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prepared by
Janice Meeking,
Mount Royal College
CHAPTER
28
Pregnancy
and Human
Development:
Part A
Copyright © 2010 Pearson Education, Inc.
Pregnancy
• Pregnancy: events that occur from fertilization
until the infant is born
• Conceptus: the developing offspring
• Gestation period: time from the last menstrual
period until birth (~280 days)
• Embryo: conceptus from fertilization through
week 8
• Fetus: conceptus from week 9 through birth
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1-week
conceptus
Fertilization
Embryo
3-week
embryo
(3 mm)
5-week embryo
(10 mm)
8-week embryo
(22 mm)
12-week fetus
(90 mm)
Copyright © 2010 Pearson Education, Inc.
Figure 28.1
From Egg to Zygote
• The oocyte is viable for 12 to 24 hours
• Sperm is viable 24 to 48 hours after
ejaculation
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From Egg to Zygote
• For fertilization to occur, coitus must occur no
more than
• Two days before ovulation
• 24 hours after ovulation
• Fertilization: when the sperm’s chromosomes
combine with those of a secondary oocyte to
form a fertilized egg (zygote)
Copyright © 2010 Pearson Education, Inc.
Accomplishing Fertilization
• Ejaculated sperm
• Leak out of the vagina immediately after
deposition
• Are destroyed by the acidic vaginal environment
• Fail to make it through the cervix
• Are dispersed in the uterine cavity or destroyed by
phagocytes
• Few (100 to a few thousand) reach the uterine
tubes
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Accomplishing Fertilization
• Sperm must become motile
• Sperm must be capacitated before they can
penetrate the oocyte
• Secretions of the female tract weaken
acrosome membrane
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Acrosomal Reaction and Sperm Penetration
• Sperm must breach oocyte coverings
• Corona radiata and zona pellucida
• Sperm binds to the zona pellucida and
undergoes the acrosomal reaction
• Enzymes are released to digest holes in the
zona pellucida
• Hundreds of acrosomes release their enzymes
to digest the zona pellucida
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Acrosomal Reaction and Sperm Penetration
• Sperm head approaches the oocyte
• An acrosomal process forms and binds to
receptors
• Oocyte and sperm membranes fuse
• Only one sperm is allowed to penetrate the
oocyte (monospermy)
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Blocks to Polyspermy
• Upon entry of a sperm, Ca2+ surge from the
ER causes the cortical reaction
• Cortical granules release enzymes (zonal
inhibiting proteins, or ZIPs)
• ZIPs destroy sperm receptors
• Spilled fluid binds water and swells, detaching
other sperm (slow block to polyspermy)
Copyright © 2010 Pearson Education, Inc.
Sperm
Granulosa
cells of
corona radiata
Zona pellucida
ZP3 molecules
Oocyte plasma
membrane
Oocyte sperm-binding
membrane receptors
Cortical
granules
Acrosomal
process
Cortical reaction
Sperm nucleus
Extracellular
space
Copyright © 2010 Pearson Education, Inc.
1 Aided by surface hyaluronidase
enzymes, a sperm cell weaves its
way past granulosa cells of the
corona radiata.
2 Binding of the sperm to ZP3
molecules in the zona pellucida
causes a rise in Ca2+ level within
the sperm, triggering the
acrosomal reaction.
3 Acrosomal enzymes digest
holes through the zona pellucida,
clearing a path to the oocyte
membrane.
4 The sperm forms an acrosomal
process, which binds to the
oocyte’s sperm-binding receptors.
5 The sperm and oocyte plasma
membranes fuse, allowing sperm
contents to enter the oocyte.
6 Entry of sperm contents (tail
and plasma membrane remain
behind) causes a rise in the Ca2+
level in the oocyte’s cytoplasm,
triggering the cortical reaction
(exocytosis of cortical granules).
The result is hardening of the zona
pellucida and clipping off of sperm
receptors (slow block to
polyspermy).
Figure 28.2
Completion of Meiosis II and Fertilization
• As sperm nucleus moves toward the oocyte
nucleus it swells to form the male pronucleus
• The Ca2+ surge triggers completion of meiosis
II  ovum + second polar body
• Ovum nucleus swells to become a female
pronucleus
• Membranes of the two pronuclei rupture and
the chromosomes combine
Copyright © 2010 Pearson Education, Inc.
