Ectoderm dermomyotome signaling through filopodia

San Francisco State University, San Francisco CA

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Abstract

The development of the somitic myotome is a multistage process that is dependent on extrinsic signals (BMP4, Shh, Wnt proteins) for the regulation of somite growth, pattern formation and tissue differentiation (myotome development). Although many studies are found on the role of these signaling molecules in myogenesis, the results have not been in complete agreement and in some cases are controversial. There is at present a gap in our knowledge of somite and dermomyotome cellular activities during critical times when extrinsic signals are released into the somite environment. However, our preliminary research show that long, thin, plasma membrane structures called filopodia extend from the dermomyotome to stably interact with its overlying ectoderm. That dermomyotome-derived filopodia might mediate signaling activities between ectoderm and dermomyotome was investigated using live somite cross-sections from 2 day-old chicken embryos labeled with lipophilicfluorescent vital dyes (dil, Bodipy-FL sphingomyelin) and analyzed by confocal microscopy. Our results show for the first time that dermomyotome cells develop large numbers of filopodia that span the subectodermal space to establish stable membrane contact with ectoderm. In contrast, epithelial cells of the newly formed somites (stages I-III) do not possess fllopodia. Also, dermomyotome-derived filopodia show spatial-temporal preference for growth in the dorsomedial half of the dermomyotome to coincide with the area most active in myotome formation (epaxial myotome formation). Finally, filopodia transport, in an ectoderm-to-dermomyotome direction, large amounts of cellular membrane materials that contain lipid rafts. We propose that these transported lipid rafts with their trapped protein cargo may include extrinsic signals or their receptors and that lipid rafts facilitate the signaling activities of these extrinsic molecules previously shown to be regulators of somitic myogenesis. Thus, the regulated development of filopodia by the dermomyotome suggest a similar role to that described for cytonemes in Drosophilia embryo wing disc development. My collaboration with Dr. Holleran is providing greatly needed expertise in lipid membrane biochemistry and ultrastructural analysis to facilitate investigation of filopodia ultrastructure and growth from the dermomyotome and stable interaction with its overlying ectoderm layer. We show that the ectoderm is a 2 layer cellular structure with and outer tightly associated thin cell layer and an inner larger cell population that is separated by cavities that appear to be formed by these cells that border it. It is intriguing to speculate that signaling molecules are released into the subectoderm space for signaling function by the transient opening caused by cell-cell disassociation. My investigation with Dr. Holleran will focus on the dermomyotome-ectoderm interaction and the mechanism for the transport of cellular membrane materials through filopodia from ectoderm-to-dermomyotome.