FERTILIZATION

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

5
(No Transcript)
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

8
(No Transcript)
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

11
(No Transcript)
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
• Sperms 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 24-48 hrs
• It forms the basis of a pregnancy test during the
  first 10 days of development

17
(No Transcript)
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

23
(No Transcript)
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

25
(No Transcript)
26
Morula

• When there are 12-32 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

29
(No Transcript)
30
(No Transcript)