Download Decidua capsularis Extraembryonic coelom Placenta

Chapter Opener 28
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Figure 28.1 Diagrams showing the approximate size of a human conceptus from fertilization to the early fetal stage.
Embryo
Fertilization 1-week
conceptus
3-week
embryo
(3 mm)
5-week
embryo
(10 mm)
8-week embryo
(22 mm)
12-week fetus
(90 mm)
© 2013 Pearson Education, Inc.
Figure 28.2 Sperm use acrosomal enzymes and receptors to approach, bind, and enter the oocyte. The cortical
reaction prevents further sperm entry, ensuring that only two copies of each chromosome are present in the fertilized
ovum.
5 Block of polyspermy.
4 Fusion.
Sperm, delivered to the vagina and
capacitated in the female reproductive
tract, stream toward a secondary
oocyte.
2 Acrosomal reaction.
Binding of the sperm to sperm-binding
receptors in the zona pellucida causes
the 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.
3 Binding.
The sperm’s
membrane
binds to the
oocyte’s
Sperm-binding
receptors.
The sperm and
oocyte plasma
membranes
fuse, allowing
sperm contents
to enter the
oocyte.
1 Approach. Aided by
enzymes on its surface, a
sperm cell weaves its way
past granulosa cells of the
corona radiata.
Entry of the sperm’s
contents causes Ca2+ levels
in the oocyte’s cytoplasm
to rise, triggering the
cortical reaction
(exocytosis of cortical
granules). As a result,
the zona pellucida hardens
and the sperm receptors
are clipped off (slow block
to polyspermy).
Extracellular
space
Sperm
Sperm
Zona
pellucida
Polar body
Oocyte nucleus
arrested in
meiotic
metaphase II
Granulosa cells
of corona radiata
Zona pellucida
Sperm-binding
receptors
Oocyte
spermbinding
membrane
receptors
Cortical
granules
Microtubules
from sperm
flagellum
Mitochondria
Zona pellucida
Extracellular space
Oocyte plasma membrane
© 2013 Pearson Education, Inc.
Sperm
nucleus
Figure 28.2 Sperm use acrosomal enzymes and receptors to approach, bind, and enter the oocyte. The cortical
reaction prevents further sperm entry, ensuring that only two copies of each chromosome are present in the fertilized
ovum. (1 of 2)
Sperm, delivered to the vagina and capacitated in the
female reproductive tract, stream toward a secondary
oocyte.
1 Approach. Aided by
enzymes on its surface, a
sperm cell weaves its way
past granulosa cells of the
corona radiata.
Extracellular
space
Sperm
Sperm
Zona
pellucida
Polar body
Granulosa cells
of corona radiata
Oocyte nucleus
arrested in
Meiotic
metaphase II
Zona pellucida
Extracellular space
Oocyte plasma membrane
© 2013 Pearson Education, Inc.
Figure 28.2 Sperm use acrosomal enzymes and receptors to approach, bind, and enter the oocyte. The cortical
reaction prevents further sperm entry, ensuring that only two copies of each chromosome are present in the fertilized
ovum. (2 of 2)
4 Fusion. The
5 Block of
2 Acrosomal reaction.
Binding of the sperm to spermbinding receptors in the zona
pellucida causes the 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.
Zona pellucida
Sperm-binding
receptors
© 2013 Pearson Education, Inc.
sperm and
oocyte plasma
3 Binding.
membranes
The sperm’s
membrane binds fuse, allowing
to the oocyte’s sperm contents
sperm-binding to enter the
oocyte.
receptors.
Oocyte
sperm-binding
membrane
receptors
Cortical
granules
polyspermy. Entry
of the sperm’s contents
causes Ca2+ levels in
the oocyte’s cytoplasm
to rise, triggering the
cortical reaction
(exocytosis of cortical
granules). As a result,
the zona pellucida
hardens and the sperm
receptors are clipped off
(slow block to
polyspermy).
Microtubules
from sperm
flagellum
Sperm Mitochondria
nucleus
Figure 28.3 Events of fertilization.
Extracellular space
Corona radiata
Zona pellucida
Second meiotic
division of oocyte
Second meiotic division
of first polar body
1 After the sperm penetrates the
secondary oocyte, the oocyte completes
meiosis II, forming the ovum and second
polar body.
Male pro-nucleus
Female pro-nucleus
(swollen ovum nucleus)
Polar bodies
2 Sperm and ovum nuclei swell, forming
pronuclei.
