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The embryonic development of the triclad Schmidtea polychroa

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Abstract

Triclad flatworms are well studied for their regenerative properties, yet little is known about their embryonic development. We here describe the embryonic development of the triclad Schmidtea polychroa, using histological and immunocytochemical analysis of whole-mount preparations and sections. During early cleavage (stage 1), yolk cells fuse and enclose the zygote into a syncytium. The zygote divides into blastomeres that dissociate and migrate into the syncytium. During stage 2, a subset of blastomeres differentiate into a transient embryonic epidermis that surrounds the yolk syncytium, and an embryonic pharynx. Other blastomeres divide as a scattered population of cells in the syncytium. During stage 3, the embryonic pharynx imbibes external yolk cells and a gastric cavity is formed in the center of the syncytium. The syncytial yolk and the blastomeres contained within it are compressed into a thin peripheral rind. From a location close to the embryonic pharynx, which defines the posterior pole, bilaterally symmetric ventral nerve cord pioneers extend forward. Stage 4 is characterized by massive proliferation of embryonic cells. Large yolk-filled cells lining the syncytium form the gastrodermis. During stage 5 the external syncytial yolk mantle is resorbed and the embryonic cells contained within differentiate into an irregular scaffold of muscle and nerve cells. Epidermal cells differentiate and replace the transient embryonic epidermis. Through stages 6–8, the embryo adopts its worm-like shape, and loosely scattered populations of differentiating cells consolidate into structurally defined organs. Our analysis reveals a picture of S. polychroa embryogenesis that resembles the morphogenetic events underlying regeneration.

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Acknowledgements

We thank B. Sjöstrand from the UCLA CHS electron microscopy services and N. Cortadellas, A. García and A. Rivera from UB Serveis Científico-Tècnics (Microscopia Electrònica) for technical assistance in preparing and analyzing TEM samples, and the anonymous reviewers whose constructive comments greatly improved this manuscript. A.C. thanks the Hartenstein lab and the Banerjee lab at UCLA for their kind assistance in every respect, and also the Romero lab at UB for their humor, technical expertise and patience. A.C. is recipient of a FPU grant from the Ministerio de Educación, Ciencia y Deportes, Spain. This research was supported by a grant from BMC2000-0546 (to R.R.) and NSF Grant IBN-0110715 (to V.H.).

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  1. Department of Genetics, Faculty of Biology, University of Barcelona, C/Diagonal 645, 08028, Barcelona, Spain

    Albert Cardona & Rafael Romero

  2. Department Molecular, Cell and Developmental Biology, University of California Los Angeles, Los Angeles, CA, 90095, USA

    Volker Hartenstein

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  1. Albert Cardona
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Cardona, A., Hartenstein, V. & Romero, R. The embryonic development of the triclad Schmidtea polychroa. Dev Genes Evol 215, 109–131 (2005). https://doi.org/10.1007/s00427-004-0455-8

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