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prepared by
Barbara Heard,
Atlantic Cape Community
College
CHAPTER
28
Pregnancy
and Human
Development
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Pregnancy
• Pregnancy - events that occur from
fertilization until infant born
• Conceptus - developing offspring
• Gestation period - time from last
menstrual period until birth (~280 days)
• Embryo - conceptus from fertilization
through week 8
• Fetus - conceptus from week 9 through
birth
© 2013 Pearson Education, Inc.
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.
From Egg to Zygote
• Oocyte viable for 12 to 24 hours
• Sperm viable 24 to 48 hours after
ejaculation
© 2013 Pearson Education, Inc.
From Egg to Zygote
• For fertilization to occur, coitus must occur
no more than
– Two days before ovulation
– 24 hours after ovulation
• Fertilization - sperm's chromosomes
combine with those of secondary oocyte to
form fertilized egg (zygote)
© 2013 Pearson Education, Inc.
Accomplishing Fertilization
• Ejaculated sperm
– Leak out of vagina immediately after
deposition
– Destroyed by acidic vaginal environment
– Fail to make it through cervix
– Dispersed in uterine cavity or destroyed by
phagocytes
– Few (100 to a few thousand) reach uterine
tubes
© 2013 Pearson Education, Inc.
Accomplishing Fertilization
• Sperm must become motile
• Sperm must be capacitated before they
can penetrate oocyte
– Motility must be enhanced; membranes must
become fragile to release hydrolytic enzymes
• Secretions of female tract weaken
acrosome membrane
• Sperm follow "olfactory trail" to reach
oocyte
© 2013 Pearson Education, Inc.
Acrosomal Reaction and Sperm Penetration
• Sperm must breach oocyte coverings
– Corona radiata and zona pellucida
• Sperm weaves through corona radiata,
then binds to zona pellucida and
undergoes acrosomal reaction
– Enzymes released to digest holes in zona
pellucida
– Hundreds of acrosomes release enzymes to
digest zona pellucida
© 2013 Pearson Education, Inc.
Figure 28.2 Sperm Penetration and the Cortical Reaction.
Sperm, delivered to the vagina and
capacitated in the female reproductive
tract, stream toward a secondary
oocyte.
Slide 1
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.
4 Fusion.
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.
5 Block of 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).
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
Acrosomal Reaction and Sperm Penetration
• Sperm head approaches oocyte
• Rear portion of acrosomal membrane
binds to oocyte plasma membrane 
– Oocyte and sperm membranes fuse
– Gametes fuse as sperm's cytoplasmic
contents enter oocyte
• Only one sperm allowed to penetrate
oocyte (monospermy)
© 2013 Pearson Education, Inc.
Block to Polyspermy
• Upon entry of sperm, Ca2+ surge from ER
causes 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)
© 2013 Pearson Education, Inc.
Figure 28.2 Sperm Penetration and the Cortical Reaction.
Sperm, delivered to the vagina and
capacitated in the female reproductive
tract, stream toward a secondary
oocyte.
Slide 7
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.
4 Fusion.
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.
5 Block of 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).
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
Completion of Meiosis II and Fertilization
• As sperm nucleus moves toward oocyte
nucleus it swells to form male pronucleus
• The Ca2+ surge triggers completion of
meiosis II  ovum + second polar body
• Ovum nucleus swells to become female
pronucleus
• Fertilization – moment when membranes
of two pronuclei rupture and chromosomes
combine
© 2013 Pearson Education, Inc.
Figure 28.3a Events of fertilization.
Extracellular space
Corona radiata
Zona pellucida
Second meiotic
division of oocyte
Second meiotic
division of first
polar body
Sperm nucleus
1 After the sperm
penetrates the
secondary oocyte, the
oocyte completes
meiosis II, forming the
ovum and second
polar body.
Male pronucleus
Female pro-nucleus
(swollen ovum
nucleus)
Polar bodies
2 Sperm and ovum
nuclei swell, forming
pronuclei.
Male pronucleus
Mitotic spindle
Centriole
Female pronucleus
3 Pronuclei approach
each other and mitotic
spindle forms between
them.
Zygote
© 2013 Pearson Education, Inc.
4 Chromoomes of the
pronuclei intermix. Fertilization
is accomplished. Then, the DNA
replicates in preparation for the
first cleavage division.
Slide 1
Figure 28.3b Events of fertilization.
Male and female
pronuclei
Polar bodies
© 2013 Pearson Education, Inc.