Male pronucleus
Mitotic spindle
Centriole
Female pronucleus
Zygote
Sperm nucleus
3 Pronuclei approach each other and
mitotic spindle forms between them.
4 Chromosomes of the pronuclei intermix.
Fertilization is accomplished. Then, the DNA
replicates in preparation for the first cleavage
division.
Male and female
pronuclei
Polar bodies
© 2013 Pearson Education, Inc.
Figure 28.3a Events of fertilization. (1 of 4)
Sperm nucleus
Extracellular
space
Corona
radiata
Zona
pellucida
Second meiotic
division of oocyte
Second meiotic
division of first
polar body
© 2013 Pearson Education, Inc.
1 After the sperm
penetrates the
secondary oocyte,
The oocyte
Completes meiosis II,
forming the ovum
and second polar
body.
Figure 28.3a Events of fertilization. (2 of 4)
Male pronucleus
Female pronucleus (swollen
ovum nucleus)
Polar bodies
© 2013 Pearson Education, Inc.
2 Sperm and ovum
nuclei swell, forming
pronuclei.
Figure 28.3a Events of fertilization. (3 of 4)
Male
pronucleus
Mitotic spindle
Centriole
Female
pronucleus
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3 Pronuclei approach
each other and mitotic
spindle forms between
them.
Figure 28.3a Events of fertilization. (4 of 4)
Zygote
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4 Chromoomes of
the pronuclei intermix.
Fertilization is
accomplished. Then,
the DNA replicates in
preparation for the first
cleavage division.
Figure 28.3b Events of fertilization.
Male and female
pronuclei
Polar bodies
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Figure 28.4 Cleavage: From zygote to blastocyst.
4-cell stage
2 days
Zygote
(fertilized egg)
Morula (a solid ball
of blastomeres).
3 days
Zona
pellucida
Degenerating
zona
pellucida
Sperm
Blastocyst
cavity
Uterine
tube
Fertilization
(sperm
meets and
enters egg)
Early blastocyst
(Morula hollows out,
fills with fluid, and
“hatches” from the
zona pellucida).
4 days
Implanting blastocyst
(Consists of a sphere
of trophoblast cells and
an eccentric cell cluster
called the inner cell
mass). 7 days
Ovary
Oocyte
(egg)
Trophoblast
Ovulation
Uterus
Endometrium
Cavity of
uterus
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Blastocyst
cavity
Inner cell
mass
Zygote (fertilized egg)
Figure 28.4a Cleavage: From zygote to blastocyst.
Zona
pellucida
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Figure 28.4b Cleavage: From zygote to blastocyst.
4-cell stage 2 days
© 2013 Pearson Education, Inc.
Morula (a solid ball of
blastomeres). 3 days
Figure 28.4c Cleavage: From zygote to blastocyst.
© 2013 Pearson Education, Inc.
Figure 28.4d Cleavage: From zygote to blastocyst.
Early blastocyst (Morula
hollows out, fills with fluid,
and “hatches” from the
zona pellucida). 4 days
Degenerating
zona
pellucida
Blastocyst
cavity
© 2013 Pearson Education, Inc.
Figure 28.4e Cleavage: From zygote to blastocyst.
Implanting blastocyst
(Consists of a sphere of trophoblast cells and an eccentric cell
cluster called the inner cell mass).
7 days
Inner cell
mass
Trophoblast
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Blastocyst
cavity
Figure 28.5 Implantation of the blastocyst.
Endometrium
Uterine endometrial
epithelium
Inner cell mass
Trophoblast
Blastocyst cavity
Lumen of uterus
Endometrial stroma
with blood vessels
and glands
Syncytiotrophoblast
Cytotrophoblast
Blastocyst cavity
Lumen of uterus
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Figure 28.5a Implantation of the blastocyst.
Endometrium
Uterine endometrial
epithelium
Inner cell mass
Trophoblast
Blastocyst cavity
Lumen of uterus
© 2013 Pearson Education, Inc.
Figure 28.5b Implantation of the blastocyst.
Endometrium
Uterine endometrial
epithelium
Inner cell mass
Trophoblast
© 2013 Pearson Education, Inc.
Figure 28.5c Implantation of the blastocyst.
Endometrial stroma
with blood vessels
and glands
Syncytiotrophoblast
Cytotrophoblast
Blastocyst cavity
Lumen of uterus
© 2013 Pearson Education, Inc.
Figure 28.5d Implantation of the blastocyst.
