Early Development 2

Question 1

What happens day 13+?

Answer 1

Notochord formation

Question 2

What is the notochord?

Answer 2

a rod-like tube structure formed of cartilage-like cells

Question 3

Where does the notochord form?

Answer 3

along the embryo midline, under the ectoderm

Question 4

What does the notochord act as?

Answer 4

key organizing centre for neurulation and mesoderm development - releases growth factor signals

Question 5

How is the neural tube and CNS formed?

Answer 5

1. Notochord signals direct the neural plate ectoderm to invaginate forming neural groove
2. Creates two ridges (neural folds) running along the cranio-caudal axis
3. Neural crest cells specified in neural folds
   As days progress:
4. Neural folds move together over neural groove
5. Ultimately neural folds fuse, forming a hollow tube
6. Neural tube overlaid with epidermis (ectoderm)
7. Migration of the neural crest cells from folds

Question 6

Is the neural tube initially open at each end?

Answer 6

yes

Question 7

When is there closure at the head end?

Answer 7

around day 23

Question 8

When is there closure at the tail end?

Answer 8

around day 27

Question 9

What does closure at the head end precede?

Answer 9

formation of brain structures

Question 10

When is failure of neural tube closure is a common developmental defect?

Answer 10

1. Anencephaly

2. Spina bifida

Question 11

What is anencephaly?

Answer 11

(absence of most of the skull and brain) arises from failure to close at the head end (1/10,000 births)

Question 12

What is spinal bifida?

Answer 12

open neural tube at birth, usually lower spine due to failure to close tail end– varying severity – (0.4-5/1000 births

Question 13

What are neural crest cells?

Answer 13

Ectoderm-derived, plastic and migrate extensively during development

Question 14

What happens to neural crest cell that migrate and are cranial neural crest cells?

Answer 14

1. cranial neurones
2. glia
3. lower jaw
4. middle ear bones (ossicles)
5. facial cartilage

Question 15

What happens to neural crest cell that migrate and are cardiac neural crest cells?

Answer 15

1. aortic arch/pulmonary artery septum

3. large arteries wall musculoconnective tissue

Question 16

What happens to neural crest cell that migrate and are trunk neural crest cells?

Answer 16

1. dorsal root ganglia
2. sympathetic ganglia
3. adrenal medulla
4. aortic nerve clusters
5. melanocytes

Question 17

What happens to neural crest cell that migrate and are vagral and sacral neural crest cells?

Answer 17

1. parasympathetic ganglia

2. enteric nervous system ganglia

Question 18

What do defects of neural crest migration/specification lead to?

Answer 18

diverse birth defects including pigmentation disorders, deafness, cardiac and facial defects and failure to innervate the gut

Question 19

What is somitogenesis?

Answer 19

formation of somites

Question 20

What are somites?

Answer 20

arise from paired blocks of paraxial mesoderm flanking the neural tube and notochord

Question 21

How are somites formed?

Answer 21

1. Blocks of paraxial mesoderm condense and bud off in somite pairs
2. One of each pair either side of the neural tube.

Question 22

Where does somitogenesis commence?

Answer 22

at the head end and progresses down the long axis of the embryo

Question 23

What is the rate of ‘budding’ or appearance of somite pairs?

Answer 23

is species-specific, as is the number of pairs.

Humans 1 pair/90 min, 44 pairs

Question 24

What two types of embryonic tissue do somites initially form?

Answer 24

1. Sclerotome

2. Dermomyotome

Question 25

What is sclerotome?

Answer 25

vertebrae and rib cartilage

Question 26

What does the dermomyotome divide into?

Answer 26

1. Dermatome

2. Myotome

Question 27

What is the dermatome?

Answer 27

gives rise to dermis of the skin, some fat and connective tissues of neck and trunk

Question 28

What is the myotome?

Answer 28

forms the muscles of the embryo

Question 29

What two types of folding in the embryo does the primitive gut arise from?

Answer 29

• Ventral folding

- Lateral folding

Question 30

What is ventral folding?

Answer 30

where the head and tail ends curl together

Question 31

What is lateral folding?

Answer 31

where the two sides of the embryo roll

Question 32

What is pinched to form the primitive gut?

Answer 32

folding pinches off part of the yolk sac to form the primitive gut

Question 33

What is the primitive gut patterned into?

Answer 33

foregut, midgut and hindgut

Question 34

What are the derivatives of the foregut?

Answer 34

1. esophagus
2. stomach
3. upper duodenum
4. liver
5. gallbladder
6. pancreas
   (endoderm derived)

Question 35

What are the derivatives of the midgut?

Answer 35

1. lower duodenum
2. remainder of small intestine
3. ascending colon and first two-thirds of transverse colon

Question 36

What are the derivatives of the hindgut?

Answer 36

1. last third of the transverse colon
2. descending colon
3. rectum
4. upper anal canal

Question 37

What is the development of the heart like?

Answer 37

1. Begins as tube of mesoderm around day 19
2. beating and pumping blood commences around day 22
3. Fetal heartbeat detectable from ~6 weeks gestational age

Question 38

What is the development of the lungs like?

Answer 38

1. Arise from the lung bud, and endodermal structure adjacent to the foregut, in the 4th week of development
2. Lung bud splits into two at the end of the 4th week, and progressively branches through development

Question 39

What do the gonads form from?

Answer 39

mesoderm as bipotential (i.e. not committed to testis or ovary) structures known as gonadal/genital ridges

Question 40

How do XY embryos form?

Answer 40

1. presence of SRY gene on Y chromosome directs gonadal cells to become Sertoli cells
2. triggering testis development, Leydig cell formation and testosterone production

Question 41

How do XX embryos form?

Answer 41

1. absence of SRY leads to gonadal cells adopting a granulosa cell fate and ovary development
2. requires reinforcement by FOXL2

Question 42

What does the notochord control?

Answer 42

Controls Neural plate (thickened ectoderm) and directs to form neural tube

Question 43

What else does the notochord control?

Answer 43

Signal organsier for neurulation