This lecture explains - the menstrual cycle First week of development Ovulation to Implantation

1. Title of Lecture: ovulation and
fertilization
By:-
Jwan Ali Ahmed AlSofi

2. First week of development:-
Ovulation to Implantation
List of Contents:
 Objectives
 Contents:
i. Ovarian cycle
ii. Ovulation
iii. Gametes transport
iv. Fertilization
 Summary or conclusion
 Question

3. Objectives:
1- Describe ovarian cycle
2- Clarify the relationship between corpus luteum and
pregnancy.
3- Identify the course of transportation of gametes
(oocyte and sperms).
4- Describe the phases of fertilization.
5- Enumerate the results of fertilization.

4. Ovarian cycle:
 At puberty, the female begins to
undergo regular monthly cycles.
 These sexual cycles are
controled by the hypothalamus.
 Gonadotropin- releasing
hormone (GnRH), produced by
the hypothalamus, acts on cells
of the anterior lobe
(adenohypophysis) of the
pituitary gland, which in turn
secrete gonadotropins.
 These hormones, (FSH) and
luteinizing hormone (LH),
stimulate and control cyclic
changes in the ovary.

7. Ovarian cycle:
- During each normal cycle, 15-20 primary-stage (preantral)
follicles are stimulated to grow under the influence of FSH.
(The hormone is not necessary to promote development of
primordial follicles to the primary follicle stage, but without it,
these primary follicles die and become atretic.)
- Thus, FSH rescues 15 to 20 of these cells from a pool of
continuously forming primary follicle, but only one of
them develops into a mature follicle and ruptures through
the surface of the ovary to discharge only one oocyte.
- When a follicle becomes atretic, the oocyte and surrounding
follicular cells degenerate and are replaced by CT, forming a
corpus atreticum.

9. Ovarian cycle:
- At the mid cycle there is an LH surge that:
1. elevates concentrations of maturation-promoting
factor, causing oocytes to complete meiosis I and
initiate meiosis II.
2. causes follicular rupture and ovulation.
3. stimulates production of progesterone by follicular
stromal cells. ( luteinization )

10. Ovarian cycle:
- FSH stimulates maturation of follicular (granulosa) cells surrounding
the oocyte.
- In turn proliferation of these cells is mediated by growth
differentiation factor 9, a member of the transforming growth factor-
B3 (TGF-B3) family..
- During maturation of the ovarian follicle, both the granulosa and
thecal cells produce estrogen that :
1- cause the uterine endometrium to enter the follicular or proliferative
phase.
2- cause thinning of the cervical mucous to allow passage of sperm.
3- stimulate pituitary gland to secrete LH.

11. Ovulation
• In the days immediately preceding ovulation, under the influence of FSH and LH, the vesicular fo
grows rapidly to a diameter o f 25 mm to become a mature vesicular (graafian) follicle.
• Coincident with final development of the vesicular follicle, there is an abrupt increase in LH that
1. the primary oocyte to complete meiosis I
2. the follicle to enter the preovulatory mature vesicular stage.
• Meiosis II is also initiated, but the oocyte is arrested in metaphase 3 hours before ovulation.
• In the meantime, the surface of the ovary begins to bulge locally, and at the apex, an avascular s
the stigma, appears.
• The high concentration of LH (LH surge ) causes increase in :
1. collagenase activity  resulting in digestion of collagen fibers surrounding the follicle.
2. Prostaglandin level  cause local muscular contractions in the ovarian wall.
• Those contractions extrude the oocyte, which together with its surrounding granulosa cells from
region of the cumulus oophorus breaks free (ovulation) and floats out of the ovary .
• Some of the cumulus oophorus cells then rearrange themselves around the zona pellucida to for
corona radiata. Follicular cells remaining inside the collapsed follicle differentiate into lutein cells

14. Clinical correlates:
 During ovulation some women feel slight pain (midcycle
pain , mittelschmerz - german)
 Ovulation is also accompanied by a rise in basal
tempreture.
 Anovulation (cessation of ovulation):
Means the woman is not able to ovulate sometime because of
an inadequate release of gondotropins, as a result they are
unable to become pregnant. In these cases treatment will
be? administration of an agent to stimulate gonadotropin in
release, and hence ovulation. Although such drugs are
effective, they often produce multiple ovulations ( likelihood
10 times higher than normal women)

15. Ovulation:
- At this time surface of the ovary begin to bulge locally
and an avascular spot(stigma) appears.
- High concentration of LH increase collagenase activity
resulting in? and increase prostaglandin level causing?
- So extruding oocyte with surrounding granulosa cells
- As the oocyte floats out of the ovary. Some of the
cumulus oophorus cells then rearrange themselves
around the ZP to form corona radiata and the
remaining follicular cells become corpus leutum.