Endometrial stroma
with blood vessels
and glands
Syncytiotrophoblast
Cytotrophoblast
Lumen of uterus
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Figure 28.6 Hormonal changes during pregnancy.
Relative blood levels
Human chorionic
gonadotropin
Estrogens
Progesterone
0
4
8
Ovulation
and fertilization
© 2013 Pearson Education, Inc.
12
16
24
20
28
Gestation (weeks)
32
36
Birth
Figure 28.7 Events of placentation, early embryonic development, and extraembryonic membrane formation.
Endometrium
Lacuna (intervillous
space) containing
maternal blood
Maternal
blood vessels
Amniotic
cavity
Primary
germ layers
Chorionic villus
• Ectoderm
Chorion
• Mesoderm
Amnion
• Endoderm
Proliferating
syncytiotrophoblast
Forming
umbilical
cord
Allantois
Cytotrophoblast
Amniotic cavity
Yolk sac
Bilayered
embryonic disc
• Epiblast
• Hypoblast
Endometrial
epithelium
Implanting 71/2 -day blastocyst.
The syncytiotrophoblast is eroding
the endometrium. Cells of the
embryonic disc are now separated
from the amnion by a fluid-filled
space.
Extraembryonic
mesoderm
Chorion
being formed
Lumen of uterus
12-day blastocyst. Implantation
is complete. Extraembryonic
mesoderm is forming a discrete
layer beneath the cytotrophoblast.
Extraembryonic
coelom
16-day embryo. Cytotrophoblast and associated
mesoderm have become the chorion, and
chorionic villi are elaborating. The embryo exhibits
all three germ layers, a yolk sac, and an allantois,
which forms the basis of the umbilical cord.
Placenta
Decidua basalis
Decidua basalis
Maternal blood
Chorionic villi
Chorionic villus
Yolk sac
Umbilical blood
vessels in
umbilical cord
Amnion
Amniotic
cavity
Amnion
Amniotic cavity
Yolk sac
Umbilical
cord
Extraembryonic
coelom
Lumen
of uterus
Chorion
Decidua
capsularis
Decidua
capsularis
Extraembryonic
41/2 -week embryo. The decidua capsularis, decidua basalis, amnion, and coelom
yolk sac are well formed. The chorionic villi lie in blood-filled intervillous
spaces within the endometrium. The embryo is nourished via the umbilical
13-week fetus.
vessels that connect it (through the umbilical cord) to the placenta.
© 2013 Pearson Education, Inc.
Uterus
Lumen of
uterus
Figure 28.7a–c Events of placentation, early embryonic development, and extraembryonic membrane formation.
Endometrium
Lacuna (intervillous
space) containing
maternal blood
Maternal
blood vessels
Proliferating
syncytiotrophoblast
Chorionic villus
• Ectoderm
Chorion
• Mesoderm
Amnion
• Endoderm
Cytotrophoblast
Amniotic cavity
Yolk sac
Implanting 71/2 -day blastocyst.
The syncytiotrophoblast is eroding
the endometrium. Cells of the
embryonic disc are now separated
from the amnion by a fluid-filled
space.
© 2013 Pearson Education, Inc.
Forming
umbilical
cord
Allantois
Bilayered
embryonic disc
• Epiblast
• Hypoblast
Endometrial
epithelium
Amniotic
cavity
Primary
germ layers
Extraembryonic
mesoderm
Chorion
being formed
Lumen of uterus
12-day blastocyst. Implantation
is complete. Extraembryonic
mesoderm is forming a discrete
layer beneath the cytotrophoblast.
Extraembryonic
coelom
16-day embryo. Cytotrophoblast and associated
mesoderm have become the chorion, and
chorionic villi are elaborating. The embryo exhibits
all three germ layers, a yolk sac, and an allantois,
which forms the basis of the umbilical cord.
Figure 28.7d 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
Lumen
of uterus
Chorion
Decidua
capsularis
41/2 -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.
© 2013 Pearson Education, Inc.
Figure 28.7e Events of placentation, early embryonic development, and extraembryonic membrane formation.
Placenta
Decidua basalis
Chorionic villi
Yolk sac
Amnion
Amniotic
cavity
Umbilical
cord
Decidua
capsularis
Extraembryonic
coelom
13-week fetus.
© 2013 Pearson Education, Inc.
Uterus
Lumen of
uterus
Figure 28.8 Detailed anatomy of the vascular relationships in the mature decidua basalis.
