Oogenesis, spermatogenesis, and embryogenesis are the processes of gamete and embryo formation. Oogenesis involves the growth of oogonia in females from fetal development through puberty, forming primary oocytes arrested in meiosis. Spermatogenesis in males involves spermatogonia differentiating into spermatocytes through meiosis and spermiogenesis. Embryogenesis begins with fertilization and cleavage, forming a blastocyst through implantation and gastrulation, establishing the three germ layers. Over 8 weeks, all major organ systems begin developing as the embryo undergoes folding and segmentation.

1. OOGENESIS &
SPERMOTOGENESS
EMBROGENESIS
Dr Hauwa Shitu B.
MBchB, Resident (Gen.Surg.)

2. OUTLINE
Definitions
Oogenesis
Spermatogenesis
Embryogenesis
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3. Definitions
• Oogenesis: the process of growth and maturation of the oogonia into ovum.
• Spermatogenesis: is the process by which the male gametes called spermatozoa
(sperms) are formed from the primitive spermatogenic cells (spermatogonia) in
the testis.
• Embryogenesis : the first eight weeks of development after fertilization, is an
incredibly complicated process.
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4. Oogenesis
Gametogenesis, in females, oogenesis process of growth and maturation of the
oogonia into ovum.
Begins with the ovarian stem (oogonia) migrating into the ovaries to multiply
during early embryonic development.
Most die prenatally, remaining begin meiosis toward end of gestation called
primary oocytes (still diploid).
Oogenesis is arrested at prophase I of 1st meiotic division during fetal growth
Number of primary oocytes in the ovaries declines from about 2 million in
infant, to approximately 400,000 at puberty, to zero by the end of menopause.

5. There are Two types of oogenesis in female;
Fetal Oogenesis
Mitotic division
At the 2 - 5 months of gestation the oogonia divide rapidly by mitosis
to form about 6 to 7 million primordial follicles in the two ovaries.
Each follicle is formed of oocyte(immature ovum containing 44
autosomes and 2x chromosomes).
Meiotic division
Then meiosis begins at 5th months of fetal life.
The primordial oocytes (germ cells) formed from oogonia enter the
long prophase I of the first meiotic division.
This continues each month at puberty to menopause within the
primary follicle
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6. 2.Pubertal oogenesis
Starts at puberty and continue throughout the female reproductive life.
During this period meiosis continues in two principal stages:
First meiotic division:
Begins and is completed just before ovulation
Primary oocyte divides into two daughter haploid cells, one will be the
secondary oocyte (22 autosomes + X chromosome).
While the other is the first polar body that degenerate.
Second meiotic division:
Also begins immediately but its arrested in metaphase II.
And just before ovulation, it is completed only if the mature ovum is fertilized by
a sperm, forming a zygote while the second polar body also degenerates.
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8. Note:
•In contrast to male who produces spermatogonia and primary spermatocytes
continuously throughout life.
•The female cannot form oogonia after birth, its formed before birth.
•Oogenesis in the female begins during intra-uterine life i.e. before birth (At the
5th month of gestation the ovaries contain about 6 to 7 million oogonia are
formed)
•While in the male spermatogenesis is stopped at the spermatogonia stage and
commences at puberty.
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9. SPERMATOGENESIS: MALE
GAMETOGENESIS
Spermatogenesis is classically divided into three phases:
1.Spermatocytogenesis
2.Meiosis
3.Spermiogenesis
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10. • Spermatocytogenesis
1.Primordial germ cells (46, 2N)
from wall of the yolk sac arrive in
the testes at week 6 and remain
dormant until puberty. At puberty,
primordial germ cells differentiate
into type A spermatogonia (46,
2N).
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11. • 2. Type A spermatogonia undergo
mitosis to provide a continuous
supply of stem cells throughout the
reproductive life of the male. Some
type A spermatogonia differentiate
into type B spermatogonia (46, 2N).
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12. • Meiosis
1. Type B spermatogonia enter
meiosis I and undergo DNA
replication to form primary
spermatocytes (46, 4N).
2. Primary spermatocytes complete
meiosis I to form secondary
spermatocytes (23, 2N).
3. Secondary spermatocytes
complete meiosis II to form four
spermatids (23, 1N).
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13. SPERMATOGENESIS: MALE GAMETOGENESIS
• Spermiogenesis
1. Spermatids undergo a postmeiotic series of morphological changes to form
sperm (23, 1N).
These changes include the
(a) formation of the acrosome,
(b) condensation of the nucleus, and
(c) formation of head, neck, and tail.
The total time of sperm formation (from spermatogonia to spermatozoa) is about
64 days.
