Follows the formation of the primitive streak on the dorsal side of the embryo.
Paraxial mesoderm (from the epiblast in the streak) give rise to somites. These form rostrally to caudally.
Paraxial Mesoderm forms?
Axial skeleton, trunk and limb muscles, dermis, tendons. Do not become sternum.
Inhibitor in somitogenesis
Comes from the extending tail end of the notochord in the caudal region. As the embryo elongates and pushes the caudal region further from the rostral region, the inhibitor is reduced until the newest somite is allowed to form. Inhibition is due to WNT3a,FGF8. When these growth factors aren't there, the somites are allowed to pinch off.
Clock in somitogenesis
Delta binds notch, produces Lnfg and Hes1. Lnfg inhibits notch binding; Hes1 inhibits Lnfg and itself.
When Notch is activated, gives the pinchy signal
Notch defects leading to missing vertebrae
Lunatic fringe mutants have messed up and fewer ribs
Derm, myo, sclero, syndet.
Scelerotome: Vertebrae (top and bottom half of each derive from different vertebrae to allow segmental nerves to run through. Blood vessels get trapped between the two fusing halves and end up within each rather than between).
Syndetome: Between myotome and sclerotome, give rise to tendons and need scleraxis (SCX)
Inductive signalling of sclerotome
Notocord secretes Shh and noggin (blocks BMP). Induces expression of Pax1
Inductive signalling of dermatome/myotome
Dorsal neural tube secretes Wnt; interacts with low levels of Shh and noggin from notochord and induces Pax3.
NT-3 ~> Dermatome
Homeodomain: protein sequence that can act as a transcription factor
Homeobox: DNA seq that encodes a homeodomain
Organization of Hox genes
4 groups, up to 13 in each.
Aligned along chromosome in manner of body segmentation
Paralogs are related genes (A9, B9,C9,D9)
Must remove all members of a paralog to really mess things up.