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Question 1

What is a somite? How is it formed? What are the three major components of a somite?

Answer 1

A block of segmented tissue formed from the paraxial mesoderm on either side of the neural tube.
Somitogenesis forms it rostral-caudally in pairs on either side of the neural tube.
1. Sclerotome (vertebrae and ribs)
2. Myotome (skeletal muscle of back, abdomen, ribs, limbs
3. Dermatome (dermis of back)

Question 2

When do the first somites appear in humans? When is somitogenesis complete?

Answer 2

Day 20. Over around day 30. (note this is the same time as neurulation). A pair made every 5-6 hours.

Question 3

How many somite pairs are formed? How many persist? Why?

Answer 3

42-44 made, 37 persist because we do not have tails. So the rest degenerate.

Question 4

What is the wavefront of somitogenesis?

Answer 4

An FGF8 gradient (from RNA decay) at the posterior (only actively transcribed at tailbud) of the PSM. Somites do not form unless the FGF8 gradient is below a certain threshold, and the gradient moves posteriorly at a constant rate because of axis elongation as the embryo grows.

Question 5

What is the clock of somitogenesis?

Answer 5

Cyclic activation of Notch signaling (leading to oscillating gene expression) and downstream genes (sets location and timing of somite separation from the PSM). Because many Notch cells are touching, it is propagated anteriorly through Hairy1(Hes1) targeting.

Question 6

What is the wave/determination front?

Answer 6

Where FGF8 is low enough for somites to form.

Question 7

How does epithelialization during somite boundary occur?

Answer 7

Through ephrin signaling. Notch activates MESP (MESP2 in mice/humans) which activated Epha4 expression in the anterior domain of each forming somite S0.

Question 8

How is each vertebra formed?

Answer 8

Through resegmentation. Each sclerotome splits into anterior/posterior parts: the posterior part of each fuses with the most anterior part of the next caudal sclerotome to form a vertebra. Since this is happening on both sides of the embryo = 4 sclerotomes. Nerve tissue forms between each vertebra.

Question 9

Summarize the developmental origins of cartilage and bone from somites, somatic lateral plate mesoderm, cranial neural crest, and head mesoderm

Answer 9

1. Somites (sclerotome): form veterbrae/ribs + occipital bone
2. Somatic Lateral Plate mesoderm: form limb and girdle bones + sternum
3. Cranial neural crest forms cranial facial bones, bones of the middle ear.
4. Head mesoderm (prechordal plate + paraxial); forms part of the skull (e.g. parietal bones)