#140 Formation of the fetus - fertilisation, implantation
After sexual intercourse, sperms swim through the cervix and the uterus into the oviducts, where they meet an egg. One sperm may fertilise the egg to produce a zygote.
After ovulation, the egg is caught in the funnel of the oviduct. Very slowly, the egg travels towards the uterus. If the egg is not fertilised by a sperm within 8-24 hours after ovulation, it will die. By this time, it has only travelled a short way along the oviduct. So a sperm must reach an egg while it is quite near the top of the oviduct if fertilization is to be successful.
1. Sexual intercourse involves inserting the erect penis into the vagina.
2. Fertilisation happens in the oviduct
- When stimulated, spongy tissue in the penis filled with blood and becomes erect.
- At the climax, semen is ejaculated from the penis into the neck of the vagina.
- Muscles in the wall of the sperm duct help to propel the semen forward
- The sperms with their tails swim from the vagina, through the cervix and uterus, into an oviduct.
2. Fertilisation happens in the oviduct
- ovum/egg pass down in oviduct
- a single sperm penetrates the membrane of ovum by secreting a protease enzyme; only the head of the
- the sperm nucleus and the egg nucleus fuse to form a diploid zygote = fertilization
- sperm can remain active in the oviduct for at least 2 days and the ovum may take a day to pass from the ovary to the uterus, so there is a fertile period of 3 to 4 days around ovulation when fertilization can happen.
3. The zygote implants in the uterus wall
- the zygote moves slowly down the oviduct. As it goes, it divides by mitosis.
- it takes several hours for the embryo to reach the uterus, and by this time it is a ball of 16 or 32 cells (a blastocyst).
- the uterus has a thin, spongy lining, and the embryo sinks into it = implantation.