Light and electron microscopic observations of sperm entry in the chum salmon egg

Abstract

AbstractSperm entry into the egg of the chum salmon, Oncorhynchus keta, was observed in living material as well as by electron microscopy. When the egg was inseminated in tap water, actively moving spermatozoa entered the micropylar canal and filled its lumen. When a narrow perivitelline space appeared in the egg (about 150 seconds after insemination), supernumerary spermatozoa were invariably discharged from the canal lumen. A similar discharge was observed in a few batches of eggs that were inseminated in Salmon‐Ringer solution. In response to the penetration of a fertilizing spermatozoon into the ooplasm, a narrow perivitelline space was formed in a restricted area of the animal pole. We suggest that the discharge is induced by the spouting of perivitelline material through the micropylar canal.A fertilizing spermatozoon was observed in contact with the apex of the ooplasmic papilla in the micropylar canal and was seen to enter the ooplasm. When a fertilizing spermatozoon entered the egg, contractile activity was seen in the ooplasm, and a “protrusion” at the animal pole was formed. The papilla extending from the upper surface of the “protrusion” increased in length. These changes did not occur in unfertilized eggs stimulated by tap water in the absence of participation by the spermatozoon. It is, therefore, clear that the “protrusion” and the elongated papilla are analogous to the fertilization cone of invertebrate eggs.The presence of the fertilization cone delayed the separation of the egg envelope from the ooplasm at the animal pole of the egg and inhibited penetration of any supernumerary spermatozoa into the perivitelline space. When the fertilization cone disappeared from the animal pole, 300 seconds after insemination, the micropylar canal decreased in diameter near the lower end. Super‐numerary spermatozoa were, thus, unable to pass through the thick egg envelope. Meanwhile, a transparent structure was seen to form at the micropylar vestibule and to serve as a barrier to entry by spermatozoa into the micropylar canal. We conclude that the formation of the fertilization cone and the spouting of perivitelline material through the micropylar canal play an important role in the prevention of polyspermy of the chum salmon egg.