Abstract
In mammals, development begins at the time of fertilization when a mature sperm fuses with an ovulated egg to form a one-cell zygote. Each gamete contains specific macromolecules that mediate the carefully orchestrated events of fertilization. These molecules must ensure the proper approach and recognition of sperm and egg as well as their subsequent fusion. Following fertilization there must be mechanisms to prevent polyspermy and to protect the embryo during preimplantation development. Over the past decade there has been an explosive growth in understanding some of the macromolecules involved in mammalian fertilization (1, 2). Because much of this information has been ascertained in the mouse, it will serve as the focus of this review.
Oogenesis in the mouse results in the formation of a 70 μm in diameter oocyte surrounded by an 7-μm thick extracellular matrix, the zona pellucida. During ovulation, the egg completes the first meiotic division and begins its passage through the oviduct. The mature epididymal sperm is approximately 120-μm long and consists of a trail, a mid-piece, and a head. The sperm acrosome lies on the anterior portion of the head and contains a mixture of lytic enzymes, including acrosin. The epididymal sperm ejaculated during coitus undergo capacitation (a poorly characterized process that results in increased sperm membrane fluidity) in the female reproductive tract. It appears that fewer than 100 sperm successfully arrive in the oviduct where only a few penetrate the outer investments (cummulus oophorus) of the ovulated egg. Following an initial loose attachment of the sperm to the zona, a tight binding triggers the sperm acrosome reaction, thereby releasing lytic enzymes that may be important for the penetration of the sperm through the zona pellucida.
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