Dividing stem cells

Health

How to have extreme longevity: 2. Add more stem cells

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By Juman Hijab

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Original date: November 1, 2022  

Updated: February 5, 2023

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Table Of Contents
1
Why are stem cells important?
1.1
 Stem cells: Having a significant supply
1.1.1
Hydra
1.1.2
Sponges
1.1.3
Sea Urchins
1.1.4
Giant deep-sea worms (tube worms)
1.1.5
Corals
1.2
Long lived animals have the ability to transform specialized cells into  stem cells
1.2.1
Jellyfish
1.2.2
Dedifferentiation, Regeneration, and Extreme longevity
1.3
Next article: How to have extreme longevity: 3. Keep adult stem cells young
1.4
Picture credits:
1.5
References:
Dividing stem cells

Dividing stem cells

Why are stem cells important?


Stem cells provide new "blood" for tissues.  In any successful project, it is important to have a mixture of younger and more vibrant entities as well as the "old hands" on deck. 


One of the main reasons for aging in animals is due to aging of the stem cells (1, 2, 3, 4). When stem cells age, they exhibit decreased proliferative ability. Tissues no longer have access to young and healthy dividing cells. 


In terms of stem cell aging, there are factors within the stem cell as well as factors outside the cell (for instance, the environment surrounding the stem cells)(4). Organisms that start off with a large abundance of stem cells have a leg up in terms of maintaining a long life span. 

In this article - the second in the series -  I will show the stark difference in the abundance of stem cells in animals that have  extreme longevity versus humans (see (5) for an exceptional review).


Hydra - stem cells and longevity

Hydra - presumably immortal

 Stem cells: Having a significant supply

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The second thing you have to do to enjoy extreme longevity is to have a lot of stem cells. Only a minute fraction of the 30 Trillion cells (6) in a human body are stem cells (estimate 0.003% - 7; 0.001–0.01% (5). Contrast this to the 5 animals shown in the table below:

Animal 

Total number of cells 

% of cells that are stem cells

Maximal life span

Hydra

100, 000 (8)

20% -30%(5, 9, 10); 50% (estimate based on 9, 11); 

1400 years (12), but very likely immortal if not eaten by a predator

Sponges

0.050 Trillion (estimate; see below) 

20%, at least, estimate based on 13); see below

11000 years (14)

Sea Urchins

0.24Trillion (see below)

1-5% (estimate based on 17)

200 years

Humans

30 Trillion (6) 

0.003% (7)

122 years (20)

Tube Worms

probably similar to sponges

50% (see below)

1,000 years

Corals

Trillions (colonies as large as 26,000 km span of the Barrier Reef - 24)

Distributed throughout the body (26)

thousands of years 

The non-human animals listed in the table (hydra, sponges, sea urchins, tube worms, and corals) have a significant supply of stem cells as a percentage of their overall body. Let's go over the  non-human animals listed in the table.

Hydra

The Hydra is basically a stem cell system. 80% of its tubular structure (its body) is made up of stem cells. Its head/foot/tentacles have relatively few stem cells (9). Given that a significant portion of the hydra's specialized structures are mostly lacking in stem cells, I am giving an estimate of 50% of the total of the hydra's  cells are stem cells. Others give an estimate of 20% -30%(5, 9, 10). Both estimates show that the hydra has an overabundance of stem cells. Because of this plethora of stem cells, a hydra can regenerate its whole self (all 100,000 cells - 8) every 20 days (3).  

Sponges

Giant barrel sponges live millennia (14). Their cells have an incredible capacity to regenerate and reproduce.  (3, 13, 15). Of note, sponges can be small (under 1 cm) or very large (3.5 meters/11.4 feet - 16). My estimate of the number of cells in a sponge comes from a small sponge Leuconia that measures 10cm (4 in) by 1cm (0.4 in) and has 2 million chambers of cells within its structure. Assuming this small creature has about 20 cells per chamber, this would make it have at least 40 million cells, though there are many other types of cells than those lining the chambers. Thus, a barrel sponge which can be 1 meter high could easily have billions of cells.

There are at least 10 types of cells in sponges. Two important ones are the archeocytes and the choanocyte. The former is clearly a stem cell producing new cells that can differentiate into specialized cells. The latter is also believed to be a potential stem cell, since it can trans-differentiate into an archeocyte (13). Since these cells are significant elements of a sponge (based on diagrams - 13), I think it is fair to say that they make up at least 20% of the total cells in a sponge.  Other estimates are 3-14% (Desmospongia) and 50 - 80% (Calcarea) (5).