Sperm nucleus
Extracellular space
Corona radiata
Zona pellucida
Second meiotic
division of oocyte
Second meiotic
division of first
polar body
Male pronucleus
Female pronucleus
(swollen ovum
nucleus)
Polar bodies
Male pronucleus
Mitotic spindle
Centriole
Female pronucleus
1 After the sperm penetrates
the secondary oocyte, the
oocyte completes meiosis II,
forming the ovum and second
polar body.
2 Sperm and ovum nuclei
swell, forming pronuclei.
3 Pronuclei approach each
other and mitotic spindle
forms between them.
4 Chromosomes of the pronuclei
Zygote
(a)
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intermix. Fertilization is
accomplished. Then, the DNA
replicates in preparation for the
first cleavage division.
Figure 28.3a
Embryonic Development
• Cleavage
• Mitotic divisions of zygote
• First cleavage at 36 hours  two daughter
cells (blastomeres)
• At 72 hours  morula (16 or more cells)
• At day 3 or 4, the embryo of ~100 cells
(blastocyst) has reached the uterus
Copyright © 2010 Pearson Education, Inc.
Embryonic Development
• Blastocyst: fluid-filled hollow sphere
• Trophoblast cells
• Display factors that are immunosuppressive
• Participate in placenta formation
• Inner cell mass
• Becomes the embryonic disc ( embryo and
three of the embryonic membranes)
Copyright © 2010 Pearson Education, Inc.
(a) Zygote
(fertilized egg)
(b) 4-cell stage
2 days
Zona
pellucida
(c) Morula (a solid ball
of blastomeres).
3 days
Degenerating
zona
pellucida
Blastocyst
cavity
Sperm
Uterine
tube
Fertilization
(sperm
meets and
enters egg)
Oocyte
(egg)
(e) Implanting
blastocyst
(Consists of a
sphere of trophoblast cells and an
eccentric cell cluster called the inner
cell mass). 7 days
Ovary
Ovulation
Uterus
Endometrium
Cavity of
uterus
Copyright © 2010 Pearson Education, Inc.
(d) Early blastocyst (Morula
hollows out, fills with fluid,
and “hatches” from the
zona pellucida). 4 days
Blastocyst
cavity
Trophoblast Inner
cell
mass
Figure 28.4
Implantation
• Blastocyst floats for 2–3 days
• Implantation begins 6–7 days after ovulation
• Trophoblast adheres to a site with the proper
receptors and chemical signals
• Inflammatory-like response occurs in the
endometrium
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Endometrium
Uterine endometrial
epithelium
Inner cell mass
Trophoblast
Blastocyst cavity
Lumen of uterus
(a)
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Figure 28.5a
Implantation
• Trophoblasts proliferate and form two distinct
layers
1. Cytotrophoblast (cellular trophoblast): inner
layer of cells
2. Syncytiotrophoblast: cells in the outer layer
lose their plasma membranes, invade and
digest the endometrium
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Endometrial stroma
with blood vessels
and glands
Syncytiotrophoblast
Cytotrophoblast
Inner cell mass
(future embryo)
Lumen of uterus
(c)
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Figure 28.5c
Implantation
• The implanted blastocyst is covered over by
endometrial cells
• Implantation is completed by the twelfth day
after ovulation
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Endometrial stroma
with blood vessels
and glands
Syncytiotrophoblast
Cytotrophoblast
Lumen of uterus
(d)
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Figure 28.5d
Hormonal Changes During Pregnancy
• Human chorionic gonadotropin (hCG)
• Secreted by trophoblast cells, later the chorion
• Prompts corpus luteum to continue secretion
of progesterone and estrogen
• hCG levels rise until the end of the second
month, then decline as the placenta begins to
secrete progesterone and estrogen
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Human chorionic
gonadotropin
Estrogens
Progesterone
Gestation (weeks)
Ovulation
and fertilization
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Birth
Figure 28.6
Placentation
• Formation of the placenta from embryonic and
maternal tissues
1. Embryonic tissues
• Mesoderm cells develop from the inner cell
mass and line the trophoblast
• Together these form the chorion and
chorionic villi
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Placentation
2. Maternal tissues
• Decidua basalis (stratum functionalis
between chorionic villi and stratum basalis
of endometrium) develops blood-filled
lacunae
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Placentation
• The chorionic villi
• Grow into blood-filled lacunae (intervillous
spaces)
• Vascularized by umbilical arteries and veins
• Lie immersed in maternal blood
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Endometrium
Lacuna (intervillous
space) containing
maternal blood
Maternal
blood vessels
Proliferating
syncytiotrophoblast
Chorionic villus
• Ectoderm
Chorion
• Mesoderm
Amnion
• Endoderm
Forming
body stalk
Cytotrophoblast
Amniotic cavity
Bilayered
embryonic disc
• Epiblast
• Hypoblast
Endometrial
epithelium
Yolk sac
Allantois
Extraembryonic
mesoderm
Chorion
being formed
Lumen of uterus
(a) Implanting 71/2-day blastocyst. (b) 12-day blastocyst. Implantation
The syncytiotrophoblast is eroding
is complete. Extraembryonic
the endometrium. Cells of the
mesoderm is forming a discrete
embryonic disc are now separated
layer beneath the cytotrophoblast.
