Search

Development of the vertebrate skull has been studied intensively for more than 150 years, yet many essential features remain unresolved. One such feature is the extent to which embryonic derivation of individual bones is evolutionarily conserved or labile. We perform long-term fate mapping using GFP-transgenic axolotl and Xenopus laevis to document the contribution of individual cranial neural crest streams to the osteocranium in these amphibians. Here we show that the axolotl pattern is strikingly similar to that in amniotes; it likely represents the ancestral condition for tetrapods. Unexpectedly, the pattern in Xenopus is much different; it may constitute a unique condition that evolved after anurans diverged from other amphibians. Such changes reveal an unappreciated relation between life history evolution and cranial development and exemplify 'developmental system drift', in which interspecific divergence in developmental processes that underlie homologous characters occurs with little or no concomitant change in the adult phenotype.

Recent years have witnessed renewed interest in defining the embryonic cell populations that directly contribute to the bony skull. This question lies at the intersection of several important developmental, clinical and evolutionary interests. Until recently, our collective understanding of the embryonic origin of the vertebrate osteocranium has been based on a small number of reports published solely using avian models. As data gradually accumulates from other, distantly related species (e.g., mouse and frog), we can begin to evaluate long-standing assumptions regarding the behavior of osteogenic (bone-forming) neural crest cells within a wider phylogenetic and comparative context. In this review, we summarize data collected to date in three major vertebrate taxa: amphibians, birds and mammals. We highlight three largely unexplored topics within the field of osteogenic neural crest development: 1) disagreements in bone tissue origin within and across current model systems; 2) whether the pattern of neural crest cell contribution to skull bone is evolutionarily conservative or labile; and 3) how our understanding of development and morphology will benefit from fate maps using currently unexamined animal models.

Contemporary studies of vertebrate cranial development document the essential role played by the embryonic neural crest as both a source of adult tissues and a locus of cranial form and patterning. Yet corresponding and basic features of cranial evolution, such as the extent of conservation vs. variation among species in the contribution of the neural crest to specific structures, remain to be adequately resolved. Investigation of these features requires comparable data from species that are both phylogenetically appropriate and taxonomically diverse. One key group are amphibians, which are uniquely able to inform our understanding of the ancestral patterns of ontogeny in fishes and tetrapods as well as the evolution of presumably derived patterns reported for amniotes. Recent data support the hypothesis that a prominent contribution of the neural crest to cranial skeletal and muscular connective tissues is a fundamental property that evolved early in vertebrate history and is retained in living forms. The contribution of the neural crest to skull bones appears to be more evolutionarily labile than that of cartilages, although significance of the limited comparative data is difficult to establish at present. Results underline the importance of accurate and reliable homology assessments for evaluating the contrasting patterns of derivation reported for the three principal tetrapod models: mouse, chicken and frog.

As a step toward resolving the developmental origin of the ossified skull in adult anurans, we performed a series of cell labeling and grafting studies of the cranial neural crest (CNC) in the clawed frog, Xenopus laevis. We employ an indelible, fixative-stable fluorescent dextran as a cell marker to follow migration of the three embryonic streams of cranial neural crest and to directly assess their contributions to the bony skull vault, which forms weeks after hatching. The three streams maintain distinct boundaries in the developing embryo. Their cells proliferate widely through subsequent larval (tadpole) development, albeit in regionally distinct portions of the head. At metamorphosis, each stream contributes to the large frontoparietal bone, which is the primary constituent of the skull vault in adult anurans. The streams give rise to regionally distinct portions of the bone, thereby preserving their earlier relative position anteroposteriorly within the embryonic neural ridge. These data, when combined with comparable experimental observations from other model species, provide insights into the ancestral pattern of cranial development in tetrapod vertebrates as well as the origin of differences reported between birds and mammals., (Copyright 2005 Wiley-Liss, Inc.)

