Gametogenesis and fertilization are complex biological processes. Gametogenesis involves the formation of gametes, or sex cells, through meiosis. This results in haploid cells containing half the number of chromosomes. In males, spermatogenesis occurs in the testes to produce sperm. In females, oogenesis occurs in the ovaries to produce eggs. Fertilization usually takes place when an egg is fertilized by sperm in the fallopian tubes, restoring the diploid number of chromosomes. Any errors during gametogenesis or fertilization can result in chromosomal abnormalities.

1. OJORA K.A
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GAMETOGENESIS AND FERTILIZATION

2. GAMETOGENESIS
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 The sperm and oocyte, the male and female gametes,
are highly specialized sex cells.
 Each of these cells contains half the number of
chromosomes (haploid number) that are present in
somatic (body) cells.
 The number of chromosomes is reduced during
meiosis, a special type of cell division that occurs
during gametogenesis.
 Gamete maturation is called spermatogenesis in males
and oogenesis in females.
 Gametogenesis (gamete formation) is the process of
formation and development of specialized
generative cells, gametes.
 During gametogenesis, the chromosome number is

3. MITOSIS
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4. MEIOSIS
 Meiosis is a special type of cell division that involves two
meiotic cell divisions; it takes place in germ cells only.
 Diploid germ cells give rise to haploid gametes
(sperms and oocytes).
 The first meiotic division is a reduction division because
the chromosome number is reduced from diploid to
haploid by pairing of homologous
chromosomes(synapsis) in prophase and their
segregation at anaphase.
 The spindle connects to the chromosome at the
centromere. At this stage, they are double-chromatid
chromosomes.
 By the end of the first meiotic division, each new cell
formed (secondary spermatocyte or secondary oocyte)
has the haploid chromosome number (double-chromatid
chromosomes), i.e., half the number of chromosomes of
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6. Crossover
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 Crossovers, critical events in meiosis I, are the
interchange of chromatid segments between paired
homologous chromosomes.
 Segments of chromatids break and are exchanged as
homologous chromosomes separate.
 As separation occurs, points of interchange are
temporarily united and form an X-like structure, a
chiasma.
 The approximately 30 to 40 crossovers (one or two per
chromosome) with each meiotic I division are most
frequent between genes that are far apart on a
chromosome.

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 As a result of meiotic divisions:
Genetic variability is enhanced through
● crossover, which redistributes genetic material
● random distribution of homologous chromosomes
to the daughter cells
● Each germ cell contains a haploid number of
chromosomes, so that at fertilization the diploid
number of 46 is restored.

9. MEIOSIS
 The second meiotic division follows the first division
without a normal interphase.
 Each chromosome divides and each half, or chromatid,
is drawn to a different pole; thus, the haploid number of
chromosomes (23) is retained and each daughter cell
formed by meiosis has the reduced haploid number of
chromosomes, with one representative of each
chromosome pair (now a single-chromatid
chromosome).
 The second meiotic division is similar to an ordinary
mitosis except that the chromosome number of the cell
entering the second meiotic division is haploid.
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11.  Abnormal Gametogenesis
 Disturbances of meiosis during gametogenesis,
e.g., nondisjunction, result in the formation of
chromosomally abnormal gametes. If involved in
fertilization, these gametes with numerical
chromosome abnormalities cause abnormal
development such as occurs in infants with Down
syndrome
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12. SPERMATOGENESIS
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 Sequence of events by which spermatogonia are
transformed into spermatozoa.
Begins at puberty.
Spermatogonia, which have been dormant in the
seminiferous tubules of the testes since the fetal period,
begin to increase in number at puberty.
After several mitotic divisions, the spermatogonia grow
and undergo changes.

13. SPERMATOGONIA
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14. PHASES
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1. SPERMATOCYTOGENESIS
2. MEIOSIS
3. SPERMIOGENESIS
4. SPERMIATION
5. CAPACITATION?