Events of Embryonic Development: Zygote
to Blastocyst Implantation
• Cleavage
– Occurs while zygote moves toward uterus
– Mitotic divisions of zygote
– First cleavage at 36 hours  two daughter
cells (blastomeres)
– At 72 hours  morula (16 or more cells)
• At day 4 or 5, blastocyst (embryo of ~100
cells) reaches uterus
© 2013 Pearson Education, Inc.
Embryonic Development
• Blastocyst - fluid-filled hollow sphere
composed of
– Trophoblast cells
• Display immunosuppressive factors
• Participate in placenta formation
– Inner cell mass
• Becomes embryonic disc ( embryo and three of
embryonic membranes)
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Blastocyst
cavity
Inner cell
mass
Implantation
• Blastocyst floats for 2–3 days
– Nourished by uterine secretions
• Implantation begins 6–7 days after
ovulation
– Trophoblast cells adhere to site with proper
receptors and chemical signals
– Inflammatory-like response occurs in
endometrium
• Uterine blood vessels more permeable and leaky;
inflammatory cells invade area
© 2013 Pearson Education, Inc.
Figure 28.5a Implantation of the blastocyst.
Endometrium
Uterine endometrial
epithelium
Inner cell mass
Trophoblast
Blastocyst cavity
Lumen of uterus
© 2013 Pearson Education, Inc.
Implantation
• Trophoblasts proliferate and form two
distinct layers
– Cytotrophoblast (cellular trophoblast) inner layer of cells
– Syncytiotrophoblast (syncytial
trophoblast) - cells in outer layer lose plasma
membranes, invade and digest endometrium
• Blastocyst burrows into lining surrounded
by pool of leaked blood; endometrial cells
cover and seal off implanted blastocyst
© 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.
Implantation
• Implantation completed by twelfth day
after ovulation
– Menstruation must be prevented
– Corpus luteum maintained by hormone
human chorionic gonadotropin (hCG)
© 2013 Pearson Education, Inc.
Figure 28.5d Implantation of the blastocyst.
Endometrial stroma
with blood vessels
and glands
Syncytiotrophoblast
Cytotrophoblast
Lumen of uterus
© 2013 Pearson Education, Inc.
Hormonal Changes During Pregnancy
• Human chorionic gonadotropin (hCG)
– Secreted by trophoblast cells; later chorion
– Prompts corpus luteum to continue secretion
of progesterone and estrogen
– Promotes placental development via its
autocrine growth factor activity
– hCG levels rise until end of second month,
then decline as placenta begins to secrete
progesterone and estrogen; low values at 4
months and rest of pregnancy
© 2013 Pearson Education, Inc.
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
Placentation
• Formation of placenta from embryonic
and maternal tissues
– Temporary organ
– Embryonic tissues
• Mesoderm cells develop from inner cell mass; line
trophoblast
• Together these form chorion and chorionic villi
© 2013 Pearson Education, Inc.
Placentation
• Cores of chorionic villi invaded by new
blood vessels; extend to embryo as
umbilical arteries and vein
• Erosion  blood-filled lacunae (intervillous
spaces) in stratum functionalis
• Villi lie in intervillous spaces, immersed in
maternal blood
© 2013 Pearson Education, Inc.
Placentation
• Maternal portion of placenta
– Decidua basalis (stratum functionalis
between chorionic villi and stratum basalis of
endometrium)
• Fetal portion of placenta
– Chorionic villi
© 2013 Pearson Education, Inc.
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.
Placentation
• Decidua capsularis - part of endometrium
at uterine cavity face of implanted embryo
• Placenta fully formed and functional by
end of third month
– Nutritive, respiratory, excretory, endocrine
functions
• Placenta also secretes human placental
lactogen, human chorionic thyrotropin, and
relaxin
© 2013 Pearson Education, Inc.
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
Placentation
• If placental hormones inadequate,
pregnancy aborted
• Throughout pregnancy blood levels of
estrogens and progesterone increase
• Prepare mammary glands for lactation
© 2013 Pearson Education, Inc.
Placentation
• Maternal and embryonic blood supplies
normally do not intermix
• Embryonic placental barriers include
– Membranes of chorionic villi
– Endothelium of embryonic capillaries
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
Events of Embryonic Development: Gastrula
to Fetus
• Germ Layers
– During implantation, blastocyst begins
conversion to gastrula
• Inner cell mass develops into embryonic disc
(subdivides into epiblast and hypoblast)
• Three primary germ layers form; extraembryonic
membranes develop
© 2013 Pearson Education, Inc.
