1. Pregnancy and Human Development: Part B28
Pregnancy and Human Development: Part B

2. 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

3. 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

4. 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

5. (a) 17 days. The flat three-layered embryo has completed
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

6. (b) 20 days. The neural folds form by folding of the
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

7. (c) 22 days. The neural folds have closed,
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

8. (d) End of week 4. Embryo undercutting is complete. Somites
Dermatome
Neural tube
(ectoderm)
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

9. 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)

10. Tail Head Amnion Yolk sac (a) Ectoderm Mesoderm Trilaminar
embryonic disc
Endoderm
Figure 28.11a

11. Future gut
(digestive
tube)
Lateral
fold
(b)
Figure 28.11b

12. Somites (seen through ectoderm) Tail fold Head fold Yolk sac (c)
Figure 28.11c

13. Neural tube Notochord Primitive gut Foregut Yolk Hindgut sac (d)
Figure 28.11d

14. 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

15. 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

16. 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)

17. Specialization of Mesoderm
Intermediate mesoderm forms gonads and kidneys
Lateral mesoderm consists of somatic and splanchnic mesoderm

18. 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

19. (a) 17 days. The flat three-layered embryo has completed
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

20. (b) 20 days. The neural folds form by folding of the
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

21. (c) 22 days. The neural folds have closed,
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

22. (d) End of week 4. Embryo undercutting is complete. Somites
Dermatome
Neural tube
(ectoderm)
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

23. Epiblast
ECTODERM
MESODERM
ENDODERM
Notochord
Somite
Intermediate
mesoderm
Lateral plate
mesoderm
Somatic
mesoderm
Splanchnic
mesoderm
• 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)
Nucleus
pulposus
of inter-
vertebral
discs
• Sclerotome:
vertebrae
and ribs
• Dermatome:
dermis of
dorsal body
region
• Myotome:
trunk and
limb
musculature
• Kidneys
• Gonads
• 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

24. 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

25. Development of Fetal Circulation
Unique vascular modifications
Umbilical arteries and umbilical vein
Three vascular shunts
All are occluded at birth

26. 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)

27. Figure 28.14a 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

28. Figure 28.14b Aortic arch Newborn 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
Umbilical arteries
Low oxygenation
Very low oxygenation
Medial umbilical ligaments
Urinary bladder
(b)
Figure 28.14b

29. Events of Fetal Development
Fetal period: weeks 9 through 38
Time of rapid growth of body structures established in the embryo

30. 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

31. Table 28.1 (1 of 3)

32. Table 28.1 (2 of 3)

33. Table 28.1 (3 of 3)

34. 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)

35. 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

36. (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

37. 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

38. 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

39. 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

40. 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

41. Parturition
Parturition giving birth to the baby
Labor events that expel the infant from the uterus

42. During the last few weeks of pregnancy
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

43. 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

44. Maternal emotional and physical stress
Initiation of Labor
Maternal emotional and physical stress
Activates the hypothalamus, causing oxytocin release from posterior pituitary
Positive feedback mechanism occurs

45. vigorous contractions of uterus
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

46. 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

47. Umbilical cord Placenta Uterus Cervix Vagina (a) Dilation (early)
Figure 28.18a

48. Pubic
symphysis
Sacrum
(b) Dilation (late)
Figure 28.18b

49. 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

50. Perineum
(c) Expulsion
Figure 28.18c

51. 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

52. Uterus
Placenta
(detaching)
Umbilical
cord
(d) Placental
Figure 28.18d

53. 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
Heart rate
Respiration
Color
Muscle tone
Reflexes

54. Respiratory rate: ~45 per minute for first two weeks, then declines
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

55. Unstable period lasting 6–8 hours after birth
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

56. 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

57. Occlusion of Fetal Blood Vessels
Umbilical vein becomes the ligamentum teres
Ductus venosus  ligamentum venosum
Foramen ovale  fossa ovalis
Ductus arteriosus  ligamentum arteriosum

58. Production of milk by the mammary glands Toward the end of pregnancy
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

59. Suckling initiates a positive feedback mechanism
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

60. 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

61. 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

62. Advantages of Breast Milk
Natural laxative effect helps eliminate bile-rich meconium, helping to prevent physiological jaundice
Encourages bacterial colonization of the large intestine

63. Assisted Reproductive Technology
Surgical removal of oocytes following hormone stimulation
Fertilization of oocytes
Return of fertilized oocytes to the woman’s body

64. 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

65. 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