15. SPERMATOCYTOGENESIS
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 Spermatogonia grow and undergo changes after several
mitotic divisions.
 Spermatogonia are transformed into primary
spermatocytes, the largest germ cells in the seminiferous
tubules.
MEIOSIS
 Each primary spermatocyte subsequently undergoes a
reduction division-the first meiotic division-to form two haploid
secondary spermatocytes, which are approximately half the
size of primary spermatocytes.
 Subsequently, the secondary spermatocytes undergo a
second meiotic division to form four haploid spermatids,
which are approximately half the size of secondary
spermatocytes.

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17. MEIOSIS
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18. SPERMIOGENESIS
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The spermatids are gradually transformed into four
mature sperm by a process known as spermiogenesis .
The entire process of spermatogenesis, which includes
spermiogenesis, takes approximately 2 months.
When spermiogenesis is complete, the sperms enter the
lumina of the seminiferous tubules.

19. SPERMIOGENESIS INVOLVES:
I. Formation of the acrosome, which covers half of the
nuclear surface and contains enzymes to assist in
penetration of the egg and its surrounding layers
during fertilization.
II. Condensation of the nucleus
III. Formation of neck, middle piece, and tail.
IV. Shedding of most of the cytoplasm as residual bodies
that are phagocytized by sertoli cells.
 In humans, the time required for a spermatogonium to
develop into a mature spermatozoon is approximately
74 days, and approximately 300 million sperm cells are
produced daily.
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20. SPERMIOGENESIS
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21. SPERMIATION
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22.  Mature sperms are free-swimming, actively motile cells
consisting of a head and a tail.
 The neck of the sperm is the junction between the head and
tail.
 The head of the sperm forms most of the bulk of the sperm
and contains the haploid nucleus.
 The anterior two thirds of the nucleus is covered by the
acrosome, a caplike saccular organelle containing several
enzymes.
 When released, these enzymes facilitate dispersion of the
follicular cells of the corona radiata and sperm penetration of
the zona pellucida during fertilization.
 The tail of the sperm consists of three segments: middle
piece, principal piece, and end piece.
 The tail provides the motility of the sperm that assists its
transport to the site of fertilization.
 The middle piece of the tail contains mitochondria, which
provide the adenosine triphosphate necessary for activity.
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25. MATURATION OF SPERMS
 Freshly ejaculated sperms are unable to fertilize
oocytes.
 Sperms must undergo a period of conditioning-
capacitation-lasting approximately 7 hours.
 During this period, a glycoprotein coat and seminal
proteins are removed from the surface of the sperm's
acrosome.
 The membrane components of the sperms are
extensively altered.
 Capacitated sperms show no morphologic changes,
but they are more active.
 Sperms are usually capacitated in the uterus or
uterine tubes by substances secreted by these
parts of the female genital tract.
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26. OOGENESIS
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SEQUENCE OF EVENTS BY WHICH OOGONIA ARE
TRANSFORMED INTO MATURE OOCYTES
BEGINS BEFORE BIRTH, COMPLETED AFTER
PUBERTY
OOGENESIS CONTINUES TO MENOPAUSE
OOGONIA UNDERGO A SERIES OF MITOTIC
DIVISIONS
ARRANGED IN CLUSTERS SURROUNDED BY
FOLLICULAR CELLS BY THE END OF THE THIRD

27. OOGENESIS - PRENATAL
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During early fetal life, oogonia proliferate by mitosis.
Oogonia enlarge to form primary oocytes before birth
As a primary oocyte forms, connective tissue cells
surround it and form a single layer of flattened,
follicular epithelial cells.
The primary oocyte enclosed by this layer of cells
constitutes a primordial follicle.
Primary oocytes begin the first meiotic division before
birth, but completion of prophase does not occur until
adolescence. (diplotene stage)

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29. OOGENESIS - POSTNATAL
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 Beginning during puberty, usually one follicle matures
each month and ovulation occurs, except when oral
contraceptives are used.
 No primary oocytes form after birth in females.
 The primary oocytes remain dormant in the ovarian
follicles until puberty.
 As a follicle matures, the primary oocyte increases in
size and, shortly before ovulation, completes the first
meiotic division to give rise to a secondary oocyte and
the first polar body.
 The polar body is a small, nonfunctional cell that soon
degenerates.

