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Fertilization of the Egg

Once the sperm have undergone capacitation they have the ability to fertilize the egg, but they must travel through the five layers of the oocyte in order to enter and fuse with the female pro nuclei. The five layers of the oocyte that the sperm must pass through are:
  1. Cumulus Oophorous
  2. Corona Radiata
  3. Zona Pellucida
  4. Perivitelline Space
  5. Vitelline Membrane

These layers are highlighted in picture 7 below.

Picture 7: Layers of the Oocyte

The cumulus oophorous is the peripheral layer of the oocyte and consists of several layers of cuboidal cells. Between these cells are cell junctions that provide protection for the egg. On the plasma membrane of the sperm is the enzyme hyaluronidase. Hyaluronidase will begin to break down the cell junctions in the cumulus oophorous (Jones and Lopez, 2006). Once the cell junctions of the cumulus oophorous have been broken apart, the sperm will encounter the corona radiata. The corona radiata is a single layer of cuboidal cells which also consist of cell junctions. A second enzyme, the corona radiata penetrating enzyme present on the plasma membrane of the sperm, will be able to break apart these cell junctions (Jones and Lopez, 2006).

The sperm have now reached the zona pellucida. This layer of the oocyte is a glycoprotein and has a ZP3 receptor present (Jones and Lopez 2006). The outer membrane of the sperm binds to the ZP3 receptor and will begin the acrosomal reaction, the process that allows sperm to be activated. The fusion of the plasma membrane with the outer acrosomal membrane of the sperm will form a composite membrane. This type of membrane forms openings and allows for the release of enzymes from the acrosomal space of the sperm to break down the zona pellucida. Two enzymes which are within the acrosomal space are neuramidase and acrosin. These two enzymes will break down the zona pellucida allowing the sperm to reach the vitelline membrane of the oocyte (Jones and Lopez, 2006). Picture 8 demonstrates this process during implantation.

Picture 8: Sperm binding to ZP3 receptor on Zona Pellucida

The vitelline membrane is the last layer that the sperm must pass through in order to fertilize the egg. On the vitelline membrane is a ZP2 receptor. The inner acrosomal membrane of the sperm will fuse to the ZP2 receptor (Jones and Lopez, 2006). The picture 9 below shows this binding between the sperm and oocyte. This binding will allow the sperm pronucleus to enter the oocyte. Once the sperm’s pronucleus has entered, the oocyte will complete the second meiotic division (Jones and Lopez, 2006). This will allow for the female and male pronuclei to fuse. This fusion is therefore considered fertilization. Fertilization of the oocyte typically occurs in the ampullary-isthmic portion of the oviducts (Jones and Lopez, 2006).

Picture 9: Inner Acrosomal Membrane binds ZP2 Receptor
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