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Lab Activity 35
Embryology
Portland Community College
BI 233
Spermatogenesis
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Oogenesis
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Fertilization
• An egg must be
fertilized within 12 to
24 hours of ovulation
• It takes about 72 hours
for an egg to reach the
uterus.
• The sperm must
encounter the egg
somewhere in the
uterine tube
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Fertilization
• A sperm can survive for 48-72 hours in the female
reproductive tract
• It takes about 10 hours to navigate into the
ampulla of the fallopian tube where fertilization
occurs.
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Fertilization
• When the sperm
encounters an egg, it
undergoes an acrosomal
reaction- exocytosis of the
acrosome, releasing the
enzymes needed to
penetrate the egg.
• Sperm must penetrate the
granulosa cells and zona
pellucida that surrounds
the egg
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Zygote
• A zygote is formed when the
nucleus of the sperm and
ovum combine to create a
cell with 23 pairs of
chromosomes. (prior to the
start of cleavage)
• Fertilization occurs in the
ampulla of the uterine tube
within 24 hours after
ovulation (oocyte dies after
24 hours)
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From Fertilization to Implantation
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Formation of the Morula
• Cleavage: Rapid mitotic
cell division (without cell
growth) that subdivides the
cytoplasm of the zygote
• Cleavage produces
daughter cells called
blastomeres
• The zona pellucida remains
around the entire group of
cells.
• About 3 days
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Morula
• After 3 days of
cleavage, a solid ball
of cells has formed
that resembles a
mulberry (similar to
a Marion berry)
• This Morula contains
16 or more cells
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Blastocyst
• By the fourth or fifth day the morula develops
into a a fluid-filled hollow sphere called a
blastocyst
• Blastocyst : contains 3 parts
1. Trophoblast: The outer layer of cells
 Takes part in placenta formation
2. Inner cell mass: A group of cells clustered
at one side of the blastocyst
 Becomes the embryo
3. Blastocoele: The fluid filled cavity
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Blastocyst
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Implantation
• By day six or seven the trophoblast will
adhere to the endometrium with the inner
cell mass side facing the uterine wall
• The trophoblast cells proliferate and form
two distinct layers
• Syncytiotrophoblast: Cells on the
perimeter touching the endometrium.
• Cytotrophoblast: Cells on the interior of
the trophoblast that retain their cell
boundaries
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Implantation
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Implantation
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Syncytiotrophoblast
• At the point of contact between the trophoblast
and the endometrium, the dividing cells lose
their plasma membranes
• This creates a syncytium in which there is a
layer of cytoplasm containing multiple nuclei
• The syncytiotrophoblast erodes a path
through the uterine endometrium by secreting
hyaluronidase
• The implanted blastocyst will eventually be
covered over by endometrial cells
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Implantation
• Viability of the corpus luteum is maintained by
human chorionic gonadotropin (hCG) secreted
by the trophoblast
• hCG prompts the corpus luteum to continue to
secrete progesterone
• Between month 2 & 3, the placenta assumes the
role of progesterone and estrogen production
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Chorionic Villi
• Chorionic villi are
extensions of the cytotrophoblast covered
with syncytiotrophoblast that extend into
the maternal blood filled spaces (lacunae)
• Maternal blood will exchange nutrients
through these villi that will eventually
contain embryonic vessels.
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Embryonic Disc
• The inner cell mass organizes into an oval sheet
with two layers (the embryonic disc)
• Ectoderm: The layer closest to the attachment of
the trophoblast
• Endoderm: The layer closest to the blastocoele
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Amniotic Cavity
• The inner cell mass
will separate from the trophoblast during
implantation
• The ectoderm cells will migrate and form a
fluid-fill chamber (the amniotic cavity)
• Amnion: Mesodermal cells will soon create
the outer second layer of the amnion
• The fluid and membranes will surround and
cushion the developing embryo and fetus
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Yolk Sac
• Begins as the endodermal
cells spread around the
outer edges of the
blastocoele.
• The yolk sac is completed as the mesoderm
cells migrate around the pouch.
• Site of embryonic blood cells and gonadal
stem cells
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Review & Clarify
Implantation
https://www.youtube.com/watch?v=bIdJOiXpp9g
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Gastrulation
• This is the term for the process by which a third
layer of cells called the mesoderm, forms
between the ectoderm and endoderm.
