The air-filled cavity and ossicles of the mammalian middle ear conduct sound to the cochlea. Usin... more The air-filled cavity and ossicles of the mammalian middle ear conduct sound to the cochlea. Using transgenic mice, we show that the mammalian middle ear develops through cavitation of a neural crest mass. These cells, which previously underwent an epithelial-to-mesenchymal transformation upon leaving the neural tube, undergo a mesenchymal-to-epithelial transformation to form a lining continuous with the endodermally derived auditory tube. The epithelium derived from endodermal cells, which surrounds the auditory tube and eardrum, develops cilia, whereas the neural crest–derived epithelium does not. Thus, the cilia critical to clearing pathogenic infections from the middle ear are distributed according to developmental derivations. A different process of cavitation appears evident in birds and reptiles, indicating that this dual epithelium may be unique to mammals.
Background: The contribution of the endoderm to the oral tissues of the head has been debated for... more Background: The contribution of the endoderm to the oral tissues of the head has been debated for many
years. With the arrival of Cre/LoxP technology endoderm progenitor cells can now be genetically labeled and tissues derived from the endoderm traced. Using Sox17-2A-iCre/Rosa26 reporter mice we have followed the fate of the endoderm in the teeth, glands, and taste papillae of the oral cavity. Results: No
contribution of the endoderm was observed at any stage of tooth development, or in development of the major salivary glands, in the reporter mouse during development. In contrast, the minor mucous glands of the tongue were found to be of endodermal origin, along with the circumvallate papilla and foliate pap- illae. The mucous minor salivary glands of the palate, however, were of mixed ectodermal and endodermal origin. Conclusions: In contrast to urodele studies, the epithelium of murine teeth is derived solely from
the ectoderm. The border between the ectoderm- and endoderm-derived epithelium may play a role in determining the position of the lingual glands and taste buds, and may explain differences observed between taste buds in the anterior and posterior part of the tongue.
Background: The stapes, an ossicle found within the middle ear, is involved in transmitting sound... more Background: The stapes, an ossicle found within the middle ear, is involved in transmitting sound waves to
the inner ear by means of the oval window. There are several developmental problems associated with this ossicle and the oval window, which cause hearing loss. The developmental origin of these tissues has not been fully elucidated. Results: Using transgenic reporter mice, we have shown that the stapes is of dual ori-
gin with the stapedial footplate being composed of cells of both neural crest and mesodermal origin. Wnt1cre/Dicer mice fail to develop neural crest-derived cartilages, therefore, have no middle ear ossicles. We have shown in these mice the mesodermal stapedial footplate fails to form and the oval window is induced but underdeveloped. Conclusions: If the neural crest part of the stapes fails to form the mesoder-
mal part does not develop, indicating that the two parts are interdependent. The stapes develops tightly associated with the otic capsule, however, it is not essential for the positioning of the oval window, suggest- ing that other tissues, perhaps within the inner ear are needed for oval window placement.
Since their recruitment in the oral cavity, approximately 450 million years ago, teeth have been ... more Since their recruitment in the oral cavity, approximately 450 million years ago, teeth have been subjected to strong selective constraints due to the crucial role that they play in species survival. It is therefore quite surprising that the ability to develop functional teeth has subsequently been lost several times, independently, in various lineages. In this review, we concentrate our attention on tetrapods, the only vertebrate lineage in which several clades lack functional teeth from birth to adulthood. Indeed, in other lineages, teeth can be absent in adults but be functionally present in larvae and juveniles, can be absent in the oral cavity but exist in the pharyngeal region, or can develop on the upper jaw but be absent on the lower jaw. Here, we analyse the current data on toothless (edentate) tetrapod taxa, including information available on enamel-less species. Firstly, we provide an analysis of the dispersed and fragmentary morphological data published on the various living taxa concerned (and their extinct relatives) with the aim of tracing the origin of tooth or enamel loss, i.e. toads in Lissamphibia, turtles and birds in Sauropsida, and baleen whales, pangolins, anteaters, sloths, armadillos and aardvark in Mammalia. Secondly, we present current hypotheses on the genetic basis of tooth loss in the chicken and thirdly, we try to answer the question of how these taxa have survived tooth loss given the crucial importance of this tool. The loss of teeth (or only enamel) in all of these taxa was not lethal because it was always preceded in evolution by the pre-adaptation of a secondary tool (beak, baleens, elongated adhesive tongues or hypselodonty) useful for improving efficiency in food uptake. The positive selection of such secondary tools would have led to relaxed functional constraints on teeth and would have later compensated for the loss of teeth. These hypotheses raise numerous questions that will hopefully be answered in the near future.
