#115 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 traveled 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.

Credit: Mayo Foundation

1. Sexual intercourse involves inserting the erect penis into the vagina. 

• 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 sperm goes in, the tail is left outside.

Credit: Web MD

• 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). 

Egg implanting in uterus. Credit: Web MD

• the uterus has a thin, spongy lining, and the embryo sinks into it = implantation.

Credit: soc.hawaii.edu