Basic Embryology

Question 1

1. What is the prenatal period?
2. Gynecologic timing has been from LMP therefore refers to what?

Answer 1

1. 38 weeks from conception to birth (average) “fetal” age
2. Gynecologic timing has been from LMP therefore refers to 40 weeks “gestational” age: LMP is on average two weeks before ovulation

Question 2

Traditional (artificial) division:

1. “Embryonic” period is when and what occurs here?

2. Fetal period is when and what occurs here?

Answer 2

1. “Embryonic” period:

First 8 weeks

All major organs formed

2. “Fetal” period:

Remaining 30 weeks

Organs grow larger and become more complex

Question 3

Describe the 5 steps in implantation?

Answer 3

See picture

Question 4

Define the following:

1. Ovulation?
2. Fertilization?
3. Zygote?
4. Cell division?

Answer 4

1. Ovulation: egg released into the peritoneal cavity
2. Fertilization: occurs as egg travels down fallopian tube
3. Zygote: the cell that forms when the ova and sperm fuse
4. Cell division: begins immediately as the zygote travels down the fallopian tube & into the uterus

Question 5

Week 1 post conception:

1. What divides repeatedly moving down tube toward uterus (cleavage)?
2. What are the daughter cells called?
3. What is the morula?
4. What occurs by day 4?

Answer 5

1. zygote
2. The daughter cells are called blastomeres
3. Morula: the solid cluster of 12-16 blastomeres at about 72 hours
4. Day 4: 60 cell morula enters uterus, taking up fluid becoming blastocyst

Question 6

1. Blastocyst stage: has two distinct cell types. What are they and what is their function?
2. This floats around for how many days?
3. Implantation is what day post conception?
4. How does implantation occur?
5. How long does this take?
6. What would have to happen to produce monozygotic twins?

Answer 6

1. Two distinct types of cells:
   - Inner cell mass: forms the embryo
   - Trophoblast: layer of cells surrounding the cavity which helps form the placenta
2. Floats for about 3 days
3. Implantation: day 6 post conception:
4. Trophoblast erodes into uterine wall
5. Takes 1 week to complete
6. If inner cell mass of a single blastocyst divides: —monozygotic (identical) twins

Question 7

1. What does the inner cell mass divide into in week 2?
2. what are the two fluid filled sacs and what do they arise from? 2
3. What structure gives rise to the whole body?

Answer 7

1. Inner cell mass divides:
   - Epiblast
   - Hypoblast
2. 2 fluid filled sacs:
   - Amniotic sac from epiblast
   - Yolk sac from hypoblast
3. Bilaminar embryonic disc: (gives rise to the whole body!)

Question 8

Describe the 12 steps between the fertilized egg and the start of the amniotic sac growing

Answer 8

See picture

Question 9

1. In week three what the bilaminar disc turn into?
2. Three primary “germ” layers:[body tissues develop from these]

Answer 9

1. Bilaminar to trilaminar disc
2. Three primary “germ” layers:[body tissues develop from these]
   - Ectoderm
   - Endoderm
   - Mesoderm

Question 10

What kind of tissue do the following arise from:

1. Ectoderm and endoderm?
2. Mesoderm?
3. Mesenchyma?

Answer 10

1. Ectoderm and endoderm are epithelial tissue (form sheets of tissue)
2. Mesoderm is a mesenchyme tissue
3. Mesenchyme cells are star shaped and do not attach to one another, therefore migrate freely

Question 11

FORMATION OF THE 3 GERM LAYERS

1. Primitive streak (groove) is located where?
2. Grastrulation is what?
3. Days 14-15: they replace hypoblast becoming _______
4. Day 16: _________ (a new third layer)
5. Epiblast cells remaining on surface and are what?

Answer 11

1. on dorsal surface of epiblast
2. invagination of epiblast cells
3. endoderm
4. mesoderm
5. ectoderm

Question 12

1. What develops in days 16-18?
2. Migrate where with some endoderm cells?
3. WHAT IS FORMED IN THIS PERIOD?
4. Future site of what?

Answer 12

1. 1. Primitive node epiblast cells invaginate
2. 2. migrate anteriorly with some endoderm cells
3. 3. Rod defining the body axis is formed!!!
4. 4. Future site of the vertebral column

Question 13

What happens during neurulation?

3 steps

Answer 13

1. Notochord signals overlying ectoderm
2. Formation begins of spinal cord and brain (neurulation)
3. Neural plate to neural groove to neural tube: pinched off into body

Question 14

What happens during week three?

Answer 14

Mesoderm begins to differentiate:

Lateral to notochord, week 3

Extends cranially and caudally (from head to tail or crown to rump)

Question 15

Division of mesoderm into three regions: What are they?

Answer 15

1. Somites: 40 pairs of body segments (repeating units, like building blocks) by end week 4
2. Intermediate mesoderm: just lateral to somites
3. Lateral plate: splits to form coelom (“cavity”)

Question 16

1. What happens at the end of week 3?
2. When is this completed?
3. Definition of the neural crest?

Answer 16

1. Closure of neural tube: begins at end of week 3;
2. complete by end of week 4 (folic acid important for this step)
3. Neural crest: lateral ectodermal cells, pulled along and form sensory nerve cells and other structures

Question 17

Divisions of the mesodermal lateral plate: 3

Answer 17

1. Somatic mesoderm: apposed to the ectoderm
2. Splanchnic mesoderm: apposed to the endoderm
3. Coelom in between will become the serous cavities of the ventral body cavity:

Question 18

Coelom in between will become the serous cavities of the ventral body cavity

3

Answer 18

1. Peritoneal
2. Pericardial
3. Pleural

Question 19

1. What does the ectoderm become? 3
2. Endoderm? 1

Answer 19

See picture

Question 20

When can you hear the heart of the baby and how big is the embryo?

Answer 20

See picture

Question 21

By 8 weeks, about 2 months, all major organs are in place in at least a rudimentary form. Why is this important to understand?

Answer 21

this is why drugs early in pregnancy are so important to avoid – many cause birth defects; baby is a little over 1” long (below right)

Question 22

1. Week 3-8 is called what?
2. What do we need to recognize about this time period?

Answer 22

1. Organogenesis

2.

This is the period when MOST gross structural birth defects are induced!!!

3rd and 4th weeks are particularly vulnerable and mother may not even know she is pregnant

Somites which produce the majority of the head mesenchyme starts in the cervical region at DAY 20 and spreads caudally at the rate of 3 pairs per day

Significance is that mother MUST avoid environmental, dietary, and drug exposure during this period!