The pharyngeal arches form the face and neck of the developing embryo. The pharyngeal tissue is d... more The pharyngeal arches form the face and neck of the developing embryo. The pharyngeal tissue is divided into distinct arches by the formation of clefts and pouches in between the arches. These clefts and pouches form at the juxtaposition between the ectoderm and endoderm and develop into a variety of essential structures, such as the ear drum, and glands such as the thymus and parathyroids. How these pouches and clefts between the arches form and what structures they develop into is the subject of this review. Differences in pouch derivatives are described in different animals and the evolution of these structures are investigated. The implications of defects in pouch and cleft development on human health are also discussed.
The air-filled cavity and ossicles of the mammalian middle ear conduct sound to the cochlea. Usin... more The air-filled cavity and ossicles of the mammalian middle ear conduct sound to the cochlea. Using transgenic mice, we show that the mammalian middle ear develops through cavitation of a neural crest mass. These cells, which previously underwent an epithelial-to-mesenchymal transformation upon leaving the neural tube, undergo a mesenchymal-to-epithelial transformation to form a lining continuous with the endodermally derived auditory tube. The epithelium derived from endodermal cells, which surrounds the auditory tube and eardrum, develops cilia, whereas the neural crest–derived epithelium does not. Thus, the cilia critical to clearing pathogenic infections from the middle ear are distributed according to developmental derivations. A different process of cavitation appears evident in birds and reptiles, indicating that this dual epithelium may be unique to mammals.
Background: The contribution of the endoderm to the oral tissues of the head has been debated for... more Background: The contribution of the endoderm to the oral tissues of the head has been debated for many
years. With the arrival of Cre/LoxP technology endoderm progenitor cells can now be genetically labeled and tissues derived from the endoderm traced. Using Sox17-2A-iCre/Rosa26 reporter mice we have followed the fate of the endoderm in the teeth, glands, and taste papillae of the oral cavity. Results: No
contribution of the endoderm was observed at any stage of tooth development, or in development of the major salivary glands, in the reporter mouse during development. In contrast, the minor mucous glands of the tongue were found to be of endodermal origin, along with the circumvallate papilla and foliate pap- illae. The mucous minor salivary glands of the palate, however, were of mixed ectodermal and endodermal origin. Conclusions: In contrast to urodele studies, the epithelium of murine teeth is derived solely from
the ectoderm. The border between the ectoderm- and endoderm-derived epithelium may play a role in determining the position of the lingual glands and taste buds, and may explain differences observed between taste buds in the anterior and posterior part of the tongue.
Background: The stapes, an ossicle found within the middle ear, is involved in transmitting sound... more Background: The stapes, an ossicle found within the middle ear, is involved in transmitting sound waves to
the inner ear by means of the oval window. There are several developmental problems associated with this ossicle and the oval window, which cause hearing loss. The developmental origin of these tissues has not been fully elucidated. Results: Using transgenic reporter mice, we have shown that the stapes is of dual ori-
gin with the stapedial footplate being composed of cells of both neural crest and mesodermal origin. Wnt1cre/Dicer mice fail to develop neural crest-derived cartilages, therefore, have no middle ear ossicles. We have shown in these mice the mesodermal stapedial footplate fails to form and the oval window is induced but underdeveloped. Conclusions: If the neural crest part of the stapes fails to form the mesoder-
mal part does not develop, indicating that the two parts are interdependent. The stapes develops tightly associated with the otic capsule, however, it is not essential for the positioning of the oval window, suggest- ing that other tissues, perhaps within the inner ear are needed for oval window placement.
Since their recruitment in the oral cavity, approximately 450 million years ago, teeth have been ... more Since their recruitment in the oral cavity, approximately 450 million years ago, teeth have been subjected to strong selective constraints due to the crucial role that they play in species survival. It is therefore quite surprising that the ability to develop functional teeth has subsequently been lost several times, independently, in various lineages. In this review, we concentrate our attention on tetrapods, the only vertebrate lineage in which several clades lack functional teeth from birth to adulthood. Indeed, in other lineages, teeth can be absent in adults but be functionally present in larvae and juveniles, can be absent in the oral cavity but exist in the pharyngeal region, or can develop on the upper jaw but be absent on the lower jaw. Here, we analyse the current data on toothless (edentate) tetrapod taxa, including information available on enamel-less species. Firstly, we provide an analysis of the dispersed and fragmentary morphological data published on the various living taxa concerned (and their extinct relatives) with the aim of tracing the origin of tooth or enamel loss, i.e. toads in Lissamphibia, turtles and birds in Sauropsida, and baleen whales, pangolins, anteaters, sloths, armadillos and aardvark in Mammalia. Secondly, we present current hypotheses on the genetic basis of tooth loss in the chicken and thirdly, we try to answer the question of how these taxa have survived tooth loss given the crucial importance of this tool. The loss of teeth (or only enamel) in all of these taxa was not lethal because it was always preceded in evolution by the pre-adaptation of a secondary tool (beak, baleens, elongated adhesive tongues or hypselodonty) useful for improving efficiency in food uptake. The positive selection of such secondary tools would have led to relaxed functional constraints on teeth and would have later compensated for the loss of teeth. These hypotheses raise numerous questions that will hopefully be answered in the near future.
