Fertilisation Process in Human Biology

Study Notes

During ejaculation, sperm are released into the vagina at the cervix.
Sperm travel through the cervix and body of the uterus and into the fallopian tubes.
The fallopian tubes are lined with cilia which propels the sperm forward.
Fertilisation usually occurs when the ovum is 1/3 of the way down the fallopian tube.
The secondary oocyte (released at ovulation) is surrounded by two layers:
- Corona radiata: an outer layer of follicle cells held together by an acid.
- Zona pellucida: an inner layer of glycoprotein surrounding the oocyte’s plasma membrane.
Sperm contain an enzyme in their acrosome that breaks down the acid holding the corona radiata together.
Thousands of sperm are required to loosen the cells and allow the entry of one sperm.
Once the sperm has penetrated the corona radiata, it releases digestive enzymes to break down the glycoprotein matrix of the zona pellucida.
Only one sperm can enter the ovum, which initiates the formation of a fertilisation membrane.
The plasma membrane of the sperm and ovum fuse, and the nucleus of the sperm (male pronucleus) moves through the cytoplasm of the ovum.
Sperm entry stimulates the secondary oocyte to finish the second meiotic division, and its nucleus becomes the female pronucleus.
The male and female pronuclei then fuse to form a single nucleus with a diploid chromosome number, which forms a zygote.

After fertilisation, the zygote travels down the fallopian tube and begins to divide by mitosis.
Rapid cell division is called cleavage and results in a multicellular embryo.
A morula is formed during the cleavage phase (day 3-4).
By day 6 after fertilisation, a blastocyst has formed and reached the uterus.
A blastocyst is a hollow ball of cells surrounding a cavity filled with fluid.
An inner cell mass is a structure of about 30 pluripotent stem cells on one side of the cavity in the blastocyst.
The blastocyst is free in the uterus for 2-3 days before sinking into the endometrium and becoming attached to the wall of the uterus (implantation).

During implantation, the inner cell mass undergoes changes to form 3 layers of cells (primary germ layers).
These layers will differentiate into all the tissues that makeup the body.
The three primary germ layers are:
- Ectoderm: the outer layer, which gives rise to the skin and the brain and nervous system.
- Mesoderm: the middle layer, which gives rise to skeletal and muscle tissues, as well as heart and circulatory systems.
- Endoderm: the inner layer, which gives rise to the inner lining of many organs and the digestive system.

Early in embryonic development, four embryonic membranes develop, which lie outside the embryo to protect and nourish it during development.
The four embryonic membranes are:
- Amnion: the first to develop, which surrounds the embryo by the 8th day after fertilisation.
- Amniotic fluid is produced by the amnion, which acts as a shock absorber, maintains a constant temperature, and allows for movement of the embryo/foetus.
- Chorion: formed from the outer cells of the blastocyst and a layer of cells from the mesoderm.
- The chorion surrounds the embryo and other membranes and becomes the foetal part of the placenta.
- The other two membranes become part of the umbilical cord.