Gamete interactions during in vitro fertilization of lobster (Homarus americanus) Oocytes

Abstract

Preovulatory lobster oocytes were mechanically removed from follicles and fertilized in vitro with sperm from the vas deferens of males or the seminal receptacle of females. Oocytes were fixed for ultrastructural analysis at various times after insemination. Stages in gamete interaction beginning with sperm binding to envelope 1A and extending through nuclear envelope breakdown of the fertilizing sperm were examined microscopically. Numerous sperm bind to envelope 1A of mature oocytes. Only properly oriented sperm undergoing normal acrosome reactions penetrate envelopes 1A and 1B (together these form the vitelline envelope). During the acrosome reaction, the inner acrosomal material comes in direct contact with envelope 1B and does not appear to release a lysin for penetration of envelopes 1A and 1B. The acrosomal membrane overlying the acrosomal filament appears to be the first membrane of the sperm to contact the oolemma and is probably the initial site of gamete membrane fusion. Following fusion, egg cytoplasm flows around the sperm nucleus forming a small fertilization cone. The acrosomal contents are left outside the oocyte in the pervitelline space. Once gamete membrane fusion has occurred, the nuclear envelope fragments, and by 120 min after insemination, it has completely disappeared around some sperm nuclei. As sperm are incorporated into the ooplasm, the microtubules associated with the membrane/microtubule complex of the nucleus depolymerize, and the membranes of this complex are less frequently observed. After gamete membrane fusion, the acrosomal filament is contiguous with oocyte cytoplasm, and it remains structurally unchanged by 120 min after insemination. It is probable that the oocyte releases some high density cortical granules within the first 120 min of insemination; however, there was no evidence for the release of low density vesicles, ring vesicles, and moderately dense vesicles during this time. © 1991.