from the amnion by a fluid-filled
space.
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Amniotic
cavity
Primary
germ layers
Extraembryonic
coelom
(c) 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 28.7 (a-c)
Placentation
• Decidua capsularis: part of the endometrium
at the uterine cavity face of the implanted
embryo
• Placenta is fully formed and functional by the
end of the third month
• Placenta also secretes human placental
lactogen, human chorionic thyrotropin, and
relaxin
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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
(d) 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 now receiving its nutrition via the umbilical vessels
that connect it (through the umbilical cord) to the placenta.
Copyright © 2010 Pearson Education, Inc.
Figure 28.7d
Placenta
Decidua basalis
Chorionic villi
Yolk sac
Amnion
Amniotic
cavity
Umbilical
cord
Decidua
capsularis
Uterus
Extraembryonic
coelom
Lumen of
uterus
(e) 13-week fetus.
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Figure 28.7e
Placentation
• Maternal and embryonic blood supplies do not
intermix
• Embryonic placental barriers include:
• Membranes of the chorionic villi
• Endothelium of embryonic capillaries
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Placenta
Chorionic
villi
Maternal
arteries
Decidua
basalis
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 28.8
Embryonic Development: Gastrula to Fetus
• Germ Layers
• During implantation, the blastocyst starts to
convert to a gastrula
• Inner cell mass develops into the embryonic
disc (subdivides into epiblast and hypoblast)
• The three primary germ layers and the
extraembryonic membranes develop
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Extraembryonic Membranes
1. Amnion: epiblast cells form a transparent sac
filled with amniotic fluid
• Provides a buoyant environment that protects the
embryo
• Helps maintain a constant homeostatic temperature
• Allows freedom of movement and prevents parts from
fusing together
• Amniotic fluid comes from maternal blood, and later,
fetal urine
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Extraembryonic Membranes
2. Yolk sac: a sac that hangs from the ventral
surface of the embryo
• Forms part of the digestive tube
• Source of the earliest blood cells and blood
vessels
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Extraembryonic Membranes
3. Allantois: a small outpocketing at the caudal
end of the yolk sac
• Structural base for the umbilical cord
• Becomes part of the urinary bladder
4. Chorion: helps form the placenta
• Encloses the embryonic body and all other
membranes
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Endometrium
Lacuna (intervillous
space) containing
maternal blood
Maternal
blood vessels
Proliferating
syncytiotrophoblast
Chorionic villus
• Ectoderm
Chorion
• Mesoderm
Amnion
• Endoderm
Forming
body stalk
Cytotrophoblast
Amniotic cavity
Bilayered
embryonic disc
• Epiblast
• Hypoblast
Endometrial
epithelium
Yolk sac
Allantois
Extraembryonic
mesoderm
Chorion
being formed
Lumen of uterus
(a) Implanting 71/2-day blastocyst. (b) 12-day blastocyst. Implantation
The syncytiotrophoblast is eroding
is complete. Extraembryonic
the endometrium. Cells of the
mesoderm is forming a discrete
embryonic disc are now separated
layer beneath the cytotrophoblast.
from the amnion by a fluid-filled
space.
Copyright © 2010 Pearson Education, Inc.
Amniotic
cavity
Primary
germ layers
Extraembryonic
coelom
(c) 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 28.7 (a-c)
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
(d) 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 now receiving its nutrition via the umbilical vessels
that connect it (through the umbilical cord) to the placenta.
