Reproductive Science - Clinical Embryology images of human preimplantation embryos - 2nd polar body extrusion

These five sequential Images showhow a fertilised human oocyte extrudes a second polar body.

On the far left image, you can see an oocyte within a shell called the zona pellucida. The space between the oocyte and the zona is called the peri-vitelline space (PVS). An unfertilised oocyte is arrested at the Metaphase II stage of meiosis. An embryologist can see this by looking for the presence of a single polar body within the PVS. You can see this at the 5 o'clock posiiton.

This oocyte has been fertilised by a single sperm. In response to the entry of the sperm into the cytoplasm, the oocyte completes meiosis II. This involves extrusion of half of the female genetic material within the oocyte in the form of a second polar body. As the images progress to towards the right, you can see the extrusion start at the 6 o'clock position. The oocyte slowly extrudes the excess genetic material in a small portion of cytoplasm into the PVS.

The process is completed by the image on the far right, where two polar bodies are visible within the PVS.

This stage of fertilisation takes place a few hours after entry of the sperm and before the formation of pronuclei within the cytoplasm.