16. Formation of the corpus luteum:
 After ovulation, the remaining granulosa cells and theca interna
collapse and thrown into folds and are vascularized by
surrounding vessels.
 Under the influence of LH, these cells develop yellowish pigment
and change into lutein cells, which form the corpus luteum. the
process is known as ‘luteinization’.
 CL produce progesterone and some estrogen.
 These two hormone causes the uterine mucosa to enter the
progestational (secretory) phase in preparation for
implantation of embryo.
 the large size of the corpus luteum, caused by hypertrophy and
accumulation of lipid in granulosa and theca interna cells.
The remaining cavity of the follicle is filled with fibrin.

18. Fate of corpus luteum
If fertilization does not occur 
o the corpus luteum reaches maximum development approximately 9 days after
ovulation.
o Subsequently, the corpus luteum shrinks because of degeneration of lutein cells
(luteolysis) and forms a mass of fibrotic scar tissue, the Corpus albicans.
o Simultaneously, progesterone production decreases, precipitating menstrual bleeding.
If the oocyte is fertilized 
o degeneration of the corpus luteum is prevented by HCG , a hormone secreted by the
syncytiotrophoblast of the developing embryo.
o The corpus luteum continues to grow and forms the corpus luteum of pregnancy
(corpus luteum graviditatis).
o By the end of the third month, this structure may be one third to one half of the total
size of the ovary.
o Yellowish luteal cells continue to secrete progesterone until the end of the fourth
month; thereafter, they regress slowly as secretion of progesterone by the
trophoblastic component of the placenta becomes adequate for maintenance of
pregnancy.
o Removal of the corpus luteum of pregnancy before the fourth month usually leads to
abortion.

19. Oocyte transport:
 Shortly before ovulation, fimbriae of the uterine tube sweep over the
surface of the ovary, and the tube itself begins to contract rhythmically.
 the oocyte, surrounded by some granulosa cells , is carried into the
tube by these sweeping movements of the fimbriae and by motion
of cilia on the epithelial lining.
 Once the oocyte is in the uterine tube, it is propelled by peristaltic
muscular contractions of the tube and by cilia in the tubal mucosa.
 the rate of transport regulated by the endocrine status during and
after ovulation. In humans, the fertilized oocyte reaches the uterine
lumen in approximately 3 to 4 days.
 Once in the tube, cumulus cells withdraw their cytoplasmic processes
from the zona pellucida and lose contact with the oocyte.

21. Sperm transport:
 When the sperms get their full motility in the
epididymis, the sperms are rapidly transported
to the urethra by peristaltic contractions of the
thick muscular coat of the ducts deferens.
 The accessory glands produce secretions that
are added to the sperm containing fluid in the
ducts deferens and urethra.

22. Sperm transport:
 -spermatozoa may remain viable in the female reproductive tract for ?
 Only 1%of sperm deposited in the vagina enter the cervix
 Movement of sperm from the cervix to uterine tube is mainly by
muscular contractions of the uterus and uterine tube And very
little by their own propulsion.
 The trip from the cervix to oviduct can occur
as rapidly as 30 minutes or as slow as 6 days
 After reaching the isthmus, sperm become less motile and cease
their migration.
 Sperm returns motility during ovulation because of
chemoattractants produced by cumulus cells surrounding the egg,
then sperms swim to the ampulla, where fertilization usually occurs.

23.  200-300 million spermatozoa in the
female genital tract deposit
 Only 300-500 reach the site of fertilization
 Only one capacitated sperm fertilize the
egg and passes through corona cells
 Spermatozoa are not able to fertilize the
oocyte immediately upon arrival in the
female genital tract but must undergo
(1) capacitation
(2) the acrosome reaction to acquire
this capability.

24. • Capacitation is a period of conditioning in the female reproductive
tract that in the human lasts approximately 7 hr. During this time, a
glycoprotein coat and seminal plasma proteins are removed from
the plasma membrane that overlies the acrosomal region of the
spermatozoa.
• Thus, speeding to the ampulla is not an advantage because
capacitation has not yet occurred and such sperm are not capable
o f fertilizing the egg.
• Much of this conditioning during capacitation occurs in the uterine
tube and involves epithelial interactions between the sperm and
the mucosal surface of the tube.
• Only capacitated sperm can pass through the corona cells and
undergo the acrosome reaction.
• The acrosome reaction, which occurs after binding to the zona
pellucida, is induced by zona proteins.
• This reaction culminates in the release of enzymes needed to
penetrate the zona pellucida, including acrosin- and trypsin-like
substances .

