• Neural crest: autonomic and sensory ganglia and glia and melanocytes (for reviews, see Francis and Landis 1999; Gershon 1999a,b; LaBonne and Bronner-Fraser 1999).

  • Neural crest: midfacial and branchial connective tissues and teeth (for reviews, see Francis-West et al. 1998; Peters and Balling 1999; Schneider et al. 1999; Tucker and Sharpe 1999; Vaglia and Hall 1999).

  • Paraxial mesoderm: somites, skeletal muscle, vertebrae, and ribs (for reviews, see Brand-Saberi and Christ 1999; Relaix and Buckingham 1999; Burke 2000; Rawls et al. 2000; Summerbell and Rigby 2000).

  • Intermediate mesoderm: kidneys, gonads, reproductive ducts, and sex determination (for reviews, see Sariola and Sainio 1998; Horster et al. 1999; Parker et al. 1999; Swain and Lovell-Badge 1999).

  • Cardiovascular system: heart, angiogenesis, and hematopoiesis (for reviews, see Baldwin and Artman 1998; Mercola 1999; Morales-Alcelay et al. 1998; Tallquist et al. 1999).

  • Limb: growth and specification of axes (for reviews, see Martin 1998; Ng et al. 1999; Vogt and Duboule 1999).

  • Pharyngeal endoderm: thyroid and thymus (for reviews see Bodey et al. 1999; Missero et al. 1998).

  • Gut tube: lungs, liver, pancreas, stomach, and intestines (for reviews, see Gretchen 1999; St-Onge et al. 1999; Warburton and Lee 1999).

The familiar conserved signaling pathways are used over and over in many different contexts in organogenesis and other steps of development, as listed in Table 6-3. For example, the Sonic Hedgehog (SHH) signaling pathway is involved in establishing asymmetry in the early gastrula, inducing floor plate and motor neurons, separating the single eye field into paired optic primordia, maintaining proliferation in migrating neural crest cells, establishing patterning of the medial and lateral nasal prominences and tooth induction, inducing sclerotome segregation and epaxial muscle formation in somites, development of the prostate gland, determining left-right asymmetry, establishing the anteroposterior (rostrocaudal) axis of the limbs, delineating the tracheo-esophageal diverticulum, and establishing sites of formation and branching patterns in lung and pancreatic epithelia. Comparable matches could be made for members of the fibroblast growth factor (FGF), TGFβ, BMP, and WNT signaling families. Although the signaling pathways involve the same or closely related signaling molecules, the responses made by cells are distinct because of the genes and gene products they express prior to and in response to the many different combinations of these signaling factors.

The Vertebrate Limb: The Best Known Organogenesis Model

The limb is by far the most studied organ rudiment of vertebrates, supported by over 50 years of experimental embryology and intensive recent molecular

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