Extraembryonic Membranes
• Amnion - epiblast cells form transparent
sac filled with amniotic fluid
– Provides buoyant environment that protects
embryo
– Helps maintain constant homeostatic
temperature
– Allows freedom of movement; prevents parts
from fusing together
– Amniotic fluid comes from maternal blood,
and later, fetal urine
© 2013 Pearson Education, Inc.
Extraembryonic Membranes
• Yolk sac - sac that hangs from ventral
surface of embryo
– Forms part of digestive tube
– Source of earliest blood cells and blood
vessels
© 2013 Pearson Education, Inc.
Extraembryonic Membranes
• Allantois - small outpocketing at caudal
end of yolk sac
– Structural base for umbilical cord
– Becomes part of urinary bladder
• Chorion - helps form placenta
– Encloses embryonic body and all other
membranes
© 2013 Pearson Education, Inc.
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.
Gastrulation
• Occurs in week 3
• Embryonic disc  three-layered embryo
with primary germ layers present
– Ectoderm, mesoderm, and endoderm
• Begins with appearance of primitive
streak, raised dorsal groove; establishes
longitudinal axis of embryo
© 2013 Pearson Education, Inc.
Gastrulation
• Cells begin to migrate into groove
– First cells form endoderm
– Cells that follow push laterally, forming
mesoderm
• Notochord - rod of mesodermal cells that serves
as axial support
– Cells that remain on embryo's dorsal surface
form ectoderm
© 2013 Pearson Education, Inc.
Gastrulation
• Ectoderm, mesoderm, endoderm - primitive
tissues from which all body organs derive
• Epithelia cells
– Ectoderm  nervous system; skin epidermis
– Endoderm  epithelial linings of digestive,
respiratory, urogenital systems; associated glands
• Mesenchyme cells
– Mesoderm  everything else
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
16 days Mesoderm Endoderm
Organogenesis
• Gastrulation sets stage for organogenesis
– Formation of body organs and systems
• At eighth week
– All organ systems recognizable
– End of embryonic period
© 2013 Pearson Education, Inc.
Organogenesis
• Embryo begins as flat plate 
• Cylindrical body resembling three stacked
sheets of paper folding laterally into tube,
and at both ends
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Yolk
sac
Foregut
Specialization of Endoderm
• Primitive gut formed from endodermal
folding
– Forms epithelial lining of GI tract
– Organs of GI tract become apparent, and oral
and anal openings perforate
• Mucosal lining of respiratory tract forms
from pharyngeal endoderm (foregut)
• Glands arise further along tract
© 2013 Pearson Education, Inc.
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.
Specialization of Ectoderm
• Neurulation
– First major event of organogenesis
– Gives rise to brain and spinal cord
– Induced by chemical signals from notochord
– Ectoderm over notochord thickens, forming
neural plate
– Neural plate folds inward as neural groove
with neural folds
© 2013 Pearson Education, Inc.
Specialization of Ectoderm
• By 22nd day, neural folds fuse into neural
tube
– Anterior end  brain; rest  spinal cord
• Neural crest cells migrate widely 
cranial, spinal, and sympathetic ganglia
and nerves; adrenal medulla; pigment
cells of skin; contribute to some
connective tissues
• Brain waves recorded by end of second
month
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
• 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.
Specialization of Mesoderm
• First evidence - appearance of notochord
– Eventually replaced by vertebral column
• Three mesoderm aggregates appear
lateral to notochord
– Somites, intermediate mesoderm, and double
sheets of lateral plate mesoderm
© 2013 Pearson Education, Inc.
Specialization of Mesoderm
• Somites (40 pairs) each have three
functional parts
– Sclerotome cells - produce vertebra and rib
at each level
– Dermatome cells - form dermis of skin on
dorsal part of body
– Myotome cells - form skeletal muscles of
neck, trunk, and limbs (via limb buds)
© 2013 Pearson Education, Inc.
Specialization of Mesoderm
• Intermediate mesoderm forms gonads
and kidneys
• Lateral plate mesoderm consists of
somatic and splanchnic mesoderm
© 2013 Pearson Education, Inc.
Specialization of the Mesoderm
• Somatic mesoderm forms
– Dermis of skin in ventral region
– Parietal serosa of ventral body cavity
– Most tissues of limbs
• Splanchnic mesoderm forms
– Heart and blood vessels
– Most connective tissues of body
– ~ Entire wall of digestive & respiratory organs
© 2013 Pearson Education, Inc.
