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Regrowing a missing limb is no big deal — to a starfish or salamander, creatures that are well-known for using regenerative "superpowers" to replace missing arms and tails. But they aren't the only animals that can rebuild body parts that are destroyed or damaged. Deer can re-sprout as much as 66 lbs. (30 kilograms) of antlers in only three months. Zebrafish can regrow their hearts, while flatworms have demonstrated that they can regenerate their own heads.
For humans, though, what's lost is lost — or is it?
Individual cells in your body are constantly being replaced as they wear out, a process that slows with aging but continues throughout the human lifetime. You can even observe this frequent and visible regeneration in one of your organs: your skin. In fact, humans shed their entire outer layer of skin every two to four weeks, losing about 18 ounces (510 grams) of skin cells per year, according to the American Chemical Society.
However, regenerating complete organs and body parts, a common practice among "Doctor Who's" Time Lords, is beyond the scope of human biology. But in recent years, scientists have successfully cultivated a range of human body structures, similar structures that have been successfully tested in animals, and small-scale human organs known as "organoids," which are used to study human organ function and structure at a level of detail that was previously impossible. Here are some recent examples:
Fallopian tubesSlide 2 of 23
Using stem cells, scientists from the Max Planck Institute for Infection Biology in Berlin grew the innermost cellular layer of human fallopian tubes, the structures that connect the ovaries and uterus. In a statement released on Jan. 11, the researchers describe the resulting organoids as sharing the features and shapes that are particular to full-size fallopian tubes.Slide 3 of 23
MinibrainSlide 4 of 23
A lab-grown brain the size of a pencil eraser was cultivated from skin cells by The Ohio State University (OSU) scientists, and is structurally and genetically similar to the brain of a 5-week-old human fetus. Described as "a brain changer" by OSU representatives in an Aug. 18 statement, the organoid has functioning neurons with signal-carrying extensions like axons and dendrites. In the photo of the minibrain, labels identify structures that are typically found in a fetal brain.Slide 5 of 23
MiniheartSlide 6 of 23
Researchers prompted stem cells to develop into heart muscle and connective tissue, and then organize into tiny chambers and "beat." In a video of the achievement, the heart muscle cells (indicated by red at the center) are beating while connective tissue (green ring) secures the miniheart to the dish where it grew. Kevin Healy, a University of California, Berkeley, professor of bioengineering and co-senior author of the study, said in a statement. "This technology could help us quickly screen for drugs likely to generate cardiac birth defects, and guide decisions about which drugs are dangerous during pregnancy." The research was published March 2015 in the journal Nature CommunicationsSlide 7 of 23
MinikidneySlide 8 of 23