Placenta
Chorionic
villi
Decidua
basalis
Maternal
arteries
Umbilical
cord
Decidua
capsularis
Uterus
Lumen of
uterus
Chorionic villus
containing fetal
capillaries
Maternal blood
in lacuna
(intervillous
space)
Fetal arteriole
Fetal venule
Amnion
Umbilical cord
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Maternal
veins
Myometrium
Stratum
basalis of
endometrium
Maternal
portion of
placenta
(decidua
basalis)
Fetal portion
of placenta
(chorion)
Umbilical arteries
Umbilical vein
Connection to
yolk sac
Figure 28.9 Formation of the three primary germ layers.
Amnion
Bilayered
embryonic disc
Head end of
bilayered
embryonic disc
Yolk sac
Frontal
section
3-D view
Section
view in (e)
Primitive
streak
Head end
Cut edge
of amnion
Epiblast
Yolk sac
(cut edge)
Right
Left
14-15 days
Hypoblast
Endoderm
Ectoderm
Primitive
streak
Tail end
Bilayered embryonic disc, superior view
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16 days Mesoderm Endoderm
Figure 28.10 Folding of the embryonic body, lateral views.
Head
Tail
Amnion
Yolk sac
Ectoderm
Mesoderm
Endoderm
Trilaminar
embryonic
disc
Future gut
(digestive
tube)
Lateral
fold
Somites
(seen
through
ectoderm)
Tail
fold
Head
fold
Yolk sac
Neural
tube
Notochord
Primitive
gut
Hindgut
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Yolk
sac
Foregut
Figure 28.10a Folding of the embryonic body, lateral views.
Head
Tail
Amnion
Yolk sac
Ectoderm
Mesoderm
Endoderm
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Trilaminar
embryonic
disc
Figure 28.10b Folding of the embryonic body, lateral views.
Lateral
fold
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Future gut
(digestive
tube)
Figure 28.10c Folding of the embryonic body, lateral views.
Somites
(seen
through
ectoderm)
Tail
fold
Head
fold
Yolk sac
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Figure 28.10d Folding of the embryonic body, lateral views.
Neural
tube
Notochord
Primitive
gut
Hindgut
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Yolk
sac
Foregut
Figure 28.11 Endodermal differentiation.
Pharynx
Parathyroid
glands and
thymus
Thyroid
gland
Esophagus
Trachea
Connection
to yolk sac
Right and
left lungs
Stomach
Liver
Umbilical
cord
Pancreas
Gallbladder
Small intestine
Allantois
Large intestine
5-week embryo
© 2013 Pearson Education, Inc.
Figure 28.12 Neurulation and early mesodermal differentiation.
Head
Amnion
Amniotic cavity
Left
Right
Neural plate
Primitive
streak
Cut
edge of
amnion
Tail
17 days. The flat three-layered
embryo has completed
gastrulation. Notochord and
neural plate are present.
Ectoderm
Mesoderm
Notochord
Endoderm
Yolk sac
Neural
crest
Neural
groove
Neural
fold
Somite
Intermediate
mesoderm
Lateral plate
mesoderm
Coelom
20 days. The neural folds form
by folding of the neural plate, which
then deepens, producing the
neural groove. Three mesodermal
aggregates form on each side of
the notochord (somite,
intermediate mesoderm, and
lateral plate mesoderm).
Surface
ectoderm
22 days. The neural folds have
closed, forming the neural tube
which has detached from the
surface ectoderm and lies
between the surface ectoderm
and the notochord. Embryonic
body is beginning to undercut.
Neural
crest
Neural
tube
Somite
Notochord
Neural tube
(ectoderm)
Somite
Dermatome
Myotome
Sclerotome
Kidney and gonads
(intermediate
mesoderm)
Epidermis
(ectoderm)
Gut lining
(endoderm)
Lateral plate
mesoderm
• Limb bud
• Smooth
muscle of gut
• Visceral serosa
Peritoneal cavity
(coelom)
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• Parietal serosa
• Dermis
End of week 4. Embryo
undercutting is complete. Somites
have subdivided into sclerotome,
myotome, and dermatome, which
form the vertebrae, skeletal
muscles, and dermis respectively.
Body coelom present.
Figure 28.12a Neurulation and early mesodermal differentiation.
Head
Amnion
Amniotic cavity
Left
Right
Cut
edge of
amnion
Primitive
streak
Tail
Neural plate
Ectoderm
Mesoderm
Notochord
Endoderm
Yolk sac
© 2013 Pearson Education, Inc.
