1. FERTILIZATION
By: Dr. Mujahid Khan

2. Fertilization Is a complex sequence of coordinated molecular events
Begins with a contact between sperm and ovum
Ends up with intermingling of maternal and paternal chromosomes

3. Site Usual site is the ampulla of uterine tubes
Ampulla is the longest and widest part
If the oocyte is not fertilized here, it slowly passes to the uterus, where it degenerates and absorbed
Fertilization may occur in other parts of uterine tubes

4. Site Does not occur in the uterus
Chemical signals from oocyte attract the gamete specific proteins on the surface of the sperms
The fertilization process takes about 24 hours

6. Phases of Fertilization
Passage of sperm through corona radiata
Penetration of zona pellucida
Fusion of the plasma membrane of the oocyte and sperm
Completion of second meiotic division & formation of female pronucleus
Formation of male pronucleus

7. Passage of Sperm Through Corona Radiata
Dispersal of the follicular cells of the corona radiata surrounding the oocyte and zona pellucida
This results mainly from the action of hyaluronidase enzyme released from the acrosome
Tubal mucosal enzymes also appear to assist in dispersal

9. Penetration of Zona Pellucida
Passage of a sperm through the zona pellucida is the important phase in the initiation of fertilization
The enzymes estrases, acrosin and neuraminidase appear to cause lysis of zona pellucida
Forming a path for the sperm to follow the oocyte

10. Penetration of Zona Pellucida
Zona reaction occurs once the sperm penetrates the zona pellucida
It makes zona pellucida impermeable to other sperms
The composition of this extracellular glycoprotein coat changes after fertilization

12. Fusion of the Plasma Membrane of the Oocyte and Sperm
The plasma membranes of the oocyte and sperm fuse and break down at the area of fusion
The head and tail of the sperm enter the cytoplasm of oocyte
Sperm’s plasma membrane remains behind

13. Completion of Second Meiotic Division
Penetration of the oocyte by a sperm activates the oocyte into completing the second meiotic division
Mature oocyte and a second polar body are formed
Nucleus of the mature oocyte becomes a female pronucleus

14. Formation of Male Pronucleus
Cytoplasm of oocyte and nucleus of the sperm enlarges to form the male pronucleus
Tail of the sperm degenerates
Morphologically the male and female pronuclei are indistinguishable
The oocyte containing two haploid pronuclei is called an ootid

15. Formation of a Zygote
As the pronuclei fuse into a single diploid aggregation of chromosomes, the ootid becomes a zygote

16. Early Pregnancy Factor
Is an immunosuppressant protein
Secreted by trophoblast
Appears in maternal serum within hrs
It forms the basis of a pregnancy test during the first 10 days of development

18. Chromosome Zygote is genetically unique
Half of chromosomes from father and half from mother
New combination is formed which is different from either of parents
This mechanism forms biparental inheritance and variation of human species

19. Sex of the Embryo
Embryo's chromosomal sex is determined at fertilization
Sex is determined by the type of sperm (X or Y) fertilizes the oocyte
It is father whose gamete decides the sex

20. Results of Fertilization
Stimulates the penetrated oocyte to complete 2nd meiotic division
Restores the normal diploid number of chromosomes
Determines chromosomal sex of embryo
Initiates cleavage (cell division) of zygote

21. Cleavage of Zygote Consists of repeated mitotic divisions of zygote
Rapid increase in the number of cells
These smaller embryonic cells are called Blastomeres
Normally occurs in the uterine tube

22. Cleavage of Zygote Zygote divides first into 2 then 4 and 8
Zygote is within the thick zona pellucida during cleavage
Zona pellucida is translucent under light microscope
Begins about 30 hrs after fertilization

24. Compaction Blastomere change their shape after 9 cell stage
Tightly align themselves against each other
Form a compact ball of cells
Probably mediated by cell surface adhesion glycoprotein
Permits greater cell to cell interaction

26. Morula
When there are blastomeres the developing human is called MORULA
Enters the uterus at this stage
Spherical morula forms about 3 days after fertilization
Has outer and inner cell layer
Resembles mulberry or blackberry

27. Formation of Blastocyst
Morula enters the uterus 4 days after fertilization
A fluid-filled space called the blastocystic cavity appears inside the morula
The fluid passes from the uterine cavity through the zona pellucida to form this space

28. Formation of Blastocyst
Blastomere is separated into two parts as the fluid increases in the blastocystic cavity
A thin outer cell layer called trophoblast
Inner cell mass that gives rise to embryo called embryoblast