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Figure 28.7d
Gastrulation
• Occurs in week 3, in which the embryonic disc
becomes a three-layered embryo with
ectoderm, mesoderm, and endoderm
• Begins with appearance of primitive streak, a
raised dorsal groove that establishes the
longitudinal axis of the embryo
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Amnion
Bilayered
embryonic
disc
Yolk sac
(a)
Head end of bilayered
embryonic disc
(b) Frontal
section
(c) 3-D
view
(d) Section
view in (e)
Primitive streak
Head end
Cut edge
of amnion
Epiblast
Yolk sac
(cut edge)
Right
(f) 14-15 days
Endoderm
Hypoblast
Left
Ectoderm
Primitive
streak
Tail end
(e) Bilayered embryonic disc,
superior view
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(g) 16 days
Mesoderm
Endoderm
Figure 28.9
Gastrulation
• Cells begin to migrate into the groove
• The first cells form the endoderm
• Cells that follow push laterally, forming the
mesoderm
• Cells that remain on the embryo’s dorsal
surface form the ectoderm
• Notochord: rod of mesodermal cells that
serves as axial support
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Amnion
Bilayered
embryonic
disc
Yolk sac
(a)
Head end of bilayered
embryonic disc
(b) Frontal
section
(c) 3-D
view
(d) Section
view in (e)
Primitive streak
Head end
Cut edge
of amnion
Epiblast
Yolk sac
(cut edge)
Right
(f) 14-15 days
Endoderm
Hypoblast
Left
Ectoderm
Primitive
streak
Tail end
(e) Bilayered embryonic disc,
superior view
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(g) 16 days
Mesoderm
Endoderm
Figure 28.9
Primary Germ Layers
• The primitive tissues from which all body
organs derive
• Ectoderm  nervous system and skin
epidermis
• Endoderm  epithelial linings of the digestive,
respiratory, and urogenital systems
• Mesoderm  forms all other tissues
• Endoderm and ectoderm are considered
epithelia
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Organogenesis
• Gastrulation sets the stage for organogenesis:
formation of body organs and systems
• At eighth week
• All organ systems are recognizable
• End of the embryonic period
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Specialization of Ectoderm
• Neurulation
• First major event of organogenesis
• Gives rise to brain and spinal cord
• Ectoderm over the notochord forms the neural
plate
• Neural plate folds inward as a neural groove
with neural folds
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Specialization of Ectoderm
• By the 22nd day, neural folds fuse into a
neural tube
• Anterior end  brain; the rest  spinal cord
• Neural crest cells  cranial, spinal, and
sympathetic ganglia, and adrenal medulla
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Head
Amnion
Amniotic cavity
Neural plate
Left
Right
Cut
edge of
amnion
Primitive
streak
Tail
Ectoderm
Mesoderm
Notochord
Endoderm
Yolk sac
(a) 17 days. The flat three-layered
embryo has completed
gastrulation. Notochord and
neural plate are present.
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Figure 28.10a
Neural groove
Neural
fold
Neural
crest
Coelom
Somite
Intermediate
mesoderm
Lateral plate
mesoderm
• Somatic
mesoderm
• Splanchnic
mesoderm
(b) 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).
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Figure 28.10b
Surface ectoderm
Neural
crest
Neural
tube
Somite
Notochord
(c) 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.
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Figure 28.10c
Somite
Dermatome
Myotome
Sclerotome
Kidney and gonads
(intermediate
mesoderm)
Splanchnic
mesoderm
• Visceral serosa
• Smooth muscle of gut
Peritoneal cavity
(coelom)
Neural tube
(ectoderm)
Epidermis
(ectoderm)
Gut lining
(endoderm)
Somatic
mesoderm
• Limb bud
• Parietal
serosa
• Dermis
(d) 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.
