Egg with Corona Radiata
Fertilization is the process of the union of two gametes. The important events that occur in fertilization are: completion of the meiotic cycle for the egg, creation of an offspring with a diploid number of chromosomes, mixing of genetic material from both gametes to produce a unique individual with a unique genetic code. In humans, fertilization takes place in the upper areas of the female uterine tube at the meeting place of egg and sperm.
After ovulation, the egg expelled from the ovaries into the body cavity
and is taken by the fimbriae into the uterine tube. At this point, the
egg is covered in follicle cells now known as the corona radiata. From
the male reproductive tract of the ductus deferens, the spermatazoa are
mixed with other fluids upon ejaculation. The sperm are deposited in the
upper vagina of the female reproductive tract and are transported up into
the uterine tube, mostly movement of the uterine smooth muscle rather
than by sperm motility.
The sperm and the egg normally meet
somewhere in the upper uterine tube. The first layer surrounding the egg
that the sperm encounters is the corona radiata, a multicellular layer
with an intercellular matrix of proteins, hyaluronic acid, and carbohydrates.
Once the sperm has cut through this layer using sperm motility, it encounters
the zona pellucida. At this point, there is a change in the sperm surface
so that the plasma membrane of the sperm head binds to specific activators,
such as ZP3, on the zona pellucida. Through an acrosomal reaction, hydrolase
enzymes released from the sperm digest a path through the layer. The sperm
makes its way through the zona pellucida, leaving behind the remains of
the acrosomal matrix.
Fertilized Egg with Two Pronuclei
In order to prevent polyspermy, a fatal event where more than one sperm fertilizes the egg, exocytosis of cortical granules occurs around the egg, beginning at the site of the sperm entrance. This produces two important results. First, there is a depolarization of the plasma membrane that destroys all sperm receptors on the plasma membrane. Second, the zona pellucida becomes hardened to prevent any sperm from entering.
Once the sperm enters the egg, the sperm nucleus, now highly compacted
chromatin material, begins to take in fluid and proteins from the egg.
It decondenses and forms the male pronucleus. At the same time, the meiotic
arrest is lifted for the egg, and it completes its second meiotic division.
It creates another polar body and the female pronucleus.
As the pronuclei approach each other, DNA replication occurs in each site.
Once they come together, the membranes degenerate and the contents of
the pronuclei are allowed to mix along a single spindle, forming one cell.
The cell then goes through a normal mitotic division, where the two daughter
cells have genetic material from both parents. After this point, the embryo
begins to develop, traveling down the uterine tube into the uterus, where
it comes out of the zona pellucida layer and imbeds itself in the lining
of the uterine wall. (41)