Fertilisation

Challenges to clear

• Walk the whole path from ejaculation to zygote.
• Reach the moment when a single sperm penetrates the egg.
• Reach the stage where the chromosome number is restored to 23 pairs.

The task

Step through the journey from ejaculation to zygote.

What this lesson covers

• Sperms released — During copulation the sperms are released into the vagina near the cervix of the uterus. A single ejaculation is 2 to 3 ml of semen and contains about four hundred million sperms.
• Through the cervix — The sperms swim actively with the help of their tails, passing through the cervix into the cavity of the uterus at a rate of roughly 1.5 mm per minute.
• Up the oviduct — From the uterus they climb up to reach the oviducts. Very few make it: the rest die on the way and are absorbed.
• The acrosome opens the way — If an ovum is present in the oviduct, the acrosome at the top of a sperm head secretes the enzyme hyaluronidase, which dissolves the wall of the ovum.
• One sperm enters — Several sperms may surround the egg, but only one enters it. The tail is left behind and only the head, which contains the nucleus, penetrates the egg.
• The door shuts — As soon as one sperm is in, a chemical barrier forms and prevents the entry of any other sperm.
• Zygote formed — The sperm nucleus, carrying 22 + X or 22 + Y chromosomes, fuses with the egg nucleus carrying 22 + X. This restores the normal number of 23 pairs. The fertilised egg is the zygote.

Hint

Fertilisation is the fusion of the male gamete and the female gamete to form a zygote, and it happens in the oviduct. Sperms may stay alive in the female passage for a few days, but their fertilising power lasts only one or two days.