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Figure 28.10d
Specialization of Endoderm
• Embryonic folding begins with lateral folds
• Next, head and tail folds appear
• Endoderm tube forms epithelial lining of the
GI tract
• Organs of the GI tract become apparent, and
oral and anal openings perforate
• Mucosal lining of respiratory tract forms from
pharyngeal endoderm (foregut)
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Tail
Head
Amnion
Yolk sac
(a)
Ectoderm
Mesoderm
Endoderm
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Trilaminar
embryonic disc
Figure 28.11a
Lateral
fold
Future gut
(digestive
tube)
(b)
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Figure 28.11b
Somites (seen
through ectoderm)
Tail
fold
Head
fold
(c)
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Yolk sac
Figure 28.11c
Hindgut
Yolk
sac
Neural tube
Notochord
Primitive
gut
Foregut
(d)
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Figure 28.11d
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
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5-week embryo
Large intestine
Figure 28.12
Specialization of Mesoderm
• First evidence is appearance of the notochord
• Three mesoderm aggregates appear lateral to
notochord
• Somites, intermediate mesoderm, and double
sheets of lateral plate mesoderm
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Specialization of Mesoderm
• Somites (40 pairs) each have three functional
parts
1. Sclerotome cells: produce vertebra and rib at
each level
2. Dermatome cells: form dermis of the skin on
the dorsal part of the body
3. Myotome cells: form skeletal muscles of the
neck, trunk, and limbs (via limb buds)
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Specialization of Mesoderm
• Intermediate mesoderm forms gonads and
kidneys
• Lateral mesoderm consists of somatic and
splanchnic mesoderm
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Specialization of the Mesoderm
• Somatic mesoderm forms the:
• Dermis of the skin in the ventral region
• Parietal serosa of the ventral body cavity
• Bones, ligaments, and dermis of limbs
• Splanchnic mesoderm forms:
• The heart and blood vessels
• Most connective tissues of the body
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Head
Amnion
Amniotic cavity
Neural plate
Left
Right
Cut
edge of
amnion
Primitive
streak
Tail
Ectoderm
Mesoderm
Notochord
Endoderm
Yolk sac
(a) 17 days. The flat three-layered
embryo has completed
gastrulation. Notochord and
neural plate are present.
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Figure 28.10a
Neural groove
Neural
fold
Neural
crest
Coelom
Somite
Intermediate
mesoderm
Lateral plate
mesoderm
• Somatic
mesoderm
• Splanchnic
mesoderm
(b) 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).
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Figure 28.10b
Surface ectoderm
Neural
crest
Neural
tube
Somite
Notochord
(c) 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.
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Figure 28.10c
Somite
Dermatome
Myotome
Sclerotome
Kidney and gonads
(intermediate
mesoderm)
Splanchnic
mesoderm
• Visceral serosa
• Smooth muscle of gut
Peritoneal cavity
(coelom)
Neural tube
(ectoderm)
Epidermis
(ectoderm)
Gut lining
(endoderm)
Somatic
mesoderm
• Limb bud
• Parietal
serosa
• Dermis
(d) 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.
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Figure 28.10d
Epiblast
ECTODERM
MESODERM
Notochord
• Epidermis, hair,
nails, glands of
skin
• Brain and
spinal cord
• Neural crest
and derivatives
(sensory nerve
cells, pigment
cells, bones
and blood
vessels of the
head)
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Nucleus
pulposus
of intervertebral
discs
Somite
• Sclerotome:
vertebrae
and ribs
• Dermatome:
dermis of
dorsal body
region
• Myotome:
trunk and
limb
musculature
Intermediate
mesoderm
• Kidneys
• Gonads
ENDODERM
Lateral plate
mesoderm
Somatic
mesoderm
Splanchnic
mesoderm
• Parietal
serosa
• Dermis of
ventral body
region
• Connective
tissues of
limbs (bones,
joints, and
ligaments)
• Wall of
digestive
and
respiratory
tracts
(except
epithelial
lining)
• Visceral
serosa
• Heart
• Blood
vessels
Epithelial
lining and
glands of
digestive
and
respiratory
tracts
Figure 28.13
Development of Fetal Circulation
• First blood cells arise in the yolk sac
• By the end of the third week
• Embryo has a system of paired vessels
• Vessels forming the heart have fused
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Development of Fetal Circulation
• Unique vascular modifications
• Umbilical arteries and umbilical vein
• Three vascular shunts
• All are occluded at birth
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Development of Fetal Circulation
• Vascular shunts
• Ductus venosus: bypasses liver (umbilical vein
 ductus venosus  IVC)
• Foramen ovale: opening in interatrial septum;
bypasses pulmonary circulation
• Ductus arteriosus: bypasses pulmonary
circulation (pulmonary trunk  ductus
arteriosus  aorta)
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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
(a)
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High oxygenation
Moderate oxygenation
Low oxygenation
Very low oxygenation
Figure 28.14a
Superior vena cava
Aortic arch
Newborn
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
High oxygenation
Moderate oxygenation
Low oxygenation
Very low oxygenation
Medial umbilical ligaments
Urinary bladder
(b)
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Figure 28.14b
Events of Fetal Development
• Fetal period: weeks 9 through 38
• Time of rapid growth of body structures
established in the embryo
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Umbilical cord
Chorionic
villi
Umbilical vein
Amniotic sac
Yolk sac
(a) Embryo at week 7,
about 17 mm long.
