Figure 1.
Planarian anatomy and body axes.
(A) Dorsal side of the planarian Schmidtea mediterranea. (B) Planarian diagram showing the brain lobes, nerve cords, and secondary nerves (green); the two eyes (black and white); the gastrovascular tract (gray); and the pharynx (light brown). (C) The three main axes of the planarian anatomy: anterior-posterior (AP), dorsal-ventral (DV), and medial-lateral (ML).
Figure 2.
Diagrams of the main planarian regeneration experiments found in the literature.
(A) Cutting experiments amputate part of the planarian body (shadowed); normally, a complete regenerated worm results within 1–2 weeks. (B) Transplantation of diverse parts also regenerates into a complete worm. (C) Planarians degrowth when starved; they restore their original size upon feeding. (D) Octanol blocks gap junction communication between the worm cells; a trunk fragment treated with octanol regenerates into a double-headed worm. (E) A post-pharyngeal fragment treated with octanol and with the nerve cords partially amputated regenerates into a quadruple-headed worm. (F) An external electric field applied to a trunk fragment disturbs AP polarity during regeneration when the anode is located in the head wound; low-intensity currents cause double-headed worms, whereas high-intensity currents cause reversed-polarity worms. (G) The drugs ivermectin (IVM) and SCH-28080 (SCH) disturb the ion pumps in the worm cells, altering their membrane voltage. IVM causes cell depolarization (more positive) and trunk fragments to regenerate into double-headed worms; SCH causes cell hyperpolarization (more negative) and trunk fragments to regenerate into worms with no heads.
Figure 3.
Planaria restore their AP polarity similarly to bar magnets.
(A) A bar magnet restores the original polarity after being cut transversally. (B) Similarly, after bisecting a worm, polarity is restored correctly in each fragment. Note that even though cells on either side of the amputation plane were direct neighbors before the cut, the ones facing posterior make a completely different structure (a tail) than the ones facing anterior (a head). The drastically different fates of cells with essentially the same positional information suggest models based on non-local control of orientation [8].
Figure 4.
Hypothetical experiment illustrating the concept of target morphology.
(A) S. mediterranea (left, rounded head) and P. felina (right, hammerhead) are planarian species with different head morphology. (B) Half the neoblasts of the rounded-head worm are killed by using irradiation and a lead shield. (C) Half the neoblasts of the hammerhead worm are transplanted to the rounded-head worm. (D) After the neoblasts have diffused, the head of the rounded-head worm is amputated. Without a model of how target morphology is determined, it is impossible to predict what shape will regenerate.