12.7.2
Fertilization Process 
When activated animal sperms approach the ovum surface, they are blocked by special matrices called the zona pellucida or the vitelline membrane. The zona pellucida mainly consists of three glycoproteins, Z1, Z2 and Z3. The Z3 protein species-specifically binds to Z3 receptors on the plasma membrane of sperms (Fig. 12-15). This binding triggers the acrosome reaction (Fig. 12-15), in which hydrolase stored in the acrosomal vesicle of a sperm is released to the zona pellucida and degrades it, allowing the sperm to penetrate. Fusion of the ovum plasma membrane and the sperm plasma membrane then occurs, and the sperm nucleus enters the ovum.
In addition to introducing a paternal gene set to an ovum, the sperm also activates the ovum. In mammals, when a sperm fuses with the surface of an ovum, there is a local increase in the level of intracellular Ca2+, which is then transmitted to the entire ovum in a wave-like motion (known as a calcium wave; see Fig. 9-4A in Chapter 9) and activates the ovum. In the activated ovum, hydrolase is secreted from the cortical granules in the cell to the zona pellucida, thus changing its nature to block the entry of other sperms. On the other hand, the Ca2+ signal drives the developmental program of the ovum, leading the fertilized egg to the next stages of development. These sperm roles can be replaced with physical or chemical treatment. As an example, a frog ovum can be activated by a pinprick. In some organisms, the ovum is activated without a sperm and develops into a complete organism. This type of development is called parthenogenesis.
In addition to introducing a paternal gene set to an ovum, the sperm also activates the ovum. In mammals, when a sperm fuses with the surface of an ovum, there is a local increase in the level of intracellular Ca2+, which is then transmitted to the entire ovum in a wave-like motion (known as a calcium wave; see Fig. 9-4A in Chapter 9) and activates the ovum. In the activated ovum, hydrolase is secreted from the cortical granules in the cell to the zona pellucida, thus changing its nature to block the entry of other sperms. On the other hand, the Ca2+ signal drives the developmental program of the ovum, leading the fertilized egg to the next stages of development. These sperm roles can be replaced with physical or chemical treatment. As an example, a frog ovum can be activated by a pinprick. In some organisms, the ovum is activated without a sperm and develops into a complete organism. This type of development is called parthenogenesis.
댓글 없음:
댓글 쓰기