2. EMBRYOLOGY
ā¢Deals with the period of development
prior to birth (prenatal)
ā¢Study of development of the embryo
ā¢Period of the embryo = first 8 weeks
3. A. Processes involved
1. Ovulation: release of ovum
2. Fertilization: union of sperm and
ovum
3. Mitosis of fertilized ovum
[zygote (2N)]
4.
5. Processes, conāt: Cleavage
4. Cleavage: Early cell division
a. Zygote undergoes mitosis
b. Initiated by fertilization
c. Rapid process
6.
7. Processes, conāt: 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
8. Processes, conāt: Cleavage
5. During cleavage:
a. Zygote moves along the
fallopian tube toward uterus
b. Takes approximately 3 days.
9.
10. Processes, conāt:
B. Differentiation:
1. āProcess of becoming differentā
a. All cells start out the same
b. Four tissue types at
c. 200 kinds of cells birth
2. Follows morula formation
11. Processes, conāt: Differentiation
3. Cell division continues
a. morula modified into two parts
b. trophoblast
- outer sphere of cells
c. inner cell mass
15. Differentiation, continued ā¦
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
16.
17. Embryology, cont ā¦
C. Implantation:
1. Begins ~day 6
a. blastocyst contacts wall
b. adheres to endometrium
c. āinner cell mass firstā
18.
19. Implantation, cont ā¦
2. Cells of trophoblast:
a. Secrete enzymes
b. Digest their way into
endometrium
3. Completed by 11th day after
fertilization
21. Embryology, conāt.
D. Germ Layer Formation:
1. Takes place during implantation
2. Inner cell mass differentiates into:
a. ectoderm (amniotic cavity)
b. endoderm (yolk sac)
22. Embryology, conāt.
ā¢ Ectoderm is shown
in blue (and will give
rise to the amnion)
ā¢ Endoderm is
shown in yellow
(and will give rise to the
yolk sac)
23. Germ layer formation, conāt
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
24. Embryology, conāt.
E. Embryonic Disc:
1. Formed at contact between
ectoderm and endoderm
2. In place by Day 14
3. Mesoderm arises between layers
26. Embryonic Disc, contā¦
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
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. 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
34. Embryology, conāt.
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
36. Embryology, conātā¦
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
38. 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
39. II. Events in Development, by Trimester
End of first month
40. Events by trimester, 1st, conāt.
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
42. Events by trimester, 1st, conāt.
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
44. II. Events in Development, by Trimester
End of second month
45. Events by trimester, 1st, conāt.
3. Third month
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, conāt.
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, conāt.
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, conāt.
a. Drugs, viral diseases, radiation
b. Alcohol use, smoking,
measles, polio, HIV, etc.
2. Exposure can produce
congenital anomalies
52. Events by trimester, conāt
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.
53. Events by trimester, 2nd, conāt.
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
56. Events by trimester, 2nd, conāt.
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.
57.
58. Events by trimester, 2nd, conā.
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, conāt.
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, conāt.
j. Fingernails and toenails develop
k. In some organs, cells replacement
begins (by mitosis!)
l. Movement becomes constant
61. Events by trimester, 2nd, conāt.
m. Can identify elbows, knees and
buttocks
n. Occasionally hiccoughs
o. Load on motherās heart, lungs, and
kidneys increases
62. Events by trimester, 2nd, conāt.
3. End of 6th month
a. Weight = 1 - 1/2 lbs,
b. Length = 12 - 14 inches
c. Well-proportioned āminiature humanā
d. Skin is red and wrinkled
e. Hair follicles, glands, sense receptors
develop
f. Active skeletal formation
63. Events by trimester, conāt
C.The Third Trimester
1. Most difficult period for mother
a. Added weight of fetus
b. Increased pressure on her
organs
64. Events by trimester, 3rd, conāt.
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, conāt.
3. 16% of motherās blood is in
uterus & placenta
4. Increased blood pressure
5. Breathing difficulties
66. Events by trimester, 3rd, conāt.
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, conāt.
3. Needs of Fetus
a. Calcium: 85% of motherās
intake goes to fetal bone
formation
b. Iron: 85% of motherās iron
intake used to make fetal
hemoglobin
68. Events by trimester, 3rd, conāt.
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, conāt.
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, conāt.
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, conāt.
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, conāt.
7. Preceding Birth
a. Evidence that aging placenta may
signal childbirth (more laterā¦)
b. āForces fetus to seek new
environmentā