Early Embryonic development 3

Question 0

What is neurulation?

Answer 0

-The development of the nervous system driven by a series of differentiation steps

Question 1

At what stage is the embryo by the end of week 3?

Answer 1

• Gastrulation has occurred -> bilaminar now trilaminar with all three germ layers established
• The axis has been set

Question 2

What is the important signalling role of the notochord?

Answer 2

• Releases signalling molecules which stimulate the overlaying ectoderm to become neuroectoderm;ectoderm which is committed to becoming nervous tissue
• The overlying tissue begins to thicken and forms the neural plate

Question 3

Why does only the ectoderm which overlays the notochord becomes neuroectoderm?

Answer 3

-Notochord signalling molecules are diffusion limited and therefore only effects the tissues close to it

Question 4

How does the neural plate develop into the neural tube?

Answer 4

-Thickening of the neural plate, relative to the rest of the ectoderm, results in the edges of the neural plate elevating out of the plane of the trilaminar disc and curling towards each other to form the neural tube

Question 5

As the neural tube develops, the mesoderm also differentiates and develops into…

(draw if possible)

Answer 5

• Paraxial mesoderm (zone of mesoderm at the sides of the axis defined by the notochord)
• Intermediate mesoderm
• Somatic mesoderm
• Splanchnic mesoderm

Question 6

What happens to the layers of the ectoderm which are not involved in the formation of the neural tube?

Answer 6

-They become suborganised into layers

Question 7

What is the lateral plate during mesoderm differentiation?

Answer 7

-Somatic and splanchnic mesoderm

Question 8

What are somites?

Answer 8

-Organised segments of paraxial mesoderm

Question 9

In what type of pattern do somites appear?

Answer 9

-Regular and predictably, early in development

Question 10

When do the first pair of somites appear?

Answer 10

-Day 20

Question 11

In what type of sequence/direction do somites appear?

Answer 11

-Craniocaudal 3 pairs at a time until 42-44 pairs are present by the end of week 5

Question 12

How many pairs of somites are present by the time they have finished developing?

Answer 12

-31 as some disappear

Question 13

Where abouts in the embryo do somites lie?

Answer 13

-In pairs, one at both sides of the neural tube

Question 14

What is the structure of somites?

Answer 14

-Regular block of mesoderm arranged around a small cavity

Question 15

What happens in organised degeneration of somites?

Answer 15

• Ventral wall (wall facing endoderm) of somite breaks down, leading to the formation of the scleotome
• Organisation of the dorsal portion of the somite forms the dermomyotome

Question 16

What is sclerotome?

Answer 16

-Hard tissue zone which develops into bone

Question 17

What is the dermomyotome?

Answer 17

-Tissue which develops into muscle and skin-like tissue

Question 18

What eventually happens to the myotome?

Answer 18

-Proliferates and migrates eventually forming the muscles

Question 19

What eventually happens to the dermotome?

Answer 19

-Disperses widely to the ectodermal covering and forms the dermis of the skin

Question 20

What does the medial demomyotome develop into?

Answer 20

-Back muscles

Question 21

What does the lateral dermomyotome develop into?

Answer 21

• Ventral body wall muscles

- Contributes to limb muscles

Question 22

What does the medial sclerotome develop into?

Answer 22

• Vertebral body

- Proximal portion of the ribs

Question 23

What does the lateral sclerotome develop into?

Answer 23

• Vertebral arch

- Distal portion of the ribs

Question 24

What is the epimere?

Answer 24

-Dorsal myotome which is innervated by dorsal rami

Question 25

What is the hypomere?

Answer 25

-Ventral myotome innervated by ventral rami

Question 26

How does 31 somites relate to full grown human structure?

Answer 26

• 31 somites
• 31 segments
• 31 pairs of spinal nerves

Question 27

What is the clinical use of the term dermatome?

Answer 27

-A strip of skin supplied by a single spinal nerve

Question 28

What is the clinical use of the word myotome?

Answer 28

-A muscle or group of muscles supplied by a single spinal nerve

Question 29

List the derivatives of the paraxial mesoderm

Answer 29

• Axial skeleton (Vertebrae column and ribs)
• Dermis
• Muscles of body wall
• some limb muscles

Question 30

List the derivatives of the intermediate mesoderm

Answer 30

-Urogenital system (kidneys, ureters and gonads)

Question 31

List the derivatives of the somatic mesoderm

Answer 31

-Connective tissue of the limbs

Question 32

List the derivatives of the splanchnic mesoderm?

Answer 32

• Smooth musculature
• Connective tissue
• Vasculature of the gut

Question 33

What drives the folding of the embryo?

Answer 33

-Processes of differentiation such as the formation of the neural tube

Question 34

In what two directions does the embryo fold?

Answer 34

• Cephalocaudal (Head and tail folding)

- Lateral folding

Question 35

Before folding, where is the cardiogenic area located?

Answer 35

-Above the buccopharangeal membrane

Question 36

Describe cephalocaudal folding

Answer 36

• Growth of the ectoderm (thickening and neural tube) leads to a top-heavy ectoderm layer
• This results in folding at the head and tail end of the trilaminar disc in such a way that endoderm becomes less visible and the entire embryo is covered in ectoderm, except one opening for the umbilical cord

Question 37

What effect does cephalocaudal folding have on the cardigenic field?

Answer 37

-After folding the cardiogenic field lies within the embryo itself

Question 38

What effect does cephalocaudal folding have on the yolk sac?

Answer 38

-As folding occurs the amniotic cavity also folds in the same direction and some yolk sac is pulled up into the embryo

Question 39

Describe lateral folding

Answer 39

• Differentiation of the neural tube, somites and mesoderm drive lateral folding
• There is folding of the amniotic sac ventrally in such a way that the ectoderm and amniotic sac are beginning to surround the embryo. The space between the splanchnic and somatic mesoderm (lateral plate) becomes wider as the two lateral plates move towards each other, beginning to create the coelom.
• The endoderm and yolk sac fold in such a way that it begins to be drawn up into the embryo and forms the gut tube and umbilical cord
• The two lateral plates, with ectoderm overlying somatic mesoderm, fuse together forming the embryonic cavity (coelom) and ectoderm now surrounds the entire embryo
• The amniotic cavity also now surrounds the entire embryo