Red Sea Urchin - average lifespan

Red Sea urchin - extreme longevity (200 years)

Sea Urchins

Red sea urchins are among some of the longest living marine animals. Some are believed to be 200 years old, with little signs of aging. Sea urchins have incredible regenerative ability. They can lose significant portions of their body and regenerate those (17, 18).

A sea urchin has an estimated 1500 tube feet (somewhat like tentacles, except they have suction cups at the end and help the urchin move along the ocean floor - 19). Studies have suggested the localization of  multi-potent stem cells in diverse areas of the sea urchin's body, including its tube feet and spines (17). This latter study estimated the % of cells that stained for Vasa protein (a marker of multi-potency); 1 - 5% of the cells of spines and tube feet stained positive for this protein.  Given that a significant volume of cells is housed in those two structures, it is fair to say that an average sea urchin has a sizable quantity of stem cells within its body.

In terms of estimating the number of cells in a sea urchin, the size of the urchin's gametes are very similar to those of human gametes. This would extrapolate to an estimate for the sea urchin of 0.8 % number of cells that a human does (since an average sea urchin weight 1 lb).


Giant deep-sea worms (tube worms)

Tubeworms live 1 - 3 km under water in cold environments. Some are thought to live 1000 years old or more. Tubeworms live in groups forming bushes as large as cars living almost 2 km (~6,000 feet) underwater. Similar to the hydra, the tubeworm has a central body structure (the trophosome) that houses  tube contains its actively dividing cells. There are both adult stem cell-type cells (bacteriocytes) and neoblasts (21, 22, 23). Given that the former make up 70% of the trophosome's volume, it is fair to say that at least 50% of the the organism is made up of stem cells.


Corals

Corals are made up of colonies of animals. They can live for thousands of years. The interesting fact about coral stem cells is that they seem to be multi-potent their whole lifespan (25) and seem to be distributed through the coral body (26). This allows coral fragments to regenerate into new corals (26) . 

Any of those 5 animals have at least 1,000 x as many stem cells as humans. And, clearly, creatures with a significant portion of their body made up of stem cells seem to live thousands  of years (see table above). 

Jellyfish - some species are virtually immortal

Jellyfish - regenerative cells and immortality

Long lived animals have the ability to transform specialized cells into  stem cells

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Animals with extreme longevity (hydra, corals, sponges, tube worms, etc) have another trick up their sleeve. Not only do they have an abundance of stem cells; they are also able to transform one cell into another. Not enough stem cells? No problem. 


Jellyfish

I did not mention jellyfish in the table above, as it is unclear whether there are  functional stem cells within jellyfish (27). But in terms of extreme longevity, some species of jellyfish are thought to be immortal. The Turritopsis dohrnii species can change from an adult back to an infantile stage when threatened and repeat that cycle again and again. Even more impressive, the mature cells of the jellyfish can dedifferentiate into an immature cell type (24, 27). 

Dedifferentiation is a key strength of animals that are able to regenerate significant parts of their body (5, 24, 27, 28). This gives the organism's cells incredible plasticity. Interestingly, animals that have high regenerative capacity also exhibit high longevity (see table in this article). Thus,  some turtles, salamanders, certain fish species (rougheye rockfish), some flatworms, quahog clams, and red sea urchins don't show signs of aging (29, 30, 31). 

Dedifferentiation, Regeneration, and Extreme longevity

All the invertebrates described above are aquatic creatures. Their stem cells constitute a significant part of the animal's body and are directly in charge of regeneration of tissues as well as asexual reproduction (5).  Clearly animals that have extreme longevity  have a large trove of stem cells and can utilize them - as well as recruit other cells - to their heart's content.

Next article: How to have extreme longevity: 3. Keep adult stem cells young

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How do animals with extreme longevity keep their stem cells vibrant?


This is the subject of the next article, the last one of the series. This series of articles sets the stage for describing elements that keep stem cells healthy and vibrant - the key to a longer and healthier life.

Picture credits:

  1. Kateryna Kon. Dividing stem cells, Shutterstock, Illustration ID: 2053227584. 
  2. Choksawatdikorn. Hydra is a genus of small, fresh-water animals of the phylum Cnidaria and class Hydrozoa.. Shutterstock.com, Stock Photo ID: 634524308.
  3. brewbooks. Red sea urchin. Mesocentrotus franciscanus. FromFlickr.com, taken on June 15, 2018.
  4. Richard. JellyFish. Flickr.com, taken on Jan 26, 2013. 

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