The history and enduring contributions of planarians to the study of animal regeneration

Abstract

Having an almost unlimited capacity to regenerate tissues lost to age and injury, planarians have long fascinated naturalists. In the Western hemisphere alone, their documented history spans more than 200 years. Planarians were described in the early 19th century as being “immortal under the edge of the knife,” and initial investigation of these remarkable animals was significantly influenced by studies of regeneration in other organisms and from the flourishing field of experimental embryology in the late 19th and early 20th centuries. This review strives to place the study of planarian regeneration into a broader historical context by focusing on the significance and evolution of knowledge in this field. It also synthesizes our current molecular understanding of the mechanisms of planarian regeneration uncovered since this animal’s relatively recent entrance into the molecular-genetic age.

Genome-wide analysis of the bHLH gene family in planarians identifies factors required for adult neurogenesis and neuronal regeneration

“…Although it has been demonstrated that planarians possess pluripotent stem cells (Baguñà et al, 1989;Wagner et al, 2011;Guedelhoefer and Sánchez Alvarado, 2012), several studies support the hypothesis that the stem cell population is heterogeneous (Elliott and Sánchez Alvarado, 2013;Reddien, 2013;Rink, 2013). Analyses of the planarian photoreceptor, excretory and serotonergic cells have shown that tissue-specific transcription factors are detected in the stem cells in intact (Lapan and Reddien, 2012) and regenerating tissues (Lapan and Reddien, 2011;Scimone et al, 2011;Currie and Pearson, 2013); these studies have identified the first sets of precursor cells in planarians outside of the germ cells (Newmark et al, 2008) and have generated a working model in which planarians possess diverse lineage-committed progenitors that contribute to the maintenance and regeneration of tissues (Reddien, 2013;Rink, 2013).…”

“…Freshwater planarians have emerged as an excellent model to examine the molecular mechanisms underlying stem cell biology and tissue replacement (Elliott and Sánchez Alvarado, 2013;King and Newmark, 2012). Following amputation, planarians are capable of restoring lost body parts from a population of adult pluripotent stem cells called neoblasts (Baguñà, 2012;Elliott and Sánchez Alvarado, 2013).…”

“…Following amputation, planarians are capable of restoring lost body parts from a population of adult pluripotent stem cells called neoblasts (Baguñà, 2012;Elliott and Sánchez Alvarado, 2013). Planarian stem cells share conserved pluripotency determinants with mammalian stem cells (Labbé et al, 2012;Önal et al, 2012;Solana et al, 2012) and serve to replace cells lost during normal physiological cell turnover or after amputation.…”

Specialized progenitors and regeneration

“…Because a small fragment of the planarian body can regenerate an entire animal (Reddien and Sánchez Alvarado, 2004) mechanisms must exist in the adult for the production of all of these cell types and organ systems. These dramatic regenerative abilities, combined with the successful development of molecular tools and resources for planarian research, have helped planarians emerge as a powerful molecular genetic system for the investigation of regeneration (Newmark and Sánchez Alvarado, 2002;Sánchez Alvarado, 2006;Reddien, 2011;Elliott and Sánchez Alvarado, 2012).…”

Inhibition of LTA4H expression promotes Staphylococcus aureus elimination by planarians

“…The planarian, a non-vertebrate, is actively used to investigate developmental and regeneration processes [1]. In addition, this platyhelminth is a model used to investigate the evolutionarily conserved mechanism of antibacterial response because of its resistance to infection [2].…”