2. Newly ejaculated sperm are incapable of fertilization until they undergo
capacitation, which occurs in the female reproductive tract and involves the
unmasking of sperm glycosyltransferases and the removal of adherent plasma
proteins coating the surface of the sperm.
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15. EMBRYOGENESIS
• Embryogenesis, the first eight weeks of development after fertilization, is an
incredibly complicated process.
• Step 1: a zygote is the single cell formed when an egg and a sperm cell fuse;
the fusion is known as fertilization
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16. • Let’s Start at the Very Beginning
Step 2: the first 12-to 24-hours after a zygote is formed are spent
in cleavage – very rapid cell division.
Cleavage is a series of mitotic divisions of the zygote where the plane
of the first mitotic division passes through the area of the cell
membrane where the polar bodies were previously extruded.
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17. CLEAVAGE AND BLASTOCYST FORMATION
2. The process of cleavage eventually forms
a blastula consisting of cells called
blastomeres.
3. A cluster of blastomeres (16–32
blastomeres) forms a morula.
4. Blastomeres are totipotent up to the
eight-cell stage (i.e., each blastomere can
form a complete embryo by itself ).
Totipotency refers to a stem cell that can
differentiate into every cell within the
organism, including extraembryonic tissues.
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18. CLEAVAGE AND BLASTOCYST FORMATION
• Blastocyst formation involves fluid secreted within the morula that forms the
blastocyst cavity.
• The conceptus is now called a blastocyst.
1. The inner cell mass of the blastocyst is called the embryoblast (becomes the
embryo). The embryoblast cells are pluripotent. Pluripotency refers to a stem cell
that can differentiate into ectoderm, mesoderm, and endoderm.
2. The outer cell mass of the blastocyst is called the trophoblast (becomes the
fetal portion of the placenta).
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19. • Zona pellucida degeneration occurs by day 4 after conception. The zona
pellucida must degenerate for implantation to occur
• IMPLANTATION
• The blastocyst usually implants within the posterior superior wall of the uterus
by day 7 after fertilization.
• Implantation occurs in the functional layer of the endometrium during the
progestational (secretory) phase of the menstrual cycle.
• The trophoblast proliferates and defernites into the cytotrophoblast and
syncytiotrophoblast. Failure of implantation may involve immune rejection
(graftversus- host reaction) of the antigenic conceptus by the mother.
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20. Week 2 of Human Development (Days 8–14)
• FURTHER DEVELOPMENT OF THE
EMBRYOBLAST
• The embryoblast FORM epiblast layer
(columnar cells) and the ventral
hypoblast layer (cuboidal cells).
• Together form bilaminar embryonic disk.
• Epiblast will give us amniotic cavity and
Hypoblast will form exocoelomic
membrane,
• Syncytiotrophoblast the outer
multinucleated zone of the trophoblast
where no mitosis occurs (i.e., it arises
from the cytotrophoblast). And continues
its invasion of the endometrium, thereby
eroding endometrial blood vessels and
glands.
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21. • Lacunae forms within the
syncytiotrophoblast and become filled
with maternal blood and glandular
secretions.
• Cytotrophoblast. The cytotrophoblast
is mitotically active as new
cytotrophoblastic cells migrate into
the syncytiotrophoblast, thereby
fueling the growth of the
syncytiotrophoblast. In addition,
cytotrophoblastic cells also produce
local mounds called primary chorionic
villi that bulge into the surrounding
syncytiotrophoblast.
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22. Embryonic Period (Weeks 3–8)
• GENERAL CONSIDERATIONS
• By the end of the embryonic period, all major organ systems begin
development, although functionality may be minimal.
• During the embryonic period, the uteroplacental circulation cannot satisfy the
increasing nutritional needs of the rapidly developing embryo, so
development of the cardiovascular system is essential.
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23. • During the embryonic period, folding of the embryo occurs in two distinct
planes. Craniocaudal folding progresses due to the growth of the central
nervous system (CNS) and the amnion.
• Lateral folding progresses due to the growth of the somites, amnion, and other
components of the lateral body wall.
• Both the craniocaudal folding and lateral folding change the shape of the
embryo from a twodimensional disk to a three-dimensional cylinder.
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24. • Both the craniocaudal folding and lateral folding change the shape of the
embryo from a two dimensional disk to a three-dimensional cylinder.
• By the end of week 8, the embryo has a distinct human appearance.
• During the embryonic period, the basic segmentation of the human embryo
in the craniocaudal direction is controlled by the Hox (homeobox) complex
of genes.
