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FERTILIZATION Presented by Priya Gupta M.Sc. final


Fertilization is a complex sequence of events that begins with contact between a sperm and a secondary oocyte and ends with the intermingling of the maternal and paternal chromosomes at metaphase of 1 st cleavage of Zygote.

Site of fertilization:

Site of fertilization External fertilization- occur outside the body of parent e.g. In invertebrates and lower chordates Internal fertilization- occur inside the body of female e.g. In terrestrial forms in which copulatory organs are developed

Mechanism of fertilization:

Mechanism of fertilization Approach of sperm to ova Capacitation and contact Acrosomal reaction and penetration Activation of ovum Amphimixis

Oocyte Transport:

Oocyte Transport At the time of ovulation, the fimbriae move over the ovary and sweep the ovulatory mass containing secondary oocyte , into the infundibulum . The ovulatory mass is then guided into the ampulla through the movements of cilia, and by muscular contraction of uterine tube wall (Tubal Peristalsis). It has been estimated that the ovulatory mass takes about 25 minutes to reach the site of fertilization.


SPERM TRANSPORT Usually 200 to 500 million sperms are deposited. Sperms pass by movements of their tails through uterus & tubes; assisted by muscular contraction. Prostaglandins present in the seminal plasma may stimulate uterine motility at the time of intercourse. It is not known how long it takes sperms to reach the fertilization site but the time of transport is short 5-45minutes.


CONDITIONING OF THE SPERMS The sperms in the female genital tract, before fertilization undergo Capacitation Acrosome reaction

Capacitation :

Capacitation Starts in uterus& continues in to tubes. Follicular fluid enhances the process. It takes about 7 hours. Glycoproteins are removed from plasma membrane over acrosome. Capacitated sperms show no morphological change, but more active Completion of capacitation permits acrosome reaction to occur.


Fertilizin and antifertizin reaction Fertilizin is glycoprotein,produced by cells of zona pellucida Makes sperm sticky adhere into clumps and to egg surface Antifertizin secreted by surface membrane of sperm it is acid protein

Acrosome reaction:

Acrosome reaction Occurs during passage of sperm through corona radiata . Outer membrane of the acrosome fuses at many places with overlying cell membrane of sperm head. fused membranes then rupture, producing multiple perforations through which the enzymes ( Hyaluronidaze ) in the acrosome escape and ooze out.


Hyaluronidase : needed to assist in penetration of the corona radiata barrier; Trypsin -like substances: needed for the digestion of the zona pellucida ; Acrosin : also needed to help the sperm cross the zona pellucida . Progesterone (present in follicular fluid) seems to stimulate the acrosome reaction . ACROSOME REACTION


Penetration Passage of sperm through corona radiata Hyaluronidase from acrosome Sperm tail Tubal mucosal enzymes Penetration of zona pellucida facilitated by Acrosin Neurominidase Esterases Zona reaction Lysosomal enzymes of cortical granules contd.

Zona Pellucida :

Zona Pellucida Wassermann and co-workers (1980, 1985, 1987, 1988) Found that zona pellucida is composed of 3 glycoproteins ZP1, ZP2, ZP3 Repeating subunits of ZP2 and ZP3 form filaments that are bound together by ZP1


Wassermann et al. found that when the sperm binds to ZP3 it causes a change in Ca+2 and Na+ flux across the sperm plasmalemma that results in the acrosome reaction. (ZP1 and ZP2 will not cause this to happen). The sperm actually binds to an o-linked oligosaccharide that is part of ZP3 - (a carbohydrate component of the glycoprotein)


ZONA REACTION As one sperm passes through zona pellucida, the permeability of zona pellucida changes and zona pellucida becomes impermeable to others sperms. This is called zona reaction. It is believed that granules released from the secondary oocyte, which contain lysosomal enzymes, produce this zona reaction.

Activation of ovum:

Activation of ovum Cytoplasmic fusion Fertilization cone fo rmation -at the region of fusion cytoplasm of egg form projection Cortical reaction - rupture of cortical granules that occurs concurrently with the Ca++ release. Contents of granules are released into perivitelline space and cause “hardening” of the zona pellucida . Causes fertilization membrane to rise away from surface of egg in some species.


Metabolic activation :-adenyl cyclase activity increased Change in permeability of plasma membrane NAD-kinase activated Ionic concentration of Na+,K+,Ca++ changed Rate of protein synthesis increased

Oocyte Cell Membrane :

Oocyte Cell Membrane After passing through zona pellucida , sperm becomes attached to cell membrane of secondary oocyte . The head and tail of the sperm enter the cytoplasm of oocyte , leaving the sperm’s plasma membrane attached to the plasma membrane of oocyte . Oocyte is usually fertilized within 12 hours after ovulation .


The secondary oocyte completes the second meiotic division, forming a mature ovum and the second polar body. The nucleus of the ovum is known as the female pronucleus . Once within the cytoplasm of the ovum the tail of sperm degenerates & its head enlarges to form male pronucleus .

Amphimixis & Zygote formation:

Amphimixis & Zygote formation The male and female pronuclei come into contact and loose their nuclear envelopes. The maternal and paternal chromosomes intermingle at equatorial plate of metaphase of the first mitotic division of the zygote.

Chemistry of fertilization:

Chemistry of fertilization Gamone Gamete Function Androgamone-1 Sperm Conserve sperm activity Androgamone-2 Sperm Dissolve vitelline membrane Gynogamone-1 Ovum Neutralizes androgamone-1 Gynogamone-2 Ovum Makes sperm head sticky to facilate attachment of sperm to egg surface


Restoration of diploid chromosomes Species variation Shuffling of genes. The zygote contains a new combination of chromosomes, different from the parents. Bases of biparental inheritance and results in variation of the human species. Results of Fertilization


ART IVF( in vitro fertlization )  The process involves hormonally controlling the ovulatory process, removing  ova  (eggs) from the woman's  ovaries  and letting  sperm  fertilise them in a fluid medium. The fertilised egg ( zygote ) is then transferred to the patient's  uterus  with the intent to establish a successful pregnancy .  GIFT( gamte intrafallopian transfer ) the sperm and eggs are mixed together and immediately inserted.  ZIFT (zygote intrafallopian transfer ) ZIFT, the fertilized eggs --”zygotes”-- are inserted within 24 hours of the mixing.


REFERANCES Developmental Biology-Veer Bala Rastogi & M.S.Jayaraj Reproduction-Steve Parker  Essential Reproduction-M. Johnson



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