The pharyngeal arches form the face and neck of the developing embryo. The pharyngeal tissue is d... more The pharyngeal arches form the face and neck of the developing embryo. The pharyngeal tissue is divided into distinct arches by the formation of clefts and pouches in between the arches. These clefts and pouches form at the juxtaposition between the ectoderm and endoderm and develop into a variety of essential structures, such as the ear drum, and glands such as the thymus and parathyroids. How these pouches and clefts between the arches form and what structures they develop into is the subject of this review. Differences in pouch derivatives are described in different animals and the evolution of these structures are investigated. The implications of defects in pouch and cleft development on human health are also discussed.
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years. With the arrival of Cre/LoxP technology endoderm progenitor cells can now be genetically labeled and tissues derived from the endoderm traced. Using Sox17-2A-iCre/Rosa26 reporter mice we have followed the fate of the endoderm in the teeth, glands, and taste papillae of the oral cavity. Results: No
contribution of the endoderm was observed at any stage of tooth development, or in development of the major salivary glands, in the reporter mouse during development. In contrast, the minor mucous glands of the tongue were found to be of endodermal origin, along with the circumvallate papilla and foliate pap- illae. The mucous minor salivary glands of the palate, however, were of mixed ectodermal and endodermal origin. Conclusions: In contrast to urodele studies, the epithelium of murine teeth is derived solely from
the ectoderm. The border between the ectoderm- and endoderm-derived epithelium may play a role in determining the position of the lingual glands and taste buds, and may explain differences observed between taste buds in the anterior and posterior part of the tongue.
the inner ear by means of the oval window. There are several developmental problems associated with this ossicle and the oval window, which cause hearing loss. The developmental origin of these tissues has not been fully elucidated. Results: Using transgenic reporter mice, we have shown that the stapes is of dual ori-
gin with the stapedial footplate being composed of cells of both neural crest and mesodermal origin. Wnt1cre/Dicer mice fail to develop neural crest-derived cartilages, therefore, have no middle ear ossicles. We have shown in these mice the mesodermal stapedial footplate fails to form and the oval window is induced but underdeveloped. Conclusions: If the neural crest part of the stapes fails to form the mesoder-
mal part does not develop, indicating that the two parts are interdependent. The stapes develops tightly associated with the otic capsule, however, it is not essential for the positioning of the oval window, suggest- ing that other tissues, perhaps within the inner ear are needed for oval window placement.
years. With the arrival of Cre/LoxP technology endoderm progenitor cells can now be genetically labeled and tissues derived from the endoderm traced. Using Sox17-2A-iCre/Rosa26 reporter mice we have followed the fate of the endoderm in the teeth, glands, and taste papillae of the oral cavity. Results: No
contribution of the endoderm was observed at any stage of tooth development, or in development of the major salivary glands, in the reporter mouse during development. In contrast, the minor mucous glands of the tongue were found to be of endodermal origin, along with the circumvallate papilla and foliate pap- illae. The mucous minor salivary glands of the palate, however, were of mixed ectodermal and endodermal origin. Conclusions: In contrast to urodele studies, the epithelium of murine teeth is derived solely from
the ectoderm. The border between the ectoderm- and endoderm-derived epithelium may play a role in determining the position of the lingual glands and taste buds, and may explain differences observed between taste buds in the anterior and posterior part of the tongue.
the inner ear by means of the oval window. There are several developmental problems associated with this ossicle and the oval window, which cause hearing loss. The developmental origin of these tissues has not been fully elucidated. Results: Using transgenic reporter mice, we have shown that the stapes is of dual ori-
gin with the stapedial footplate being composed of cells of both neural crest and mesodermal origin. Wnt1cre/Dicer mice fail to develop neural crest-derived cartilages, therefore, have no middle ear ossicles. We have shown in these mice the mesodermal stapedial footplate fails to form and the oval window is induced but underdeveloped. Conclusions: If the neural crest part of the stapes fails to form the mesoder-
mal part does not develop, indicating that the two parts are interdependent. The stapes develops tightly associated with the otic capsule, however, it is not essential for the positioning of the oval window, suggest- ing that other tissues, perhaps within the inner ear are needed for oval window placement.