Sperm Migration
Sperm Structure
The head of a mature sperm is around 4-5 µm long, 3 µm wide at its broadest point and is composed of 3 parts;
1. Nucleus – this fills most of the head and contains a haploid set of condensed, genetically inactive chromosomes.
2. Acrosome – a lysosome in the form of a thin cap covering the apical half of the nucleus. It contains enzymes that are used to penetrate the egg if the sperm is successful.
3. Basal body of the tail flagellum – this is nestled in an indentation at the basal end of the nucleus.
Fig. 4. Sperm Structure. Image courtesy of Wikimedia Commons.
The tail is also divided into 3 regions;
1. Midpiece – a cylinder around 5-9 µm long and half as wide as the head. It is the thickest part of the tail and contains numerous mitochondria that spiral tightly around the axoneme of the flagellum. These mitochondria produce ATP needed for the beating of the tail.
2. Principal piece – this is around 40-45 µm long. It constitutes most of the tail and consists of the axoneme surrounded by a sheath of fibres.
3. Endpiece – this region is approximately 4-5 µm long and is made up of the axoneme only, making it the narrowest part of sperm.
Sperm Migration
Ejaculate contains around 150-600 million sperm which enter the vagina. An egg must be fertilised within 12-24 hours of ovulation for it to survive but it takes 72 hours for the egg to reach the uterus from the ovary. Importantly then, the sperm meets the egg in the distal 1/3rd of the uterine tube, i.e. the ovarian end. Sperm get there within 5-10 minutes due to their motility, the forceful contractions of the uterus and the prostaglandins in the fluid.
Only a few 100 sperm (out of the 100’s of millions ejaculated) reach the egg due to;
- Destruction by the vaginal acid.
- Drainage out of the vagina.
- Failure to penetrate the cervical mucus (those that penetrate may be destroyed by
leukocytes in the uterus).
- Entrance up the wrong uterine tube (normally only one ovary produces an egg each
month).
The female tract, the cervix, uterus and uterine tubes, essentially filters the sperm.
Sperm migrate by ‘lashing’ their tails in a snake-like way along the female tract assisted by uterine contractions during the female orgasm; this ‘sucks’ the semen from the vagina and spreads it through the uterus.
Sperm swim to heat and progesterone, a chemotaxin, and the egg may also release a chemical that attracts the sperm a short distance.
Capacitation
Sperm undergo capacitation whilst migrating through the female tract and this is required for penetration of the egg. Before ejaculation the sperm head membrane contains a large amount of cholesterol, which toughens it and prevents premature release of the acrosomal enzymes. This protects the sperm and stops enzymatic damage to the spermatic ducts.
Fig. 5. Sperm Migration. Image courtesy of Wikimedia Commons.
After ejaculation, fluids of the female scrape off molecules, including this cholesterol, and wash away other inhibitory factors in the semen. The membrane of the sperm head becomes more fragile and more permeable to calcium ions, which diffuse into the sperm and cause more powerful movements of the tail. After capacitation the sperm is able to penetrate the egg due to this modification of the protective protein coat of the sperm cell membrane.
Most sperm are fertile for a maximum of 48 hours after ejaculation and so there is a low chance of fertilisation if intercourse occurs more than 48 hours before ovulation. Fertilisation is also unlikely if intercourse takes place more than 14 hours after ovulation as the egg would no longer be viable by the time sperm is capacitated. Thus, for the chance of conception the ‘window of opportunity’ is between 48 hours prior to ovulation and 14 hours after it. However, to avoid pregnancy it is important to allow a wider margin due to variations in;
- Sperm and egg longevity.
- Capacitation time.
- Time of ovulation.
Although sperm can reach the distal uterine tube soon after ejaculation it cannot fertilise the egg for around 10 hours due to the requirement of this process of capacitation.