The maintenance and regeneration of the planarian excretory system are regulated by EGFR signaling.

The maintenance of organs and their regeneration in case of injury are crucial to the survival of all animals. High rates of tissue turnover and nearly unlimited regenerative capabilities make planarian flatworms an ideal system with which to investigate these important processes, yet little is known about the cell biology and anatomy of their organs. Here we focus on the planarian excretory system, which consists of internal protonephridial tubules. We find that these assemble into complex branching patterns with a stereotyped succession of cell types along their length. Organ regeneration is likely to originate from a precursor structure arising in the blastema, which undergoes extensive branching morphogenesis. In an RNAi screen of signaling molecules, we identified an EGF receptor (Smed-EGFR-5) as a crucial regulator of branching morphogenesis and maintenance. Overall, our characterization of the planarian protonephridial system establishes a new paradigm for regenerative organogenesis and provides a platform for exploring its functional and evolutionary homologies with vertebrate excretory systems.

Whole-Mount In Situ Hybridization in Large Sexual Schmidtea mediterranea.

Whole-mount in situ hybridization (WISH), colorimetric or fluorescent (FISH), allows for the visualization of endogenous RNA. For planarians, robust WISH protocols exist for small-sized animals (>5 mm) of the model species Schmidtea mediterranea and Dugesia japonica. However, the sexual strain of Schmidtea mediterranea studied for germline development and function reaches much larger body sizes in excess of 2 cm. The existing whole-mount WISH protocols are not optimal for such large specimens, owing to insufficient tissue permeabilization. Here, we describe a robust WISH protocol for 12-16 mm long sexually mature Schmidtea mediterranea individuals that could serve as a starting point for adapting WISH to other large planarian species.

Emergence of a Bilaterally Symmetric Pattern from Chiral Components in the Planarian Epidermis.

Most animals exhibit mirror-symmetric body plans, yet the molecular constituents from which they are formed are often chiral. In planarian flatworms, centrioles are arranged in a bilaterally symmetric pattern across the ventral epidermis. Here, we found that this pattern is generated by a network of centrioles with prominent chiral asymmetric properties. We identify centriole components required for establishing asymmetric connections between centrioles and balancing their effects to align centrioles along polarity fields. SMED-ODF2, SMED-VFL1, and SMED-VFL3 affect the assembly of centriole appendages that tether cytoskeletal connectors to position the centrioles. We further show that the medio-lateral polarization of centrioles relies on mechanisms that are partly distinct on the left and right sides of the planarian body. Our findings shed light on how bilaterally symmetrical patterns can emerge from chiral cellular organizations.

Dynamic Polarization of the Multiciliated Planarian Epidermis between Body Plan Landmarks.

Polarity is a universal design principle of biological systems that manifests at all organizational scales, yet its coordination across scales remains poorly understood. Here, we make use of the extreme anatomical plasticity of planarian flatworms to probe the interplay between global body plan polarity and local cell polarity. Our quantitative analysis of ciliary rootlet orientation in the epidermis reveals a dynamic polarity field with head and tail as independent determinants of anteroposterior (A/P) polarization and the body margin as determinant of mediolateral (M/L) polarization. Mathematical modeling rationalizes the global polarity field and its response to experimental manipulations as superposition of separate A/P and M/L fields, and we identify the core PCP and Ft/Ds pathways as their molecular mediators. Overall, our study establishes a framework for the alignment of cellular polarity vectors relative to planarian body plan landmarks and establishes the core PCP and Ft/Ds pathways as evolutionarily conserved 2D-polarization module.

Self-organization in development, regeneration and organoids.

Self-organization of cells is a fundamental design principle in biology, yet the inherent non-linearity of self-organizing systems often poses significant challenges in deciphering the underlying mechanisms. Here, we discuss recent progress in this respect, focusing on examples from development, regeneration and organoid differentiation. Together, these three paradigms emphasize the active material properties of tissues that result from the functional coupling between individual cells as active units. Further, we discuss the challenge of obtaining reproducible outcomes on the basis of self-organizing systems, which development and regeneration, but not the current organoid culture protocols, achieve.