The human body is composed of over 37 trillion cells , each with a limited lifespan. These cells are continuously replaced to maintain organ and system function. Yet over time, or as a result of damage, the number of functioning cells can decrease to a level that causes symptoms or even organ failure.
Author
- Adam Taylor
Professor and Director of the Clinical Anatomy Learning Centre, Lancaster University
Regeneration of organs and systems is a scientific holy grail that relies on stem cells , but due to their limited number and slow division rate, this isn't a practical route to organ regeneration. It would take many years to repopulate all the cell types needed.
However, some people see organs "reappear", like Katy Golden who had her tonsils removed for a second time as an adult after they grew back over 40 years.
One reason that tonsils may grow back is that one of the operations to remove them is a partial tonsillectomy . Only removing part of the tonsils leads to a quicker recovery and fewer complications, but around 6% of children may see regrowth , which may require further surgery in later life.
Most people associate organ regrowth and regeneration with the liver. As little as 10% of the liver can regrow into a fully functioning liver. This is also how partial liver transplants allow the donor to "regrow" a normal sized and fully functioning liver.
One organ that has a surprising capacity to regenerate is the spleen and sometimes it can regenerate without people realising.
The spleen is a high-risk organ for injury and is the most commonly injured organ in blunt abdominal trauma during traffic collisions, sporting injuries or trivial activities such as bumping into furniture .
The spleen is at high risk because it has lots of blood vessels and hence lots of blood, but is only surrounded by a thin capsule that can tear in trauma, allowing blood to leak out. This can result in death if not treated promptly.
What may also happen is small pieces of the spleen - sometimes just a few cells - can become free in the abdomen and go on to "grow" where they settle - termed splenosis , going on to have similar functional activity to a mature, normally located spleen. This can be beneficial for those who have to have their spleen removed due to traumatic injury, with some reports suggesting regeneration in up to 66% of patients .
In the last few years, our lungs have also been shown to have regenerative capacity. It is well known that smoking and other pollutants destroy the alveoli (tiny air sacs) where oxygen is passed to the blood. Stopping smoking has been shown to allow cells that have avoided damage from the cancer-causing chemicals in tobacco smoke to help regenerate and repopulate the lining of parts of the airways with healthy cells.
Where a lung has been removed, the remaining lung has to adapt to support the tissues of the body and ensure enough oxygen gets to them. Studies have shown that the remaining lung increases the number of alveoli it has, rather than the remaining alveoli compensating by getting bigger to take up more oxygen.
It isn't just organs inside that regenerate. One organ that constantly does so on a humongous scale is the skin.
As the largest organ, it has multiple barrier functions to keep things such as water in and germs out . With a surface area of almost 2m² , the skin requires a significant amount of regeneration to replenish the 500,000,000 cells that are lost each day - that's over 2g of skin cells per day .
Tissue regeneration is much more common
One of the most active regenerative tissues is the endometrial lining of the uterus which is shed every 28 days as part of the menstrual cycle and goes through about 450 cycles of this during a woman's life.
This layer varies between 0.5 and 18mm in thickness depending on the stage of the menstrual cycle, the functional cells that are lost along with the blood from vessels that support a fertilised egg if it implants.
Men's genitalia can also show regeneration. Vasectomy , which removes a piece of the tube ( vas deferens ) connecting the testes to the openings in the urethra, is used to reduce the chance of pregnancy by preventing sperm moving from the testes out of the penis.
However, the cut ends of the ducts have shown regenerative capacity and reconnected. Some sections, where up to 5cm has been restricted or removed, have shown regeneration, even through scar tissue . This "recanalisation" can result in unexpected pregnancies.
Bone is another tissue that can regenerate. If you've ever broken a bone, you'll know that it repairs so that (eventually) you will regain function.
This process of repairing the break takes six to eight weeks. But the process of regenerating the bone architecture and strength continues for months and years beyond this date.
However, with increasing age and in post-menopausal woman , this process slows and the bone may not regenerate to its previous strength or structure .
Where paired organs exist and one is lost, there is good evidence that the remaining organ can increase its functional ability to help the body cope with maintaining function. For example, when one kidney is removed, the remaining kidney enlarges to handle the extra workload, filtering blood and eliminating waste efficiently.
Although organ regeneration is rare, it does happen and typically takes years to manifest because organs are complex structures. Work continues to try to understand how scientists can develop this knowledge to help with the shortage of donor organs. Thankfully, tissue regeneration happens much more often than many people might suppose, and it is a much-needed part of staying alive.
Adam Taylor does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
