Fertilization
After sexual intercourse, why do millions of sperm enter the vagine?
- Only a small percentage will ever reach the location of the egg
- It takes many sperm to penetrate the follicle cell layer and coating called the zona pellucida
What do the motile sperm do in order to have enough "fuel" for their potentially long journey to the egg?
They absorb some of the sugar in semen
Where does fertilization occur?
Inside one of the Fallopian tubes
What is the zona pellucida?
A protective glycoprotein layer that surrounds the egg.
How do sperm penetrate the zona pellucida?
- They release hydrolytic enzymes from their acrosomes to help break down the zona pellucida
- Cortical granules are released immediately after one sperm binds itself to the egg's plasma membrane, they release enzymes which the change the zona pellucida to prevent more sperm from entering the egg
- This ensures that polyspermy is avoided
What part of the sperm enter the egg?
Only the nucleus- the flagellum and mitochondria are destroyed by vesicles released from the egg
Describe the acrosome reaction.
- It involves exocytosis: the release of digestive enzymes from the acrosome of the sperm which allows sperm to penetrate the zona pellucida
What happens after fertilization?
- The nuclei (containing the maternal and paternal sets of chromosomes) remain seperate initially
- They are haploid and called pronuclei. While in the pronuclei stage, the DNA undergoes replication in preperation for mitosis
- The two pronucli then come together and the nuclear membranes dissolve
- A spindle apparatus forms as the chromosomes prepare for the first mitotic division of the newly formed diploid cell
- It takes about 30 hrs after fertilization for this first mitotic division to be completed
What is an acrosome?
A cap-liked organelle on the top of the head of a sperm cell that contains enzymes essential for fertilization
What is the purpose of the sperm's flagellum during fertilization?
To provide motility, or movement, allowing it to propel itself to reach and fertilize the egg
How long does it typically take for the fertilized egg to complete its first mitotic division?
30 hrs
Why is polyspermy bad?
- Too many chromosomes from the male will disrupt cell division and development
- Combination of abnormal chromosomes and disrupted cell division prevents the embroyo from developing correctly, leading to embroyonic death