We summarize the work that has been published to date, especially nodal expression in the invertebrate deuterostomes, and suggest future experiments to better understand the evolution of nodal signaling and deuterostome gastrulation. Expression is in the ectoderm in most of the invertebrate deuterostomes, and in the mesoderm in vertebrates. The expression of nodal is in different germ layers in embryos of different phyla. In most phyla, the blastopore becomes the mouth animals with this type of development are called protostomes, meaning 'first (or primary) mouth'. Nodal is expressed asymmetrically on the left side in chordates and on the presumptive oral side of the embryo in echinoid echinoderms. All triploblastic animals go through an embryonic stage known as a gastrula, which has a hollow interior (archenteron) and a single opening (blastopore) to the outside. We review gastrulation in the deuterostomes, then examine nodal expression early during mesoderm formation and later during the establishment of asymmetries in both vertebrates and invertebrates. Nodal has been shown to be particularly important in determining left-right asymmetries in vertebrate embryos, but less information is available for its developmental role in the invertebrate deuterostomes. Nodal has also been found in other invertebrate deuterostomes, such as ascidians and sea urchins, but, so far, is missing in protostomes. Nodal is a member of the TGF-beta superfamily of signaling molecules that are important in signaling between cells during many embryonic processes in vertebrate embryos. All holoblastic deuterostomes gastrulate at the vegetal pole and the blastopore becomes the anus, while a mouth is formed at the anterior or to the oral side. Chordates, including vertebrates, evolved within a group of animals called the deuterostomes.
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