6R: Mesoderm

Question 1

3 segments of the mesodermal germ layer at days 17-19

Answer 1

1. paraxial mesoderm: medium portion closest to notochord
2. intermediate mesoderm: in between the paraxial and LPM
3. Lateral plate mesoderm: very thin. outermost portion

Question 2

Paraxial mesoderm thickening helps lead to ____.

Answer 2

lateral folding

Question 3

On day 21, the mesoderm splits into what two layers?

Answer 3

1. somatic (parietal) mesoderm: (outer)

2. splanchnic (visceral) mesoderm: (inner, closest to yolk sac)

Question 4

What do the three segments of the mesoderm give rise to?

Answer 4

1. paraxial mesoderm: somites (skeletal muscle)
2. intermediate mesoderm: urogenital structures
3. lateral plate mesoderm: heart and blood vessels, body wall (cardiac and smooth muscle)

Question 5

What structures are formed from somites?

Answer 5

PM

vertebrae and ribs, dermis of the dorsal skin, skeletal muscles of the back , skeletal muscles of the body wall, limbs

Question 6

somitomere

Answer 6

PM
somite precursor mesodermal cells arranged in concentric whorls
- their formation proceeds craniocaudally
- in head, the somitomeres form structures right away
- in caudal region, somitomeres form somites

Question 7

Somite pairs in the:

a. occipital
b. cervial
c. thoracic
d. lumbar
e. sacral
f. coccygeal

Answer 7

a. 4
b. 8
c. 12
d. 5
e. 5
f. 8-10
somite pairs are used for staging (estimating EGA)

Question 8

Define:

a. sclerotome
b. myotome
c. dermatome
d. ventrolateral cells (subset)

Answer 8

PM

a. sclerotome: ribs, vertebrae, rib cartilage
b. myotome: muscles of the back, body wall (intercostals), and some limb muscles
c. dermatome: dermis of the back
d. ventrolateral cells: most of the musculature for body wall (obliques, transversus abdominus), and limbs

Question 9

Germ layers of the pharyngeal arches

Answer 9

1. ectodermal: between the arches externally
2. mesodermal: the core of the arches derived from paraxial mesoderm
3. endodermal: outpocketings of the pharyngeal guy

Question 10

kidney formation

Answer 10

IM
occurs in three different systems. cranial to caudal
1. pronephric system
2. mesonephric system (involved in gonad formation)
3. metanephric system (future permanent kidney)

Question 11

genital formation

Answer 11

IM
primordial germ cells migrate from the yolk sac, around the wall of the hindgut, to the genital ridge.
- there they induce the formation of the ovaries or testes from the genital ridges

Question 12

What does the lateral plate mesoderm split into? (3rd and 4th week)

Answer 12

1. the parietal layer (lines the intraembryonic cavity)

2. the visceral layer (surrounds the organs)

Question 13

heart formation

Answer 13

LPM
as lateral folding occurs, the endocardial progenitor cells form the endocardial tube, and the mycardial progenitors form the myocardium. happens concurrently with gut formation

Question 14

blood vessel formation

Answer 14

LPM and extraembryonic mesoderm

• FGF2 and FGFR induce mesodermal cells to form hemangioblasts (precursor for blood cells and endothelial cells)
• VEGF and VEGFR2 induce the hemangioblasts to form blood islands with bood precursors and endothelial precursors
• VEGF and VEGFR1 induce the precursor cells to become the blood cells and endothelial cells

Question 15

Define:

a. angiogenesis

b. vasculogenesis

Answer 15

a. angiogenesis: sprouting of tubes off of existing vessels

b. vasculogenesis: formation of new endothelial tubes

Question 16

where does blood vessel formation first occur?

Answer 16

extraembryonically! in the wall of the yolk sac and the wall of the connecting stalk

Question 17

Capillary hemangiomas

Answer 17

unusual conglomeration of blood vessels that shouldn’t have formed. Happens most often in the craniofacial region. occur in about 10% of all births

Question 18

what germ layer do blood cells form from?

Answer 18

splanchnic mesoderm

Question 19

Limb formation

Answer 19

• form from limb buds that are outpocketings comprised of LPM. Forelimnbs form before the hindlimbs
• covered on the outside by ectoderm

Question 20

Digit formation

Answer 20

programmed cells death at apical ectodermal ridge to go from webbed digits to actual fingers and toes.

Question 21

Syndactyly

Answer 21

fused digits due to improper apoptosis in the limbs