11 - Embryology II

Question 1

Which period of foetal development has the greatest changes?

Answer 1

The embryonic period

• All major structures and systems are formed

Question 2

At what stage is there the greatest risk of congenital malformation?

Answer 2

The embryonic period as it is highly sensitive.

Pre-embryonic malformations are lethal, so pregnancy is lost.

Question 3

The germ layers (trilaminar disc) are all formed from the epiblast. What happens to the hypoblast?

Answer 3

No longer needed so degenerates

Question 4

Following gastrulation, holes develop towards the caudal and cranial ends of the mesoderm (the ectoderm and endoderm are still intact), what do these develop into?

Answer 4

Cranial end - future mouth

Caudal end - future anus

Question 5

What is situs inversus? When does this cause a problem?

Answer 5

• Complete mirror image of viscera
• Common result of immotile cilia in embryonic development
• No morbidity but problems arise if there is normal and mirror-image disposition

Question 6

How is left-right asymmetry developed in gastrulation?

Answer 6

• Ciliated cells at the primitive node results in left-sided flow of signalling molecules (absence of signals on right)
• Side-specific signalling cascades are activated

Question 7

What is the next stage of embryonic development after gastrulation?

Answer 7

Neurulation

Question 8

What is the first stage of neurulation?

Answer 8

• Formation of the notochord

- Notochord is a solid rod of cells that forms along the midline of the mesoderm

Question 9

What does the notochord do?

Answer 9

• Thickens and converts the overlying cells in the ectoderm into neurectoderm
• Endoderm below is not affected as it doesn’t contain the receptors required

Question 10

What happens to the neurectoderm formed by the notochord?

Answer 10

• Neurectoderm forms the neural plate

- Edges elevate out of the plane of the disc, curl towards each other and fuse, creating the neural tube

Question 11

After neurulation, what different parts of the mesoderm are formed and where are they?

Answer 11

• Paraxial mesoderm - surrounds the neural tube
• Intermediate mesoderm - lateral to the paraxial mesoderm
• Somatic mesoderm - at the edge of the mesoderm layer on the ectoderm (upper) border
• Splanchnic mesoderm - at the edge of the mesoderm layer on the endoderm (lower) border

Question 12

What is the space between the somatic and splanchnic mesoderm known as?

Answer 12

Intraembryonic coelom

Question 13

The paraxial mesoderm splits into 31 pairs of blocks of mesoderm along its length, these are called ………….

Answer 13

Somites

Question 14

Somites degenerate at their ………. wall. This portion forms the ………. which is the basis of the skeletal system.

Answer 14

Ventral

Sclerotome

Question 15

The dorsal portion of the somite forms the ………………….. which splits into the ………….. and ……………….

Answer 15

Dermomyotome

Dermatome and myotome

Question 16

The dermatome forms the ……………..
The myotome forms the ……………….
The sclerotome forms the ……………..

Answer 16

The dermatome forms the dermis of the skin
The myotome forms the muscles
The sclerotome forms the bones

Question 17

The organisation of the mesoderm into somites gives rise to repeating structures such as:

Answer 17

• Vertebrae
• Ribs
• Intercostal muscles
• Spinal cord segments

Question 18

How do the germ layers fold?

Answer 18

• Cephalocaudal folding - cephalic and caudal ends fold over toward the midline (ectoderm on the outside)
• Lateral folding - lateral sides fold over toward the midline (ectoderm on the outside)
• Small opening left in the middle to the yolk sac
• Like a drawstring purse - only ectoderm left on the outside

Question 19

Describe how the embryo folds and how the position of key structures such as the amniotic and yolk sacs change.

Answer 19

• Amniotic sac folds down over the embryo
• Embryo curls over, keeping ectoderm on the outside
• Yolk sac brought up into the space created by the folding embryo, part is pinched off and degenerates
• Heart is brought into the centre

Question 20

The yolk sac that is brought up into the embryonic cavity forms the ….. ………

Answer 20

GI tract

Question 21

Folding results in the embryo’s suspension within the ……. …….. surrounded by the ……….. …………..

Answer 21

Amniotic sac

Amniotic membrane