15.4 - Development of Human Foetus

The Process of Fertilization

Of the millions of sperms ejaculated, only one sperm will successfully penetrate and fertilize the ovum. Once the penetration is successful, a fertilization membrane will form on the outer surface of the secondary oocyte, preventing other sperms from entering the ovum. The union of the nuclei of a sperm and an ovum will then result in a diploid zygote.

Early Development of an Embryo until Implantation

Figure 8 - After fertilization, the human zygote undergoes repeated mitosis to divide as it travels along the Fallopian tube. The first division form a two-cell embryo. Further divisions result in the formation of mass of cells called morula. The morula then grows into a blastocyst. Implantation of the blastocyst to the endometrium occurs 7 days after fertilization. The blastocyst continues its growth to become embryo.

Role of Human Chorionic Gonadotropin (HCG) Hormone

During the early stage of pregnancy, the placenta will release a very high level of the HCG hormone which can be easily detected with a pregnancy test kit. HCG hormone plays a major role in ensuring that the corpus luteum continues to release estrogen and progesterone to maintain the pregnancy.

Role of Placenta and Umbilical Cord in Fetal Development

During pregnancy, the formation of placenta connects to the fetus from the mother by the umbilical cord. The umbilical cord is a tube with one umbilical vein and two umbilical arteries.

• The umbilical arteries carry deoxygenated blood and other waste products from the fetus to the placenta.
• The umbilical vein carries oxygenated blood and nutrients from the placenta to the fetus.

Figure 9 - The placenta is the site of the exchange of nutrients, respiratory gases and wastes between the mother and fetus. Nutrients, oxygen, antibodies and hormones are transported from the maternal arteriole to the fetus. Meanwhile, carbon dioxide and nitrogenous waste products are transported from the fetus to the maternal vein. It also acts as an endocrine gland that secretes estrogen and progesterone to replace the degenerative corpus luteum. Both hormones help in maintaining a thick, blood-enriched endometrium throughout the pregnancy.

Fetal and Maternal Blood Circulatory Systems

Even though the blood capillaries of the fetus and mother are very close to one another, their blood do not mix. This is important because it

• Prevents the mixing of blood groups which can cause agglutination if incompatible.
• Ensures that the thin blood vessels of the fetus do not burst due to the high pressure of blood flowing from the mother's side.
• Prevents certain harmful bacteria or toxins from entering the fetus.

However, the membranes of the capillaries and chorionic villi only provide a partial protection as some of the substances such as drugs, alcohol, nicotine, HIV virus and rubella virus can still penetrate the barrier and affect the fetal development.