Cortical interneurons in rodents are generated in the ventral telencephalon and migrate tangentially into the cortex. This process requires the coordinated action of many intrinsic and extrinsic factors. Here we show that Robo1 and Robo2 receptor proteins are dynamically expressed throughout the period of corticogenesis and colocalize with interneuronal markers, suggesting that they play a role in the migration of these cells. Analysis of Robo mutants showed a marked increase in the number of interneurons in the cortices of Robo1(-/-), but not Robo2(-/-), animals throughout the period of corticogenesis and in adulthood; this excess number of interneurons was observed in all layers of the developing cortex. Using BrdU incorporation in dissociated cell cultures and phosphohistone-3 labeling in vivo, we demonstrated that the increased number of interneurons in Robo1(-/-) mice is, at least in part, due to increased proliferation. Interestingly, a similar increase in proliferation was observed in Slit1(-/-)/Slit2(-/-) mutant mice, suggesting that cell division is influenced by Slit-Robo signaling mechanisms. Morphometric analysis of migrating interneurons in Robo1(-/-), Robo2(-/-) and Slit1(-/-)/Slit2(-/-), but not in Slit1(-/-) mice, showed a differential increase in neuronal process length and branching suggesting that Slit-Robo signaling also plays an important role in the morphological differentiation of these neurons.

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.

The mammalian ear is a complex structure divided into three main parts: the outer; middle; and inner ear. These parts are formed from all three germ layers and neural crest cells, which have to integrate successfully in order to form a fully functioning organ of hearing. Any defect in development of the outer and middle ear leads to conductive hearing loss, while defects in the inner ear can lead to sensorineural hearing loss. This review focuses on the development of the parts of the ear involved with sound transduction into the inner ear, and the parts largely ignored in the world of hearing research: the outer and middle ear. The published data on the embryonic origin, signalling, genetic control, development and timing of the mammalian middle and outer ear are reviewed here along with new data showing the Eustachian tube cartilage is of dual embryonic origin. The embryonic origin of some of these structures has only recently been uncovered (Science, 339, 2013(Science, 339, , 1453 Development, 140, 2013, 4386), while the molecular mechanisms controlling the growth, structure and integration of many outer and middle ear components are hardly known. The genetic analysis of outer and middle ear development is rather limited, with a small number of genes often affecting either more than one part of the ear or having only very small effects on development. This review therefore highlights the necessity for further research into the development of outer and middle ear structures, which will be important for the understanding and treatment of conductive hearing loss.

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 papillae. 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. Developmental Dynamics 241:1183-1191, 2012. V C 2012 Wiley Periodicals, Inc.Key words: endoderm; oral cavity; lineage labeling; mouse; tooth; gland; taste buds Key findings:The epithelium of murine teeth is derived from the ectoderm. The epithelium of the major salivary glands is derived from the ectoderm. A V-shaped border at the base of the tongue separates the ectoderm-and endoderm-derived tissue. The minor mucous salivary glands of the tongue, circumvallate papillae, and foliate papillae are derived from endoderm.

An early step in the formation of the optic pathway is the directed extension of retinal ganglion cell (RGC) axons into the optic fiber layer (OFL) of the retina in which they project toward the optic disc. Using analysis of knock-out mice and in vitro assays, we found that, in the mammalian retina, Slit1 and Slit2, known chemorepellents for RGC axons, regulate distinct aspects of intraretinal pathfinding in different regions of the retina. In ventral and, to a much lesser extent, dorsal retina, Slits help restrict RGC axons to the OFL. Additionally, within dorsal retina exclusively, Slit2 also regulates the initial polarity of outgrowth from recently differentiated RGCs located in the retinal periphery. This regional specificity occurs despite the fact that Slits are expressed throughout the retina, and both dorsal and ventral RGCs are responsive to Slits. The gross morphology and layering of the retina of the slit-deficient retinas is normal, demonstrating that these distinct guidance defects are not the result of changes in the organization of the tissue. Although displaced or disorganized, the aberrant axons within both dorsal and ventral retina exit the eye. We also have found that the lens, which because of its peripheral location within the developing eye is ideally located to influence the initial direction of RGC axon outgrowth, secretes Slit2, suggesting this is the source of Slit regulating OFL development. These data demonstrate clearly that multiple mechanisms exist in the retina for axon guidance of which Slits are an important component.

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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 origin 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 mesodermal 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, suggesting that other tissues, perhaps within the inner ear are needed for oval window placement. Developmental Dynamics 241:1396-1404, 2012. V C 2012 Wiley Periodicals, Inc.Key words: stapes; oval window; neural crest; lineage labeling; mesoderm Key Findings:The stapedial footplate is of neural crest and mesoderm origin. The otic capsule contributes to the stapedial footplate. The neural crest component of the stapedial footplate is needed for development of the mesodermal component and for proper formation of the oval window. The position of the oval window appears independent of the stapes.

RGC axons extend in the optic tracts in a manner that correlates with the expression in the hypothalamus and epithalamus of a soluble factor inhibitory to RGC axon outgrowth. Additionally, although the RGC axons extend adjacent to the telencephalon, they do not normally grow into this tissue. Here, we show that slit1 and slit2, known chemorepellents for RGC axons expressed in specific regions of the diencephalon and telencephalon, help regulate optic tract development. In mice lacking slit1 and slit2, a subset of RGC axons extend into the telencephalon and grow along the pial surface but not more deeply into this tissue. Surprisingly, distinct guidance errors occur in the telencephalon of slit1 -/-; slit2 +/- and slit1/2 -/- embryos, suggesting that the precise level of Slits is critical for determining the path followed by individual axons. In mice lacking both slit1 and slit2, a subset of RGC axons also project aberrantly into the epithalamus, pineal and across the dorsal midline. However, many axons reach their primary target, the superior colliculus. This demonstrates that Slits play an important role in directing the guidance of post-crossing RGC axons within the optic tracts but are not required for target innervation.