Embryology deals with human development from fertilization through the first 8 weeks, known as the embryonic period. Key processes include fertilization, cleavage, differentiation of the germ layers (ectoderm, endoderm, mesoderm), implantation, formation of the embryonic disc and germ layers, and development of the major organ systems. During the first trimester, the embryo develops most of its major structures. In the second trimester, growth and development of organs continues. The third trimester places increased demands on the mother as the fetus grows and develops further in preparation for birth.

1. EMBRYOLOGY

2. I. EMBRYOLOGY:
•Deals with the period of development
prior to birth (prenatal)
•Study of development of the embryo
•Period of the embryo = first 8 weeks

3. The beginning…
Fertilization = the joining of
the sperm and the egg in
the fallopian tube to form a
unique human being.
For the first 8 weeks it is
known as an Embryo.
After 8 weeks it is known
as a Foetus.
an Embryo
This is a fertilized
egg only 30 hours
after conception

4. Embryology
A. Processes involved
1. Ovulation: release of ovum
2. Fertilization: union of sperm and
ovum
3. Mitosis of fertilized ovum
[zygote (2N)]

6. Processes, con’t: Cleavage
4. Cleavage: Cleavage is a series of
rapid mitotic divisions (without cell
growth)
Early cell division
a. Zygote undergoes mitosis
b. Initiated by fertilization
c. Rapid process

8. Processes, con’t: Cleavage
d. Continuing cleavage results in
formation of Morula
(morula - 32 cells)
1. A solid ball of cells
2. Same size as ovum
e. Complete by ~day 3

9. 5. During cleavage:
a. Zygote moves along the
fallopian tube toward uterus
b. Takes approximately 3 days.

11. B. Differentiation:
1. “Process of becoming different”
a. All cells start out the same
b. Four tissue types at
c. 200 kinds of cells birth
2. Follows morula formation

12. 3. Cell division continues
a. morula modified into two parts
b. trophoblast
- outer sphere of cells
c. inner cell mass

13. Inner Cell Mass

14. 4. Trophoblast cells:
a. Outer layer of sphere
b. Secrete fluid into interior
of hollow sphere
c. Grows rapidly

15. Trophoblast Cells
Trophoblast Cells

16. 5. Inner cell mass attached to one
side of trophoblast
6. Blastocoele (cavity) is full of fluid
7. Blastocyst:
a. Trophoblast + inner cell mass
b. Floats free in uterus until
implantation

18. C. Implantation:
1. Begins ~day 6
a. blastocyst contacts wall
b. adheres to endometrium
c. “inner cell mass first”

20. Implantation, cont …
2. Cells of trophoblast:
a. Secrete enzymes
b. Digest their way into
endometrium
3. Completed by 11th day after
fertilization

21. Implantation:
Initial events, 5-7
days after fertilization

22. D. Germ Layer Formation:
1. Takes place during implantation
2. Inner cell mass differentiates into:
a. ectoderm (amniotic cavity)
b. endoderm (yolk sac)

23. Embryology, con’t.
 Ectoderm is
shown in blue
(and will give rise to the
amnion)
 Endoderm is
shown in yellow
(and will give rise to the
yolk sac)

24. 3. Initial placental formation
a. Extraembryonic mesoderm
[from trophoblast cell division]
b. Trophoblast + mesoderm =
chorion
c. Projections grow from chorion
1. Chorionic villi
2. Form fetal part of placenta

25. E. Embryonic Disc:
1. Formed at contact between
ectoderm and endoderm
2. In place by Day 14(third week)
3. Mesoderm arises between layers

26. Embryonic Disc with Mesoderm Formation
• Embryonic Disc
(Day 14)
• Ectoderm
• Notochord
• Primitive node
• Primitive streak
• Intraembryonic mesoderm

27. Embryonic Disc, cont…
4. Day 16(4week)
a. constriction starts between
amniotic sac and yolk sac
b. causes disc to become tubular
c. head grows most rapidly
d. is being folded over
e. constriction forms gut

