02 Reproduction in Lower and Higher Animals - part 08 - Fertilization / Syngamy

02 Reproduction in Lower and Higher Animals - part 08 - Fertilization / Syngamy


Fertilization / Syngamy: 
  • Sexual reproduction primarily involves formation and fusion of gametes. 
  • Fertilization is the later process which involves fusion of the haploid male and female gametes resulting in the formation of a diploid zygote (2n). 
  • Like in other mammals, in humans the process of fertilization is internal and it usually takes place in the ampulla of the fallopian / uterine tube. 
  • The fertilized egg or zygote will develop into an embryo and this process occurs within the uterus.


Mechanism of fertilization : 

  • Semen released during ejaculation has sperms and some secretions. 
  • The coagulated semen now undergoes liquification and sperms become active. 
  • The mechanism of fertilization is - 
  1. Movement of sperm towards egg
  2. Entry of sperm into the egg 
  3. Activation of ovum
  4. Fusion of egg and sperm
1. Movement of sperm towards egg : 
  • It involves capacitation of sperms reaching the vagina. 
  • Here as many as 50% are demotilised / broken / destroyed. Remaining sperms undergo capacitation. This process requires 5-6 hours. 
  • Acrosome membrane becomes thin, Ca++ enters the sperm and their tails begin to show rapid whiplash movements.
  • As a result of capacitation, sperms become extra active and begin to start moving upwards from vagina to uterus and to the oviducts. 
  • The prostaglandins activate the sperms. 
  • The vestibular secretions of the female also enhance sperms motility. 
  • The sperms swim at an average speed of 1.5 to 3.0 mm/min. 
  • Sperms reach upto the ampulla as a result of their own swimming and partly by contraction of uterus and fallopian tubes stimulated by oxytocin of female. 
  • After capacitation the sperms may reach ampulla within 5 minutes. 
  • Sperms can remain viable for 24-48 hours (Ovum for about 24 hours).
2. Entry of sperm into the egg : 

  • Out of 200 to 400 million sperms, only few hundred manage to reach the ampulla. 
  • Though many sperms reach the ampulla but only a single sperm fertilizes the ovum. 
  • A sperm after reaching the egg / ovum comes to lie against it.
  •  Its acrosome releases lysins : hyaluronidase and corona penetrating enzymes. 
  • They separate and dissolve the cells of corona radiata, so the sperm head passes through the zona pellucida of egg. 
  • The zona pellucida has fertilizin receptor proteins (ZP3, ZP2). 
  • The fertilizin binds to specific acid protein- antifertilizin of sperm. 
  • It brings about attraction of sperms to the egg to enhance fertilization. 
  • Fertilizin-antifertilizin interaction is species specific. Thus, the fertilizin-antifertilizin reaction is also called compatibility reaction.
Acrosome reaction : 

  • As the sperm head touches the zona pellucida in the animal pole region, its acrosome covering ruptures to release lytic enzymes, acrosin or zona lysin. 
  • They act on the zona pellucida at the point of contact. 
  • This causes egg reaction - A small fertilization cone / cone of reception is formed on the egg membrane. 
  • The sperm head comes in contact with this cone. It results in production of a weak wave of depolarisation. 
  • Plasma membrane of the both cells dissolve at the point of contact. The sperm nucleus and the centrioles enter the egg, while other parts remain outside. 
  • As soon as the sperm head touches the vitelline membrane, a cortical reaction gets activated changing the vitelline membrane into a fertilization membrane by deactivating the sperm receptors of zona pellucida. 
  • A distinct perivitelline space is created around the fertilization membrane. This prevents any further entry of other sperms into the egg i.e. polyspermy is avoided
3. Activation of ovum : 
  • The ovum before fertilization was at metaphase II stage. 
  • With a contact of sperm head to the vitelline membrane of egg, it gets activated to resume and complete its meiosis II. 
  • With this it gives out the second polar body. 
  • The germinal vesicle organises into female pronucleus. 
  • At this stage, it is the true ovum or egg. 
4. Fusion of egg and sperm : 
  • The coverings of male and female pronuclei degenerate allowing the chromosomal pairing. This results in the formation of a synkaryon by the process called syngamy or karyogamy. 
  • The zygote is thus formed. 
  • The proximal centriole recieved from the sperm helps in formation of the synkaryon spindle and cleavage of cell into two blastomeres.

Significance of fertilization : 
  1. Secondary oocyte completes the process of oogenesis and is transformed into a mature ovum (n). 
  2. The diploid chromosome number is restored in the zygote by the process of syngamy. 
  3. The ovum lacks the centrioles necessary for further divisions, are received from the sperm during fertilization. 
  4. Fertilization involves fusion of male and female gametes from the two parents. 
  5. It results in variations which are significant to evolution. 
  6. Sex of the offspring is determined.
Always Remember · 
  • Secondary oocyte (egg) is ovulated after LH surge at about the middle of menstual cycle i.e. day 14. 
  • Egg (arrested at metaphase II) reaches the ampulla of uterine tube in 12-24 hours after ovulation. 
  • The cilia and the fimbriae of the fallopian tube help, direct the egg to ostium. 
  • During coitus/ intercourse semen is deposited into the vagina of the female. This process is called insemination. 
  • Human male during ejaculation gives out about 2-4ml of semen with an average count of 200-400 million sperms.

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