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(Embryology) Fertilization - Quick Revision
  • Fertilization involves spermatozoa penetration of the corona radiata and zona pellucida, and fusion of the male and female pronuclei.
Week 1: Fertilization to Early Implantation
  • The normal site of fertilization is the ampulla of the oviduct.
  • The ejaculate (3- 4 ml) normally contains 200 million to 600 million spermatozoa, but only 200 to 300 reach the oocyte.
  • The spermatozoa can remain viable in the female genital tract for 48 hours. 
  • Freshly ejaculated spermatozoa are not able to fertilize the oocyte. 
  • When deposited in the female genital tract, spermatozoa undergo two events, capacitation and acrosome reaction.
  • Capacitation involves the removal of glycoproteins from the surface of the acrosome region of the spermatozoa that allows the sperm to penetrate the corona cells. 
  • Capacitation lasts about seven hours, with most of the process occurring in the oviduct.

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Acrosome Reaction
  • Acrosome reaction occurs quickly when the sperm reach the zona pellucida. 
  • Apertures occur on the acrosome surface, and hydrolytic enzymes are released that are used to penetrate the zona pellucida.
  • A cortical reaction occurs on the zona pellucida that mostly prevents penetration of more than one spermatozoon into the oocyte (polyspermy).
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First-Week Events
  • Cleavage is a series of rapid mitotic divisions of the zygote for the first four days. During cleavage, the cells become smaller and compacted, and are called blastomeres.
  • Day 4 - the embryo is 32 celled and called the morula (mulberry). The embryo is still contained in the zona pellucida at the morula stage.
  • Day 5 - fluid begins to seep in and forms a blastocyst cavity. The embryo, now called a blastocyst, enters the uterine cavity and floats freely for about a day. 
  • The zona pellucida breaks down and disappears at this stage. The blastocyst consists of two important types of cells.
  1. The inner cell mass (embryoblast) forms at the embryonic pole and gives rise to the embryo proper.
  2. A peripheral single layer of cells called the trophoblast forms around the blastocyst cavity and contributes to the formation of the placenta.
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  • Day 7 - the blastocyst begins to attach itself to the upper posterior wall of the uterus to begin early implantation.
  • Before implantation begins, trophoblast cells differentiate and form cytotrophoblasts (active in mitosis) and syncytiotrophoblast (lose cell membranes and no mitosis). 
  • The syncytiotrophoblasts cells form finger-like processes that invade the endometrium and release enzymes that are responsible for implantation.
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