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From: lincoln@randvax.UUCP (Tom Lincoln)
Newsgroups: sci.bio,sci.med,comp.ai,sci.misc
Subject: Re: Models of biological aging
Message-ID: <309@iris.randvax.UUCP>
Date: Mon, 22-Jun-87 02:26:19 EDT
Article-I.D.: iris.309
Posted: Mon Jun 22 02:26:19 1987
Date-Received: Thu, 25-Jun-87 06:24:30 EDT
References: <622@unicus.UUCP> <1343@sigi.Colorado.EDU> <1756@ttrdc.UUCP> <300@iris.randvax.UUCP> <5491@think.UUCP>
Reply-To: lincoln@iris.UUCP (Tom Lincoln)
Organization: Rand Corp., Santa Monica
Lines: 42
Xref: mnetor sci.bio:469 sci.med:2502 comp.ai:564 sci.misc:367

In article <5491@think.UUCP> craig@godot.think.com.UUCP (Craig Stanfill) writes:
>> Hayflick limit: In actual practice, it is not thought that any
>> human cell approaches 50 divisions during the human lifetime.
>
>> Is this true even for skin cells? ....that intestinal cells continually
>> regenerate and get sloughed off during the normal digestive process.
>> That's a lot of cell division, or am I mistaken?)
>***
>> 2^50 is a very large number, if every cell had two progeny (which of course
>> they don't)
>***
>
>If I am correctly informed, in the skin, intestines, and other areas of
>the body which are constantly renewed, there is a thin layer of rapidly
>reproducing cells, covered by a layer of non-reproducing but living
>cells, followed by a layer of dead cells.  In essence, after each
>division one daughter will cease to divide and one will continue to
>divide.  Thus, the number of rapidly reproducing cells stays more or
>less constant, and the production of new tissue is steady.  Of course,
>sometimes the mechanism (whatever that may be) malfunctions, producing
>tumors of various sorts.

What happens is somewhat inbetween. Blood cells are the best example of
a system that has a very active cell cascade, where stem cells divide ..
and one waits.. as you suggest.. but then the other divides a number of
times (about 6) with all progeny headed for mature cells. The rate of
these divisions are modulated by numerous factors.

Skin cells and gut cells behave in a somewhat similar way, with bursts of
activity and multiple divisions when the erosions become greater than
normal. The rest of the time a process is in force that is linear in
overall impact, but consists, nevertheless, in bursts of cascaded growth.

As Sean Eddy has noted in another article, 2^60 would cover all
contingencies, so more than 60 divisions, the upper Haflick limit need
not be violated.

 p q
 \|/
 /|\   TOM LINCOLN  lincoln@rand-unix.ARPA
 \|/  "Life is short, art is long, opportunity fugitive,
 /|\   experimenting dangerous, reasoning difficult."