The human liver is renowned for its regenerative capabilities. Some researchers describe it as “one of the most amazing tissue injury response.”
“Liver regeneration […] has fascinated clinicians, surgeons, and scientists who have observed this apparently supernatural process and studied its mechanisms for many years,” write a group of researchers in a 2019 paper.
The not-so-supernatural process has cells called hepatocytes that split and proliferate in response to injury or loss. Even if 90 percent of the liver is lost, the repairing process will eventually recover the liver to its full size, though not necessarily its original shape.
Though the liver is most famous for regeneration and recovery, other cells, tissues and organs go through their own regular renewal; these parts of our body are in a continual state of repair, taking a variety of timespans.
Regeneration refers to the biological process through which living organisms, tissues or cells replace or repair damaged or lost body parts or structures.
This phenomenon occurs in various forms across different organisms, and it allows them to recover from injuries, heal wounds and, in some cases, even regrow entire body parts.
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In addition to the liver cells, skin cells frequently replace themselves over time. Specifically, the epidermis, or top layer of the skin, does so regularly over a time span of around 40 to 56 days, according to research. This process happens when skin is damaged from a sun burn, cut or abrasion, and allows self-healing that doesn’t leave a scar.
Similarly, hair follicles regenerate themselves 10 times on average during a human’s lifespan. The intestinal lining also changes on a daily basis to keep up with the demands of processing and cleansing; this lining fully replaces itself in around one week.
Research published in 2023 indicates that the kidneys, for example, can also regenerate to an extent. Stem cells in the kidney can form new kidney cells, but not to the same degree as in the liver.
“If you had a piece of the kidney cut away, it will not grow back like with the liver,” says Jie Zheng, a distinguished chair in Natural Sciences and Mathematics at the University of Dallas. But that’s not to say it doesn’t go through its own form of self-healing.
Zheng was part of a team that discovered a “housekeeping” method whereby the kidneys’ proximal tubule cells – which make up a large portion of the kidney’s mass – are in a constant state of self-renewal.
Since these cells cannot easily divide like liver cells, Zheng explains, they have their own way of keeping their shape during our lifetime. It’s called an extrusion-mediated self-renewal mechanism. Essentially, old cells and any intrusive particles – Zheng’s team used gold nanoparticles in their experiment – are enclosed in the cell lysosome and eventually get pushed out from the kidney’s surface, while the contents of the cells are renewed.
But they also found that this process happens whether there is a foreign body or not.
“It’s another way to regenerate the cell without going through the division cycles,” Zheng says, adding that it’s possible a similar mechanism is used within other organs “to help them stay healthy and keep their normal functions without the traditional regeneration mechanism.”
Researchers have studied regeneration in the animal kingdom for decades. Salamanders can regrow their entire tail within a matter of weeks. Another species, Polycarpa mytiligera, a marine animal, is capable of regenerating all of its organs, even when they are cut up into three pieces. When the head or tail of flat worm is cut off, it can regrow in as little as a week. The axolotl, a species of salamander, can regrow its limbs, multiple organs and even its spinal cord.
Scientists hope to learn from such species, with the goal of one day enabling human beings to regrow limbs. Researchers are also exploring ways to mimic our own liver’s remarkable regenerative properties and apply it to other organs, such as the human heart. Zheng hopes his team’s discovery of the kidney’s self-healing mechanism could eventually play its own part in improving human health.
“If we can monitor this self-renewal process and its functions in a non-invasive way, we could intervene early to keep the kidneys healthy, which could be important for many people who have high risk of kidney disease,” he explains.