Human Development (Chapter 29)
Lecture # 18: Early Human
Development
Objectives
1- Describe the process of fertilization.
2- Explain how the egg prevents
fertilization by more than one sperm.
3- Describe the major events that
transform a fertilized egg into an embryo.
4- Describe the implantation of the
embryo in the uterine wall.
5- Identify the major tissues derived from
the primary germs layers.
6- Describe the formation and functions
of the embryonic membranes.
7- Explain
nourished.
how
the
conceptus
is
Oogenesis
Oogonia
Diploid
OOGENESIS
MITOSIS
Primary
oocyte
Diploid
MEIOSIS I
It stops in
prophase
Before birth
MEIOSIS I
Completed
First
polar
body
After puberty
Secondary
oocyte
Haploid
MEIOSIS II
It stops in
metaphase
1st
&
2nd
polar
body
MEIOSIS II
Completed
Before ovulation
After ovulation
If fertilization
occurs
Secondary
oocyte
Haploid
Ovulation of mature
(graafian) follicle
Oocyte at Ovulation
Egg
First polar
body
Secondary oocyte
(arrested in metaphase
of meiosis II)
If not fertilized
If fertilized
It completes
meiosis II
Dies
Second polar
body (dies)
Zygote
Embryo
Zona
pellucida
It is a layer of glycoprotein gel secreted by
granulosa cells around
the oocyte.
Corona
radiata
It is composed of
several layers of
granulosa cells.
Mature Spermatozoon
Acrosome
Head
It contains enzymes that break down material
surrounding the oocyte.
Nucleus It contains one set of 23 chromosomes (haploid).
Basal body
It is the first part of the flagellum nestled in an
indentation in the base of the nucleus.
Mitochondrion They provide the ATP needed for the
beating of the tail.
Axoneme It is the core of the flagellum consisting of
microtubules.
Midpiece of tail
Tail
It propels the
sperm during
migration in
the
female
reproductive
system.
Principal piece of tail
Endpiece of tail
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Fertilization
There are two mechanisms to prevention of
polyspermy (fertilization by two or more sperm):
1- Fast block: Binding of the sperm to the egg opens
Na+ channels in egg membrane. Inflow of Na+
depolarizes the membrane and inhibits the attachment
of any more sperm.
4
Rejected sperm
3
Fertilization membrane
2
Cortical reaction
2- Slow block : Sperm penetration releases an inflow
of Ca2+, which stimulates the cortical granules to release
their secretion beneath the zona pellucida.
The secretion swells with water, pushes any remaining
sperm away and creates an impenetrable fertilization
membrane between the egg and the zona pellucida.
1
Acrosomal reaction
It is the exocytosis of the acrosome,
releasing the enzymes needed to penetrate
the egg.
Two acrosomal enzymes are released:
Hyaluronidase,
which
digests
the
hyaluronic acid that binds granulosa cells
together.
Acrosin, a protease similar to trypsin.
When a path has been cleared, a sperm
binds to the zona pellucida.
Cortical
granules
The Preembryogenic Stage
1- Cleavage
It comprises the first 16 days of development,
culminating with the existence of an embryo.
3- Embryogenesis
It refers to the mitotic divisions that occurs
1-Cleavage: in the first 3 days, while the conceptus
migrates down the uterine tubes.
Zygote
Egg
pronucleus
Morula
(72 hours)
It is a solid ball of 16 cells
that resemble a mulberry.
Blastomeres
2-celled stage 4-celled
(30 hours)
stage
2- Implantation
8-celled
stage
The morula lies free in
uterine cavity for 4-5 days
and divides into a 100 cells
or so.
Blastocyst
Sperm
pronucleus
Fertilization
(0 hours)
Migration of the Conceptus
The zona pellucida disintegrates and releases
conceptus: blastocyst.
Implanted blastocyst
(6 days)
2- Implantation
Embryoblast
Blastocyst:
Blastocoel
Trophoblast:
Embryoblast
Trophoblast
Cytotrophoblast
Syncytiotrophoblast
(6-7 days)
Endometrium:
Epithelium
Endometrial
gland
The blastocyst attaches to uterine wall 6
days after ovulation, usually on the
fundus or the posterior wall of the
uterus.
2- Implantation:
It is the process of attachment to uterine
wall that begins when blastocyst adheres
to the endometrium.
The trophoblast secrets human
chorionic gonadotropin (HCG),
which stimulates the corpus luteum
to secret estrogen and progesterone
(it suppresses menstruation).
(8 days)
The trophoblast on the attachment side separates into two
layers:
The superficial layer in contact with the endometrium. The
plasma membranes break down and trophoblastic cells fuse
into a multinucleate mass called syncytiotrophoblast.
The deep layer, close to embryoblast, retains the individual
cells divided by membranes and is called cytotrophoblast.
3- Embryogenesis
Embryogenesis:
It is the arrangement of the
embryoblast into three primary
germ
layers:
ectoderm,
mesoderm, and endoderm.
