Fertilization and Development
• Pregnancy: events that occur from fertilization
until infant is born
• Conceptus
• Gestation period
– time from last menstrual period until birth
– fertilization approx. 14 d into gestation
– ~280 days
• Embryo: fertilization through week 8
• Fetus: week 9 through birth
Diagram shows the approximate size of a human conceptus from fertilization to the early fetal stage.
Fertilization 1-week
conceptus
3-week
embryo
(3 mm)
5-week embryo
(10 mm)
Embryo
8-week embryo
(22 mm)
12-week fetus
(90 mm)
Fertilization
• Sperm must reach secondary oocyte
– Oocyte viable for 12 to 24 hours
– Sperm viable 24 to 48 hours after ejaculation
• Only a few thousand out of millions of
ejaculated sperm make the trip through the
uterine tubes
– 12-cm (5-inch) journey
– Most are lost
• Fertilization: sperm’s chromosomes combine
with those of secondary oocyte to form fertilized
egg, called a zygote
Sperm Capacitation
• Secretions of female tract help to weaken and thin
out acrosome membrane
– Motility enhanced
– Sperm cell membranes begin releasing hydrolytic
enzymes (delaying until needed at corona radiata)
– Occurs after several hours exposed to female
reproductive secretions
– Incapacitated sperm can’t fertilize
• Sperm have olfactory receptors that can follow
chemical trail released by egg or surrounding cells
Acrosomal Reaction and Sperm Penetration
• Oocyte protective layers must be digested
• Corona radiata and zona pellucida slowly degraded
by the action of hundreds of sperm
• Cells displaced by sperm enzymes degrading cell
to cell connections
• Zona degraded by releases of many acrosomes
• The single sperm reaching the membrane binds
acrosomal markers to oocyte membrane receptors
• Sperm is bound and contents ‘emptied’ into oocyte
Sperm, delivered to the vagina and capacitated in the female reproductive
tract, stream toward a secondary oocyte.
Sperm
Zona pellucida
Oocyte nucleus
arrested in meiotic
metaphase II
Polar body
Granulosa
cells of corona
radiata
Sperm
Extracellular
space
Approach. Aided by enzymes
on its surface, a sperm cell weaves
its way past granulosa cells of the
corona radiata.
1
Zona pellucida
Extracellular space
Oocyte plasma membrane
2
3 4
5
Zona pellucida
sperm-binding
receptors
Oocyte sperm-binding
membrane receptorsCortical
granulesSperm
nucleus
Mitochondria
Microtubules from
sperm flagellum
Acrosomal reaction. Binding of
the sperm to receptors in the zona
pellucida causes Ca2+ levels within the
sperm to rise, triggering the acrosomal
reaction. Acrosomal enzymes from many
sperm digest holes through the zona
pellucida, clearing a path to the
oocyte membrane.
Binding. The
sperm’s membrane
binds to the oocyte’s
sperm-binding
receptors.
Fusion. Sperm
and oocyte plasmamembranes fuse.Sperm contents enterthe oocyte.
Blocks to polyspermy.
Oocyte sperm-binding membrane
receptors are shed. Ca2+ levels
in the oocyte’s cytoplasm rise,
triggering the cortical reaction
(exocytosis of cortical granules).
As a result, the zona pellucida
hardens and the zona pellucida’s
sperm-binding receptors
are clipped off.
Blocks to Polyspermy
• Polyspermy prevented in humans
– One-sperm-per-oocyte condition
• Two mechanisms ensure monospermy
– first encounter between sperm and oocyte
membrane receptors causes destruction of
remaining receptors
– Zygote destroys zona receptors shedding
remaining sperm from the area
2
1
3
4
Extracellular
space
Corona
radiata
Zona
pellucida
Second meiotic
division of oocyte
Second meiotic
division of first
polar body
Male pro-
nucleus
Female pro-
nucleus (swollen
ovum nucleus)
Polar bodies
Male
pronucleus
Mitotic spindle
Centriole
Female
pronucleus
Zygote
Male and female
pronuclei
Polar bodies
Sperm nucleus
After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and second polar body.
