Planarian
Planarians are freshwater flatworms best known for their extraordinary ability to regenerate their entire body. Unlike most animals, which can only repair small parts of tissue, planarians can regenerate a complete organism from tiny body fragments. Even a piece as small as 1/279 of the original body could regrow the entire body back, including the brain, nervous system, gut, and pharynx. This whole-body regeneration is rare among animals and makes planarians a valuable model for regeneration research.
Planarians have very complex anatomy. A bilobed brain composed of diverse neurons and glia connects to two ventral nerve cords for longitudinal integration. Simple eyes (ocelli) detect light intensity and direction. The body-wall musculature contains longitudinal, circular, and diagonal fibers that coordinate bending and gliding. A highly branched intestine distributes nutrients and connects to a centrally located, muscular, eversible pharynx that serves as both mouth and anus. Protonephridia—a ciliated excretory network—handle waste removal and osmoregulation. The epidermis secretes mucus and is densely ciliated on the ventral surface to power locomotion.
Parenchyma, a mesenchymal tissue compartment that surrounds internal organs and harbors the adult’s only proliferative somatic cells, the neoblasts. Neoblasts are crucial in turnover as well as supplying new cells during regeneration.
Regeneration
Planarians regenerate through two mechanisms: epimorphosis and morphallaxis. Epimorphosis involves the formation of blastema, where neoblasts proliferate and differentiate into specialized tissues, for example, the head. Morphallaxis, in contrast, relies on the repositioning and resizing of pre-existing tissues, often by cellular turnover. Morphallaxis usually happens in early stage of missing tissue injury (major) and incision (non-major)
(new cell production plus selective cell loss). In the early phase after major tissue loss injury, small body fragments cannot feed until structures like the pharynx and brain are regenerated; regeneration therefore proceeds using pre-existing resources. The missing tissues are regenerated at reduced scale, and since body fragment is of course smaller than the entire organism, the retained tissues are now overabundant relative to the smaller body fragment and subsequently resize and reposition to match the regenerating fragment. Not all repair requires a blastema: for example, skin incisions typically heal by morphallactic remodeling without overt blastema formation.
Neoblasts
The key players in planarian regeneration are neoblasts. Before getting into the function of neoblasts, we first need to understand the difference between cNeoblasts and tissue-specialized Neoblasts.
Big names:
