Differential requirements of Sonic hedgehog for the formation of somite-derived vertebrae and growth of rib primordia into the somatic lateral mesoderm

Interactions between developing muscles, resident connective tissue, ribs and tendons provide the scaffold for normal body wall morphogenesis and function. Formation and integration between the above structures are to a large extent unknown.

Sonic Hedgehog (Shh) is essential for patterning and differentiation of neural and mesodermal progenitors. Here we address the nature and timing of Shh functions in late development of ribs and vertebrae using conditional loss of function in avian embryos.

Shh activity was abrogated by electroporation of Hedgehog interacting protein (Hhip) at E2. Hhip activity was activated in sclerotome either from E2 until E5, E2 to E3, E3 to E5 and compared to control GFP. At E3, lack of Shh activity yielded smaller sclerotomes, an effect accounted for by enhanced cell death but not proliferation. Later on, all treatments reduced the size of vertebral bodies and caused a lateral deviation of the enclosed notochord, effects for which a one-day Shh deprivation (E2-3) was sufficient without significant regeneration thereafter.

At E5, the extent of colonization of the somatic mesoderm by the rib primordia was unaffected in all treatments, yet the area of Sox9 expression was reduced, suggesting that Shh is necessary throughout the entire time period examined for rib size but not extension into the somatic mesoderm. In contrast, we monitored no defects in the lateralmost sternal primordium.

Thus, Shh acts both early and later during morphogenesis of sclerotome derivatives. Shh operates in a medial to lateral gradient to affect vertebrae and rib development with differential temporal requirements.