Sperm and ovum nuclei swell, forming pronuclei.
The DNA in each pronucleus replicates. The pronucleiapproach each other and a mitotic spindle forms between them.
Chromosomes of the pronuclei intermix. Fertilization is accomplished and the cell, now called a zygote, is ready for the first cleavage division.
Completion of Meiosis II and Fertilization
Cleavage
• Zygote is slowly moving toward uterus
• Rapid mitotic divisions
– No cell growth: Cell size optimized for exchange
– First occurs after ~36 hours
• Two daughter cells called blastomeres
• After 72 hours, cluster of cells contains 16 or
more cells and is referred to as a morula
Zygote to Blastocyst Implantation
Figure 28.3 Cleavage: From zygote to blastocyst.
Zygote
(fertilized egg)
4-cell stage
2 days
Morula (a solid ball
of blastomeres)
3 days
Early blastocyst
(Morula hollows out, fills with fluid, and “hatches” from the zona pellucida)4 days
Implanting blastocyst
(Consists of a sphereof trophoblast cells andan eccentric cell cluster called the inner cell mass) 7 days
Degenerating
zona
pellucida
Blastocyst
cavity
Zona
pellucida
Sperm
Uterine
tube
Fertilization
(sperm meets and enters egg)
Oocyte
(egg)
Ovary
Ovulation
Uterus
Endometrium
Cavity of
uterus
Trophoblast
Blastocyst
cavity
Inner cell
mass
Blastocyst Formation
• Around day 4 or 5 blastocyst reaches uterus
– Embryo now consists of ~100 cells
• Trophoblast cells
– Participate in placenta formation
• Inner cell mass: cluster of 20–30 rounded cells
– Becomes embryonic disc, which will form embryo
and three or four extraembryonic membranes
• Fourth extraembryonic membrane (chorion) is formed
by trophoblast
– Now called embryoblast
Implantation and Placentation
Implantation
• Blastocyst floats for about 2–3 days
– Nourished by uterine secretions
• Begins 6–7 days after ovulation
– Trophoblast cells grow favorable connection to
endometrium penetrating it
– Blastocyst incorporated under endometrial epithelium
– Endometrium still being supported by ovarian hormones
and hCG
• Complete by day 12
Failed implantation may be as high as 2 out of every
three
Figure 28.4a Implantation of the blastocyst.
Endometrium
Uterine endometrialepithelium
Inner cell mass
Trophoblast
Blastocyst cavity or blastocoele
Lumen of uterus
Figure 28.4b Implantation of the blastocyst.
Endometrial stromawith blood vesselsand glands
Syncytiotrophoblast
Cytotrophoblast
Blastocyst cavity
Lumen of uterus
Human Chorionic Gonadotropin
• Embryo dependent on endometrium at this time
• Secreted by trophoblast cells and later the chorion
• Prompts corpus luteum to continue secretion of
progesterone and estrogen
• Promotes placental development
• hCG levels begin decline (2 months) as placenta
begins to secrete progesterone and estrogen
• Low values occur at 4 months and continue for rest
of pregnancy
Figure 28.5 Hormonal changes during pregnancy.
Human chorionic
gonadotropin
Estrogens
Progesterone
Re
lati
ve
blo
od
leve
ls
Gestation (weeks)
Ovulation
and fertilization
Birth
0 4 8 12 16 20 24 28 32 36
Figure 28.6a Events of placentation, early embryonic development, and extraembryonic membrane formation.
Maternal
blood vessels
Cytotrophoblast
Amniotic cavity
Proliferating
syncytiotrophoblast
Bilayeredembryonic disc
• Epiblast• Hypoblast
Implanting 7½-day blastocyst. The syncytio-trophoblast is eroding the endometrium. Cells ofthe embryonic disc are now separated from the amnion by a fluid-filled space.
Endometrial
epithelium
Figure 28.6b Events of placentation, early embryonic development, and extraembryonic membrane formation.
Endometrium
Maternal
blood vessels
Proliferating
syncytiotrophoblast
Cytotrophoblast
Amniotic cavity
Bilayeredembryonic disc
• Epiblast• Hypoblast
Amnion
Yolk sac
Extraembryonic
mesoderm
Chorion
being formed
Lumen of uterus
12-day blastocyst. Implantation is complete. Extraembryonic
mesoderm is forming a discrete layer beneath the cytotrophoblast.
