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Reddien and Alvarado, 2004

Reddien and Alvarado, 2004 - 3 Sep 2004 16:54 AR...

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Annu. Rev. Cell Dev. Biol. 2004. 20:725–57 doi: 10.1146/annurev.cellbio.20.010403.095114 Copyright c 2004 by Annual Reviews. All rights reserved First published online as a Review in Advance on July 2, 2004 F UNDAMENTALS OF P LANARIAN R EGENERATION Peter W. Reddien and Alejandro S´anchez Alvarado University of Utah School of Medicine, Department of Neurobiology and Anatomy, Salt Lake City, Utah 84132-3401; email: [email protected], [email protected] Key Words stem cells, blastema, epimorphosis, morphallaxis, flatworm Abstract The principles underlying regeneration in planarians have been ex- plored for over 100 years through surgical manipulations and cellular observations. Planarian regeneration involves the generation of new tissue at the wound site via cell proliferation (blastema formation), and the remodeling of pre-existing tissues to re- store symmetry and proportion (morphallaxis). Because blastemas do not replace all tissues following most types of injuries, both blastema formation and morphallaxis are needed for complete regeneration. Here we discuss a proliferative cell population, the neoblasts, that is central to the regenerative capacities of planarians. Neoblasts may be a totipotent stem-cell population capable of generating essentially every cell type in the adult animal, including themselves. The population properties of the neoblasts and their descendants still await careful elucidation. We identify the types of structures produced by blastemas on a variety of wound surfaces, the principles guiding the re- organization of pre-existing tissues, and the manner in which scale and cell number proportions between body regions are restored during regeneration. CONTENTS INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 726 Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 726 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 728 THE PLANARIAN BLASTEMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729 Responses to Wounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729 Possible Cues for the Initiation of Regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729 Cell Proliferation, Cell Migration, and the Regeneration Blastema . . . . . . . . . . . . . 730 THE CELL BIOLOGY OF PLANARIAN REGENERATION . . . . . . . . . . . . . . . . . . 731 What Are Neoblasts? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 731 What Is the Source of Neoblasts? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733 Neoblast Migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 735 PROPERTIES OF BLASTEMA FORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 736 Tissue Polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738 Gradients of Regenerative Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738 What a Regeneration Blastema Makes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739 Differentiation in the Blastema . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744 REMODELING AND PROPORTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745 Resetting Positional Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745 1081-0706/04/1115-0725$14.00 725 Annu. Rev. Cell. Dev. Biol. 2004.20:725-757. Downloaded from arjournals.annualreviews.org by UNIVERSITY OF ILLINOIS on 01/09/06. For personal use only.
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726 REDDIEN S ´ ANCHEZ ALVARADO Morphallaxis and the Restoration of Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747 Regulation of Proportions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749 SUMMARY AND PROSPECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750 INTRODUCTION Planarians (Figure 1) are bilaterally symmetric metazoans of the phylum Platyhel- minthes commonly found in freshwater streams and ponds where they prey pre- dominantly upon insects, insect larvae, and other invertebrates. Planarians have the capacity to replace large regions of missing structures through regeneration.
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