42.3: Whole Body Regeneration
Regeneration is the process of restoring injured or lost tissues, organs, or body parts. While simpler organisms generally show greater ability to regenerate their whole body, few complex animals show similarly exceptional regeneration. For example, planarian flatworms have a unique regenerative potential making them a popular study organism among biologists to understand the mechanisms of whole body regeneration. Other organisms, such as hydra, also show extreme regeneration potential; even when cut into several pieces, each piece can turn into an individual organism.
Planarian Stem Cells
Planarians are among the few animals that maintain pluripotent stem cells throughout life. First described in the 1800s, these pluripotent stem cells are now called neoblasts. Neoblasts are essential for the regeneration of the endoderm, mesoderm, and ectoderm, thus, facilitating whole body regeneration. These neoblasts are present throughout the planarian body, including the space surrounding the gut. As much as 30 percent of cells in planaria are neoblasts. Thus, a planarian worm can regenerate a whole body even when cut into small pieces.
Regeneration Polarity in Planarians
Regeneration requires the restoration of complex anatomical structures and specific integration of organs within the body through precise control of the organs’ size, location, and identity. When a planarian flatworm is amputated transversely, two fragments are generated — a fragment containing the head that regenerates a tail; and a tail fragment that regenerates the head. The location along the anterior-posterior axis determines whether the tissue differentiates into the head or tail. The Wnt proteins are a major determinant of this axis, with high Wnt expression in the anterior cells and low Wnt expression in the posterior cells. At the wound site, posterior cells express the Wnt1 gene, which triggers tail regeneration. In contrast, anterior cells express the notum gene, which inhibits the Wnt signaling and facilitates regeneration of the head.