FERTILIZATION OF THE LAMPREY (LAMPETRA-JAPONICA) EGGS - IMPLICATION OF THE PRESENCE OF FAST AND PERMANENT BLOCKS AGAINST POLYSPERMY

We have carried out a morphological examination to analyze the mechani sm of monospermic fertilization in the lamprey, Lampetra japonica. Upo n insemination, many sperm passed through a ''tuft'' of jelly that is found only on the animal pole area of the egg and underwent the acroso me reaction after they came into contact with the outer surface of the two-layered vitelline envelope. The acrosomal filaments were admitted into the envelope and their tips touched the ooplasmic surface. The H oechst dye transfer method revealed that a single sperm showed intense fluorescence before its nucleus penetrated the vitelline envelope; th is indicated the actual fusion of the gametes. No other sperm with acr osomal filaments fluoresced, however. These findings indicate that a f ast block against polyspermy is established at the plasma membrane lev el and prevents additional sperm-egg fusion. The separation of the oop lasmic surface from the vitelline envelope (perivitelline space format ion) was initiated beneath the margin of the animal pole region and pr ogressed toward the animal and vegetal poles of the egg. At this stage , the head of a fertilizing sperm passed through the thickness of the vitelline envelope. The nuclei of supernumerary sperm were unable to e nter the inner layer of the vitelline envelope. During the early phase of the perivitelline space formation at the animal pole region, many thin threads and one, or a small number, of the thick projection(s) of the ooplasm connected the envelope with the vitellus. A fertilizing s perm invariably moved along one of the thin threads and was embedded i n the ooplasm at the animal pole region. Cytochalasin B inhibited sper m from passing through the inner layer of the vitelline envelope. When ionophore-activated eggs were inseminated, the tips of the acrosomal filaments were prevented from touching the ooplasmic surface by the in tervening presence of the perivitelline space. None of these sperm pas sed through the vitelline envelope. These findings suggest that the fu sion of the acrosomal filament with the egg plasma membrane is a prere quisite for the passage of sperm through the inner layer of the vitell ine envelope. Furthermore, actin-mediated ooplasmic activity is probab ly involved in the passage. We conclude that the formation of the peri vitelline space at the animal pole region contributes to the establish ment of a permanent block against polyspermy in the lamprey egg, (C) 1 994 Wiley-Liss, Inc.