Morphogenesis of the avian beak. (A) At embryonic stage (HH) 9.5 (Hamburger & Hamilton, 1951), neural crest cells (dorsal view) migrate from the caudal forebrain (fb) and rostral midbrain (mb) and produce frontonasal structures (pink). Cells from the caudal midbrain and first two rhombomeres (r) of the hindbrain give rise to maxillary and mandibular derivatives (light blue). The hyoid arch forms from r4 (green) and the third arch forms from r6 (brown). Neural crest cells migrate through and around mesodermal mesenchyme (m; grey). (B) By HH25, the frontonasal (fn), maxillary (mx), mandibular (ma) and hyoid (hy) primordia (sagittal view) are surrounded by surface ectoderm (se; light purple), pharyngeal endoderm (pe; yellow) and forebrain neuroepithelium (fb; dark blue), and contain contributions from the neural crest, nasal placode (np; dark blue) and cranial ganglia (V, VII, IX; dark blue). Mesoderm (m) produces muscles, vascular endothelial cells, and some skeletal tissues. (C) By HH40, neural crest cells produce cartilaginous and bony elements in the facial and jaw skeletons (pink, light blue, green and brown) whereas mesoderm forms much of the caudal cranial vault (grey) and basicranium caudal to the pituitary. Based on data and drawings from a variety of sources (Noden, 1978, 1988; Couly et al. 1993; Köntges & Lumsden, 1996; Schneider, 1999; Schneider & Helms, 2003). (D,E) At HH25, the frontonasal (fn), maxillary (mx) and mandibular (ma) primordia of quail and duck appear equivalent in proportion and shape but not size (frontal view). Scale bars = 1 mm.

The origins of species-specific feather pattern. (A) Cranial feather buds form through interactions between neural crest-derived dermis and overlying epidermis, which would be derived from donor and host, respectively. In both quail and duck at HH33, there is little histological evidence for cranial feather development, but by HH34, epithelial placodes form in the epidermis and the mesenchyme aggregates into dense dermis. By HH36, feather buds contain dermal condensations and they begin to rise above the surface of the integument. Long buds form after HH37. (B,C) Quail cranial feather buds are large and widely spaced in comparison with those of duck, which are smaller and spaced closer together, as shown schematically. (D) Quail and duck embryos stage-matched for surgery at HH9.5 subsequently diverge in stage due to their different rates of maturation. Cranial feather placodes arise in duck almost 3 days after they are observed in quail (arrows). Modified from Eames & Schneider (2005). (E) In chimeric quck, cranial feather tracts patterned by quail donor mesenchyme (left side) adjoin those patterned by duck host mesenchyme (right side). Note that the quail-like feathers are at the long bud stage whereas those derived from the duck host linger as placodes.

The cellular origins of species-specific beak morphology. (A,B) The beaks of quail embryos are short and blunt whereas those of duck are long and broad. (C) Transplants of cranial neural crest cells, which are destined to form the beak, from quail donors to duck hosts produce chimeric ‘quck’ embryos with quail-like beak morphology (Schneider & Helms, 2003). Note that the quail-like quck has webbed feet (arrow), which is indicative of the duck host.