26.  During fertilization, the spermatozoon must
penetrate:
1. The corona radiata
2. The zona pellucida
3. The oocyte cell membrana

27. Fertilization:
 Means male and femal gametes fuse.
 The usual site of fertilization is the
ampullary region of the uterine tube - the
widest part of the tube and is close to the
ovary.
 the phases of fertilization include the
following:
■ Phase 1, penetration o f the corona radiata
■ Phase 2, penetration o f the zona pellucida
■ Phase 3, fusion of the oocyte and sperm cell
membranes

28. Phase 2, penetration of the zona pellucida
• The zona is a glycoprotein shell surrounding the egg that facilitates
and maintains sperm binding and induces the acrosome reaction.
• Both binding and the acrosome reaction are mediated by the ligand
ZP3, a zona protein which is a species-specific receptor sites for
spermatozoa on the zona surface.
• Release of acrosomal enzymes (acrosin) allows sperm to penetrate
the zona, thereby coming in contact with the plasma membrane of
the oocyte.
• Permeability of the zona pellucida changes when the head of the
sperm comes in contact with the oocyte surface.
• This contact results in release of lysosomal enzymes from cortical
granules lining the plasma membrane of the oocyte. In turn, these
enzymes alter properties of the zona pellucida (zona reaction) to
prevent sperm penetration and inactivate species-specific receptor
sites for spermatozoa on the zona surface.
• Other spermatozoa have been found embedded in the zona

31. Phase 3: Fusion of the Oocyte and Sperm Cell Membranes
• The initial adhesion of sperm to the oocyte is mediated in
part by the interaction of integrins on the oocyte and their
ligands, disintegrins, on sperm.
• After adhesion, the plasma membranes of the sperm and
egg fuse .
• Because the plasma membrane covering the acrosomal
head cap disappears during the acrosome reaction, actual
fusion is accomplished between the oocyte membrane
and the membrane that covers the posterior region of the
sperm head .
• In the human, both the head and the tail of the
spermatozoon enter the cytoplasm of the oocyte, but the
plasma membrane is left behind on the oocyte surface.

32. When the spermatozoa has entered
the oocyte
 The egg responds in three ways
1- cortical and zona reactions -
The zona pellucida becomes impenetrable to
other spermatozoa
2- resumption of the 2nd meiotic
division- The oocyte finishes its second
meiotic division and forms the female
pronucleus.
3- metabolic activation of the egg
- The head o f the sperm separates from the
tail, swells, and forms the male pronucleus

33. Cortical and zona reactions.
 As a result of the release of
cortical oocyte granules, which
contain lysosomal enzymes,
1. the oocyte membrane becomes
impenetrable to other spermatozoa,
2. the zona pellucida alters its
structure and composition to
prevent sperm binding and
penetration. By inactivate species-
specific receptor sites for
spermatozoa on the zona surface
 These reactions prevent
polyspermy (penetration of more
than one spermatozoon into the
oocyte).
Resumption of the second meiotic
division.
 the oocyte finishes its second
meiotic division immediately after
entry of the spermatozoon.
 One of the daughter cells, which
receives hardly any cytoplasm, is
known as the second polar
body; the other daughter cell is
the definitive oocyte.
 Its chromosomes (22 plus X)
arrange themselves in a
vesicular nucleus known as the
female pronucleus

34. metabolic activation of the egg
• The activating factor is probably carried by the spermatozoon .
• Activation encompasses the initial cellular and molecular events associated
with early embryogenesis.
• the spermatozoon, meanwhile, moves forward until it lies close to the
female pronucleus. Its nucleus becomes swollen and forms the male
pronucleus ; the tail detaches and degenerates. Morphologically, the male
and female pronuclei are indistinguishable, and eventually, they come into
close contact and lose their nuclear envelopes .
• During growth of male and female pronuclei (both haploid), each
pronucleus must replicate its DNA. If it does not, each cell of the two-cell
zygote has only half of the normal amount of DNA.
• Immediately after DNA synthesis, chromosomes organize on the spindle in
preparation for a normal mitotic division. The23 maternal and 23 paternal
(double) chromosomes split longitudinally at the centromere, and sister
chromatids move to opposite poles, providing each cell o f the zygote with
the normal diploid number of chromosomes and DNA
• As sister chromatids move to opposite poles, a deep furrow appears on the
sur face of the cell, gradually dividing the cytoplasm into two parts

37. Results of fertilization:
1-Restoration of the diploid number of
chromosomes.
2- Determination of the sex of the new
individual.
3- Initiation of cleavage
(mitotic division of the zygote without
fertilization).

38. Clinical correlates:
 Contraceptive methods:
1- barrier methods
2- hormonal methods.
3- a male pill
4- the intrauterine device (IUD)
5- Emergency contraceptive pills
6- sterilization

39. Clinical correlates:
Cervical
3%
Uterine
11%
Tubal
23%
Hormonal
29%
Male
34%
Infertility is the inability of a sexually active non-
contracepting couple to achieve pregnancy in one year
(WHO, 2000). Is a problem for 15% to 30%of couples
Main causes of infertility

40. Conclusion:
 Ovulation is controlled by FSH and LH.
 Site of fertilization is the ampulla.
 Sperms penetrate different layers to
reach the nucleus of the egg.

41. Questions?
 What happen to the corpus luteum if
there is fertilization and if there is no
fertilization.
 Factors affect the transport of sperms
and the egg.