The neural crest is a population of multipotent stem cells unique to vertebrates. In the head, cranial neural crest (CNC) cells make an assortment of differentiated cell types and tissues, including neurons, melanocytes, cartilage, and bone. The earliest understanding of the developmental potentiality of CNC cells came from classic studies using amphibian embryos. Fate maps generated from these studies have been largely validated in recent years. However, a fate map for the most late-developing structures in amphibians, and especially anurans (frogs), has never been produced. One such tissue type, skull bone, has been among the most difficult tissues to study due to the long time required for its development during anuran metamorphosis, which in some species may not occur until several months, or even years, after hatching. We report a relatively simple technique for studying this elusive population of neural crest-derived osteogenic (bone-forming) cells in Xenopus laevis by using fluorescently labeled dextran conjugates., (Copyright 2004 Wiley-Liss, Inc.)

The contribution of cranial neural crest cells to the development and patterning of cranial muscles in amphibians was investigated in the phylogenetically basal and morphologically generalized frog, Bombina orientalis. Experimental methods included fluorescent marking of premigratory cranial neural crest and extirpation of individual migratory streams. Neural crest cells contributed to the connective tissue component, but not the myofibers, of many larval muscles within the first two branchial arches (mandibular and hyoid), and complex changes in muscle patterning followed neural crest extirpation. Connective tissue components of individual muscles of either arch originate from the particular crest migratory stream that is associated with that arch, and this relationship is maintained regardless of the segmental identity-or embryonic derivation-of associated skeletal components. These developmental relations define a pattern of segmentation in the head of larval anurans that is similar to that previously described in the domestic chicken, the only vertebrate that has been thoroughly investigated in this respect. The fundamental role of the neural crest in patterning skeleton and musculature may represent a primitive feature of cranial development in vertebrates. Moreover, the corresponding developmental processes and cell fates appear to be conserved even when major evolutionary innovations-such as the novel cartilages and muscles of anuran larvae-result in major differences in cranial form., (Copyright 2001 Academic Press.)

Based primarily on studies in the chick, it has been assumed that the zinc finger transcription factor Slug is required for neural crest migration. In the mouse, however, Slug is not expressed in the premigratory neural crest, which forms normally in Slug -/- animals. To study the role of Slug in Xenopus laevis, we used the injection of XSlug antisense RNA and tissue transplantation. Injection of Slug antisense RNA did not suppress the early expression of the related gene XSnail, but led to reduced expression of both XSlug and XSnail in later stage embryos, whereas the expression of another neural crest marker, XTwist, was not affected. Down-regulation of XSlug and XSnail was associated with the inhibition of neural crest cell migration and the reduction or loss of many neural crest derivatives. In particular, the formation of rostral cartilages was often highly aberrant, whereas the posterior cartilages were less frequently affected. The effects of Slug antisense RNA on neural crest migration and cartilage formation were rescued by the injection of either XSlug or XSnail mRNA. These studies indicate that XSlug is required for neural crest migration, that XSlug and XSnail may be functionally redundant, and that both genes are required to maintain each other's expression in the neural crest development of xenopus laevis., (Copyright 1999 Academic Press.)

Direct development is a common reproductive mode in living amphibians characterized by absence of the free-living, aquatic larval stage. In Eleutherodactylus, a species-rich genus of New World frogs, evolution of direct development from the ancestral biphasic ontogeny is correlated with a comprehensive modification in embryonic cranial patterning, including the loss of many larval-specific components and the precocious formation of many adult (postmetamorphic) structures. We use scanning electron microscopy (SEM) to examine the emergence and early migration of cranial neural crest cells in Eleutherodactylus coqui to begin to assess the possible role of the neural crest in mediating these evolutionary changes. As in metamorphosing frogs, cranial crest cells emerge prior to neural fold closure and assemble into three streams: rostral otic, and caudal otic. These streams contribute to the face and first visceral (mandibular) arch, to the second (hyoid) arch, and to posterior (branchial) arches, respectively. Rostrocaudal position, morphology, and/or migration patterns distinguish subpopulations of cells within the rostral stream and caudal otic stream. With the possible exception of the small size of the rostral otic caudal otic streams, evolution of direct development in E. coqui has not altered basic patterns of neural crest emergence or early migration as assessed by SEM. If observed evolutionary changes in embryonic cranial patterning are mediated by the neural crest, then they likely involve later aspects of crest migration or more subtle features related to pattern formation such as cell behavior and commitment, or gene expression.