D- EMBRYOLOGY

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

Embryology

A. Processes involved

1. Ovulation: release of ovum

2. Fertilization: union of sperm and ovum

3. Mitosis of fertilized ovum [zygote (2N)]

Processes, con’t: Cleavage

4.Cleavage: Early cell division

a. Zygote undergoes mitosis

b. Initiated by fertilization

c. Rapid process

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

Processes, con’t: Cleavage

5. During cleavage:

a. Zygote moves along the fallopian tube toward uterus

b. Takes approximately 3 days.

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

Processes, con’t: Differentiation

3. Cell division continues

a. morula modified into two parts

b. trophoblast

- outer sphere of cells

c. inner cell mass

Inner Cell Mass

Processes, con’t.: Differentiation

4. Trophoblast cells:a. Outer layer of sphereb. Secrete fluid into

interior of hollow sphere c. Grows rapidly

Trophoblast Cells

Trophoblast Cells

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

Embryology, cont …

C. Implantation:

1. Begins ~day 6

a. blastocyst contacts wall

b. adheres to endometrium

c. “inner cell mass first”

Implantation, cont …

2. Cells of trophoblast:

a. Secrete enzymes

b. Digest their way into endometrium

3. Completed by 11th day after fertilization

Implantation: Initial events, 5-7

days after fertilization

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)

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)

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 villi2. Form fetal part of placenta

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

Embryonic Disc with Mesoderm Formation

• Embryonic Disc (Day 14)

• Ectoderm

• Notochord

• Primitive node

• Primitive streak

• Intraembryonic mesoderm

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

Gut Formation

Day 16

Day 28

Gut and Mesentery Formation, 18 days

Ectoderm blue

Mesoderm pink

Endoderm yellow

Gut tube

Yolk sac

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

Embryonic Disc with Mesoderm Formation

• Embryonic Disc (Day 14)

• Ectoderm

• Notochord

• Primitive node

• Primitive streak

• Intraembryonic mesoderm

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

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

Somite Development

Embryonic Inner Ear

Occipital myotomes

Cervical myotomes

Thoracic myotomes

Lumbar myotomes

Sacral myotomes

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

Neurulation

•Neurectoderm Formation begins ~Day 15

•Embryo at 20 days

•Embryo at 21 days

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

Neural Tube Formation

Embryo at 24 days showing closed neural tube

II. Events in Development, by Trimester

A. First Trimester

1. End of first month: a. Nervous System and brain

developingb. Somites for muscle, skeleton,

and skin in placec. Mandible formsd. Amnion and chorion fuse

II. Events in Development, by Trimester

End of first month

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

Lung Development,First Trimester

Hyoid bone

Cricoid cartilage

Trachea

Right primary bronchus

Right lung

Events by trimester, 1st, con’t.

2. Second montha. 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

Differentiation of Reproductive Organs

Male (left) vs. Female (right) Homologs

II. Events in Development, by Trimester

End of second month

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

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

II. Events in Development, by Trimester

End of third month

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

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

Congenital Anomalies: NTDs

Cystic Hygroma

AnencephalyEncephalocele

Congenital anomalies: Cleft Palate

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.

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

Foot with Tootsies

Embryo at 16 weeks

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.

Looks Like Dad…

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

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

Events by trimester, 2nd, con’t.

j. Fingernails and toenails develop

k. In some organs, cells replacement begins (by mitosis!)

l. Movement becomes constant

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

Events by trimester, 2nd, con’t.

3. End of 6th montha. 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

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

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%

Events by trimester, 3rd, con’t.

3. 16% of mother’s blood is in uterus & placenta

4. Increased blood pressure

5. Breathing difficulties

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

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

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

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

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

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)

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”

Why Pregnant Ladies are Grumpy!!