30. OOGENESIS - POSTNATAL
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 At ovulation, the nucleus of the secondary oocyte begins
the second meiotic division, but progresses only to
metaphase, when division is arrested.
 If a sperm penetrates the secondary oocyte, the second
meiotic division is completed, and most cytoplasm is
again retained by one cell, the fertilized oocyte.
 The other cell, the second polar body, also a small
nonfunctional cell, soon degenerates.
 As soon as the polar body is extruded, maturation of the
oocyte is complete.

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32. OOGENESIS - POSTNATAL
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 There are approximately two million primary oocytes in
the ovaries of a newborn female, but most regress
during childhood so that by adolescence no more than
40,000 remain.
 Of these, only approximately 400 become secondary
oocytes and are expelled at ovulation during the
reproductive period.
 Few of these oocytes, if any, are fertilized and become
mature.
 The number of oocytes that ovulate is greatly reduced in
women who take oral contraceptives because the
hormones in them prevent ovulation from occurring.

33. OVARIAN CYCLE
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 FSH and LH produce cyclic changes in the ovaries-the
ovarian cycle- development of follicles, ovulation,
and corpus luteum formation.
 During each cycle, FSH promotes growth of several
primordial follicles into 5 to 12 primary follicles; however,
only one primary follicle usually develops into a mature
follicle and ruptures through the surface of the ovary,
expelling its oocyte.
 Follicular Development- Development of an ovarian
follicle is characterized by:
1. Growth and differentiation of primary oocyte
2. Proliferation of follicular cells
3. Formation of zona pellucida
4. Development of the theca folliculi

34. Follicular Development
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 As the primary follicle increases in size, the adjacent
connective tissue organizes into a capsule, the theca
folliculi.
 The theca soon differentiates into two layers, an internal
vascular and glandular layer, the theca interna, and a
capsule-like layer, the theca externa.
 Thecal cells are thought to produce an angiogenesis factor.
 The follicular cells divide actively, producing a stratified layer
around the oocyte .
 The ovarian follicle soon becomes oval and the oocyte
eccentric in position.
 Subsequently, fluid-filled spaces appear around the follicular
cells, which coalesce to form a single large cavity, the antrum,
which contains follicular fluid.
 After the antrum forms, the ovarian follicle is called a vesicular

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36. Follicular Development
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 The primary oocyte is pushed to one side of the follicle,
where it is surrounded by a mound of follicular cells, the
cumulus oophorus, that projects into the antrum.
 The follicle continues to enlarge until it reaches maturity
and produces a swelling on the surface of the ovary.
 The early development of ovarian follicles is induced by
FSH, but final stages of maturation require LH as well.
 Growing follicles produce estrogen, a hormone that
regulates development and function of the reproductive
organs.

37. Ovulation
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 Around midcycle, the ovarian follicle, under the
influence of FSH and LH, undergoes a sudden growth
spurt, producing a cystic swelling or bulge on the
surface of the ovary.
 Before ovulation, the secondary oocyte and some cells
of the cumulus oophorus detach from the interior of the
distended follicle.
 Ovulation is triggered by a surge of LH production.
 Ovulation usually follows the LH peak by 12 to 24
hours.
 The LH surge, elicited by the high estrogen level in the
blood.
 Expulsion of the oocyte is the result of intrafollicular
pressure.

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39. Mittelschmerz and
Ovulation
Anovulation
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 A variable amount of
abdominal pain,
mittelschmerz (German,
mittel, mid + schmerz, pain),
accompanies ovulation in
some women.
 In these cases, ovulation
results in slight bleeding into
the peritoneal cavity, which
results in sudden constant
pain in the lower abdomen.
 Mittelschmerz may be used
as a symptom of ovulation,
but there are better
symptoms, such as the
slight drop in basal body
 Some women do not
ovulate (cessation of
ovulation-anovulation)
because of an inadequate
release of gonadotropins.
 In some of these women,
ovulation can be induced
by the administration of
gonadotropins or an
ovulatory agent such as
clomiphene citrate.
 This drug stimulates the
release of pituitary
gonadotropins (FSH and
LH), resulting in
maturation of several

