Early Embryonic development 2

Question 0

What is gastrulation?

Answer 0

-The process of forming all three germ layers from the epiblast

Question 1

What are the main events which occur in week 3 of development?

Answer 1

• Gastrulation

- Defining of the axes

Question 2

What is the first event in gastrulation?

Answer 2

-Formation of the primitive streak

Question 3

Where does the primitive streak form?

Answer 3

-In any direction on the surface of the epiblast

Question 4

What is the structure of the primitive streak?

Answer 4

• A streak of cells which appears on the surface of the epiblast which has narrow bulging at the edges
• The primitive node is an elevated area located at the cranial end of the streak
• The primitive pit is a central depression in the node

Question 5

What is the first event that occurs involving the primitive streak?

Answer 5

-Invagination-> cells of the epiblast migrate towards the streak, detach from the epiblast and depress down into the streak

Question 6

What does the process of invagination into the primitive streak cause?

Answer 6

-Displacement of the hypoblast by the differentiated cells of the epiblast known as endoderm

Question 7

What happens to the primitive streak as invagination is occuring?

Answer 7

-Begins to regress towards the caudal end

Question 8

How are the mesoderm and ectoderm layers formed during gastrulation?

Answer 8

-Once the endoderm has formed, the epiblast cells begins to diferentiate into ectoderm, whilst invagination of epiblast cells which are differentiating to mesoderm fills the space inbetween the two layers

Question 9

What happens to the primitive yolk sac during gastrulation?

Answer 9

-The hypoblast regresses and endoderm now lines the primitive yolk sac

Question 10

What is the end result of gastrulation?

Answer 10

-Formation on the trilaminar disc containing the three germ layers

Question 11

Does mesoderm fill the entire space between the ectoderm and endoderm?

Answer 11

-Yes, except from 2 areas, one at the cranial end and one at the caudial end which will be the future mouth and anus

Question 12

How are the openings for the mouth and anus produced?

Answer 12

• These areas are not filled with mesoderm which gives rise to the CVS
• Without CVS these areas will die and rupture creating openings

Question 13

What is the notochord?

Answer 13

-A solid rod of cells which runs down the midline of the trilaminar disc

Question 14

How is the notochord formed?

Answer 14

-Cells migrate from the primitive node and form a line down the midline without spreading out

Question 15

What two events does the notochord permit?

Answer 15

• Defining of the axes

- Formation of the nervous system

Question 16

In what direction does the notochord form?

Answer 16

-Cranial to caudal

Question 17

Which event defines the formation of the cranial/caudal axes?

Answer 17

-Formation of the primitive streak

Question 18

Which axes does the notochord define?

Answer 18

• Dorso-ventral

- left-right

Question 19

How does the notochord define the left-right axes

Answer 19

-Signals for the beating action of cilia which produce a side-specific signalling cascade

Question 20

What is situs invertus and what causes it to occur?

Answer 20

• Complete mirror image of the organs and structures of the body-> often not associated with morbidity
• Immotile cilia

Question 21

What are the derivatives of ectoderm?

Answer 21

• Organs and structures which remain in contact with the outside world
• Epidermis
• Nervous system

Question 22

What are the derivatives of mesoderm?

Answer 22

• Supporting structures

- CVS, muscles, bone, cartilage

Question 23

What are the derivatives of the endoderm?

Answer 23

• Internal structures
• Epithelia of the GI tract and respiratory tract
• Parenchymal cells

Question 24

What are the three possible causes of monozygotic twins?

Answer 24

• Zygote separates at two cell stage
• Zygote separates at the inner cell mass
• Appearance of second primitive streak

Question 25

In twinning, why do two embryos have two placentas is separation occurs at the two cell stage?

Answer 25

-Embryo has split before any differentiation, meaning that each ambryo can then differentiate to produce a placenta of their own

Question 26

In twinning, if separation occurs at the inner cell mass, why are two embryos produced which share a placenta?

Answer 26

• The embryo has undergone differentiation into inner cell mass and outer cell mass
• Outer cell mass forms placenta, separation of the inner cell mass inside the outercell mass will produce two embryos sharing a placenta

Question 27

In twinning, why does the appearance of another primitive streak lead to embryos sharing an amniotic cavity?

Answer 27

-The primitive streak appears on the epiblast which lies inside of the amniotic cavity which has already developed

Question 28

In twinning, when can the appearance of a second primitive streak produce conjoined twins?

Answer 28

-If separation (of the bilaminar disc) is not complete

Question 29

What is teratogenesis?

Answer 29

-The disruption of embryonic development by whatever means

Question 30

Why does each organ system have a particularly sensitive window for teratogenesis?

Answer 30

-They all develop at different times

Question 31

Give an example of a chemical teratogenic agent

Answer 31

• Thalidomide
• Alchol
• Drugs

Question 32

Give an example of an infectious teratogenic agent

Answer 32

-Rubella