Reproduction in Humans
Senior 3 · Biology
Reproduction in Humans
Lesson 4 of 5
25 min
Every person reading this began as a single cell about a tenth of a millimetre across, smaller than a full stop. About nine months later that cell had become a baby of typically about 3 kilograms, with a beating heart, working kidneys and a brain ready to learn a language. This lesson follows the first steps: how the egg and sperm meet, how the embryo settles in the uterus, and how the placenta keeps it alive.
Definition
Fertilisation: the fusion of the nucleus of a sperm with the nucleus of an egg, forming a zygote with 46 chromosomes.
Implantation: the sinking of the early embryo, a hollow ball of cells, into the thick lining of the uterus.
Embryo: the developing baby during the first 8 weeks after fertilisation; from then until birth it is called a fetus.
Placenta: the organ, formed from the embryo's tissues and the uterus lining, through which materials are exchanged between the mother's blood and the fetus's blood. The umbilical cord joins the fetus to the placenta.
Amnion: the membrane that encloses the fetus in amniotic fluid.
Gestation: the length of pregnancy: about 40 weeks counted from the first day of the last period, about 38 weeks from fertilisation.
During sexual intercourse, semen is released into the vagina. Sperm swim through the cervix and the uterus into the oviducts. Of the millions released, only a few hundred typically reach the egg. If an egg is there, one sperm's head enters it; at once the egg's surface changes so that no other sperm can enter. The two nuclei fuse: this is fertilisation, and the zygote has 46 chromosomes, 23 from each parent.
Figure 1: simplified; the cells are drawn far larger than real size. Timings are typical.
As the zygote is moved along the oviduct it divides by mitosis: into 2 cells, 4, 8, and then a solid ball of cells. By the time it reaches the uterus it has become a hollow ball of cells, which sinks into the thick lining (Lesson 3): implantation, typically about a week after fertilisation. Some cells will form the embryo; others form the membranes and, with the lining, the placenta.
Twins. Identical twins form when one zygote splits into two embryos: they have the same genes and are the same sex. Non-identical twins form when two eggs are released and each is fertilised by a different sperm: they are no more alike than other brothers and sisters.
Figure 2: harmful substances, such as alcohol, nicotine, some medicines and some pathogens, including HIV and the bacterium that causes syphilis, can also cross.
In the placenta, the fetus's blood flows through capillaries inside thousands of finger-like villi, which give a very large surface area, and the mother's blood flows round them. The two bloods are kept apart by a thin barrier, so they do not mix: this protects the fetus from the mother's higher blood pressure and keeps their blood groups separate. Substances cross mainly by diffusion. The umbilical cord carries the fetus's blood to the placenta in two umbilical arteries and back in one umbilical vein. The placenta also makes hormones, including progesterone, which keeps the uterus lining in place.
The amniotic fluid cushions the fetus against bumps, lets it move and exercise its muscles, and helps keep its temperature steady.
Worked example 1: how fast does a fetus grow?
Typical masses: about 300 g at 20 weeks of pregnancy and about 3300 g at 40 weeks.
Worked example 2: why should a pregnant woman not drink alcohol or smoke?
Warning
Common mistakes. (1) Saying the mother's and fetus's blood mix in the placenta: they are kept separate; substances diffuse across a thin barrier. (2) Writing that the umbilical cord carries the mother's blood: it carries the fetus's blood. (3) Saying fertilisation happens in the uterus: it happens in the oviduct; the embryo implants in the uterus.
Pregnancy and Role of Placenta, IGCSE Biology, The Cambrige Girl. Watch for implantation and the substances that cross the placenta, and compare them with Figures 1 and 2.
Tip
Memory hook: "mother gives GOAAW, fetus returns CU". Mother to fetus: Glucose, Oxygen, Amino acids, Antibodies, Water (and salts). Fetus to mother: Carbon dioxide, Urea.
Practice: fertilisation and pregnancy
Key points
Sources and credits
Written by Ganzaa: the text, Figures 1 and 2, the worked examples (typical figures) and the practice questions.