40. OOGENESIS
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42. MATURE (GRAAFIAN) FOLLICLE
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43. COMPARISON OF GAMETES?
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SIZE
MOTILITY
CYTOPLASM
SUPPORTING CELLS
CHROMOSOMES

44. CLINICAL APPLICATION
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 THE LONG DURATION OF THE FIRST MEIOTIC DIVISION (UP TO
45 YEARS) MAY ACCOUNT IN PART FOR THE RELATIVELY HIGH
FREQUENCY OF MEIOTIC ERRORS, SUCH AS NONDISJUNCTION
THAT OCCUR WITH INCREASING MATERNAL AGE

45. ABNORMAL GAMETES
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 The ideal maternal age for reproduction is generally
considered to be from 18 to 35 years.
 The likelihood of chromosomal abnormalities in the
embryo increases after the mother is 35.
 In older mothers, there is an appreciable risk of Down
syndrome or some other form of trisomy in the infant.
 As many as 10% of sperms in an ejaculate are grossly
abnormal (e.g., with two heads), but it is generally
believed that these abnormal sperms do not fertilize
oocytes due to their lack of normal motility.

46. FERTILIZATION
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47. FERTILIZATION
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 The usual site of fertilization is the ampulla of the uterine tube, its longest
and widest part.
 Spermatozoa may remain viable in the female reproductive tract for
several days.
 If the oocyte is not fertilized here, it slowly passes along the tube to the
uterus, where it degenerates and is resorbed.
 Although fertilization may occur in other parts of the tube, it does not occur
in the uterus.
 Chemical signals (attractants), secreted by the oocyte and surrounding
follicular cells, guide the capacitated sperms (sperm chemotaxis) to the
oocyte.
 Fertilization is a complex sequence of coordinated molecular events that
begins with contact between a sperm and an oocyte and ends with the

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50. PHASES OF FERTILIZATION
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1. Passage of a sperm through the corona radiata. Dispersal of the
follicular cells of the corona radiata surrounding the oocyte and zona
pellucida appears to result mainly from the action of the enzyme
hyaluronidase released from the acrosome of the sperm.
2. Penetration of the zona pellucida. The enzymes esterases, acrosin,
and neuraminidase appear to cause lysis of the zona pellucida, thereby
forming a path for the sperm to follow to the oocyte. Once the sperm
penetrates the zona pellucida, a zona reaction-a change in the
properties of the zona pellucida-occurs that makes it impermeable to
other sperms.
3. Fusion of plasma membranes of the oocyte and sperm. The plasma
or cell 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 the
oocyte, but the sperm's plasma membrane remains behind.
4. Completion of the second meiotic division of oocyte and formation
of female pronucleus. Penetration of the oocyte by a sperm activates
the oocyte into completing the second meiotic division and forming a
mature oocyte and a second polar body. Following decondensation of the

51. PHASES OF FERTILIZATION
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1. Formation of the male pronucleus. Within the cytoplasm of the oocyte,
the nucleus of the sperm enlarges to form the male pronucleus and the
tail of the sperm degenerates.The oocyte containing two haploid
pronuclei is called an ootid.
2. As the pronuclei fuse into a single diploid aggregation of
chromosomes, the ootid becomes a zygote. The chromosomes in the
zygote become arranged on a cleavage spindle in preparation for
cleavage of the zygote.

52. FERTILIZATION
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54. RESULTS OF FERTILIZATION
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COMPLETETION OF OOCYTE 2ND MEIOTIC DIVISION
RESTORATION DIPLOID NUMBER (2N)
SPECIES VARIATION
CHROMOSOMAL SEX DETERMINATION
INITIATION OF CLEAVAGE
FORMATION OF THE ZYGOTE

55. CLINICAL APPLICATION
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INFERTILITY
Preselection of the Embryo's Sex
Assisted Reproductive Technologies
i. In Vitro Fertilization and Embryo Transfer
ii. Cryopreservation of Embryos
iii. Intracytoplasmic Sperm Injection
iv. Surrogate Mothers
BARRIER METHODS
SURGICAL CONTRACEPTION
MULTIPLE PREGNANCIES

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THE END