28. Gut Formation
Day 18
Day28
(4week)

29. Gut and Mesentery Formation, 18 days
Ectoderm blue
Mesoderm pink
Endoderm yellow
Gut tube
Yolk sac

30. Embryology, con’t.
F. Mesoderm formation:
1. Primitive groove: “dent” in disc
2. Primitive streak: cells in groove
3. Mesoderm cells proliferate from
primitive streak
4. Insert between ectoderm and
endoderm

31. Mesoderm formation, cont.
5. Mesoderm divides into two layers:
a. Somatopleure =
1. ectoderm + mesoderm
2. forms body walls
b. Splanchnopleure =
1. endoderm + mesoderm
2. forms viscera

32. c. Somites
1. Thickened mesoderm lying on
either side of notochord
[Notochord: axis of vertebral development]
2. Primary segmentation
3. Forms muscles, skeleton

33. Somite Development
Embryonic Inner Ear
Occipital myotomes
Cervical myotomes
Thoracic myotomes
Lumbar myotomes
Sacral myotomes

34. G. Neural Tube:
1. Thickened ridges appear
2. Between primitive streak, disc
3. Arise from neurectoderm
a. Neural crest cells (PNS)
b. Notochord induces formation,
differentiation of motor neurons

35. Neurulation
•Neurectoderm
Formation begins
~Day 15
•Embryo at 20 days
•Embryo at 21 days

36. Embryology, con’t…
4. Form a groove
5. Ridges join
6. Form neural tube ( CNS)
7. Develops into brain and spinal
cord
8. Ectoderm (skin, meninges)
covers it
9. In place by Day 28

37. Neural Tube Formation
 Embryo at 24 days
showing closed
neural tube

38. II. Events in Development, by Trimester
A. First Trimester(0-12week)
1. End of first month: (4week)
a. Nervous System and brain
developing
b. Somites for muscle, skeleton,
and skin in place
c. Mandible forms
d. Amnion and chorion fuse

39. II. Events in Development, by Trimester
End of first month

40. Events by trimester, 1st, con’t.
d. Lungs begin to develop
e. Heart starts beating (day 21)
f. Blood cells arise in yolk sac, liver
g. Kidneys developing
h. Arm and leg buds appear
i. All major organ systems in place

41. Lung Development,First Trimester
Hyoid bone
Cricoid cartilage
Trachea
Right primary
bronchus
Right lung

42. Events by trimester, 1st, con’t.
2. Second month(8week)
a. Eyes, ears, jaws develop
b. Reproductive organs distinct
c. Fingers and toes apparent
d. Skeletal formation begins (~6 wks)
e. First 8 weeks: period of the embryo

43. Differentiation of Reproductive Organs
Male (left) vs. Female (right) Homologs

44. II. Events in Development, by Trimester
End of second month

45. Events by trimester, 1st, con’t.
3. Third month(12 week)
a. Period of the fetus begins
b. Thumb & forefinger opposed
c. Breathing, eating, general
movements more coordinated
d. Behavior becomes individualized

46. Events by trimester, 1st, con’t.
d. Taste buds develop
e. Excreting urine into amniotic fluid
f. Extensive skeletal formation
g. Facial hair by day 70

47. II. Events in Development, by Trimester
End of third month

48. Events by trimester, 1st, con’t.
4. End of first trimester
a. All major organ systems in place
b. During organ formation:
1. Embryo is most susceptible to
outside insults

49. Events by trimester, 1st, con’t.
a. Drugs, viral diseases, radiation
b. Alcohol use, smoking,
measles, polio, HIV, etc.
2. Exposure can produce
congenital anomalies

50. Congenital Anomalies: NTDs
Cystic
Hygroma
Anencephaly
Encephalocele

51. Congenital anomalies: Cleft Palate

52. Events by trimester, con’t
B. The Second Trimester(13week-28week)
1. End of 4th month(16 week)
a. Weight = 4 - 6 oz
b. Length = 7 - 8 inches
c. Internal viscera enlarge, are
enclosed by abdominal wall.

