1. 29
Development and Inheritance

2. 29-1 Development Embryological Development Fetal Development
Occurs during first two months after fertilization
Study of these events is called embryology
Fetal Development
Begins at start of ninth week
Continues until birth

3. 29-1 Development Prenatal Development Postnatal Development
Embryological and fetal development stages
Postnatal Development
Commences at birth
Continues to maturity, the state of full development or completed growth

4. 29-2 Fertilization Fertilization Spermatozoon
Fusion of two haploid gametes, each containing 23 chromosomes
Produces zygote containing 46 chromosomes
Spermatozoon
Delivers paternal chromosomes to fertilization site
Travels relatively large distance
Is small, efficient, and highly streamlined

5. 29-2 Fertilization Fertilization
Occurs in uterine tube within a day after ovulation
Secondary oocyte travels a few centimeters
Spermatozoa must cover distance between vagina and ampulla

6. 29-2 Fertilization Acrosomes Oocyte Activation
Release hyaluronidase and acrosin
Penetrate corona radiata, zona pellucida, toward oocyte surface
Oocyte Activation
Contact and fusion of cell membranes of sperm and oocyte
Follows fertilization
Oocyte completes meiosis II, becomes mature ovum

7. 29-2 Fertilization Polyspermy Cortical Reaction
Fertilization by more than one sperm
Prevented by cortical reaction
Cortical Reaction
Releases enzymes that:
Inactivate sperm receptors
Harden zona pellucida

8. Figure 29-1a Fertilization
A secondary oocyte and
numerous sperm at the time of
fertilization. Notice the
difference in size between the
gametes. 8

9. Figure 29-1b Fertilization
Oocyte at Ovulation
Ovulation releases a secondary
oocyte and the first polar body;
both are surrounded by the corona
radiata. The oocyte is suspended in
metaphase of meiosis II.
Corona
radiata
First polar
body
Zona
pellucida 9

10. Figure 29-1b Fertilization
Fertilization and Oocyte
Activation
Acrosomal enzymes from multiple
sperm create gaps in the corona
radiata. A single sperm then makes
contact with the oocyte membrane,
and membrane fusion occurs,
triggering oocyte activation and
completion of meiosis.
Second polar
body
Fertilizing
spermatozoon 10

11. Figure 29-1b Fertilization
Pronucleus Formation
Begins
The sperm is absorbed into
the cytoplasm, and the female
pronucleus develops.
Nucleus of
fertilizing
spermatozoon
Female
pronucleus 11

12. Figure 29-1b Fertilization
Spindle Formation and
Cleavage Preparation
The male pronucleus
develops, and spindle fibers
appear in preparation for the
first cleavage division.
Male
pronucleus
Female
pronucleus 12

13. Figure 29-1b Fertilization
Amphimixis Occurs and
Cleavage Begins
Metaphase of first
cleavage division 13

14. Figure 29-1b Fertilization
Cleavage Begins
The first cleavage division
nears completion roughly
30 hours after fertilization.
Blastomeres 14

15. 29-3 Gestation Gestation Time spent in prenatal development
Consists of three integrated trimesters, each three months long

16. 29-3 Gestation First Trimester Second Trimester Third Trimester
Period of embryological and early fetal development
Rudiments of all major organ systems appear
Second Trimester
Development of organs and organ systems
Body shape and proportions change
Third Trimester
Rapid fetal growth and deposition of adipose tissue
Most major organ systems are fully functional

17. 29-4 The First Trimester First Trimester Includes four major stages
Cleavage
Implantation
Placentation
Embryogenesis

18. 29-4 The First Trimester Cleavage Implantation
Sequence of cell divisions begins immediately after fertilization
Zygote becomes a pre-embryo, which develops into multicellular blastocyst
Ends when blastocyst contacts uterine wall
Implantation
Begins with attachment of blastocyst to endometrium of uterus
Sets stage for formation of vital embryonic structures

19. 29-4 The First Trimester Placentation Embryogenesis
Occurs as blood vessels form around periphery of blastocyst and placenta develops
Embryogenesis
Formation of viable embryo
Establishes foundations for all major organ systems

20. 29-4 The First Trimester The First Trimester
Most dangerous period in prenatal life
40% of conceptions produce embryos that survive past first trimester

21. 29-4 The First Trimester Cleavage and Blastocyst Formation Blastomeres
Identical cells produced by cleavage divisions
Morula
Stage after three days of cleavage
Pre-embryo is solid ball of cells resembling mulberry
Reaches uterus on day 4

22. 29-4 The First Trimester Cleavage and Blastocyst Formation Blastocyst
Formed by blastomeres
Hollow ball with an inner cavity

23. 29-4 The First Trimester Cleavage and Blastocyst Formation Trophoblast
Outer layer of cells separate outside world from blastocoele
Cells responsible for providing nutrients to developing embryo

24. 29-4 The First Trimester Cleavage and Blastocyst Formation
Inner cell mass
Clustered at end of blastocyst
Exposed to blastocoele
Insulated from contact with outside environment by trophoblast
Will later form embryo