The embryoblast separates
slightly from the trophoblast
and creates a narrow space
between them: the amniotic
cavity.
Amniotic cavity
Ectoderm
Mesoderm
Endoderm
Once the three primary germ layers are formed,
embryogenesis is complete and the individual is
considered an embryo. It is about 2 mm long
and 16 days old.
Zygote
Morula
Blastocyst
Embryo
The ectoderm and endoderm are epithelia composed of tightly joined cells.
The mesoderm is a more loosely organized tissue which differentiates into a
loose fetal connective tissue called mesenchyme.
Several accessory organs develop along side the embryo: the placenta, the
umbilical cord and four embryonic membranes (amnion, yolk sac, allantois,
and chorion)
Amniotic cavity
Yolk sac
Chorion
Amnion
Allantois
They
are
extensions
of
Chorionic
syncytiotrophoblast into the
villi
endometrium by digestion and
growth of “roots” of tissue.
Placental
sinus
16 days
Uterus
Chorionic
villus
Placental sinus
They are lacunae filled with
maternal blood that merge
and surround villi.
Developing
placenta
Umbilical
blood vessels
Allantois
Yolk sac
Amnion
Amniotic cavity
28 days
Chorion
12 weeks
Umbilical cord
Placenta
It contains two umbilical
arteries and one umbilical
vein.
Allantois
It begins as an outpocketing of the yolk sac. It
forms the foundation for the
umbilical cord becomes part
of the urinary bladder.
It contributes to formation GI
Yolk sac tract, blood cells, and future
egg or sperm cells.
Amnion
It is a transparent sac that
grows to completely enclose
the embryo and penetrated
only by the umbilical cord fills
with amniotic fluid.
Chorion
It
is
the
outermost
membrane enclosing all the
rest of the membranes and
the embryo.
The Amniotic Fluid
At first, amniotic fluid is formed from
filtration of mother’s blood plasma.
Beginning at 8 to 9 weeks, the fetus
urinates into the amniotic cavity about
once per hour contributing substantially to
fluid volume.
The fetus swallows amniotic fluid at the
same rate. At term, the amnions contains
about 700 to 1000 mL of fluid.
12 weeks
Functions of the Amniotic Fluid:
1- It protects the embryo from trauma,
infections, and temperature fluctuations.
2- It allows freedom of movement
important to muscle development.
3- It enables the embryo to develop
symmetrically.
4- It prevents body parts from adhering
to each other.
5- It stimulates lung development as the
fetus‘ breathes’ fluid.
Amniotic cavity fills
with amniotic fluid.
Prenatal Nutrition
During gestation the conceptus is nourished in three different, overlapping ways: 1- Uterine milk
2- Trophoblastic nutrition
3- Placental nutrition
1- Uterine milk
It is a glycogen-rich secretion of the uterine tubes and endometrial
glands.
The conceptus absorbs this fluid as it travels down the tube and lies
free in the uterine cavity before implantation (6, 7 days).
2- Trophoblastic nutrition
The conceptus consumes decidual cells of the endometrium.
Progesterone from corpus luteum stimulates decidual cells to
proliferate and accumulate a store of glycogen, proteins, and lipids.
It is the only mode of nutrition for the first week after implantation and
remains the dominant source through the end of 8 weeks.
3- Placental nutrition
Nutrients diffuse from the mother’s blood through the placenta into the
fetal blood.
The placental phase is the period beginning the 9th week and it is the
sole mode of nutrition from end of 12th week until birth.
Placenta and Umbilical Cord
As the placenta grows:
The villi grow and branch and their
surface area increases.
The membrane becomes thinner
and more permeable.
The placental conductivity (the rate
at which substances diffuse through
the membrane) increases.
Materials diffuse from the side of the
membrane where they are more
concentrated to the side where they
are less concentrated.
Oxygen and nutrients pass to the
fetal blood. Fetal wastes pass the
other way and are eliminated by the
mother.
The placenta is also permeable to nicotine, alcohol, and most other drugs that may be
present in the maternal blood stream.
Blood Circulation in the Adult
Pulmonary trunk
Aorta
Superior vena cava
Right atrium
It receives O2 poor blood
returning to the heart
through the superior and
inferior vena cava and the
coronary sinus.
Left atrium
It receives O2 rich blood
returning from the lungs
through the pulmonary veins.
Pulmonary veins
Left ventricle
It pumps O2 rich blood
through the aorta artery to
every organ of the body.
Inferior vena cava
Right ventricle It pumps O2 poor blood to
the lungs through the
pulmonary arteries.
Blood Circulation in the Fetus
Pulmonary trunk
Aorta
Foramen oval
Ductus arteriosus
2 Blood bypasses the lungs
by flowing directly from the
right atrium through the
foramen oval into the left
atrium.
Right atrium
1 It receives O rich blood
2
returning from the placenta,
mixed with O2 poor returning
through the inferior vena
cava.
Inferior vena cava
3
2
1
3 Blood also bypasses
lungs by flowing from
pulmonary trunk through
ductus arteriosus to
aorta.
the
the
the
the