until the end of the second meiosis. The male and female pronuclei migrate to the centre of the oosperm and are incorporated into the zygote nucleus and then the first cleavage occurs. A two cell embryo is then formed by plasma membrane invagination. The cytological changes of sperm and oocyte nuclei during sperm entry into the oocyte and the fertilization process in P. esculenta are characterized by: (1) the mature oocyte with fertilization ability being in the metaphase of the first meiosis; (2) sperm is drawn into the oocyte by the acrosomal filament and fertilization cone; (3) some abnormal phenomena such as polyspermy, multiple pronuclei and multipolar spindle; and (4) the male pronucleus is formed earlier than the female pronucleus, at which point they will fuse to form a zygote nucleus.