Much has been made of the Hayflick limit for fibroblasts in cell
culture. For my own purposes human fibroblasts beyond 35 doublings in
cell culture are unusable, and I have seen cells grow "old" many
times. Although this senescence is ascribed as an intrinsic aspect of
these cells, I can't help but wonder if growing a single cell type in a
monolayer in media which dosen't present diurnal and shorter variations
in temperature, nutrients, hormones, immune cell survelence, trophic
factors (both circulating and directly delivered), all these and probably
a few more might actually obscure the true potential of these cells.
We don't know all the factors which lead to the differentiation
of pleupotent stem cells into fibroblasts, some of which apparently need
be delivered by cell-to-cell contact. The cells which display a Hayflick
limit may in fact be cells which need their enviromental clues for
maintenance, and in culture they are just coasting, slowly coming to a
stop.
I work on getting parasitic protozoa which infect humans cells to grow
in cell culture. It is painfully obvious that the conditions we can
generate in culture don't match those in vivo.
I am making this posting to argue that the Hayflick limit may be
an artifact of incomplete knowledge of cell vitality requirements. Then
again, the decline in the supply of those requirements in the whole
animal may be a part of the "ageing process".
Edward C. Krug e-mail =kruged at essex.hsc.colorado.edu
Univ. of Colorado Med. School