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Cut edge
of chorion
Figure 28.15
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Table 28.1 (1 of 3)
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Table 28.1 (2 of 3)
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Table 28.1 (3 of 3)
Effects of Pregnancy: Anatomical Changes
• Reproductive organs become engorged with
blood
• Chadwick’s sign: the vagina develops a
purplish hue
• Breasts enlarge and areolae darken
• Pigmentation of facial skin many increase
(chloasma)
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Effects of Pregnancy: Anatomical Changes
• The uterus expands, occupying most of the
abdominal cavity
• Lordosis occurs with the change in the center
of gravity
• Weight gain of ~13 kg (28 lb)
• Relaxin causes pelvic ligaments and the pubic
symphysis to relax to ease birth passage
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(a) Before conception
(Uterus the size of a
fist and resides in
the pelvis.)
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(b) 4 months
(Fundus of the
uterus is halfway
between the pubic
symphysis and
the umbilicus.)
(c) 7 months
(Fundus is well
above the
umbilicus.)
(d) 9 months
(Fundus reaches
the xiphoid
process.)
Figure 28.16
Effects of Pregnancy: Metabolic Changes
• Placental hormones
• Human placental lactogen (hPL), or human chorionic
somatomammotropin (hCS)
•  maturation of the breasts, fetal growth, and
glucose sparing in the mother
• Human chorionic thyrotropin (hCT)
•   maternal metabolism
• Parathyroid hormone and vitamin D levels are high
throughout pregnancy
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Effects of Pregnancy: Physiological
Changes
• GI tract
• Morning sickness due to elevated levels of
estrogen and progesterone
• Heartburn and constipation are common
• Urinary system
•  Urine production due to  metabolism and
fetal wastes
• Stress incontinence may occur as bladder is
compressed
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Effects of Pregnancy: Physiological
Changes
• Respiratory system
• Estrogens may cause nasal edema and
congestion
• Tidal volume increases
• Dyspnea (difficult breathing) may occur later in
pregnancy
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Effects of Pregnancy: Physiological
Changes
• Cardiovascular system
• Blood volume increases 25–40%
• Blood pressure and pulse rise
• Venous return from lower limbs may be
impaired, resulting in varicose veins
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Parturition
• Parturition giving birth to the baby
• Labor events that expel the infant from the
uterus
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Initiation of Labor
• During the last few weeks of pregnancy
• Fetal secretion of cortisol stimulates the
placenta to secrete more estrogen
• Causes production of oxytocin receptors by
myometrium
• Antagonizes calming effects of
progesterone, leading to Braxton Hicks
contractions in uterus
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Initiation of Labor
• Surfactant protein A (SP-A) from fetal lungs
causes softening of the cervix
• Fetal oxytocin causes the placenta to produce
prostaglandins
• Oxytocin and prostaglandins: powerful uterine
muscle stimulants
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Initiation of Labor
• Maternal emotional and physical stress
• Activates the hypothalamus, causing oxytocin
release from posterior pituitary
• Positive feedback mechanism occurs
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Estrogen
Oxytocin
(+)
from
placenta
from fetus
and mother’s
posterior pituitary
Induces oxytocin
receptors on uterus
Stimulates uterus
to contract
Stimulates
placenta to make
(+)
Prostaglandins
Stimulate more
vigorous contractions
of uterus
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Figure 28.17
Stages of Labor: Dilation Stage
• Longest stage of labor: 6–12 hours or more
• Initial weak contractions:
• 15–30 minutes apart, 10–30 seconds long
• Become more vigorous and rapid
• Cervix effaces and dilates fully to 10 cm
• Amnion ruptures, releasing amniotic fluid
• Engagement occurs: head enters the true
pelvis
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Umbilical
cord
Placenta
Uterus
Cervix
Vagina
(a) Dilation (early)
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Figure 28.18a
Pubic
symphysis
Sacrum
(b) Dilation (late)
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Figure 28.18b
Stages of Labor: Expulsion Stage
• Strong contractions every 2–3 minutes, about
1 minute long
• Urge to push increases (in absence of local
anesthesia)
