Human Embryology
GAMETOGENESIS
Dr. Ali K. Hamad
Department of Anatomy
2022
Objectives
At the end of this session, students should able to
• Define gametogenesis, spermatogenesis and
oogenesis
• Explain the mechanism of sperm and ovum
formation
• Describe the clinical importance of
gametogenesis
Gametogenesis (gamete
formation)
• Process involving conversion of germ cells into
male and female gametes
• Chromosome number is reduced by half and
shape of cells is altered
• Number of chromosomes is reduced during
meiosis
• Gametogenesis is divided into
– Spermatogenesis: Male gametes production
– Oogenesis: Female gametes production
Meiosis
Spermatogenesis
• Spermatogonia are spherical to polygonal cells
which contain diploid number of chromosomes
• Repeated mitotic division gives rise to three
types of spermatogonia
– Dark Type A spermatogonia
– Pale Type A spermatogonia
– Type B spermatogonia
• Spermatogonia will differentiate to produce
primary spermatocytes
• Each primary spermatocyte is diploid
• Undergoes a first meiotic division to give two
secondary spermatocytes
• Secondary spermatocytes have the haploid
number of chromosomes (22 + Y or 22 + X)
• Secondary spermatocytes then divide by
mitosis to give rise to spermatids
• Each secondary spermatocyte gives rise to four
spermatids
• Young spermatids occupy deep invaginations in
apical cytoplasm of cells of Sertoli
• They then undergo spermiogenesis
• Cells of Sertoli support and provide nutrients to
the spermatids
Spermiogenesis
• Morphological changes that transform a
spermatid into spermatozoon
– discarding excess cytoplasm and growing
tails
• Last phase of spermatogenesis
Spermiogenesis
Mature sperm
Transport of Spermatozoa
• Despite having flagella the young
spermatozoa are non-motile
• Transport from seminiferous tubules to the
epididymis and vas deferens is passive
• Absorption of luminal fluid resulting in suction
of spermatozoa into epididymis and vas
deferens
Seminal fluid (Semen)
• Mixture of spermatozoa and the secretions
from the accessory glands
• Slightly alkaline in reaction and is especially
rich in fructose which serves as a source of
energy for the spermatozoa
• Also, citric acid and hyaluronidase which
facilitate the passage of spermatozoa through
cervical mucus
• Examination of spermatozoa in ejaculate has
revealed the following facts:
– Spermatozoa with Y chromosome have
round and small heads, while those with X
chromosomes are avoid and larger
– Male spermatozoa (22+y) are about twice
as numerous as female spermatozoa
(22+X)
– Female spermatozoa are more tolerant to
acidic environments than male spermatozoa
– Male spermatozoa are more speedy but
have a shorter life span of not more than 24
hours
– Female spermatozoa have a life span of
about 72 hours.
Abnormal spermiogenesis
– Poorly developed acrosome hence inability
to fertilize
– No/poorly developed tail
– Absence or decreased in mitochondria hence
poor propagation of sperm
• All these end up giving sperms which can not
fertilize the ovum and hence infertility
• Clinical Application
– Azoospermia: medical condition of a man
whose semen contains no sperm.
Oogenesis
• Development of ova from primordial germ cells
or oogonia
• Oogonia are diploid and have 22 pairs of
autosomes and a pair of X chromosomes
• Before birth oogonia undergo repeated mitotic
divisions and enlarge to become primary
oocytes
• Primary oocytes enter first division of meiosis
just before birth
• For unknown reasons division is arrested at this
stage until puberty
• At birth there are about 200,000 follicles in
each ovary
• Majority of these degenerate so that only
about 10,000 survive until puberty
Maturation of primary oocyte
• Before ovulation, primary oocyte completes
first meiotic division which was started before
birth
• This is a reduction division resulting in a
secondary oocyte with the haploid number of
chromosomes
• Secondary oocyte immediately undergoes a
second meiotic division
• But is not completed until fertilization, arrests at
second metaphase.