17 days. The flat three-layered
embryo has completed
gastrulation. Notochord and
neural plate are present.
Figure 28.12b Neurulation and early mesodermal differentiation.
Neural
crest
Neural
groove
Neural
fold
Coelom
© 2013 Pearson Education, Inc.
Somite
Intermediate
mesoderm
20 days. The neural folds form
by folding of the neural plate, which
then deepens, producing the
neural groove. Three mesodermal
Lateral plate aggregates form on each side of
the notochord (somite,
mesoderm
intermediate mesoderm, and
lateral plate mesoderm).
Figure 28.12c Neurulation and early mesodermal differentiation.
Surface
ectoderm
Neural
crest
Neural
tube
Somite
Notochord
© 2013 Pearson Education, Inc.
22 days. The neural folds have
closed, forming the neural tube
which has detached from the
surface ectoderm and lies
between the surface ectoderm
and the notochord. Embryonic
body is beginning to undercut.
Figure 28.12d Neurulation and early mesodermal differentiation.
Neural tube
(ectoderm)
Somite
Dermatome
Myotome
Sclerotome
Kidney and gonads
(intermediate
mesoderm)
Epidermis
(ectoderm)
Gut lining
(endoderm)
Lateral plate
mesoderm
• Limb bud
• Smooth
muscle of gut
• Visceral serosa
Peritoneal cavity
(coelom)
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• Parietal serosa
• Dermis
End of week 4. Embryo
undercutting is complete. Somites
have subdivided into sclerotome,
myotome, and dermatome, which
form the vertebrae, skeletal
muscles, and dermis respectively.
Body coelom present.
Figure 28.13 Flowchart showing major derivatives of the embryonic germ layers.
Epiblast
ECTODERM
MESODERM
Notochord
Somite
Intermediate
mesoderm
ENDODERM
Lateral plate
mesoderm
Somatic
mesoderm
• Epidermis, hair,
nails, glands of skin
• Brain and spinal
cord
• Neural crest and
derivatives (e.g.,
cranial, spinal, and
sympathetic
ganglia and
associated nerves;
chromaffin cells of
the adrenal
medulla; pigment
cells of the skin)
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Nucleus
pulposus of
intervertebral
discs
• Sclerotome:
vertebrae and
ribs
• Dermatome:
dermis of
dorsal body
region
• Myotome:
trunk and limb
musculature
• Kidneys
• Parietal serosa
• Gonads
• Dermis of ventral
body region
• Connective
tissues of limbs
(bones, joints,
and ligaments)
Splanchnic
mesoderm
• Wall of
digestive and
respiratory
tracts (except
epithelial
lining)
• Visceral serosa
• Heart
• Blood vessels
Epithelial lining
and glands of
digestive and
respiratory
tracts
Figure 28.14 Circulation in fetus and newborn.
Newborn
Fetus
Aortic arch
Superior vena cava
Ductus arteriosus
Ligamentum arteriosum
Pulmonary artery
Pulmonary veins
Heart
Lung
Foramen ovale
Fossa ovalis
Liver
Ductus venosus
Ligamentum venosum
Hepatic portal vein
Umbilical vein
Ligamentum teres
Inferior vena cava
Umbilicus
Abdominal aorta
Common iliac artery
Umbilical arteries
Medial umbilical ligaments
Urinary bladder
Umbilical cord
Placenta
High oxygenation
Moderate oxygenation
Low oxygenation
Very low oxygenation
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Figure 28.14a Circulation in fetus and newborn.
Fetus
Aortic arch
Superior vena cava
Ductus arteriosus
Ligamentum arteriosum
Pulmonary artery
Pulmonary veins
Heart
Lung
Foramen ovale
Fossa ovalis
Liver
Ductus venosus
Ligamentum venosum
Hepatic portal vein
Umbilical vein
Ligamentum teres
Inferior vena cava
Umbilicus
Abdominal aorta
Common iliac artery
Umbilical arteries
Medial umbilical ligaments
Urinary bladder
Umbilical cord
Placenta
© 2013 Pearson Education, Inc.
High oxygenation
Moderate oxygenation
Low oxygenation
Very low oxygenation
Figure 28.14b Circulation in fetus and newborn.
