development of axial musculoskeletal system

what is a somite

• blocks of mesoderm located on either side of the neural tube in a developing embryo

how are somites formed

• paraxial mesoderm forms during gastrulation

• first forms pre somitic mesoderm, PSM

• then this becomes somites

• as the anterior end of PSM become somites, more PSM added at posterior ensuring PSM length is constant

• number of somites is constant in a species

• usually one pair somites is one vertebrae

• at physiological temp a pair of somites segment from the PSM every 90 mins, maintained by a molecular clock which is temperature dependent

segmentation of PSM

• Hairy gene regulates time on molecular clock

• each cycle of Hairy expression is 90 min

• so somite forms every 90 min

formation of dermamyotome

• dorsolateral epithelial wall of each somite remains epithelial

• ventromedial part (sclerotome) undergoes epithelial to mesenchymal transition (EMT)

• remaining epithelial roof forms the dermatomyotome

• shaped like a fitted sheet

formation of the myotome

• cells at the medial and lateral edges of the dermatomyotome undergo EMT and migrate under the demamyotome

• this forms:

• epaxial (medial) myotome which is induced by sonic hedgehog

• hypaxial (lateral) myotome which is induced by Wnt signals from the dorsal neural tube and BMP from the lateral plate mesoderm

formation of dermatome

• after myotome formation central region of dermomyotome flattens and loses epithelial nature

• forms mesenchymal dermal fibroblasts

• these produce connective tissue layer of dermis, so dermatome gives rise to dermis of skin

what is sonic hedgehog

• induces epithelial to mesenchymal transition in ventromedial somite forming the sclerotome

• represses dorsal markers eg FoxD3, maintaining separation between sclerotome and dermomyotome

FoxD3

• produced in dorsal somite particularly dermomyotome

• maintain epithelial identity and progenitor identity of dermomyotome cells

• preventing premature mesenchymal differentiation

• expression induced by Wnt signals

resegmenting

• sclerotome resegments

• fissure develops in middle of sclerotome

• posterior half of one half of somite fuses with anterior half of adjacent sclerotome

• myotome does not resegment so muscle binds adjacent segments

• sclerotome encases neural tube and differentiates first into cartilage then bone

what do myotome and dermatome form

• myotome forms skeletal muscle body and limb

• dermatome forms dermis