Specialization of the Mesoderm
• At end of embryonic period
– Bones have begun to ossify; skeletal muscles
well formed, contracting; metanephric kidneys
developing; gonads formed
– Lungs, digestive organs attaining final shape
and body position
– Blood delivery to/from placenta constant &
efficient
– Heart and liver bulge on ventral surface
© 2013 Pearson Education, Inc.
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
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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
Development of Fetal Circulation
• First blood cells arise in yolk sac
• By end of third week
– Embryo has system of paired vessels
– Two vessels forming heart have fused; bent
into "S" shape
• Heart beats by 3½ weeks
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Development of Fetal Circulation
• Unique vascular modifications
– Umbilical arteries and umbilical vein
– Three vascular shunts
• All 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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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
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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
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High oxygenation
Moderate oxygenation
Low oxygenation
Very low oxygenation
Events of Fetal Development
• Fetal period - weeks 9 through 38
• Time of rapid growth of body structures
established in embryo
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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.
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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.
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)
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Table 28.1 Developmental Events of the Fetal Period (3 of 3)
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Effects of Pregnancy on the Mother:
Anatomical Changes
• Reproductive organs become engorged
with blood
– Chadwick's sign - vagina develops purplish
hue
– Breasts enlarge and areolae darken
– Pigmentation of facial skin many increase
(chloasma)
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Effects of Pregnancy: Anatomical Changes
• Uterus expands, occupying most of
abdominal cavity
– Ribs flare  thorax widens
• Lordosis occurs with change in center of
gravity
• Relaxin causes pelvic ligaments and
pubic symphysis to relax to ease birth
passage
• Weight gain of ~13 kg (28 lb)
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Effects of Pregnancy: Anatomical Changes
• Good nutrition vital
– 300 additional daily calories
• Multivitamins with folic acid reduce fetal
risk of neurological problems, e.g., spina
bifida, anencephaly, and spontaneous
preterm birth
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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.)
Effects of Pregnancy: Metabolic Changes
• Placental hormones
– Human placental lactogen (hPL) (human
chorionic somatomammotropin (hCS))
•  maturation of breasts, fetal growth, and glucose
sparing in mother (reserving glucose for fetus)
• Parathyroid hormone and vitamin D levels
high throughout pregnancy  adequate
calcium for fetal bone mineralization
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Effects of Pregnancy: Physiological
Changes
• GI tract
– Morning sickness believed due to elevated
levels of hCG, estrogen and progesterone
– Heartburn and constipation are common
• Urinary system
–  Urine production due to  maternal
metabolism and fetal wastes
– Frequent, urgent urination; stress
incontinence may occur as bladder
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%
• Safeguards against blood loss during childbirth
– Cardiac output rises as much as 35-40%
• Propels greater volume around body
– Venous return from lower limbs may be
impaired, resulting in varicose veins
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Homeostatic Imbalance
• Preeclampsia
– Insufficient placental blood supply  fetus
starved of oxygen
– Woman  edematous, hypertensive,
proteinuria
– May be due to immunological abnormalities
• Correlated with number of fetal cells that enter
maternal circulation
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Parturition
• Giving birth to baby
• Labor
– Events that expel infant from uterus
• Increased production of surfactant protein
A (SP-A) in weeks before delivery may
trigger inflammatory response in cervix 
softening in preparation for labor
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Initiation of Labor
• Fetus determines own birth date
• During last few weeks of pregnancy
– Fetal secretion of cortisol stimulates placenta
to secrete more estrogen
• Causes production of oxytocin receptors by
myometrium
• Causes formation of gap junctions between uterine
smooth muscle cells
• Antagonizes calming effects of progesterone,
leading to Braxton Hicks contractions in uterus
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Initiation of Labor
• Fetal oxytocin causes placenta to
produce prostaglandins
• Oxytocin and prostaglandins - powerful
uterine muscle stimulants
– Due especially to prostaglandins, contractions
 more frequent and vigorous
– Anti-prostaglandins contraindicated during
labor
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Initiation of Labor
• Increasing cervical distension
– Activates hypothalamus, causing oxytocin
release from posterior pituitary
– Positive feedback mechanism occurs
• Greater distension of cervix  more oxytocin
release  greater contractile force  greater
distension of cervix  etc.
