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28. Pregnancy and Human Development: Part B. 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. Specialization of Ectoderm. Neurulation

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Pregnancy and Human Development: Part B


Organogenesis

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


Specialization of ectoderm

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


Specialization of ectoderm1

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

Ectoderm

Left

Right

Mesoderm

Cut

edge of

amnion

Primitive

streak

Notochord

Endoderm

Tail

Yolk sac

(a) 17 days. The flat three-layered

embryo has completed

gastrulation. Notochord and

neural plate are present.

Figure 28.10a


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Neural groove

Somite

Neural

fold

Intermediate

mesoderm

Neural

crest

Lateral plate

mesoderm

• Somatic

mesoderm

Coelom

• 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).

Figure 28.10b


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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.

Figure 28.10c


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Neural tube

(ectoderm)

Dermatome

Somite

Myotome

Sclerotome

Epidermis

(ectoderm)

Kidney and gonads

(intermediate

mesoderm)

Gut lining

(endoderm)

Splanchnic

mesoderm

Somatic

mesoderm

• Visceral serosa

• Limb bud

• Smooth muscle of gut

• Parietal

serosa

• Dermis

Peritoneal cavity

(coelom)

(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.

Figure 28.10d


Specialization of endoderm

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

Trilaminar

embryonic disc

Mesoderm

Endoderm

Figure 28.11a


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Future gut

(digestive

tube)

Lateral

fold

(b)

Figure 28.11b


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Somites (seen

through ectoderm)

Tail

fold

Head

fold

Yolk sac

(c)

Figure 28.11c


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Neural tube

Notochord

Primitive

gut

Foregut

Yolk

Hindgut

sac

(d)

Figure 28.11d


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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

Figure 28.12


Specialization of mesoderm

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


Specialization of mesoderm1

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 the skin on the dorsal part of the body

    • Myotome cells: form skeletal muscles of the neck, trunk, and limbs (via limb buds)


Specialization of mesoderm2

Specialization of Mesoderm

  • Intermediate mesoderm forms gonads and kidneys

  • Lateral mesoderm consists of somatic and splanchnic mesoderm


Specialization of the mesoderm

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

Ectoderm

Left

Right

Mesoderm

Cut

edge of

amnion

Primitive

streak

Notochord

Endoderm

Tail

Yolk sac

(a) 17 days. The flat three-layered

embryo has completed

gastrulation. Notochord and

neural plate are present.

Figure 28.10a


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Neural groove

Somite

Neural

fold

Intermediate

mesoderm

Neural

crest

Lateral plate

mesoderm

• Somatic

mesoderm

Coelom

• 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).

Figure 28.10b


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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.

Figure 28.10c


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Neural tube

(ectoderm)

Dermatome

Somite

Myotome

Sclerotome

Epidermis

(ectoderm)

Kidney and gonads

(intermediate

mesoderm)

Gut lining

(endoderm)

Splanchnic

mesoderm

Somatic

mesoderm

• Visceral serosa

• Limb bud

• Smooth muscle of gut

• Parietal

serosa

• Dermis

Peritoneal cavity

(coelom)

(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.

Figure 28.10d


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Epiblast

ECTODERM

MESODERM

ENDODERM

Intermediate

mesoderm

Lateral plate

mesoderm

Notochord

Somite

Somatic

mesoderm

Splanchnic

mesoderm

Nucleus

pulposus

of inter-

vertebral

discs

Epithelial

lining and

glands of

digestive

and

respiratory

tracts

• 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)

• 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

• Sclerotome:

vertebrae

and ribs

• Dermatome:

dermis of

dorsal body

region

• Myotome:

trunk and

limb

musculature

• Kidneys

• Gonads

Figure 28.13


Development of fetal circulation

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


Development of fetal circulation1

Development of Fetal Circulation

  • Unique vascular modifications

    • Umbilical arteries and umbilical vein

    • Three vascular shunts

  • All are occluded at birth


Development of fetal circulation2

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

High oxygenation

Moderate oxygenation

Low oxygenation

(a)

Very low oxygenation

Figure 28.14a


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Newborn

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

High oxygenation

Common iliac artery

Moderate oxygenation

Low oxygenation

Umbilical arteries

Very low oxygenation

Medial umbilical ligaments

Urinary bladder

(b)

Figure 28.14b


Events of fetal development

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

Amniotic sac

Umbilical vein

Yolk sac

Cut edge

of chorion

(a) Embryo at week 7,

about 17 mm long.

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

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)


Effects of pregnancy anatomical changes1

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.)

(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

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


Effects of pregnancy physiological changes

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


Effects of pregnancy physiological changes1

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


Effects of pregnancy physiological changes2

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


Parturition

Parturition

  • Parturition giving birth to the baby

  • Labor events that expel the infant from the uterus


Initiation of labor

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


Initiation of labor1

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


Initiation of labor2

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

Figure 28.17


Stages of labor dilation stage

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)

Figure 28.18a


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Pubic

symphysis

Sacrum

(b) Dilation (late)

Figure 28.18b


Stages of labor expulsion stage

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

Figure 28.18c


Stages of labor placental stage

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

Figure 28.18d


Adjustments of the infant to extrauterine life

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

    • Score of 8–10: healthy


First breath

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


Transitional period

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


Occlusion of fetal blood vessels

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


Occlusion of fetal blood vessels1

Occlusion of Fetal Blood Vessels

  • Umbilical vein becomes the ligamentum teres

  • Ductus venosus  ligamentum venosum

  • Foramen ovale  fossa ovalis

  • Ductus arteriosus  ligamentum arteriosum


Lactation

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


Lactation1

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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Inhibits hypothalamic neurons that

release dopamine. Hypothalamus

releases prolactin releasing factors

(PRF) to portal circulation.

Start

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

lactiferous glands.

Milk production

Alveolar glands

respond by

releasing milk

through ducts of

nipples.

Figure 28.19


Advantages of breast milk

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


Advantages of breast milk1

Advantages of Breast Milk

  • Natural laxative effect helps eliminate bile-rich meconium, helping to prevent physiological jaundice

  • Encourages bacterial colonization of the large intestine


Assisted reproductive technology

Assisted Reproductive Technology

  • Surgical removal of oocytes following hormone stimulation

  • Fertilization of oocytes

  • Return of fertilized oocytes to the woman’s body


Assisted reproductive technology1

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


Assisted reproductive technology2

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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