developmental anatomy notes specifically basic embryology

1. BASIC 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. Embryology
 Embryology – study of the origin and
development of single individual
 Prenatal period
 Embryonic period – first 8 weeks
 Fetal period – remaining 30 weeks

4. The Embryonic Period
 Week 1 – from zygote to blastocyst
 Conception – in lateral third of uterine
tube
 Zygote (fertilized oocyte) moves toward
the uterus
 Blastomeres – daughter cells formed from
zygote
 Morula – cluster of 12–16 blastomeres
 Blastocyst – fluid-filled structure – about
60 cells

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

6. Fertilization and the Events of the
First 6 Days of Development
Figure 3.3

8. Processes, con’t: Cleavage
4. Cleavage: Early cell division
a. Zygote undergoes mitosis
b. Initiated by fertilization
c. Rapid process

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

11. Processes, con’t: Cleavage
5. During cleavage:
a. Zygote moves along the
fallopian tube toward uterus
b. Takes approximately 3 days.

13. Processes, con’t:
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

14. Processes, con’t: Differentiation
3. Cell division continues
a. morula modified into two parts
b. trophoblast
- outer sphere of cells
c. inner cell mass

15. Inner Cell Mass

16. Processes, con’t.: Differentiation
4. Trophoblast cells:
a. Outer layer of sphere
b. Secrete fluid into interior
of hollow sphere
c. Grows rapidly

17. Trophoblast Cells
Trophoblast Cells

18. Differentiation, continued …
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

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

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

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

24. Embryology, con’t.
D. Germ Layer Formation:
1. Takes place during implantation
2. Inner cell mass differentiates into:
a. ectoderm (amniotic cavity)
b. endoderm (yolk sac)

25. 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)

26. Week 2 – The Two-Layered
Embryo
 Bilaminar embryonic disc – inner cell
mass divided into two sheets
 Epiblast and the hypoblast
 Together make up the bilaminar embryonic
disc
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.

27. Week 2 – The Two-Layered
Embryo
 Amniotic sac – formed by an extension
of epiblast
 Outer membrane – forms the amnion
 Inner membrane – forms the amniotic sac
cavity
 Filled with amniotic fluid

28. Week 2 – The Two-Layered
Embryo
 Yolk sac – formed by an extension of
hypoblast
 Digestive tube forms from yolk sac
 Tissues around yolk sac
 Gives rise to earliest blood cells and blood
vessels

29. Implantation of the Blastocyst
Figure 3.4

30. Germ layer formation, con’t
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

31. Embryology, con’t.
E. Embryonic Disc:
1. Formed at contact between
ectoderm and endoderm
2. In place by Day 14
3. Mesoderm arises between layers

32. Week 3 – The Three-Layered
Embryo
 Primitive streak – raised groove on the
dorsal surface of the epiblast
 Gastrulation – a process of
invagination of epiblast cells
 Endoderm – formed from migrating
cells that replace the hypoblast
 Mesoderm – formed between epiblast
and endoderm
 Ectoderm – formed from epiblast cells
that stay on dorsal surface

33. The Primitive Streak
Figure 3.5e-h

34. The Notochord
 Primitive node – a swelling at one end
of primitive streak
 Notochord – defines body axis
 Is the site of the future vertebral column

35. Formation of the Mesoderm
and Notochord
Figure 3.6

36. Neurulation
 Neurulation – ectoderm starts forming
brain and spinal cord
 Neural plate – ectoderm in the dorsal
midline thickens
 Neural groove – ectoderm folds inward
 Neural tube – a hollow tube pinches off into
the body
 Cranial part of the neural tube becomes
the brain
 Neural crest – forms sensory nerve
cells, ganglia, and melanocytes

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

38. Embryonic Disc, cont…
4. Day 16
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

39. Gut Formation
Day 16
Day 28

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

41. 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

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

43. 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

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

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

46. Embryology, con’t.
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

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

48. 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

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

50. Changes in the Embryo
Figure 3.7a, b

51. Changes in the Embryo
Figure 3.7c, d

52. The Germ Layers in Week
Four
Figure 3.9a-d

53. Week 4 – The Body Takes
Shape
 Derivatives of the germ layers
 Ectoderm – forms brain, spinal cord, and
epidermis
 Endoderm
 Forms inner epithelial lining of the gut tube
 Forms respiratory tubes, digestive organs, and
urinary bladder

54. Week 4 – The Body Takes
Shape
 Notochord – gives rise to nucleus
pulposus within intervertebral discs
 Mesoderm – forms muscle, bone,
dermis, and connective tissues
 Somites divides into sclerotome
(vertebrae and ribs), dermatome (dermis
of skin in dorsal part of body), and
myotome (trunk and limb muscles)
 Intermediate mesoderm – forms kidneys
and gonads
 (SEE FIGURE 3.10, PG 60)

55. Week 4 – The Body Takes
Shape
 Mesoderm
 Splanchnic mesoderm
 Forms musculature, connective tissues, and
serosa of the digestive and respiratory
structures
 Forms heart and most blood vessels
 Somatic mesoderm – forms dermis of
skin, bones, and ligaments

56. Week 5-8 – The Second
Month of Embryonic
Development
 Limb buds form
 Embryo first looks recognizably human
 Head is disproportionately large
 All major organs are in place
Figure 3.11

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

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

59. 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

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

61. Events by trimester, 1st, con’t.
2. Second month
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

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

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

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

65. 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

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

67. 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

68. 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

69. Congenital Anomalies: NTDs
Cystic
Hygroma
Anencephaly
Encephalocele

70. Congenital anomalies: Cleft Palate

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

72. 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

73. Foot with Tootsies

74. Embryo at 16 weeks

75. Events by trimester, 2nd, con’t.
2. End of the 5th month
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.

76. Looks Like Dad…

77. 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

78. 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

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

80. 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

81. Events by trimester, 2nd, con’t.
3. End of 6th month
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

82. Events by trimester, con’t
C.The Third Trimester
1. Most difficult period for mother
a. Added weight of fetus
b. Increased pressure on her
organs

83. 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%

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

85. 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

86. 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

87. 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

88. 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

89. 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

90. 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)

91. 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”

92. Why Pregnant Ladies are Grumpy!!

94. The Fetal Period
 A time of maturation and rapid growth
 Cells are differentiating during the first
half of the fetal period
 Normal births occur 38 weeks after
conception
 Premature birth is one that occurs
before 38 weeks

95. Developmental Events of the
Fetal Period
Table 3.1

96. Developmental Events of the
Fetal Period
Table 3.1

97. Developmental Events of the
Fetal Period
Table 3.1