Xenopus laevis is widely used as a model organism in biological research. Morphological descriptions of the larval cartilaginous skeleton are more than half a century old and comprehensive studies of early cartilage differentiation and development are missing. A proper understanding of early cranial skeletal development in X. laevis requires a detailed description that can function as a baseline for experimental studies. This basis makes it possible to evaluate skeletal defects produced by experiments on gene interactions, such as gain-or loss-of function experiments. In this study, we provide a detailed description of the pattern and timing of early cartilage differentiation and development in the larval head of X. laevis. Methods used include antibody staining, confocal laser scanning microscopy and 3D-reconstruction. Results were than compared to earlier studies based on classical histological approaches and clearing-and-staining. The first cartilage to chondrify is, in contrast to other vertebrates investigated so far, the ceratohyal. The components of the branchial basket chondrify in anterior-to-posterior direction as reported for other amphibians. Modern morphological as well as molecular studies refer mostly to only a few so-called model organisms (chicken, zebrafish, mouse, etc.). Xenopus laevis is often used as a model to represent anurans or even amphibians (Cannatella & De S a, 1993). Its development and morphology has been investigated for more than a century. Initial descriptions and depictions of the larval and adult skull of X. laevis (5Dactylethra capensis) were published by Parker (1876Parker ( , 1879. Further investigations were done on the development of the hypobranchial skeleton (Ridewood, 1897) followed by detailed behavioural descriptions at the beginning of the 20th century (Bles, 1906). Specimens collected by Bles were the basis for further analysis of the masticatory muscles (Edgeworth, 1930) and of external features (Peter, 1931). More detailed chondrocranial investigations were also published early (

The vertebrate head as a major novelty is directly linked to the evolutionary success of the vertebrates. Sequential information on the embryonic pattern of cartilaginous head development are scarce, but important for the understanding of its evolution. In this study, we use the oriental fire bellied toad, Bombina orientalis, a basal anuran to investigate the sequence and timing of larval cartilaginous development of the head skeleton from the appearance of mesenchymal Anlagen in post-neurulation stages until the premetamorphic larvae. We use different methodological approaches like classic histology, clearing and staining, and antibody staining to examine the larval skeletal morphology. Our results show that in contrast to other vertebrates, the ceratohyals are the first centers of chondrification. They are followed by the palatoquadrate and the basihyal. The latter later fuses to the ceratohyal and the branchial basket. Anterior elements like Meckel's cartilage and the rostralia are delayed in development and alter the ancestral anterior posterior pattern observed in other vertebrates. The ceratobranchials I-IV, components of the branchial basket, follow this strict anterior-posterior pattern of chondrification as reported in other amphibians.Chondrification of different skeletal elements follows a distinct pattern and the larval skeleton is nearly fully developed at Gosner Stage 28. We provide baseline data on the pattern and timing of early cartilage development in a basal anuran species, which may serve as guidance for further experimental studies in this species as well as an important basis for the understanding of the evolutionary changes in head development among amphibians and vertebrates.

The acquisition of a movable jaw and a jaw joint are key events in gnathostome evolution. Jaws are derived from the neural crest derived pharyngeal skeleton and the transition from jawless to jawed vertebrates consists of major morphological changes, which must have a genetic foundation. Recent studies on the effects of bapx1 knockdown in fish and chicken indicate that bapx1 has acquired such a role in primary jaw joint development during vertebrate evolution, but evidence from amphibians is missing so far. In the present study, we use Ambystoma mexicanum, Bombina orientalis, and Xenopus laevis to investigate the effects of bapx1 knockdown on the development of these three different amphibians. Using morpholinos we downregulated the expression of bapx1 and obtain morphants with altered mandibular arch morphology. In the absence of bapx1 Meckeĺs cartilage and the palatoquadrate jaw joint initially develop separately but during further development the joint cavity between both fills with chondrocytes. This results in the fusion of both cartilages and the loss of the jaw joint. Despite this the jaw itself remains usable for feeding and breathing. We show that bapx1 plays a role in jaw joint maintenance during development and that the morphants morphology possibly mirrors the morphology of the jawless ancestors of the gnathostomes.

Background The craniofacial skeleton is an evolutionary innovation of vertebrates. Due to its complexity and importance to protect the brain and aid in essential functions (e.g., feeding), its development requires a precisely tuned sequence of chondrification and/or ossification events. The comparison of sequential patterns of cartilage formation bears important insights into the evolution of development. Discoglossus scovazzi is a basal anuran species. The comparison of its chondrocranium (cartilaginous neuro- & viscerocranium) development with other basal anurans (Xenopus laevis, Bombina orientalis) will help establishing the ancestral pattern of chondrification sequences in anurans and will serve as basis for further studies to reconstruct ancestral conditions in amphibians, tetrapods, and vertebrates. Furthermore, evolutionary patterns in anurans can be studied in the light of adaptations once the ancestral sequence is established. Results We present a comprehensive overview on the chondrocranium development of D. scovazzi. With clearing and staining, histology and 3D reconstructions we tracked the chondrification of 44 elements from the first mesenchymal Anlagen to the premetamorphic cartilaginous head skeleton and illustrate the sequential changes of the skull. We identified several anuran and discoglossoid traits of cartilage development. In D. scovazzi the mandibular, hyoid, and first branchial arch Anlagen develop first followed by stepwise addition of the branchial arches II, III, and IV. Nonetheless, there is no strict anterior to posterior chondrification pattern within the viscerocranium of D. scovazzi. Single hyoid arch elements chondrify after elements of the branchial arch and mandibular arch elements chondrify after elements of the branchial arch I. Conclusions In Osteichthyes, neurocranial elements develop in anterior to posterior direction. In the anurans investigated so far, as well as in D. scovazzi, the posterior parts of the neurocranium extend anteriorly, while the anterior parts of the neurocranium, extend posteriorly until both parts meet and fuse. Anuran cartilaginous development differs in at least two crucial traits from other gnathostomes which further supports the urgent need for more developmental investigations among this clade to understand the evolution of cartilage development in vertebrates.

More than 150 years ago, in 1866, Ernst Haeckel published a book in two volumes called Generelle Morphologie der Organismen (General Morphology of Organisms) in the first volume of which he formulated his biogenetic law, famously stating that ontogeny recapitulates phylogeny. Here, we describe Haeckel's original idea as first formulated in the Generelle Morphologie der Organismen and later further developed in other publications until the present situation in which molecular data are used to test the “hourglass model,” which can be seen as a modern version of the biogenetic law. We also tell the story about his discovery, while traveling in Norway, of an unknown organism, Magosphaera planula, that was important in that it helped to precipitate his ideas into what was to become the Gastraea theory. We also follow further development and reformulations of the Gastraea theory by other scientists, notably the Russian school. Elias Metchnikoff developed the Phagocytella hypothesis for the origin of metazoans based on studies of a colonial flagellate. Alexey Zakhvatin focused on deducing the ancestral life cycle and the cell types of the last common ancestor of all metazoans, and Kirill V. Mikhailov recently pursued this line of research further.