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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
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Positive feedback
(+)
Stages of Labor: Dilation Stage
• From labor's onset to fully dilated cervix (10 cm)
• 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 true pelvis
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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
Slide 1
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
Stages of Labor: Expulsion Stage
• From full dilation to delivery of infant
• Strong contractions every 2–3 minutes,
about 1 minute long
• Urge to push increases (in absence of
local anesthesia)
• Crowning occurs when largest dimension
of head distends vulva
– Episiotomy may be done to reduce tearing
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Stages of Labor: Expulsion Stage
• Vertex position – head-first
– Skull dilates cervix; early suctioning allows
breathing prior to complete delivery
• Breech position – buttock-first
– Delivery more difficult; often forceps required,
or C-section (delivery through abdominal and
uterine wall incision)
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Stages of Labor: Placental Stage
• Strong contractions continue, causing
detachment of placenta and compression
of uterine blood vessels
– Limit bleeding; cause placental detachment
• Delivery of afterbirth (placenta and
membranes) occurs ~30 minutes after
birth
• All placenta fragments must be removed
to prevent postpartum bleeding
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Adjustments of the Infant to Extrauterine
Life
• Neonatal period - four-week period immediately
after birth
• Physical status assessed 1–5 minutes after birth
– Apgar score - 0–2 points each for
• Heart rate
• Respiration
• Color
• Muscle tone
• Reflexes
– Score of 8–10 - healthy
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First Breath
•  CO2  central acidosis  stimulates
respiratory control centers to trigger first
inspiration
– Requires tremendous effort – airways tiny;
lungs collapsed
– Surfactant in alveolar fluid helps reduce
surface tension
• Respiratory rate ~45 per minute first two
weeks, then declines
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First Breath
• Keeping lungs inflated difficult for
premature infant (< 2500 g, or 5.5 pounds,
at birth)
– Surfactant production in last months of
prenatal life
– Preemies usually on respiratory assistance
until lungs mature
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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
– Baby gags frequently as regurgitates mucus
and debris
• 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 round ligament
of liver (ligamentum teres)
• Ductus venosus  ligamentum
venosum about 30 minutes after birth
• Pressure changes from infant breathing
cause pulmonary shunts to close
– Foramen ovale  fossa ovalis up to a year
after birth
– Ductus arteriosus  ligamentum
arteriosum about 30 minutes after birth
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Lactation
• Production of milk by mammary glands
• Toward end of pregnancy
– Placental estrogens, progesterone, and
human placental lactogen stimulate
hypothalamus to release prolactin-releasing
factors (PRFs) 
– Anterior pituitary releases prolactin
• 2-3 days later true milk production begins
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Lactation
• Colostrum
– Less lactose but more protein, vitamin A,
minerals than true milk; almost no fat
– Yellowish secretion rich in IgA antibodies
• IgA resistant to digestion; may protect infant
against bacterial infection; absorbed into
bloodstream for immunity
– Released first 2–3 days
– Followed by true milk production
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Lactation
• Prolactin release wanes after birth
• Lactation sustained by mechanical
stimulation of nipples - suckling
– Suckling causes afferent impulses to
hypothalamus  prolactin  stimulates milk
production for next feeding
– Hypothalamus also  oxytocin from posterior
pituitary  let-down reflex
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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
Advantages of Breast Milk
• Fats and iron better absorbed; amino
acids more easily metabolized, compared
with cow's milk
• Beneficial chemicals
– IgA, complement, lysozyme, interferon, and
lactoperoxidase (protect from infections)
– Interleukins and prostaglandins prevent
overzealous inflammatory responses
– Glycoprotein deters ulcer-causing bacterium
from attaching to stomach mucosa
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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 large
intestine
• Women nursing 6 months lose bone calcium;
replaced after weaning if healthy diet
• Women may ovulate when nursing despite
inhibition of GnRH and gonadotropins
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Assisted Reproductive Technology
• Surgical removal of oocytes following
hormone stimulation
• Fertilization of oocytes
• Return of fertilized oocytes to woman's
body
• Disadvantages
– Costly, emotionally draining, painful for oocyte
donor
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Assisted Reproductive Technology
• In vitro fertilization (IVF)
– Oocytes and sperm incubated in culture
dishes for several days
– Embryos (two-cell to blastocyst stage)
transferred to uterus for possible implantation
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Assisted Reproductive Technology
• Zygote intrafallopian transfer (ZIFT)
– Fertilized oocytes transferred to uterine tubes
• Gamete intrafallopian transfer (GIFT)
– Sperm and harvested oocytes are transferred
together into the uterine tubes
• Cloning
– Legal, moral, ethical, political roadblocks
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