15.4 - Development of a Human Foetus

Fertilisation and Zygote Formation

One sperm penetrates the secondary oocyte in the Fallopian tube.
The secondary oocyte membrane changes and prevents other sperms from entering.
The sperm nucleus fuses with the ovum nucleus to form a diploid zygote ( $2 n$ ).

The zygote undergoes repeated mitosis while moving down the Fallopian tube.
The first division forms a two-celled embryo; subsequent divisions form a morula.
The morula develops into a blastocyst.
The blastocyst implants in the endometrium and develops into an embryo.

The placenta produces HCG during pregnancy.
HCG levels rise during early pregnancy.
HCG ensures that the corpus luteum continues to secrete oestrogen and progesterone in the early stages of pregnancy.
Pregnancy tests detect HCG in urine.

The placenta forms from the mother’s endometrial tissue and embryonic tissue.
The umbilical cord connects the placenta to the foetus.
Umbilical vein: Carries oxygen-rich, nutrient-rich blood from the placenta to the foetus.
Umbilical arteries: Carry deoxygenated blood, carbon dioxide and nitrogenous waste such as urea from the foetus to the placenta.
The placenta exchanges substances: glucose, amino acids, hormones, antibodies and oxygen move to foetal blood; carbon dioxide and nitrogenous waste move to maternal blood.
From the fourth month, the placenta replaces the corpus luteum in producing progesterone and oestrogen to maintain endometrial thickness.

Maternal and foetal blood are separated by a thin membrane.
Separation protects the foetus from some toxins and bacteria.
Separation prevents high maternal blood pressure from bursting thin foetal blood vessels.
Separation prevents agglutination if maternal and foetal blood groups differ.
Drugs, medication, cigarette smoke, alcohol, HIV and rubella viruses can cross the membrane and disrupt foetal development.