DEVBIOLEC L3: FERTILIZATION

• Fertilization is the union of the sperm cell and the egg cell to form the zygote
• There are two conditions for fertilization.

  1. Sperm and eggs have a short lifespan, they must be released within a short period of time.
  2. There must be a fluid medium for sperm to swim to the egg

  • eggs cannot move by themselves
  • sperm are specialized for fast movement (flagella push them along)
• There are two types of fertilization

  1. External fertilization
  2. Internal fertilization
• External fertilization occurs outside the body. It occurs mostly in aquatic environments where the water provides fluid environment for gametes.
• In external fertilization, only sex organs required are gonads and duct to carry gametes to water.
• Examples of external fertilization are fish and amphibians.
• The three hazard and difficulties in external fertilization are

  1. Eggs and sperm may not meet
  2. Eggs may be eaten by predators
  3. Environmental conditions may change eg ph
• The solutions for external fertilization are

  1. Many eggs and sperm are produced so some survive
  2. Offspring will develop externally too eg tadpoles
• The internal fertilization occurs inside the body (oviduct or fallopian tube). Example: chicken
• Internal fertilization requires special sex organs and mating must take place within a certain time period for fertilization to occur.
• There are three adaptations to simulate mating process

  1. Mating calls
  2. Special feathers or color patches
  3. Chemicals such as pheromones  
• In oviparous organisms, embryo is enclosed enough protective shell and released by the female. (External development - reptiles and birds)
• In viviparous organisms embryo is held in a female's body until the embryonic stages of development have been completed. ( internal development - most mammals)
• The usual site of fertilization is the ampulla of uterine tubes. Ampulla is the longest and widest part of the uterine tube.
• If the oocyte is not fertilized in the ampulla, it slowly passes the uterus where it degenerates and absorbed.
• Fertilization may occur in the other parts of uterine tubes. This results to ectopic pregnancy that occurs when a fertilized egg implants and grows outside the main cavity of the uterus. The most frequent site for ectopic pregnancy is (tubal) at fallopian tube.
• From egg to zygote. The oocyte is viable for 12 to 24 hours while sperm is viable 24 to 48 hours after ejaculation.
• For fertilization to occur, coitus must occur no more than

  • 2 days before ovulation
  • 24 hours after ovulation.
• Fates of Ejaculated Sperm

  1. Leak out of the vagina immediately after deposition,
  2. Destroyed by the acidic vaginal environment,
  3. Fail to make it through the cervix,
  4. Dispersed in the uterine cavity or destroyed by phagocytes,
  5. Few (100 or few thousand) reach the uterine tubes.
• Accomplishing Fertilization

  1. Sperms must be motile
  2. Sperms must be capacitated before they can penetrate the oocyte -secretion of the female tract weaken acrosome membrane
• Capacitation is the readying of the sperm. Sperms cannot fertilize oocytes when they are newly ejaculated. The process of capacitation takes 5-7 hours. Capacitated sperms are more active.
• Capacitation occurs in the uterus and oviduct. Facilitated by substances of the female genital tract. Acrosomal reaction cannot occur until capacitation has occured.
• Steps in fertilization are:

  1. Penetration of corona radiata
  2. Attachment to and penetration of zona pellucida
  3. Fusion of Sperm and Egg
  4. Prevention of Polyspermy
  5. Activation of the Egg
  6. Decondensation of the Sperm Nucleus
  7. Completion of Meiosis and Development of Pronuclei in the egg
• Corona radiata is highly cellular layer with an intercellular matrix consisting of proteins and high concentration of carbohydrates (hyaluronic acid). Active swimming movements of spermatozoa play a significant role in penetration of the corona radiata.
• Zona pellucida is 13 microns thick in humans. Composed of principally of sulfated glycoproteins. Functions are

  1. serves as a barrier that normally allows sperm of the same species access to the egg
  2. after fertilization the modified zona prevents additional spermatozoa from reaching the egg
• Fertilization is a multi-step process whereby multiple sperm bind to the corona radiata but only a single sperm usually fertilizes the egg.
• 1. Acrosome Reaction - sperm bind to the zona pellucida proteins in the ZP; this initiates the release of enzymes from the sperm allowing it to burrow through ZP.
• 2. Zona Reaction - binding of the sperm and egg plasma membrane initiates Calcium ions + influx into the egg and release of cortical granules from the egg that block other sperm from fertilizing the egg.
• Once the sperm penetrates zona pellucida, the zona reaction occurs.
• acrosomal enzymes are esterases, acrosin, and neuraminidase that cause lysis of the zona pellucida.
• The zona reaction makes the zona pellucida impermeable to other sperms. When more than one sperm manages to enter the ovum (dispermy, trispermy), the fetus nearly always aborts.
• 3. Fusion of the sperm and egg

  • sperm makes contact with plasma membrane of the egg
  • the plasma membrane of the oocyte and sperm fuse and break down at the area of fusion
  • the head and tail of the sperm enter the _
  • contents of the sperm (head, midpiece, tail) sink to the egg, whereas plasma membrane of the sperm becomes incorporated into the egg's plasma membrane
• 4. Prevention of Polyspermy

  1. Fast Block
  2. Slow Block or Cortical Granule reaction
• 4.1 Fast Block is immediate and causes the egg membrane resting potential to rise. Sperm can no longer fuse with the egg. It consists of rapid electrical depolarization of the plasma membrane of the egg. The resting membrane potential of the egg changes for -70mV to +10mV within 2 to 3 seconds after fusion. This change in membrane potential prevents other spermatozoa from adhering to the egg plasma membrane.
• 4.2 Slow block or cortical granule reaction is physical and is mediated by Calcium ions. A wave of Ca++ propagates from the point of sperm entry, causing the cortical granules to fuse with the egg cell membrane. The released contents of the granules cause the vitelline envelope to rise and harden into fertilization envelope. ZP2 is clipped by the protease ovastacin and lose its ability to bind sperm.
• Cortical reaction:

  Cortical granules enzymes digest ZP proteins so other sperm can no longer bind. Hyaluronic acid and other proteoglycans are also released that egg become hydrated and swell, pushing other sperm away.
• 5. Activation of the Egg

  • Ca++ is necessary for egg activation in mammals
  • the sperm induces a series of Ca++ waves that can last for hours, terminating in egg activation
  • resumption of meiosis
  • cortical granules exocytosis
  • release of the inhibition on maternal mRNAs
  • formation of the male and female pronuclei
• 6. Decondensation of Sperm Nuclei

  • the chromatin begins to spread out within the nucleus as it moves closer to the nuclear materials of the egg
• 7. Completion of Meiosis & Development of Pronuclei in the Egg

  • after penetration of the egg by the sperm, the nucleus of the egg which has been arrested in the metaphase stage
• 1. Meiosis II complete
  2. Formation of male and female pronuclei
  3. Decondensation of male chromosome
  4. Fusion of pronuclei
  5. Zygote