Final answer:
Fertilization occurs when a single sperm cell's plasma membrane fuses with the oocyte's membrane, initiating the embedding and nuclear release necessary for zygote formation. This process is preceded by the sperm's passage through protective layers and is followed by mechanisms to prevent additional sperm from fusing with the oocyte.
Step-by-step explanation:
A single sperm is drawn into the oocyte by the fusing action of its plasma membrane in the initial stages of fertilization. This remarkable process begins as the sperm penetrates the protective layers that surround the oocyte, namely the corona radiata and the zona pellucida.
Success depends on a sequence of coordinated events that culminate in the fusion of a sperm cell to the oocyte's plasma membrane.
The journey begins with capacitated sperm navigating toward the oocyte in response to chemical attractants. As they reach the corona radiata and the underlying zona pellucida, the acrosomal reaction is initiated whereby the sperm's acrosome releases enzymes to clear a path to the oocyte membrane. This enzymatic activity allows the sperm to reach and make contact with sperm-binding receptors located on the oocyte's plasma membrane.
Through this contact, the plasma membrane of the 'winning' sperm fuses with the oocyte's plasma membrane, allowing the sperm to release its nucleus into the oocyte's cytoplasm. This fusion event also triggers a fast block to prevent polyspermy, ensuring that only one sperm fertilizes the oocyte.
Following this, a slow block mechanism called the cortical reaction occurs, further preventing polyspermy and contributing to the formation of the fertilization membrane around the newly formed zygote.