tail), even though they began as adjacent neighbors with identical positional information (green circles), suggests the need for long-range communication across the fragment so that decisions at the wound could be made based on the rest of the fragment. The ability of cells on either side of a bisection to develop distinct anatomical fates (head vs. (A″) Common models of axial patterning postulate a chemical gradient that indicates positional information for cells along the AP axis. japonica shown here) regenerates a head and tail at the correct end. (A′) Planarian’s anterior-posterior (AP) axis likewise re-scales: every piece cut from a planarian ( D. (A) Bar magnets illustrate a basic property of polarity re-scaling: having a North and South pole, a magnet can be cut into pieces and each piece reorganizes its polarity to likewise have a North and South pole by orientating small magnetic domains into large-scale axial patterning. Planarian regeneration: fundamental puzzles of pattern control 1A), each piece determines head/tail identity at the wound based on an invariant axial polarity, regenerating into a worm with one head and one tail. This is most obvious in the head-tail polarity of their primary axis: like a bar magnet cut into pieces ( Fig. Every piece of a planarian is able to grow and remodel towards its species-specific large-scale pattern, stopping precisely when that specific anatomical pattern (the target morphology) is achieved. Amputated fragments of planaria regenerate a complete worm, growing precisely what is missing – no more and no less – with events at the wound edge coordinated tightly with body-wide remodeling of the intact tissue to ensure that a perfectly proportioned animal results within about a week after it is cut along any plane. Though planaria have a true brain, the real wonder of this remarkable model system is revealed most clearly when one considers the robust decision-making capabilities of its individual somatic pieces. Planaria possess rich behavioral repertoires that include sensing of a wide range of environmental cues ranging from chemicals to gravity, to weak gamma radiation. Planarian flatworms are free-living bilaterian organisms with a complex set of organ systems and cell types. We analyze existing models of planarian pattern control and highlighting recent successes and remaining knowledge gaps in this interdisciplinary frontier field.ġ.1 A primer on planarians’ functional features Finally, we focus on computational approaches that complement reductive pathway analysis with synthetic, systems-level understanding of morphological decision-making. We then review recent progress in understanding of the physiological control of an endogenous, bioelectric pattern memory that guides regeneration, and how modulating this memory can permanently alter the flatworm’s target morphology. In this review article, we first highlight several deep mysteries about planarian regeneration in the context of the current paradigm in this field. Despite recent advances in the molecular genetics of stem cell differentiation, this model organism’s remarkable anatomical homeostasis provokes us with truly fundamental puzzles about the origin of large-scale shape and its relationship to the genome. This tells us that when 5 thousand items are produced, the average cost per item is decreasing by $0.Planarian behavior, physiology, and pattern control offer profound lessons for regenerative medicine, evolutionary biology, morphogenetic engineering, robotics, and unconventional computation. Since the units on \(AC\) are dollars per item, and the units on \(x\) are thousands of items, the units on \(AC'\) are dollars per item per thousands of items.
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