1. 34.4 Fertilization and Development
SC.912.L.16.1 Describe the process of human development from fertilization to birth and major changes that occur in each trimester of pregnancy.

2. Pregnancy
Pregnancy: events that occur from fertilization until the infant is born
Gestation period: time from the last menstrual period until birth (~280 days)
Embryo: from fertilization through week 8
Fetus: from week 9 through birth

3. After 8 weeks the embryo is called a fetus
conceptus
Embryo
Fertilization
3-week
embryo
(3 mm)
5-week embryo
(10 mm)
8-week embryo
(22 mm)
After 8 weeks the embryo is called a fetus
12-week fetus
(90 mm)
Figure 28.1

4. From Egg to Zygote
The oocyte is viable for 12 to 24 hours
Sperm is viable 24 to 48 hours after ejaculation

5. For fertilization to occur, coitus must occur no more than
From Egg to Zygote
For fertilization to occur, coitus must occur no more than
Two days before ovulation
24 hours after ovulation
Fertilization: when the sperm’s chromosomes combine with those of a secondary oocyte to form a fertilized egg (zygote)

6. Fertilization

7. Embryonic Development
Cleavage
Mitotic divisions of zygote
First cleavage at 36 hours  two daughter cells (blastomeres)
At 72 hours  morula (16 or more cells)
At day 3 or 4, the embryo of ~100 cells (blastocyst) has reached the uterus

8. Embryonic Development
Blastocyst: fluid-filled hollow sphere
Inner cell mass
Becomes the embryonic disc ( embryo and three of the embryonic membranes)

9. (a) Zygote
(fertilized egg)
(b) 4-cell stage
2 days
(c) Morula (a solid ball
of blastomeres).
3 days
(d) Early blastocyst (Morula
hollows out, fills with fluid,
and “hatches” from the
zona pellucida). 4 days
Zona
pellucida
Degenerating
zona
pellucida
(e) Implanting
blastocyst
(Consists of a
sphere of tropho-
blast cells and an
eccentric cell clus-
ter called the inner
cell mass). 7 days
Sperm
Blastocyst
cavity
Uterine
tube
Fertilization
(sperm
meets and
enters egg)
Oocyte
(egg)
Ovary
Uterus
Blastocyst
cavity
Ovulation
Endometrium
Trophoblast
Inner
cell
mass
Cavity of
uterus
Figure 28.4

10. Implantation
Blastocyst floats for 2–3 days
Implantation begins 6–7 days after ovulation

11. Placentation
Formation of the placenta from embryonic and maternal tissues
Placenta is fully formed and functional by the end of the third month

12. Placenta
Organ that connection between mother and embryo
Act‘s as embryo’s organ of respiration, nourishment, and excretion.
Oxygen and nutrients diffuse from the mother to the embryo.
Carbon dioxide and metabolic waste diffuse from the embryo to the mother.
Maternal and embryonic blood supplies do not intermix

13. Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic
cavity
Umbilical
cord
Decidua
capsularis
Uterus
Extraembryonic
coelom
Lumen of
uterus
(e)
13-week fetus.
Figure 28.7e

14. Embryonic Development: Gastrula to Fetus
Germ Layers
During implantation, the blastocyst starts to convert to a gastrula
Inner cell mass develops into the embryonic disc (subdivides into epiblast and hypoblast)
The three primary germ layers and the extraembryonic membranes develop

15. Extraembryonic Membranes
Amnion: transparent sac filled with amniotic fluid
Provides a buoyant environment that protects the embryo
Helps maintain a constant homeostatic temperature
Allows freedom of movement and prevents parts from fusing together
Amniotic fluid comes from maternal blood, and later, fetal urine

16. Extraembryonic Membranes
Yolk sac: a sac that hangs from the ventral surface of the embryo
Forms part of the digestive tube
Source of the earliest blood cells and blood vessels

17. Gastrulation
Occurs in week 3, in which the embryonic disc becomes a three-layered embryo with ectoderm, mesoderm, and endoderm
Begins with appearance of primitive streak, a raised dorsal groove that establishes the longitudinal axis of the embryo

18. (e) Bilayered embryonic disc, superior view Mesoderm Endoderm
Amnion
Bilayered
embryonic
disc
Head end of bilayered
embryonic disc
Yolk sac
(b) Frontal
section
(c) 3-D
view
(a)
(d) Section
view in (e)
Primitive streak
Head end
Epiblast
Cut edge
of amnion
Yolk sac
(cut edge)
(f) days
Hypoblast
Endoderm
Right
Left
Ectoderm
Primitive
streak
Tail end
(e) Bilayered embryonic disc,
superior view
Mesoderm
Endoderm
(g) 16 days
Figure 28.9

19. Cells begin to migrate into the groove
Gastrulation
Cells begin to migrate into the groove
The first cells form the endoderm
Cells that follow push laterally, forming the mesoderm
Cells that remain on the embryo’s dorsal surface form the ectoderm
Notochord: rod of mesodermal cells that serves as axial support

20. Primary Germ Layers
The primitive tissues from which all body organs derive
Ectoderm  nervous system and skin epidermis
Mesoderm  forms all other tissues
Endoderm  epithelial linings of the digestive, respiratory, and urogenital systems

21. Building Vocabulary
Prefix and their meaning:
Ecto- “outer”
Meso- “middle”
Endo- “inner”

22. 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

23. Specialization of Endoderm
Forms the lining of organs of the
GI tract (gastro intestinal)
Respiratory tract
Excretory system

24. Specialization of the Mesoderm
Dermis of the skin in the ventral region
Parietal serosa of the ventral body cavity
Bones, ligaments, and dermis of limbs
The heart and blood vessels
Most connective tissues of the body

25. 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

26. Neurulation First step in the development of the nervous system
If the neural tube doesn’t close completely a serious birth defect known as spina bifida can result.
Folic Acid (Vitamin B 9) can prevent spina bifida
The neural tube forms before a woman knows she is pregnant.
So, folic acid should be part of any woman's diet.

27. 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

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

29. 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

30. Table 28.1 (1 of 3)

31. Table 28.1 (2 of 3)

32. Table 28.1 (3 of 3)

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

34. 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

35. 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

36. 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

37. 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

38. 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

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

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

41. 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

42. 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

43. Perineum
(c) Expulsion
Figure 28.18c

44. 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

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