Fertilization
• “Verily WE created insan from mixture of germinal
drop (fertilized egg) in order to try him: so We gave
him (the gifts) of Hearing and Sight”.
• (Surah 76, Ad-Dahr or Al-Insan, Ayat 2, Para 29)
• Fertilization is the union of mature male and
female gametes to form a viable zygote
capable of producing a new human being.
•
• Fertilization occurs in the ampullary part of
uterine tube.
• Any male can fertilize any female. But this is
forbidden in Islam.
• They will marry. Without marriage it is zina
(adultry).
• Remember zina is one of the kbair (the big sins).
The nikah will be in front of the society.
• The guardian of the lady will hand over her to the
gentleman.
• The man will accept her in front of the society and
they will become husband and wife.
• So the legal fertilization accepted in Islam, will
be the union of a mature gamete (sperm) of
husband with a mature gamete (secondary
oocyte) of wife in the ampullary part of
uterine tube.
Oocyte transport
• At the time of ovulation, the fimbriae, of the
fallopian tube, move over the ovary and
sweep the ovulatory mass containing
secondary oocyte, into the infundibulum
(infundibular part of uterine tube).
• The ovulatory mass is then guided into the
ampulla through the movements of cilia, and
by muscular contraction of uterine tube wall.
• 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 on the cervix and in the posterior
fornix of the vagina at intercourse.
•
• So 200 to 500 million sperms are for one
secondary oocyte.
• See the importance of one female mature
gamete.
• The sperms pass by movements of their tails
through the cervical canal
• but
• The passage of sperms through the uterus and
uterine tubes appears to be assisted by
muscular contraction of the walls of these
organs.
• The prostaglandins present in the seminal
plasma may stimulate uterine motility at the
time of intercourse and assist in the
movement of sperms through the uterus and
the uterine tubes to the site of fertilization.
• It is not known how long it takes sperms to
reach the fertilization site but the time of
transport is probably short.
• Only 300 to 500 sperms reach the fertilization
site.
• The secondary oocyte is usually fertilized
within 12 hours after ovulation.
• Observations have shown that in vitro the
unfertilized human secondary oocyte dies
within 12 to 24 hours.
• Most sperms probably do not survive for more
than 24 hours in the female genital tract.
• Many sperms are stored in the crypts, glands and
even in the cervical mucus.
• This storage of sperms ensures a constant realize
of sperms and increases the chances of
fertilization.
• So some sperms may be able to fertilize a
secondary oocyte even after 24 hours.
• It is also known that not all parts of the
ejaculate are equally potent in the fertilization
process.
• In the first portion of ejaculate are
spermatozoa, epididymal fluid, and the
secretions from the prostate gland.
• In the last portions of the ejaculate are the
secretions of the seminal vesicles.
• Thus spermatozoa in the initial portion of the
ejaculate have better motility and survival
than those in the later portion.
• It is generally believed that fertilization cannot
be delayed until the oocyte reaches the
uterus, since it becomes overripe and
undergoes dissolution and degeneration in the
uterus.
Conditioning of sperms
• The sperms in the female genital tract, pass
through two processes before fertilization
• 1. Capacitation
• 2. Acrosome reaction
• Capacitation is an activation process that takes
about seven hours.
• The glycoprotein coat and plasma proteins are
removed from the plasma membrane over the
acrosome.
• Capacitation starts in the uterus and continues
in the uterine tubes.
• The follicular fluid, released at ovulation,
enhances the process.
• Look at the reception given to sperms.
• Completion of capacitation permits the
acrosome reaction to occur.
• Acrosome reaction occurs during the passage
of sperm through the corona radiata.
• The outer membrane of the acrosome fuses at
many places with the overlying cell membrane
of the sperm head.
• The fused membranes then rupture,
producing multiple perforations through
which the enzymes in the acrosome escape
and ooze out.
• Progesterone seems to stimulate the
acrosome reaction (Austin, 1975).
• It is present in large amounts in follicular fluid
released at ovulation and between the
follicular cells of corona radiate.
• During the acrosome reaction following
substances (enzymes) are released:
• 1. Hyaluronidase, needed to assist in
penetration of corona radiata cells
• 2. Trypsin-like substances, needed for the
digestion of zona pellucida
• 3. Acrosin or Zona lysine is also needed to
help the sperm to cross zona pellucida.
Penetration through Corona Radiata
• Of the 200 to 500 million sperms deposited in
the female genital tract only 400 reach the site
of fertilization.
• Only one of these sperms is marked for
fertilization.
• ALLAH Himself knows the secrets of this super
selection of genetic material.
• The other sperms help the fertilizing sperm in
penetrating the corona radiata, the first
barrier protecting the female gamete.
• Probably the tails of the sperms help to
disperse the corona radiata cells.
• The enzymes released from the acrosome,
principally hyaluronidase, also help in the
dispersal of corona radiata cells.
Penetration of Zona Pellucida
• The enzyme acrosin causes lysis of zona
pellucida.
• Trypsin like substances digest zona pellucida.
• Once the sperm touches zona pellucida, it
becomes firmly attached to it and penetrates
it rapidly.
• As one sperm passes through zona pellucida,
the permeability of zona pellucida changes
and the 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.
Penetration of cell membrane
• After passing through zona pellucida, the sperm
becomes attached to the cell membrane of
secondary oocyte.
• Since the plasma membrane covering the
acrosome has already disappeared during
acrosome reaction, the remaining membrane
that covers the posterior region of the sperm
head, fuses with the oocyte plasma membrane.
Formation of Zygote
• The head and tail of the sperm enter the
cytoplasm of the oocyte, leaving the sperm’s
plasma membrane attached to the plasma
membrane of secondary oocyte.
• The secondary oocyte now completes its
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 the sperm rapidly degenerates, and its
head enlarges to form the male pronucleus.
• The male and female pronuclei approach each
other in the center of the ovum, where they
come into contact and loose their nuclear
envelopes.
•
• The maternal and paternal chromosomes
intermingle at equatorial plate of metaphase.
• Now the cell is called zygote.
• It immediately completes its first mitotic
division and the zygote reaches two-cell-stage.
• So now we can say that fertilization is the
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.
Consequences of Fertilization
• Restoration of diploid number of
chromosomes. Fusion of the two (male and
female) germ cells produces a zygote, a
diploid cell with 46 chromosomes.
•
• Restoration of diploid amount of DNA.
• Meiosis in the male and female germ cells
allows independent assortment of maternal
and paternal chromosomes.
• Crossing over reallocates and shuffles the
genes and there is a recombination of genetic
material.
• The zygote contains a new combination of
chromosomes that is different from those of
parents.
• Through this mechanism ALLAH forms the
bases of biparental inheritance and results in
variation of the human species.
• Zona pellucida and the cell membrane of
secondary oocyte become impermeable to
other sperms thereby preventing the
polyspermy.
• Fertilization initiates the development by
stimulating the zygote to undergo a series of
rapid mitotic cells divisions called cleavage.
• Fertilization by an X-bearing sperm produces
an XX zygote which will develop into a female,
whereas fertilization by a sperm with Y
chromosome produces an XY zygote, which
will develop into a male.
• Hence it is the father whose gamete
determines the sex of their offspring.