Differentiation of fibroblast stem cells

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Abstract
Primary human skin fibroblasts derived from the abdomen of 45 female donors of the four age groups 1–20, 20–40, 40–60, and 60–80 years were studied in primary explant, in primary low-density mass cultures, and in primary clonal populations in vitro. As a function of the age of the donor, primary mitotic and postmitotic fibroblasts in the three primary cell systems analysed represent heterogeneous populations with reproducible changes in the proportions of the mitotic fibroblasts MF I, MF II, MF III, and postmitotic fibroblasts PMF IV, PMF V, PMF VI, and PMF VII. These findings make it very likely that equivalent cell types exist in the connective tissue of skin in vivo, and that these cells undergo reproducible changes in the proportions of the mitotic and postmitotic counterparts in vivo as a function of the age of the donor. Secondary mitotic human skin fibroblast populations of the cell line HH-8 in vitro underwent 53.6 ± 6.0 cumulative population doublings (CPD) in 302 ± 27 days. If appropriate methods are applied, mitotic fibroblasts differentiate spontaneously into postmitotic fibroblasts which are kept in stationary cultures for up to 305 ± 41 additional days. As a function of the CPD level and of the duration of stationary culture, secondary mitotic and postmitotic fibroblast populations are heterogeneous populations with reproducible changes in the proportions of mitotic fibroblasts MF I, MF II, and MF III, and postmitotic fibroblasts PMF IV, PMF V, PMF VI, and PMF VII. The seven secondary fibroblast cell types show differentiation-dependent and cell-type specific patterns of [35S]methionine polypeptides in total soluble cytoplasmic and nuclear proteins, in secreted proteins, and in membrane bound proteins. These findings make it very likely that the morphologically recognizable primary and secondary fibroblasts differentiate spontaneously along a seven stage terminal cell lineage MF I – MF II – MF III – PMF IV – PMF V – PMF VI – PMF VII in three compartments of the fibroblast stem cell system.