25. Figure 29-2 Cleavage and Blastocyst Formation
Blastomeres
Polar bodies
4-cell stage
2-cell stage
DAY 1
DAY 2
First cleavage
division
DAY 0:
Fertilization 25

26. Figure 29-2 Cleavage and Blastocyst Formation
Early morula
DAY 3
Advanced
morula
DAY 4
Hatching
Inner cell
mass
DAY 6
Blastocoele
Days 7–10:
Implantation in
uterine wall
(See Figure 29–3)
Trophoblast
Blastocyst 26

27. 29-4 The First Trimester Implantation
Occurs seven days after fertilization
Blastocyst adheres to uterine lining
Trophoblast cells divide rapidly, creating several layers

28. 29-4 The First Trimester Implantation Cellular trophoblast
Cells closest to interior of blastocyst
Syncytial trophoblast
Outer layer
Erodes path through uterine epithelium by secreting hyaluronidase

29. Figure 29-3 Stages in Implantation
DAY 6
FUNCTIONAL ZONE
OF ENDOMETRIUM
UTERINE
CAVITY
Uterine
glands
Blastocyst
DAY 7
Trophoblast
Blastocoele
Inner cell
mass 29

30. Figure 29-3 Stages in Implantation
DAY 8
Cellular
trophoblast
Syncytial
trophoblast
DAY 9
Developing
villi
Amniotic
cavity
Lacuna 30

31. 29-4 The First Trimester Ectopic Pregnancy
Implantation occurs outside uterus
Does not produce viable embryo
Can be life threatening

32. 29-4 The First Trimester Formation of the Amniotic Cavity
Villi extend away from trophoblast into endometrium
Increase in size and complexity until day 21
Amniotic Cavity
A fluid-filled chamber
Inner cell mass is organized into an oval sheet two layers thick
Superficial layer faces amniotic cavity
Deeper layer is exposed to fluid contents of blastocoele

33. 29-4 The First Trimester
Three distinct embryonic layers, or germ layers
Ectoderm: consists of the superficial cells that did not migrate into interior of inner cell mass
Endoderm: consists of cells that face blastocoele
Mesoderm: consists of poorly organized layer of migrating cells between ectoderm and endoderm

34. Figure 29-4 The Inner Cell Mass and Gastrulation
Day 10: Yolk Sac Formation
Syncytial trophoblast
Cellular trophoblast
Amniotic cavity
Yolk sac
Blastocoele
Lacunae
Superficial layer
Deep layer 34

35. Figure 29-4 The Inner Cell Mass and Gastrulation
Day 12: Gastrulation
Yolk sac
Amnion
Ectoderm
Mesoderm
Primitive
streak
Endoderm
Blastodisc
Embryonic disc 35

36. 29-4 The First Trimester Formation of the Extraembryonic Membranes
Support embryological and fetal development
Yolk sac
Amnion
Allantois
Chorion

37. 29-4 The First Trimester The Yolk Sac
Begins as layer of cells spread out around outer edges of blastocoele to form complete pouch
Important site of blood cell formation

38. 29-4 The First Trimester The Amnion Amniotic fluid
Surrounds and cushions developing embryo or fetus

39. 29-4 The First Trimester The Allantois The Chorion
Sac of endoderm and mesoderm
Base later gives rise to urinary bladder
The Chorion
Combination of mesoderm and trophoblast
Blood vessels develop within mesoderm
Rapid-transit system for nutrients that links embryo with trophoblast
First step in creation of functional placenta

40. 29-4 The First Trimester Chorionic Villi
In contact with maternal tissues
Create intricate network within endometrium carrying maternal blood

41. Figure 29-5 Extraembryonic Membranes and Placenta Formation
Week 2
Migration of mesoderm around the inner surface of the
trophoblast creates the chorion. Mesodermal migration
around the outside of the amniotic cavity, between the
ectodermal cells and the trophoblast, forms the amnion.
Mesodermal migration around the endodermal pouch
creates the yolk sac.
Amnion
Syncytial
trophoblast
Cellular
trophoblast
Chorion
Mesoderm
Yolk sac
Blastocoele 41

42. Figure 29-5 Extraembryonic Membranes and Placenta Formation
Week 3
The embryonic disc bulges into the amniotic cavity at the
head fold. The allantois, an endodermal extension
surrounded by mesoderm, extends toward the trophoblast.
Amniotic cavity
(containing
amniotic fluid)
Allantois
Head fold
of embryo
Yolk
sac
Chorion
Syncytial
trophoblast
Chorionic villi
of placenta 42

43. Figure 29-5 Extraembryonic Membranes and Placenta Formation
Week 4
The embryo now has a head fold and a tail fold. Constriction
of the connections between the embryo and the surrounding
trophoblast narrows the yolk stalk and body stalk.
Tail fold
Body stalk
Yolk stalk
Yolk sac
Embryonic gut
Embryonic
head fold 43

44. Figure 29-5 Extraembryonic Membranes and Placenta Formation
Week 5
The developing embryo and extraembryonic
membranes bulge into the uterine cavity. The
trophoblast pushing out into the uterine lumen remains
covered by endometrium but no longer participates in
nutrient absorption and embryo support. The embryo
moves away from the placenta, and the body stalk and
yolk stalk fuse to form an umbilical stalk.
Uterus
Myometrium
Decidua
basalis
Umbilical stalk
Placenta
Yolk sac
Chorionic villi
of placenta
Decidua
capsularis
Decidua
parietalis
Uterine lumen 44

