In the previous part of this series, I talked about why some species might become ageless or retain that ability. This part of the series attacks a misinterpreted idea about “natural limits” to cell division and repair. Some of you may have heard about the Hayflick limit for cell divisions.
Let us refresh our memories-
The Hayflick limit is the number of times a normal and differentiated cell population will divide before it stops, presumably because the telomeres reach a critical length.
Irrespective of what you think about this limit for cell divisions, two issues about this ‘limit’ are almost never discussed.
1] It best applies to, and has been studied most extensively in, differentiated mammalian cells.
Another way of putting that is- the hayflick limit has been poorly studied in long-lived reptilian, archosaurian and other ageless species.
2] Even in mammals, it really applies to terminally DIFFERENTIATED cells, not stem cell (embryonic or tissue) or transformed (cancerous) cells.
So what are the implications of these two issues, as far as longevity is concerned?
The first is the most obvious- namely that species other than terrestrial mammals likely have terminally differentiated tissue cells that can (and do) exceed the Hayflick limit. Indeed, this has been documented for terminally differentiated tissue cells from lizards, turtles and tortoises. Research_Proposal_with_References_2001
The second possibility is that even simply increasing the number of tissue stem cells (semi-differentiated) could potentially slow down or stop aging in mammals- as long as we had ways to stop them from becoming cancerous. It is worth noting that ageless animal species have a negligible incidence of cancer INSPITE of superb mechanisms for tissue growth, regeneration and repair. It seem that their repair systems are either error-free or they have additional mechanisms to stop malignant transformations.
The implications of this for increasing human longevity are easy to see, and almost any method to boost the levels of tissue stem cells (semi-differentiated) could in themselves decrease tissue aging in most tissues (muscle, skin, flesh, organs, blood vessels, testes and even the brain). The eye and the ovaries are a tougher nut to crack because there are significant differences amongst these two organs between mammals and non-mammalian species.
So what is money? Paper and combinations of electrons in computer memory? What is the cost of printing more money?