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25. Gastrulation
• Gastrulation is the process that establishes
the three definitive germ layers of the
embryo (ectoderm, intraembryonic
mesoderm, and endoderm) forming a
trilaminar embryonic disk by day 21 of
development.
• These three germ layers give rise to all the
tissues and organs of the adult.
• Gastrulation is heralded by the formation
of the primitive streak and is caused by a
proliferation of epiblast cells. The primitive
streak consists of the primitive groove,
primitive node, and primitive pit.
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26. • The cloacal membrane, located caudal
to the primitive streak. is the future
site of the anus where the epiblast
and hypoblase cells fuse.
The ectoderm, intraembryonic
mesoderm, and endoderm of the
trilaminar embryonic disk are all derived
from the epiblast.
The term intraembryonic mesoderm
describes the germ layer that forms
during week 3 (gastrulation), in contrast
to the extraembryonic mesoderm,
which forms during week 2.
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27. • The caudal-most somites eventually
disappear to give a final count of
approximately 35 pairs of somites.
The number of somites is one of the
criteria for determining the age of the
embryo. Somites further differentiate
into the following components:
a. Sclerotome forms the cartilage and
bone components of the vertebral
column.
b. Myotome forms epimeric and
hypomeric muscles.
Dermatome forms dermis and
subcutaneous area of skin
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28. Changes involving intraembryonic mesoderm
• 1. Paraxial mesoderm is a thick plate of mesoderm located on each side of the
midline.
• Paraxial mesoderm becomes organized into segments known as somitomeres,
which form in a craniocaudal sequence.
• Somitomeres 1–7 do not form somites but contribute mesoderm to the
pharyngeal arches. The remaining somitomeres further condense in a
craniocaudal sequence to form 42–44 pairs of somites. The first pair of somites
forms on day 20, and new somites appear at a rate of 3 per day.
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29. 2. Intermediate mesoderm is a longitudinal dorsal ridge of mesoderm located
between the paraxial mesoderm and lateral mesoderm. This ridge develops into
the urogenital ridge, which forms the future kidneys and gonads.
3. Lateral mesoderm is a thin plate of mesoderm located along the lateral sides of
the embryo.
Large spaces develop in the lateral mesoderm and coalesce to form the
intraembryonic coelom.
• The intraembryonic coelom divides the lateral mesoderm into two layers:
a. Intraembryonic somatic mesoderm (also called somatopleure)
b. Intraembryonic visceral mesoderm (also called visceropleure or
splanchnopleure)
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30. • Notochord is a solid cylinder of mesoderm extending in the midline of the
trilaminar embryonic disk from the primitive node to the prochordal plate. It has
a number of important functions,
• which include the following:
• a. The notochord induces the overlying ectoderm to differentiate into
neuroectoderm to form
• the neural plate.
• b. The notochord induces the formation of the vertebral body of each of the
vertebrae.
• c. The notochord forms the nucleus pulposus of each intervertebral disk.
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31. 5. Cardiogenic region is a horseshoe-shaped region of mesoderm located at the
cranial end of the trilaminar embryonic disk rostral to the prochordal plate. This
region forms the future heart.
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34. CLINICAL CONSIDERATIONS
• Ectopic tubal pregnancy (ETP)
1. ETP occurs when the blastocyst implants within the uterine tube due to delayed
transport.
2. The ampulla of the uterine tube is the most common site of an ectopic
pregnancy. The rectouterine pouch (pouch of Douglas) is a common site for an
ectopic abdominal pregnancy.
3. ETP is most commonly seen in women with endometriosis or pelvic
inflammatory disease.
4. ETP leads to uterine tube rupture and hemorrhage if surgical intervention (i.e.,
salpingectomy) is not performed.
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35. CLINICAL CONSIDERATIONS
• Caudal dysplasia (sirenomelia) refers to a constellation of syndromes ranging
from minor lesions of lower vertebrae to complete fusion of the lower limbs.
• Caudal dysplasia is caused by abnormal gastrulation, in which the migration of
mesoderm is disturbed.
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36. • Human chorionic gonadotropin (hCG) is a glycoprotein produced by the
syncytiotrophoblast, which stimulates the production of progesterone by the
corpus luteum (i.e., maintains corpus luteum function).
• Essential for the maintenance of pregnancy until week 8 prior to placental
production
• hCG can be assayed in maternal blood at day 8 or urine at day 10.
• Low hCG values may predict a spontaneous abortion or indicate an ectopic
pregnancy.
• Elevated hCG values may indicate multiple pregnancy, hydatidiform mole, or
GTD
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37. REFERRENCE
• LANGMAN MEDICAL EMBRYOLOGY
• BOARD REVIEW SYSTEMIC EMBRYOLOGY
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