RESUMEN
The association between lateral-line canals and skull bones in fishes has been the subject of several studies and raised a series of controversies, particularly with regard to the hypothesized role of lateral-line organs (i.e. neuromasts) in osteogenesis and the consequences for hypotheses of homology of the bones associated with lateral-line canals. Polypteridae, a group of freshwater fishes that occupies a key phylogenetic position as the most basal extant lineage of ray-finned fishes (Actinopterygii), provides an interesting model for the study of the relationships between lateral-line canals and skull bones. We describe the development of bones associated with lateral-line canals in the Senegal Bichir, Polypterus senegalus, and use these data to re-address previous hypotheses of homology of skull bones of polypterids. We demonstrate that the lateral-line canals constitute a separate component of the dermatocranium that may interact with a membranodermal component, thereby forming compound bones in the adult. Differences in the interactions between these components determine the characteristics of the development of each independent bone in the skull of adult P. senegalus. Our results shed light on long-standing controversies about the identity of skull bones such as the rostral, preopercle, and sphenotic in Polypteridae, and suggest the presence of an ancestral two-component pattern of formation of bones associated with lateral-line canals in bony fishes. These findings reveal the need to re-address previous hypotheses of homology of bones associated with lateral-line canals in different groups of bony fishes, especially fossil taxa.
Asunto(s)
Peces/anatomía & histología , Sistema de la Línea Lateral/anatomía & histología , Cráneo/anatomía & histología , Animales , Peces/embriología , Sistema de la Línea Lateral/embriología , Osteogénesis/fisiología , Cráneo/embriologíaRESUMEN
During tissue morphogenesis and differentiation, cells must self-renew while contemporaneously generating daughters that contribute to the growing tissue. How tissues achieve this precise balance between proliferation and differentiation is, in most instances, poorly understood. This is in part due to the difficulties in dissociating the mechanisms that underlie tissue patterning from those that regulate proliferation. In the migrating posterior lateral line primordium (PLLP), proliferation is predominantly localised to the leading zone. As cells emerge from this zone, they periodically organise into rosettes that subsequently dissociate from the primordium and differentiate as neuromasts. Despite this reiterative loss of cells, the primordium maintains its size through regenerative cell proliferation until it reaches the tail. In this study, we identify a null mutation in the Wnt-pathway transcription factor Lef1 and show that its activity is required to maintain proliferation in the progenitor pool of cells that sustains the PLLP as it undergoes migration, morphogenesis and differentiation. In absence of Lef1, the leading zone becomes depleted of cells during its migration leading to the collapse of the primordium into a couple of terminal neuromasts. We show that this behaviour resembles the process by which the PLLP normally ends its migration, suggesting that suppression of Wnt signalling is required for termination of neuromast production in the tail. Our data support a model in which Lef1 sustains proliferation of leading zone progenitors, maintaining the primordium size and defining neuromast deposition rate.
Asunto(s)
Proliferación Celular , Homeostasis/genética , Sistema de la Línea Lateral/embriología , Factores de Transcripción/fisiología , Proteínas Wnt/fisiología , Proteínas de Pez Cebra/fisiología , beta Catenina/fisiología , Aletas de Animales/embriología , Aletas de Animales/crecimiento & desarrollo , Aletas de Animales/metabolismo , Animales , Animales Modificados Genéticamente , Tipificación del Cuerpo/genética , Diferenciación Celular/genética , Embrión no Mamífero , Homeostasis/fisiología , Sistema de la Línea Lateral/metabolismo , Masculino , Morfogénesis/genética , Morfogénesis/fisiología , Mutación/fisiología , Transducción de Señal/genética , Transducción de Señal/fisiología , Factores de Transcripción/genética , Factores de Transcripción/metabolismo , Proteínas Wnt/genética , Proteínas Wnt/metabolismo , Pez Cebra/embriología , Pez Cebra/genética , Pez Cebra/metabolismo , Pez Cebra/fisiología , Proteínas de Pez Cebra/genética , Proteínas de Pez Cebra/metabolismo , beta Catenina/genética , beta Catenina/metabolismoRESUMEN
BACKGROUND: Development of the posterior lateral line (PLL) system in zebrafish involves cell migration, proliferation and differentiation of mechanosensory cells. The PLL forms when cranial placodal cells delaminate and become a coherent, migratory primordium that traverses the length of the fish to form this sensory system. As it migrates, the primordium deposits groups of cells called neuromasts, the specialized organs that contain the mechanosensory hair cells. Therefore the primordium provides both a model for studying collective directional cell migration and the differentiation of sensory cells from multipotent progenitor cells. RESULTS: Through the combined use of transgenic fish, Fluorescence Activated Cell Sorting and microarray analysis we identified a repertoire of key genes expressed in the migrating primordium and in differentiated neuromasts. We validated the specific expression in the primordium of a subset of the identified sequences by quantitative RT-PCR, and by in situ hybridization. We also show that interfering with the function of two genes, f11r and cd9b, defects in primordium migration are induced. Finally, pathway construction revealed functional relationships among the genes enriched in the migrating cell population. CONCLUSIONS: Our results demonstrate that this is a robust approach to globally analyze tissue-specific expression and we predict that many of the genes identified in this study will show critical functions in developmental events involving collective cell migration and possibly in pathological situations such as tumor metastasis.