Aortic arch
Superior vena cava
Ductus arteriosus
Newborn
Ligamentum arteriosum
Pulmonary artery
Pulmonary veins
Heart
Lung
Foramen ovale
Fossa ovalis
Liver
Ductus venosus
Ligamentum venosum
Hepatic portal vein
Umbilical vein
Ligamentum teres
Inferior vena cava
Umbilicus
Abdominal aorta
Common iliac artery
Umbilical arteries
Medial umbilical ligaments
Urinary bladder
© 2013 Pearson Education, Inc.
High oxygenation
Moderate oxygenation
Low oxygenation
Very low oxygenation
Figure 28.15 Photographs of a developing fetus.
Amniotic sac Umbilical cord
Umbilical vein
Chorionic
villi
Yolk sac
Cut edge
of chorion
Embryo at week 7, about 17 mm long.
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Fetus in month 3, about 6 cm long.
Fetus late in month 5, about 19 cm long.
Figure 28.15a Photographs of a developing fetus.
Amniotic sac Umbilical cord Umbilical vein
Chorionic
villi
Yolk sac
Cut edge
of chorion
Embryo at week 7, about 17 mm long.
© 2013 Pearson Education, Inc.
Figure 28.15b Photographs of a developing fetus.
Fetus in month 3, about 6 cm long.
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Figure 28.15c Photographs of a developing fetus.
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Fetus late in month 5, about 19 cm long.
Figure 28.16 Relative size of the uterus before conception and during pregnancy.
Before conception
(Uterus the size of a
fist and resides
in the pelvis.)
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4 months
(Fundus of the
uterus is halfway
between the pubic
symphysis and
the umbilicus.)
7 months
(Fundus is well
above the
umbilicus.)
9 months
(Fundus
reaches the
xiphoid
process.)
Table 28.1 Developmental Events of the Fetal Period (1 of 3)
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Table 28.1 Developmental Events of the Fetal Period (2 of 3)
© 2013 Pearson Education, Inc.
Table 28.1 Developmental Events of the Fetal Period (3 of 3)
© 2013 Pearson Education, Inc.
Figure 28.17 Hormonal induction of labor.
Start
Estrogen
Oxytocin
from
placenta
from fetus
and mother's
posterior pituitary
Induces oxytocin
receptors on uterus
Stimulates uterus
to contract
Stimulates
placenta to release (+)
Prostaglandins
Stimulate more
vigorous contractions
of uterus
© 2013 Pearson Education, Inc.
Positive feedback
(+)
Figure 28.18 Parturition.
1a Early dilation.
Baby’s head engaged;
widest dimension Is
along left-right axis.
1b Late dilation.
Baby’s head rotates so
widest dimension is in
anteroposterior axis
(of pelvic outlet). Dilation
nearly complete
Umbilical cord
Placenta
Uterus
Cervix
Vagina
Pubic
symphysis
Sacrum
2
Expulsion.
Baby’s head extends
as it is delivered
3
Placental stage.
After baby is delivered,
the placenta detaches
and is removed.
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Perineum
Uterus
Placenta (detaching)
Umbilical cord
Figure 28.18 Parturition. (1 of 4)
1a Early
dilation. Baby’s
head engaged;
widest dimension
Is along left-right
axis.
Umbilical cord
Placenta
Uterus
Cervix
Vagina
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Figure 28.18 Parturition. (2 of 4)
1b Late
dilation. Baby’s
head rotates so
widest dimension
is in anteroposterior
axis (of pelvic
outlet). Dilation
nearly complete
Pubic
symphysis
Sacrum
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Figure 28.18 Parturition. (3 of 4)
2 Expulsion.
Baby’s head extends
as it is delivered
Perineum
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Figure 28.18 Parturition. (4 of 4)
3 Placental
stage. After baby
is delivered, the
placenta detaches
and is removed
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Uterus
Placenta
(detaching)
Umbilical
cord
Figure 28.19 Milk production and the positive feedback mechanism of the milk let-down reflex.
Hypothalamus releases prolactin
releasing factors (PRF)
to portal circulation.
Start
Positive feedback
Stimulation of
mechanoreceptors in
nipples by suckling
infant sends afferent
impulses to the
hypothalamus.
Hypothalamus
sends efferent
impulses to the
posterior
pituitary where
oxytocin is stored.
Anterior pituitary
secretes prolactin
to blood.
Oxytocin is
released from the
posterior pituitary
and stimulates
myoepithelial cells
of breasts to contract.
Prolactin targets
mammary glands
of breasts.
Let-down reflex.
Milk is ejected
through ducts
of nipples.
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Milk production
Closer Look 28.1
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