• Primitive streak : raised dorsal groove that
establishes the longitudinal axis of the embryo
• Gastrulation sets the stage for organogenesis;
the formation of body organs
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Gastrulation
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Gastrulation
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Primary Germ Layers
• Serve as primitive tissues from which all body
organs will be derived
• Endoderm:
• Epithelial lining of GI & lower respiratory tract
• All ducts entering the GI tract
• Urinary bladder
• Ectoderm:
•
•
•
•
Brain & Nervous system
Epidermis
Lining of mouth, and anus
Sense organs such as eyes
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Primary Germ Layers
• Mesoderm:
•
•
•
•
•
•
•
Muscle
Bone
Cartilage
Blood
Dermis and hypodermis
Kidneys, ovaries, testes
Lining of body cavities
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Ectoderm
Mesoderm
Endoderm
Approximately 13 days after fertilization
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Review & Clarify
Gastrulation
https://www.youtube.com/watch?v=3AOoikTEfeo
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21-23 Days After Fertilization
• Actual Size 1.0 – 1.5 mm
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28 Days After
Fertilization
Actual Size: 4-6 mm
• By the end of week 4 there’s significant
development of the nervous system, heart, body
form and placenta.
• The embryo starts receiving nutrients and
oxygen through the placenta.
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Embryonic Membranes:
Chorion and Allantois
• Allantois: a small outpocketing at the caudal end of the
yolk sac
• Structural base for the umbilical cord
• Becomes part of the urinary bladder
• Chorion
• Becomes the embryonic contribution to the placenta
• Derived from trophoblast & mesoderm lining it
• Contains the chorionic villi
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Placenta
This side faces the baby
This side touches the uterus
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2 Arteries
1 Vein
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Placenta
• The placenta is fully formed and functional by the 3rd
month of pregnancy
• Chorion of embryo & stratum functionalis layer of
uterus
• Chorionic villi extend into maternal blood filled
intervillous spaces: maternal & fetal blood vessels do
not join & blood does not mix
• Diffusion of O2, nutrients, wastes
• Stores nutrients & produces hormones
• Barrier to microorganisms, except some viruses
• AIDS, measles, chickenpox, poliomyelitis,
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encephalitis
Fetal Circulation – 3 Bypasses
1. Umbilical vein conducts
oxygenated fetal blood from
the placenta, through the
umbilical cord, to and
through the liver, and into
the ductus venosus which
drains into the inferior vena
cava, which returns blood to
the heart
2. Oxygenated blood may flow
through the foramen ovale
into the left atrium.
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Fetal Circulation
3. Some right atrial blood moves
into the right ventricle and leaves
the heart through the pulmonary
artery where another circulation
bypass is located called the
ductus arteriosus.
• The umbilical arteries return
the deoxygenated blood to
umbilical cord and placenta.
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•
•
•
Heart Sounds PDA http://depts.washington.edu/physdx/heart/demo.html
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Endocrine Placenta
• Human Chorionic Gonadotropin (hCG):
Maintains the integrity of the corpus luteum
(CL) and promotes the continued secretion
of progesterone
• Human Placental Lactogen (hPL) and
Placental Prolactin: Prepare the mammary
glands for milk production
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Endocrine Placenta
• Relaxin: which relaxes connective tissue of
pelvis and cervix in preparation for childbirth
• Suppresses the release of oxytocin by the
hypothalamus
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Endocrine Placenta
• Progesterone: Support endometrium and
suppress contraction of uterine smooth muscle
• Estrogens: Stimulate growth of myometrium
and stimulate mammary gland development
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Gestation
• Fetal period: Begins at week 9
• All major organs have been established
• This is the time of growth
• Gestation: 40 weeks measured from the first
day of the uterine cycle (day one of
menstruation)
• Actually only 38 weeks from fertilization
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Fetal
Period
• 56 days after
fertilization
• Actual Size:
30 mm
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•
• Lifes Greatest Miracle
http://www.pbs.org/wgbh/nova/body/life-greatest-miracle.html
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•
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•
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6:50
15.25
26.48
40.24
43.49
50:17
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The End
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