Sexual Reproduction in Humans - The Final Stages

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Sexual Reproduction in Humans - The Final Stages

For pregnancy to occur a sperm must fertilise the egg.

The sperm travel from the epididymis into the vas deferens by waves of contractions along the walls of the tubes.

For sex to take place, the penis needs to be inserted into the woman's vagina and thus it needs to be erect. An erection is caused by an increase in blood pressure, brought about by the dilation of arteries supplying blood to the penis, and the constriction of veins taking blood away from the penis.

Sexual stimulation triggers contractions in the wall of the vas deferens. Sperm are swept along, past the prostate gland and the seminal vesicles, which secrete fluid to maintain the sperm in a viable state. The resulting mixture, called semen, is expelled from the end of the penis during ejaculation. The alkaline semen is deposited at the top of the acidic vagina.

A layer of mucus blocks the cervix. It is thinnest, and therefore easiest for the sperm to penetrate at the beginning of the menstrual cycle. For the sperm it is an incredibly long journey from the cervix to the egg in the oviducts, so it probably manages it by a combination of swimming using their tails, muscular contractions of the uterus and/or oviducts, and the action of cilia lining the uterus and the oviducts.

It takes several hours to reach the oviducts, but this is beneficial because, before sperm can fertilise an egg they must undergo a process called capacitation. This is where enzymes from the female genital tract remove molecules from the outer surface of the sperm.

It is possible for a sperm to fuse with a secondary oocyte if there is one near the top of an oviduct. Sperm surround it and, if one manages to penetrate the outer layer so that the two cell membranes fuse, meiosis II is completed in the secondary oocyte to form the haploid nucleus that is now ready to be fertilised.

The process of fertilisation is shown below:

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When the two plasma membranes fuse the cortical reaction takes place. Cortical granules from the secondary oocyte release their contents into the space around the oocyte. They cause the zona pellucida to thicken and harden. It becomes impenetrable and so prevents the entry of more than one sperm. After fertilisation the diploid zygote divides by mitosis to form a solid ball of cells.

At this stage the embryo is called the morula. The morula moves towards the uterus, developing into a hollow ball of cells, called the blastocyst. After about 6-8 days it implants into the uterus wall.

The outer layer of the blastocyst, the chorion, forms small projections - chorionic villi - which extend into the uterus wall.

Enzymes are released by the embryo to break down arterial and venous blood vessels in the mother's endometrium. The mother's blood leaks into the space around the villi.

Foetal membranes envelop the embryo. The innermost one is the amnion, which encloses a fluid-filled cavity around the embryo to cushion and protect it.

The growing villi become invaded with capillary loops coming off the umbilical artery. Blood circulates through these so that the foetal blood flows very close to the mother's blood.

The structure of the placenta is made up of the villi and the mother's tissue in very close union. The villi provide a massive surface area over which transport of molecules can take place.

Examples include:

  • Oxygen diffuses from mother's blood to foetal blood;
  • Carbon dioxide and urea diffuse from foetal blood to mother's blood;
  • Glucose, amino acids and vitamins move by facilitated diffusion into the foetal blood;
  • Sodium, potassium and calcium are actively transported into the foetal blood.

Antibodies also pass from mother to foetus to give the foetus passive immunity during the pregnancy and for a few months after birth.

Throughout pregnancy, the placenta releases oestrogen, progesterone, HPL (Human Placental Lactogen), and CG (Chorionic Gonadotrophin).

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Waste substances from the foetus are carried by the umbilical artery. They diffuse into the blood-filled space and into the mother's vein to be carried away.

Nutrients are carried by the mother's artery to the placenta. They diffuse into the capillaries of the foetus. They are taken to the foetus by the umbilical vein.

Oestrogen:
  • Stimulates growth of uterus.
  • Inhibits release of FSH.
  • Stimulates the duct system of the breasts to develop.
  • Inhibits lactation by inhibiting the release of prolactin.
  • Increases the sensitivity of the uterus to the hormone oxytocin (this causes the uterus wall to contract).

Progesterone:
  • Maintains the endometrium.
  • Inhibits myometrium contractions (therefore decreasing the possibility of miscarriage).
  • Stimulates the development of milk glands in the breasts.
  • Inhibits the release of prolactin.
  • Inhibits the release of FSH.

HPL:
  • Stimulates the growth and development of breasts.

    Note: Without HPL oestrogen and progesterone would not affect the breasts.


CG:
  • Maintains the corpus luteum for about 3 months. This means that oestrogen and progesterone are produced before the placenta assumes this role.

After about 38 weeks, labour begins and birth follows.

Late in pregnancy, progesterone levels decrease and prostaglandins are produced. They stimulate contractions, which are strong, as oxytocin is no longer inhibited by high progesterone levels.

After labour the baby is expelled. The umbilical cord is clamped and severed and sometime afterwards the placenta is also expelled (the afterbirth).

Due to the loss of the placenta, oestrogen and progesterone levels fall, prolactin is no longer inhibited so the breasts can produce milk. Oxytocin is released from the posterior pituitary gland, which forces milk out of the nipples.

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