The morphology of anuran larvae is substantially different from that of other vertebrates owing to the presence of rostralia in the upper and lower jaw skeletons and other specialisations that decouple larval and adult ecologies. The evolutionary success (measured, for example, as species number) of frogs in contrast to other recent amphibians is probably associated with this innovation, which facilitates herbivory in the larvae. This opened up a new feeding niche for frog larvae (scraping algae), and has been suggested to be an important factor behind the evolutionary success of anurans in relation to other groups of recent amphibians (Svensson & Haas, 2005). Xenopus laevis has a highly derived tadpole stage with a filigreed structure of the gill basket necessary for filter feeding in addition to the extra mouth structures present as unique novelties in frog tadpoles, which are slightly modified in X. laevis. In this species, the infrarostral cartilages, which articulate with Meckel’s cartilage in the lower jaw, are fused medially (Sokol, 1977) and the suprarostral cartilage is a crescent-shaped plate that supports the tentacular cartilage in the upper jaw (Trueb & Hanken, 1992). The origin of the rostralia is unresolved (De Sa & Swart, 1999; Svensson & Haas, 2005). Is it the cartilages themselves that are novelties, or is it rather the articulation between them that is an evolutionary novelty? If the latter is the case, the infrarostrals are partitioned off from Meckel’s cartilage and the suprarostrals from the trabecular horns. Changes in developmental processes and mechanisms must underlie the appearance of novel anatomical structures, but the way these work remains a challenging question.
The cells composing the novel head skeletal structures are derived from the neural crest (NC) (Gross & Hanken, 2008). The neural crest is a transient population of cells that is unique to vertebrates and that gives rise to different types of tissues, including neurones, cranial ganglia, glia and pigment cells, the sclera and cornea of the eye, and skeletal and dental tissues. The cranial development and morphology of different anuran tadpoles and other amphibian larvae have been investigated (reviewed by Hall & Hörstadius, 1988; Hall, 2009), but the molecular mechanisms of incipient chondrogenesis, and the morphogenesis of the NC-derived jaw and skull elements are only partly known in recent amphibians. Based on study of tissue cultures of the neural folds and pharynx endoderm of the Alpine newt (Triturus alpestris), Epperlein & Lehmann (1975) defined three stages of the NC-derived chondrocyte lineage: (i) prechondroblasts, (ii) chondroblasts and (iii) chondrocytes. Chondrogenesis induction is mediated by different extrinsic and intrinsic signals (Francis-West et al. 1998; Santagati & Rijli, 2003). In amphibians, NC cells must be in contact with the underlying pharyngeal endoderm to evoke chondrogenesis (Seufert & Hall, 1990). Although the mechanisms and processes of chondrogenesis are known primarily from studies of limb bud development, little is known about the detailed mechanisms of chondrogenesis of the NC-derived chondrocyte lineage (Hall & Miyake, 1992; Hall, 2005).
Neural crest cells also contribute directly to cranial muscle connective tissue in some species (chicken, Noden, 1983a,b; Couly et al. 1992; Fire-bellied toad, Olsson et al. 2001; Mexican axolotl, Ericsson et al. 2004). Vertebrate cranial muscles are derived from cranial paraxial mesoderm that originates from restricted areas ventrolateral to the corresponding rhombomeres (Edgeworth, 1935; Noden, 1983a,b; Couly et al. 1992; Trainor & Tam, 1995; Schilling & Kimmel, 1997). The muscle progenitors maintain a constant permanent nearest-neighbour relation with adjacent, overlying NC cells when populations of both kinds of cells reach the sites of their morphogenesis within the branchial arches; these mesenchymal populations elongate to their final position together (Evans & Noden, 2006). The development of craniofacial structures and muscles requires intricate spatiotemporal signalling interactions among molecules, specialised cell types and tissues (Noden & Trainor, 2005; Noden & Francis-West, 2006). The detailed processes involved in the positional guidance remain unclear. Due to this, principal questions concern (i) the process of cartilage formation from NC cells and (ii) the role of NC cells in the patterning of cranial muscles. Several factors have been implicated in the migration, determination and maintenance of NC cells, including several signalling cascades, e.g. the FGF, BMP and Wnt pathways (Barembaum & Bronner-Fraser, 2005), as well as the Sox family (Sox8, Sox9, Sox10; Hong & Saint-Jeannet, 2005), the Pax family (Pax3; Sato et al. 2005), and several forkhead transcription factors (e.g. FoxD1, FoxD3; Dottori et al. 2001; Gomez-Skarmeta et al. 1999). Recently, FoxN3, another member of the forkhead box gene family was shown to be important for craniofacial development in X. laevis and in the mouse (Schuff et al. 2007; Samaan et al. 2010). Some of the effects in X. laevis of FoxN3 depletion in head cartilages and cranial nerves were described by Schuff et al. (2007). The present study thoroughly describes the effects on cranial muscle anatomy and development, and also gives a more complete account of the effects of FoxN3 knockdown on the anatomy and development of NC-derived cartilages, with a focus on evolutionarily novel structures, such as the rostralia and the complicated fine structure of the gill basket.