Somitogenesis

Question 1

What are 3 portions of embryonic mesoderm?

Answer 1

Going medial to lateral: Paraxial mesoderm, intermediate to lateral. Paraxial is most important for somite segmentation

Question 2

Which somitomeres for the cranial and postcranial structures?

Answer 2

1-7 make cranial structures and 8+ become postcranial

Question 3

Clock Wavefront Model

Answer 3

different gradients of retinoic acid and FGF/Wnt. RA is elevated in the cranium and FGF/Wnt Wavefronts move caudally during proliferation and elongation of embryo.

Question 4

What is Notch gene responsible for?

Answer 4

Formation of the next somite after it passes. Notch signals downstream to initiate the next segmentation.

Question 5

What determines # of somites?

Answer 5

the number of times the segmentation clock cycles

Question 6

What do somite derivatives of sclerotome, myotome, and dermatome become?

Answer 6

1. Sclerotome becomes vertebrae, ribs, intervertebral discs
2. Myotome becomes intrinsic back muscles, and limbs
3. Dermatome becomes dorsal dermis

Question 7

How do somites differentiate into the sclerotome and dermomyotome?

Answer 7

Wnt signaling from ectoderm to neural tube makes dermamyotome and Shh Noggin from notochord induces Pax1 to specify into sclerotome

Question 8

How do somites differentiate into the dermatome and myotome?

Answer 8

Shh and Wnt dermomyotome to become myogenic dorsomedially and BMP-4 signals the dermomyotome to become myogenic ventrolaterally. NT-3 induces dermatome formation

Question 9

Sclerotome coalesces around the notochord to give rise to bony structures, what are the five types of sclerotome?

Answer 9

1. Ventral becomes vertebral bodies and intervertebral discs
2. Lateral becomes distal ribs and tendons
3. Dorsal becomes dorsal neural arch and SP
4. Central becomes pedicles, ventral neural arch, proximal ribs, or TP
5. Medial becomes meninges and blood vessels

Question 10

How do neurala tube defect such as spina bifida arise?

Answer 10

when sclerotome cannot fully coalesce, resulting in incomplete formation of vertebrae

Question 11

How does sclerotome resegmentation occur?

Answer 11

There is polarization of the caudal region of the sclerotome due to rapid mitosis. After Resegmentation, sclerotomes recombine to make cartilaginous vertebral bodies that will ossify via endochondral ossification. Any sclerotome that don’t form vertebral bodies become annulus fibrosis and notochord becomes nucleus pulposis.

Question 12

What two genes are responsible for vertebral segment identity?

Answer 12

Hox genes and retinoic acid

Question 13

Answer 13

A. Hemivertebra B. Sagittal cleft C. Block vertebrae

Question 14

How do you get a cervical rib

Answer 14

misexpression of hox genes

Question 15

What does the dorsomedial myotome become?

Answer 15

intrinsic back muscles, extensor muscles of limbs, tendons of epaxials

Question 16

What does the ventrolateral myotome become?

Answer 16

muscles of ventrolateral body wall, flexor muscles of limbs, hypaxials

Question 17

Prune Belly Syndrome

Answer 17

myotome abnormality caused by absence of abdominal musculature

Question 18

What are derived from dermatome?

Answer 18

becomes dermis of dorsal trunk, blade of scapula, and brown adipocytes

Question 19

eWhere does ventral dermis and limbs come from?

Answer 19

Lateral plate mesoderm