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prepared by
Janice Meeking,
Mount Royal College
CHAPTER
28
Pregnancy
and Human
Development:
Part B
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
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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