45. Figure 29-5 Extraembryonic Membranes and Placenta Formation
Decidua
parietalis
Decidua basalis
Umbilical cord
Placenta
Amniotic cavity
Amnion
Week 10
The amnion has expanded
greatly, filling the uterine cavity.
The fetus is connected to the
placenta by an elongated umbilical
cord that contains a portion of the
allantois, blood vessels, and the
remnants of the yolk stalk.
Chorion
Decidua
capsularis 45

46. 29-4 The First Trimester Umbilical Cord Placental Circulation
Connects fetus and placenta
Contains allantois, placental blood vessels, and yolk stalk
Placental Circulation
Through paired umbilical arteries
Returns in single umbilical vein

47. Figure 29-6a A Three-Dimensional View of Placental Structure
Decidua
capsularis
Umbilical
cord (cut)
Amnion
Placenta
Chorion
Yolk sac
Decidua basalis
A view of the uterus after the fetus has been removed and the
umbilical cord cut. 47

48. Figure 29-6a A Three-Dimensional View of Placental Structure
Decidua
parietalis
Myometrium
Uterine
cavity
Cervical
(mucous)
plug in
cervical
canal
Cervix
Vagina
External os
A view of the uterus after the fetus has
been removed and the umbilical cord cut. 48

49. Figure 29-6a A Three-Dimensional View of Placental Structure
Chorionic
villi
Umbilical
vein
Umbilical
arteries
Area filled with
maternal blood
Amnion
Trophoblast (cellular
and syncytial layers)
Maternal
blood vessels
Arrows in the enlarged view indicate the direction
of blood flow. 49

50. Figure 29-6b A Three-Dimensional View of Placental Structure
Syncytial
trophoblast
Embryonic
connective tissue
Area filled with
maternal blood
Fetal blood
vessels
Chorionic villus
LM  280
A cross section through a chorionic villus,
showing the syncytial trophoblast exposed
to the maternal blood space. 50

51. 29-4 The First Trimester The Endocrine Placenta
Synthesized by syncytial trophoblast, released into maternal bloodstream
Human chorionic gonadotropin (hCG)
Human placental lactogen (hPL)
Placental prolactin
Relaxin
Progesterone
Estrogens

52. 29-4 The First Trimester Human Chorionic Gonadotropin (hCG)
Appears in maternal bloodstream soon after implantation
Provides reliable indication of pregnancy
Pregnancy ends if absent

53. 29-4 The First Trimester Human Placental Lactogen (hPL)
Human chorionic somatomammotropin (hCS)
Prepares mammary glands for milk production
Synergistic with growth hormone at other tissues
Ensures adequate glucose and protein is available for the fetus

54. 29-4 The First Trimester Placental Prolactin Relaxin
Helps convert mammary glands to active status
Relaxin
A peptide hormone secreted by placenta and corpus luteum during pregnancy
Increases flexibility of pubic symphysis, permitting pelvis to expand during delivery
Causes dilation of cervix
Suppresses release of oxytocin by hypothalamus and delays labor contractions

55. 29-4 The First Trimester Embryogenesis Organogenesis
Body of embryo begins to separate from embryonic disc
Body of embryo and internal organs start to form
Folding, differential growth of embryonic disc produces bulge that projects into amniotic cavity
Projections are head fold and tail fold
Organogenesis
Process of organ formation

56. Figure 29-7a The First 12 Weeks of Development
Future head
of embryo
Thickened
neural plate
(will form brain)
Axis of future
spinal cord
Somites
Neural folds
Cut wall of
amniotic cavity
Future tail
of embryo
Week 2. An SEM of the superior surface of a monkey
embryo at 2 weeks of development. A human embryo
at this stage would look essentially the same. 56

57. Figure 29-7b The First 12 Weeks of Development
Medulla
oblongata
Ear
Pharyngeal
arches
Forebrain
Eye
Heart
Somites
Body
stalk
Arm bud
Tail
Leg bud
Week 4. Fiberoptic view of human
development at week 4. 57

58. Figure 29-7c The First 12 Weeks of Development
Chorionic
villi
Amnion
Umbilical
cord
Placenta
Week 8. Fiberoptic view of human
development at week 8. 58

59. Figure 29-7d The First 12 Weeks of Development
Amnion
Umbilical
cord
Week 12. Fiberoptic view of human
development at week 12. 59

60. 29-5 The Second and Third Trimesters
Second Trimester
Fetus grows faster than surrounding placenta
Third Trimester
Most of the organ systems become ready
Growth rate starts to slow
Largest weight gain
Fetus and enlarged uterus displace many of mother’s abdominal organs

61. Figure 29-8a The Second and Third Trimesters
A four-month-old fetus, seen through a fiberoptic endoscope 61

62. Figure 29-8b The Second and Third Trimesters
Head of a six-month-old fetus, revealed
through ultrasound 62