Fine structure of the micropylar apparatus of the chum salmon egg, with a discussion of the mechanism for blocking polyspermy

Abstract

The micropylar apparatus of the eggs of the chum salmon, Oncorhynchus keta, was examined by scanning and transmission electron microscopy. The apparatus consists of a funnel-shaped vestibule, from the bottom of which extends the micropylar canal, about 5 pm, in diameter. The canal traverses the entire zona pellucida interna of the egg envelope, about 30 pm in thickness. At its inner end, the diameter of the micropylar canal is the same as the width of the sperm head, about 2 pm. The lower portion of the canal is occupied by a conspicuous outgrowth of the ooplasmic surface, 3-10 pn in length.The first contact of the ooplasmic surface with a fertilizing spermatozoon occurs at the apex of the outgrowth in the micropylar canal. The surface of the outgrowth undergoes changes immediately after penetration by a fertilizing spermatozoon that prevent supernumerary spermatozoa from penetrating the ooplasm. As the formation of the perivitelline space takes place, the outgrowth withdraws from the micropylar canal. Supernumerary spermatozoa, however, cannot enter the perivitelline space and are eliminated from the canal lumen. In addition, the micropylar canal decreases in diameter at the region near its inner end and forms a mechanical barrier to the passage of supernumerary spermatozoa.

On the relative effect of spawning asynchrony, sperm quantity, and sperm quality on paternity under sperm competition in an external fertilizer

“…Moreover, the ovarian fluid seems to represent a selective environment for sperm (Yeates et al, 2013), and there is a strong interaction effect of ovarian fluid on sperm swimming speed in charr with certain female fluids stimulating swimming speed of sperm from some males over others (Urbach et al, 2005). Eggs are fertilized after a sperm cell enters the micropyle, which is barely wide enough to allow entry of one sperm cell (Ginsburg, 1963;Kobayashi and Yamamoto, 1981;Yanagimachi et al, 1992). Thus, in salmonids, where up to 80% of the eggs can be fertilized within the first 5 s of egg and sperm interactions (Hoysak and Liley, 2001), the first sperm cell to enter the micropyle fertilizes the egg (Kobayashi and Yamamoto, 1981;Yanagimachi et al, 1992).…”

Observation of the embryonic development in Pseudoplatystoma coruscans (Siluriformes: Pimelodidae) under light and scanning electron microscopy

“…Such micropyle shape allows the entry of a single spermatozoon, according to Kobayashi & Yamamoto (1981) and Rizzo & Bazzoli (1993). Other ways of preventing polyspermy include the formation of a fertilization cone, which obstructs the micropylar canal, as well as the releasing of cortical alveoli, which increases the perivitelline space after the fertilizing spermatozoon has entered (Ganeco, 2003).…”

Structural and ultrastructural characterization of the embryonic development of Pseudoplatystoma spp. hybrids

“…Most of the Siluriform fish analyzed by Rizzo et al (2002) were also characterized by a gelatinous border around the vestibule. According to Kobayashi and Yamamoto (1981) and Hart and Donovan (1983), the diameter of the micropyle canal is equivalent to the size of the spermatozoon head, acting as a mechanical barrier to polyspermy, since just the first spermatozoon reaching the micropyle is able to establish direct contact with the plasma membrane of the oocyte. During fertilization, a large number of spermatozoa compete to fertilize a single oocyte, as observed at the micropylar region in the hybrid egg 30 seconds after fertilization.…”