• Crowning occurs when the largest dimension
of the head distends vulva
• Delivery of infant
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Perineum
(c) Expulsion
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Figure 28.18c
Stages of Labor: Placental Stage
• Strong contractions continue, causing
detachment of the placenta and compression
of uterine blood vessels
• Delivery of the afterbirth (placenta and
membranes) occurs ~30 minutes after birth
• All placenta fragments must be removed to
prevent postpartum bleeding
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Uterus
Placenta
(detaching)
Umbilical
cord
(d) Placental
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Figure 28.18d
Adjustments of the Infant to Extrauterine
Life
• Neonatal period: four-week period
immediately after birth
• Physical status is assessed 1–5 minutes after
birth
• Apgar score: 0–2 points each for
•Heart rate
•Muscle tone
•Respiration
•Reflexes
•Color
• Score of 8–10: healthy
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First Breath
•  CO2  central acidosis  stimulates
respiratory control centers to trigger the first
inspiration
• Requires tremendous effort: airways are tiny
and the lungs are collapsed
• Surfactant in alveolar fluid helps reduce
surface tension
• Respiratory rate: ~45 per minute for first two
weeks, then declines
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Transitional Period
• Unstable period lasting 6–8 hours after birth
• Alternating periods of activity and sleep
• Vital signs may be irregular during activity
• Stabilizes with waking periods occurring every
3–4 hours
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Occlusion of Fetal Blood Vessels
• Umbilical arteries and vein constrict and
become fibrosed
• Proximal umbilical arteries  superior vesical
arteries to urinary bladder
• Distal umbilical arteries  medial umbilical
ligaments
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Occlusion of Fetal Blood Vessels
• Umbilical vein becomes the ligamentum teres
• Ductus venosus  ligamentum venosum
• Foramen ovale  fossa ovalis
• Ductus arteriosus  ligamentum arteriosum
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Lactation
• Production of milk by the mammary glands
• Toward the end of pregnancy
• Placental estrogens, progesterone, and
lactogen stimulate the hypothalamus to
release prolactin-releasing factors (PRFs)
• Anterior pituitary releases prolactin
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Lactation
• Colostrum
• Yellowish secretion rich in vitamin A, protein,
minerals, and IgA antibodies
• Released the first 2–3 days
• Followed by true milk production
• Suckling initiates a positive feedback
mechanism
• Oxytocin causes the letdown reflex
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Start
Stimulation of
mechanoreceptors
in nipples by
suckling infant
sends afferent
impulses to the
hypothalamus.
Inhibits hypothalamic neurons that
release dopamine. Hypothalamus
releases prolactin releasing factors
(PRF) to portal circulation.
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
lactiferous glands.
Milk production
Alveolar glands
respond by
releasing milk
through ducts of
nipples.
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Figure 28.19
Advantages of Breast Milk
• Fats and iron are easily absorbed; amino
acids more easily metabolized, compared with
cow’s milk
• Beneficial chemicals: IgA, complement,
lysozyme, interferon, and lactoperoxidase
• Interleukins and prostaglandins prevent
overzealous inflammatory responses
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Advantages of Breast Milk
• Natural laxative effect helps eliminate bile-rich
meconium, helping to prevent physiological
jaundice
• Encourages bacterial colonization of the large
intestine
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Assisted Reproductive Technology
• Surgical removal of oocytes following
hormone stimulation
• Fertilization of oocytes
• Return of fertilized oocytes to the woman’s
body
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Assisted Reproductive Technology
• In vitro fertilization (IVF)
• Oocytes and sperm are incubated in culture
dishes for several days
• Embryos (two-cell to blastocyst stage) are
transferred to uterus for possible implantation
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Assisted Reproductive Technology
• Zygote intrafallopian transfer (ZIFT
• Fertilized oocytes are transferred to the uterine
tubes
• Gamete intrafallopian transfer (GIFT)
• Sperm and harvested oocytes are transferred
together into the uterine tubes
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