Mammalian Development: Fertilization

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I- Fertilization

Fertilization accomplishes two objectives: 1. sex and 2. reproduction. Sex is accomplished when genes from two parents are combined. Reproduction is accomplished when there is successful generation of a new organism.

A. Sex
1. Contact and recognition between sperm and egg

Mammalian sperm and egg interaction takes place inside the female reproductive tract. Fertilization takes place at the ampulla. For fertilization to happen, the sperm has to get to the ampulla. This is accomplished by means of translocation and capacitation.

a. Translocation

For sperm to travel from the vagina to the oviduct, it is assisted by several processes. These processes are sperm motility, uterine muscle contraction, and sperm rheotaxis. Sperm motility is accomplished by flagellar action. The flagellar action of the sperm alone, however, cannot transport it to the site of the egg at they right time. Translocation of sperm is further assisted by uterine muscle contractions. Sperm movement is also directed by chemical signals, migrating against the direction of the flow of calcium influx. This is known as sperm rheotaxis.

b. Capacitation

Newly ejaculated mammalian sperms are immature. For them to be able to fertilize the egg, they have to undergo physiological changes to mature. This process is called capacitation.

Capacitation involves lipid and protein changes. Lipid changes involves the alteration of the sperm cell membrane through the removal of cholesterol. When cholesterol is removed, protein receptors important for sperm-egg interaction is exposed. Protein changes involves the removal of certain proteins and carbohydrates on the surface of the sperm. This exposes recognition sites that are also important for sperm-egg interaction.

2. Regulation of sperm entry into the egg.

Only one sperm is allowed to fertilize an egg. To ensure that this happens, regulation must take place. Before the mammalian sperm can successfully bind to the oocyte, it has to penetrate the egg's zona pellucida. In general, sperm entry through the oocyte is facilitated by acrosome reaction, and the interaction of the Izumo protein on the sperm and the Junno protein on the egg. Acrosome reaction takes place when Izumo proteins are trasnlocated to the sperm cell membrane. The Izumo protein then binds to Junno protein of the oocyte. This interaction recruits the egg membrane protein CD9 which facilitates membrane fusion. This then allows the sperm to successfully penetrate the oocyte (figure 1) .

3. Fusion of the genetic material of sperm and egg.

The mammalian egg (oocyte) is arrested in the metaphase of it's second meiotic division. Upon entry of the sperm into the oocyte, the male DNA is released into the cytoplasm of the oocyte. During this time, DNA synthesis takes place separately on both male and female pronuclei. Microtubules  surrounding the male and female pronuclei assists in joining them together. Upon meeting, their nuclear envelope break down, chromatin condenses, and chromosomes orient themselves in a common mitotic spindle (figure 2).


B. Reproduction

1. Activation of egg metabolism to start development.

A series of chemical reactions takes place to allow resumption of cell division. A rise in Ca2+ is necessary for egg activation. The release of Ca2+ is apparently activated by the sperm. These Ca2+ binds to several enzymes such as calmod-ulin-activated protein kinase, MAP kinase, and synaptotagmin.

  • Calmod-ulin-activated protein kinase - eliminates the inhibitors of RNA translation.
  • MAP kinase - allows meiosis to resume.
  • Synaptotagmin - initiates cortical granule fusion.


Reference:
Scott G.F. 2016. Developmental Biology 11th Ed. Sunderland, Mass: Sinauer Associates.

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