Figure 28.6c Events of placentation, early embryonic development, and extraembryonic membrane formation.
Lacuna (intervillousspace) containingmaternal blood
Chorionic villus
Chorion
Amnion
Yolk sac
Extraembryonicmesoderm
Lumen of uterusExtraembryoniccoelom
Allantois
Formingumbilicalcord
• Endoderm
• Mesoderm
• Ectoderm
Amnioticcavity
Primarygerm layers
16-day embryo. Cytotrophoblast and associated mesoderm havebecome the chorion, and chorionic villi are elaborating. The embryoexhibits all three germ layers, a yolk sac, and an allantois, whichforms the basis of the umbilical cord.
Figure 28.9 Folding of the embryonic body, lateral views.
Tail Head
Amnion
Yolk sac
Ectoderm
Mesoderm
Endoderm
Trilaminar
embryonic disc
Future gut
(digestive
tube)Lateral
fold
Somites
(seen through
ectoderm)
Neural tube
Notochord
Primitive gut
ForegutHindgut Yolk
sac
Yolk sac
Tail
fold
Head
fold
Figure 28.6d Events of placentation, early embryonic development, and extraembryonic membrane formation.
Decidua basalis
Maternal blood
Chorionic villus
Umbilical blood
vessels in
umbilical cord
Amnion
Amniotic cavity
Yolk sac
Extraembryonic
coelom
Chorion
Decidua
capsularis
Lumen
of uterus
4½-week embryo. The decidua capsularis, decidua basalis, amnion, and
yolk sac are well formed. The chorionic villi lie in blood-filled intervillous
spaces within the endometrium. The embryo is nourished via the umbilical
vessels that connect it (through the umbilical cord) to the placenta.
Placentation
• Placenta is a temporary organ that originates
from both embryonic and maternal tissues
• Provides
– Nutrition
– Gas exchange
– Waste removal and fluid balance
– Transport for hormones
– Pathway for substances in mother’s blood to fetal
blood
• Fully functional by third month
Placentation
• Trophoblast cells proliferate and differentiate into
– Chorionic villi
– Umbilical blood vessels
– surrounded by blood-filled lacunae in the stratum functionalis of the
endometrium
• Exchange occurs here
– Maternal and embryonic blood supplies normally do not intermix
• Together chorionic villi and layers of functional endometrium
make up placenta
• Layers of cells in the inner cell mass will differentiate into
three cell layers which form the origin of all of the cells in the
body - called embryonic disc
Figure 28.6e Events of placentation, early embryonic development, and extraembryonic membrane formation.
Placenta
Decidua basalis
Chorionic villi
Yolk sac
Amnion
Amniotic
cavity
Umbilical
cord
Uterus
Lumen of
uterus
Decidua
capsularis
Extraembryonic
coelom
13-week fetus.
Figure 28.7 Detailed anatomy of the vascular relationships in the mature decidua basalis.
Decidua
capsularis
Chorion
Amnion
Amniotic
fluid
Lumen of
uterus
Mucous
plug
Yolk sac
Placenta
Umbilical cord
Uterus
Umbilical arteries
Umbilical vein
Amnion
Umbilical
cord
Connection
to yolk sac
Fetal portion
of placenta
(chorion)
Maternalportion ofplacenta(decidua basalis)
Stratumbasalis ofendometrium
Myometrium
Maternalveins
Maternalarteries
Fetal venule
Fetal arteriole
Maternal blood in lacuna(intervillous space)
Chorionic villus containingfetal capillaries
Chorionic villi
Decidua basalis
Embryonic Development: Extraembryonic
Membranes
Form during first 2–3 weeks of development and
include:
• Amnion: transparent sac filled with amniotic fluid
that envelopes embryo
– Buoyant, constant homeostatic temperature, freedom of
movement, prevents parts from fusing together
• Yolk sac
• Allantois
• Chorion - Encloses embryonic body and all other
membranes