Ovulation
• Follicle approaches surface of ovary and
causes a stigma
• Fluid-filled clefts appear in the cumulus
oophorus and freeing ovum
• Further increase in follicular fluid results in
rupture of the stigma
• Ovum is released with liquor folliculi into the
peritoneal cavity
• Discharged ovum usually passes into fimbriated
end of uterine tube
• Two main factors bring this about namely:
– movement of ciliated epithelium at
fimbriated end
– contraction of the smooth muscle of tube
• Ovum is viable for only 24 hours
• If fertilization does not occur within 24 hours
ovum starts to degenerate
• After ovulation remaining follicle forms the
corpus luteum
• If fertilization does not occur it attains its
greatest development at about 9th days after
ovulation
• Then begins to degenerate
• This is the corpus leteum of menstruation
• At 14 days after ovulation is completely
degenerated and forms corpus
albicans
• If pregnancy: corpus luteum of
pregnancy
– produce progesterone
– Thereafter its function is taken up by
placenta
Cyclical changes in the Female Genital
System
• Undergoes structural changes every
month
• Occur in the ovary, uterine tubes, uterus
and vagina
• Occur from puberty to the onset of
menopause
Ovarian cycles
• Cyclical changes in the ovary determine the
cyclic changes in the uterus
• Ovarian cycles takes an average of 28 days
• Divided into three phases
– follicular phase
– ovulatory phase
– luteal phase
Follicular Phase (6 – 14 days)
• Commences under the influence of the follicle
stimulating hormone (FSH)
• Several primordial follicles develop into
primary follicles and proceed to develop into
graafian follicles
• Developing follicles secrete mostly oestrogens
and small amounts of progesterone
• Rising levels of oestrogen stimulate the anterior
lobe of the pituitary to produce luteinizing
hormone (LH)
• LH levels gradually increase, at around day 14
there is a surge of the LH secretion, leading to
ovulation
Functions of oestrogens
1. Development and maintenance of female
secondary sexual characters
2. Prime the female so that she is attracted to
the opposite sex
3. Stimulate the repair and proliferation of the
endometrium before ovulation
4. Stimulate the motility of the uterus and uterine
tubes
5. Stimulate growth of the uterus and its
musculature during pregnancy
6. Prevent keratinisation of the vagina by
increasing the glycogen content of the
epithelial cells
7. Stimulate the proliferation of the duct system
of the breast during pregnancy
8. Provide a feed-back mechanism which controls
the outflow of gonadotrophic hormones
Ovulation
• Ovulation occurs at about day 14 +/- 2
• Triggered by a surge of LH production
• Oocyte is released into the peritoneal cavity
• Occurrence of ovulation can be estimated by
– Measurement of pregnanediol in urine
– Taking daily body temperature recordings
Luteal Phase (14-28 days)
• Lasting approximately 13 days
• Coincides with formation, functioning and
growth of corpus luteum
• Corpus luteum produces progesterone for the
following functions
– Causes secretory phase of uterine cycle
– Increases the tone of uterine wall
If fertilization does not occur
• The corpus luteum degenerates
• Estrogen and progesterone levels fall and the
secretory endometrium enters an ischemic
phase
• Menstruation occurs
Uterine Cycle
• Also known as the
menstrual cycle
• Divided into three
phase
– Proliferative
– Secretory
– Menstrual
phases
Proliferative Phase
• Starts at the end of menstruation
• Lasting approximately 9 days
• Characterized by endometrial repair brought
by proliferation of epithelial cells
• These changes in the proliferative phase are
due to oestrogens
• Endometrium grows up to about 5 mm
Secretory Phase
• Commences after ovulation under the influence
of progesterone produced by the corpus luteum
• Glands become wide, tortuous and saccular
• Endometrium thickens up to about 7 mm due to:
– influence of progesterone and estrogen
– increased fluid in the connective tissue
• If pregnancy occurs the corpus luteum persists
and continues to secrete progesterone
• If pregnancy does not occur the corpus luteum
starts to degenerate at about day 23
• Fall in progesterone levels resulting in the
commencement of menstruation
Menstrual Phase
• Characterized by shading of blood and
endometrial tissue per vaginum
• Lasts about 4 to 5 days
• About 50 – 60ml of blood are lost
• Stratum spongiosum and part of the stratum
compactum is shed during this phase
• Mechanism of Menstrual phase
– Vasoconstrictions of the spiral arteries
– Vasodilatation causing bleeding from the
damaged superficial vessels
• Menstrual blood does not clot due to the
presence of proteolytic enzymes
Anovulatory Cycles
• No ovarian follicle ruptures to release oocyte
• No corpus luteum, endometrium does not
progress to luteal (secretary) phase
• Remains in proliferative phase until menstruation
begins
• Common in young girls starting to menstruate or
women approaching end of their reproductive
capacity