Fertilisation & Implantation (DP IB Biology)

Fertilisation is the fusion of one sperm cell and one ovum; this fusion of two haploid nuclei gives rise to a diploid zygote

During sexual reproduction, many sperm are released, and the sperm cells are attracted towards the secondary oocyte by chemical signals

• Remember that at this point in the oogenesis process, meiosis II has not yet been completed, so the female gamete is still a secondary oocyte and not yet an ovum

When the sperm cells reach the secondary oocyte, the process that takes place at its cell surface prevents more than one sperm from passing through its cell surface membrane

• The entry of more than one sperm into a single oocyte is known as polyspermy

The oocyte is surrounded by a layer of follicle cells, as well as a layer of glycoproteins known as the zona pellucida

The sperm cells need to digest the glycoproteins of the zona pellucida in order to reach the oocyte cell surface membrane; they do this by releasing digestive enzymes from a structure called the acrosome

• This is known as the acrosome reaction

When the first sperm cell digests its way through the zona pellucida, it reaches the oocyte cell surface membrane; complementary receptors on the head of the sperm bind with proteins on the oocyte cell surface membrane, enabling the cell surface membranes of the two gametes to fuse together and the sperm nucleus to enter the oocyte

• At this point the process of meiosis II continues, leading to the release of the second polar body and the formation of the mature ovum

The fusion of the two cell surface membranes triggers the movement of a series of vesicles called cortical granules; the vesicles move from the outer layer of the ovum cytoplasm to the cell surface membrane, releasing enzymes that digest the binding proteins on the cell surface of the ovum and that cause the zona pellucida to harden

• This is the cortical reaction

• The enzymes are released by the process of exocytosis

• Once the binding proteins have been broken down and the zona pellucida has hardened, no more sperm cells can enter the ovum; polyspermy is prevented

Inside the ovum the male and female nuclei fuse together and fertilisation is completed

Following human fertilisation, the newly fertilised ovum divides by mitosis to form two diploid nuclei (i.e. each nucleus contains two sets of chromosomes) and the cytoplasm divides equally to form a two-cell embryo

Mitosis continues to form a four-cell embryo and this process continues until eventually, the embryo takes the shape of a hollow ball called a blastocyst (with an internal group of cells called blastomeres)

• Blastomeres will eventually develop into the foetus

The embryo is now referred to as a blastocyst and up until this point is found in the oviduct

After about seven days it consists of around 125 cells and will have reached the uterus

During the embryo stage and up until this point the blastocyst is surrounded by a protective extracellular coat called the zona pellucida; at around seven days of age this coat breaks down and is lost

The blastocyst has used up the nutrient supplies of the egg cell and now needs an external supply of nutrients, which it obtains by implanting into the endometrium (uterus lining)

The outer layer of the blastocyst develops finger-like projections that allow it to penetrate the endometrium

At this stage, there is already an exchange of nutrients and oxygen with the mother’s blood

The embryo continues to grow and develop rapidly after this point