If An Earthworm Can Grow A New Body, Why Can’t A Human Grow New Cartilage For Their Knee Or Hip?

By Nathan Wei

If a tadpole’s tail is cut off, it will grow a new one. If a salamander’s leg is cut off, it will grow a new one. If an earthworm is cut in half, it will eventually grow a new half. If a snails head is cut off, it will grow a new one. (Osborn HF, Wilson EB. Regeneration. Columbia University Biological Series. Macmillan Company. 1901)

Regeneration in lower forms of animals has been known about for at least a century in the scientific community. The primary reason lower forms of animals are able to regenerate lost limbs while higher forms such as humans can’t, has been the subject of much conjecture. The most reasonable explanations include:

1. Stem cells in lower forms of animals are plentiful and available in sufficient concentrations to hasten regeneration.

2. Stem cells are in closer proximity- meaning that these animals have very simple organs that aren’t nearly as compartmentalized as those of higher animal forms. What this means is that once a cell has differentiated into a heart cell or lung cell, it is committed. It cannot reverse the process. And, since stem cells are not that plentiful in the normal bloodstream of higher animal forms such as humans, the ability to regenerate lost tissues is limited.

3. Finally, the amount of differentiation of cells into more complex and different types of organs is not quite so pronounced in earthworms and salamanders as compared with higher forms of animals. Brains, heart, and lung tissue are highly developed, very complex organs. And… these organs are located far away from a large supply of stem cells since stem cells are rarely found in the peripheral blood. An earthworm or salamander doesn’t have to worry about this problem!

Mammals do have the ability to ‘regenerate’ to the extent that most wounds will heal over time.

However, the ability to heal degenerative processes or manufacture new organs has been an elusive target.


At least until recently… It turns out that stem cells, the progenitor cells of all cells in the body, are what allow massive healing to take place. (By progenitor, I mean that stem cells are capable of differentiating into any type of organ cell).

In higher mammals such as humans, the number of stem cells required for massive regeneration is not available under normal circumstances.

Stem cells are sparse in the general blood circulation. They are,though, present within bone marrow where they are called “mesenchymal stem cells.”

However, in the adult, even in the bone marrow, stem cells need to be concentrated in order to be effective for tissue regenerative processes.

The lack of sufficient stem cells in peripheral tissues as well as the absence of triggering mechanisms which can send the stem cells into the “warp speed’ required to help with regeneration also aren’t present under normal circumstances.

It is this relative lack and access to stem cells that has presented researchers with the problem of how to produce the same kind of healing in human beings as is seen in lower forms of animals.

Recent development in stem cell biology has shown much promise.

For instance, autologous stem cells (cells obtained directly from the patient) have been used to help heal an ailing heart (National Geographic, July 2008). And in a more recent article (Parade Magazine, September 28, 2008), investigators at the Hospital for Special Surgery in New York City have mentioned the possible application of stem cells and platelet rich plasma in the treatment of arthritis.

It appears that a number of requirements must be met in order for the magic of stem cells to take hold for conditions such as osteoarthritis. First, a large number of stem cells are required. This involves taking stem cells from the bone marrow of the patient and concentrating them. Second, the stem cells must be viable, meaning the concentration process cannot damage the stem cells. Third, a ‘trigger’ such as platelet rich plasma must be provided in order for certain substances such as growth hormone to stimulate the stem cells to replicate and differentiate.

Fourth, the arthritis area must be prepared properly. That means ‘injury’ must be induced. Injury should be sufficient to induce stem cells to proliferate and differentiate but not enough to render stem cell therapy useless. In other words, special techniques need to be applied.

Fifth, the stem cells and platelet rich plasma must be introduced in such a fashion that a framework or scaffold is prepared so that stem cells have a place to ‘latch onto to’ so they can accomplish what needs to be done.

A small number of centers in the U.S. are working on such as procedure. Early results look promising.

At the Arthritis and Osteoporosis Center of Maryland, exciting work in the field of stem cells and their application to treating osteoarthritis is ongoing.

According to Clinical Director, Dr. Nathan Wei, ‘Stem cell research is probably the future of treatment for osteoarthritis. We have to work on newer and better techniques… and also provide data that can be used to assess the efficacy of this type of therapy. To date we have treatments that help with symptoms… but we need therapies that can actually regrow cartilage and other damaged connective tissue.

Currently, we are in the middle of some very interesting work that looks very encouraging. The results are in the preliminary stages, so while the early evidence is extremely promising, longer term data is needed.

We are harvesting and concentrating mesenchymal stem cells. We are also providing a scaffold for stem cells to proliferate and grow.

Unfortunately, cartilage tissue grows relatively slowly… but we have to be patient. If the objective evidence- thickening… or growth of cartilage- matches the symptomatic improvement we’ve seen, this will be a major breakthrough!’

About the Author: Nathan Wei, MD FACP FACR is a nationally known board-certified rheumatologist. For more info:

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