53. Events by trimester, 2nd, con’t.
e. Can hold head erect
f. Skin well-developed enough to
distinguish finger and foot prints
g. Spontaneously stretches arms,
legs
h. Some reflex ability

54. Foot with Tootsies

55. Embryo at 16 weeks

56. Events by trimester, 2nd, con’t.
2. End of the 5th month(20 week)
a. Weight = 1/2 lb (8 oz)
b. Freely mobile in amniotic sac
1. ~ 350 ml/hr amniotic fluid
made/hour (8.5 liters/day)
2. Fluid protects against
trauma and temperature
changes.

57. Looks Like Dad…

58. Events by trimester, 2nd, con’.
c. Fetus looks fully formed
d. Only rarely survive on their own
e. Lungs completely formed, not
functional
f. Skin covered by vernix
1. Cheesy substance
2. Protective function

59. Events by trimester, 2nd, con’t.
g. Digestive organs formed, but not
yet functional
h. Infants born < 1000 grams need
complete life support
i. Reduction or loss of vestigial
structures occurs
-remnants of ~125 remain in adult

60. Events by trimester, 2nd, con’t.
j. Fingernails and toenails develop
k. In some organs, cells replacement
begins (by mitosis!)
l. Movement becomes constant

61. Events by trimester, 2nd, con’t.
m. Can identify elbows, knees and
buttocks
n. Occasionally hiccoughs
o. Load on mother’s heart, lungs, and
kidneys increases

62. Events by trimester, 2nd, con’t.
3. End of 6th month(24 week)
a. Weight = 1 - 1/2 lbs,
b. Length = 12 - 14 inches
c. Well-proportioned “miniature human”
d. Skin is red and wrinkled
e. Hair follicles, glands, sense receptors
develop
f. Active skeletal formation

63. Events by trimester, con’t
C.The Third Trimester(28week-40week)
1. Most difficult period for mother
a. Added weight of fetus
b. Increased pressure on her
organs

64. Events by trimester, 3rd, con’t.
c. Increased demands on mom:
1. Must breathe, digest, excrete &
circulate blood for two
2. Blood volume at 7 months:
increased 30%

65. Events by trimester, 3rd, con’t.
3. 16% of mother’s blood is in
uterus & placenta
4. Increased blood pressure
5. Breathing difficulties

66. Events by trimester, 3rd, con’t.
2. Premature births
a. 10% of 7 month babies survive
b. 70% of 8 month babies survive
c. 95% of 9 month babies survive

67. Events by trimester, 3rd, con’t.
3. Needs of Fetus
a. Calcium: 85% of mother’s
intake goes to fetal bone
formation
b. Iron: 85% of mother’s iron
intake used to make fetal
hemoglobin

68. Events by trimester, 3rd, con’t.
c. Nitrogen: Required for
nervous system development
d. Evidence that low intellect
is correlated to low protein
maternal diet in 3rd trimester

69. Events by trimester, 3rd, con’t.
4. Immune Transferal to Fetus
a. Infections contracted by
mother cause her to develop
antibodies
b. These are passed to fetus via fetal
circulation.
c. Antibodies will protect infant for up
to 6 months post-partum

70. Events by trimester, 3rd, con’t.
5. Activity of Fetus
a. Baby seems less active because it
is more confined (less space)
b. Fetal activity slows prior to birth

71. Events by trimester, 3rd, con’t.
6. Uterine changes
a. 60 times original volume at term
b. Amniotic fluid volume varies
1. 500 - 1000 ml (normal limits)
2. Volumes over 2500 ml are
abnormal (polyhydramnios)

72. Events by trimester, 3rd, con’t.
7. Preceding Birth
a. Evidence that aging placenta may
signal childbirth (more later…)
b. “Forces fetus to seek new
environment”

73. Why Pregnant Ladies are Grumpy!!