Somitic and head mesoderm contribute to cartilage and bone and deliver the entire skeletal musculature. Studies on avian somite patterning and cell differentiation led to the view that these processes depend solely on cues from surrounding tissues. However, evidence is accumulating that some developmental decisions depend on information within the somitic tissue itself. Moreover, recent studies established that head and somitic mesoderm, though delivering the same tissue types, are set up to follow their own, distinct developmental programmes. With a particular focus on the chicken embryo, we review the current understanding of how extrinsic signalling, operating in a framework of intrinsically regulated constraints, controls paraxial mesoderm patterning and cell differentiation.

ABBREVIATIONS aer apical ectodermal ridge of the limbs cm cardiac mesoderm di diencephalon d dermatome da dorsal aorta dm dermomyotome drg dorsal root ganglion ect surface ectoderm end endoderm fg foregut fge foregut endoderm fl forelimb fp floor plate of the neural tube g gut hl hind limb hm head mesoderm hn Hensen's node (avian organizer) hp head process ht heart hy hyoid (2nd) branchial arch ift inflow tract of the heart im intermediate mesoderm lb lung bud lm lateral mesoderm m myotome ma mandibular (1st) branchial arch mes mesencephalon mx maxillary aspect of the 1st branchial arch ncc neural crest cells not notochord np neural plate nt neural tube oft outflow tract of the heart ov otic vesicle pchpl prechordal plate ps primitive streak pV trigeminal placode r1, r2 etc rhombomere 1, rhombomere 2 etc. rp roof plate of the neural tube som somite som 0 epithelialising somite sp segmental plate (presomitic mesoderm) spc spinal cord tb tail bud tel telencephalon 3rd, 4th 3rd or 4th branchial arch V trigeminal (5th cranial nerve) VI facial (7th) cranial nerve IX glossopharyngeal (9th) cranial nerve

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COVER PHOTOGRAPH: Chicken embryo at HH9 of development, dorsal view, anterior to the top, displaying expression of the novel Dpr2 gene in cranial neural crest cells and the somites. See Alvares et al., Developmental Dynamics 238:1166–1178. © 2009 Wiley‐Liss, Inc.