Embryology ii

Question 1

When do teratogens have their greatest effect?

Answer 1

the earlier the exposure
pre-embryonic = lethal
embryonic = significant effects on system development
fetal- systems already established only CNS vulnerable

Question 2

What is the sequence of events of early embryonic development?

Answer 2

fertilisation and implantation 
gastrulation 
neurulation 
segmentation 
embryonic folding

Question 3

Where is the primitive streak found?

Answer 3

dorsal surface of the embryonic disc

Question 4

What does it mean if two primitive streaks are identified?

Answer 4

conjoined twins

Question 5

What happens in gastrulation? What is formed?

Answer 5

migration and invagination

trilamanar disc produced

Question 6

What are the three layers of the trilamanar disc and what are their fates?

Answer 6

ectoderm- skin/ tissues in contact with outside
mesoderm- supporting tissues
endoderm- internal structures

Question 7

What causes situs inversus?

Answer 7

immotile cilia

cilia not present to waft signalling molecules to the left affecting L/R signalling cascades in development

Question 8

What is the notochord?

Answer 8

solid rod of cells in midline of mesoderm layer

Question 9

How is neural tube created?

Answer 9

notochord release molecules that diffuse a limited distance–>overlying ectoderm thickens and edges elevate out of plane of disc and curl towards each other

Question 10

What part of the mesoderm is segmented?

Answer 10

paraxial

Question 11

When does the first pair of somites appear?

Answer 11

day 20 occipital region

Question 12

What rate do somites appear?

Answer 12

3 pairs/day till 42-44 pairs cranio-caudal sequence

Question 13

How many somite pairs in total?

Answer 13

31 as some disappear

Question 14

What does the initial somite look like?

Answer 14

block of mesoderm cells around small cavity

Question 15

What are the sections of a somite and their fates?

Answer 15

dermatome- dermis
myotome- muscle section
sclerotome- hard tissue section ( bone)

Question 16

Implications of segmentation?

Answer 16

rise of repeating structures such as ribs, vertebrae and intercostal muscles

Question 17

What are the two types of embryonic folding? Where does the pressure that causes it come form?

Answer 17

cephalocaudal folding- growth of neural tube–> pressure downwards
lateral folding - somites put lateral pressure on disc

Question 18

What does the part of the yolk sac pinched of in folding become?

Answer 18

midgut

Question 19

What membrane surrounds embryo?

Answer 19

amniotic