Skin stem cell hypotheses and long term clone survival--explored using agent-based modelling

Abstract

Epithelial renewal in skin is achieved by the constant turnover and differentiation of keratinocytes. Three popular hypotheses have been proposed to explain basal keratinocyte regeneration and epidermal homeostasis: 1) asymmetric division (stem-transit amplifying cell); 2) populational asymmetry (progenitor cell with stochastic fate); and 3) populational asymmetry with stem cells. In this study, we investigated lineage dynamics using these hypotheses with a 3D agent-based model of the epidermis. The model simulated the growth and maintenance of the epidermis over three years. The offspring of each proliferative cell was traced. While all lineages were preserved in asymmetric division, the vast majority were lost when assuming populational asymmetry. The third hypothesis provided the most reliable mechanism for self-renewal by preserving genetic heterogeneity in quiescent stem cells, and also inherent mechanisms for skin ageing and the accumulation of genetic mutation.

Figure 1. Illustration of the three division hypotheses.

(a) Keratinocytes represented by spheres. (b) The three division hypotheses are illustrated. The strength of adhesion to the basement membrane is qualitatively represented by green lines resembling adhesive molecules (e.g. β1 integrin). In the asymmetry division case, the adhesive strength reduces progressively after each division.

Figure 2. Agent based model of the epidermis.

The initial seeding (day zero) and the mature epidermis (three years) are shown above. Cells shown in yellow are preparing for mitosis. The mature epidermis has similar appearance across the three scenarios with individual corneocytes shedding on top.

Figure 3. Evolution of a single lineage over three years in scenarios (1) and (3).

In the asymmetric division case, the offspring forms a column around the stem cell similar to that described as an EPU. The stem cell is surrounded by several progeny in the basal compartment. The differentiated cells are localised in this area. In contrast, individual colony shape changes dramatically in the PAS hypothesis, with substantial lateral movements.

Figure 4. Asymmetric division colony dynamics.

(a) The evolution of four largest colonies at the end of year three with the total number of cells (blue) plotted along the secondary y-axis. (b) The mean colony size (n = 38 colonies) during the first three months of development. (c) Colony size over three years. Error bars show standard deviation.

Figure 5. Colony dynamics for populational asymmetry.

(a) The evolution of four remaining colonies over three years. The total number of cells (blue line) is plotted along the secondary y-axis. (b) The percentage of colonies lost during the first year. (c) The cumulative plot of colonies lost during the first year.

Figure 6. Colony dynamics for the PAS hypothesis.

(a) Colonies lost during the first year. (b) At year three, a small wound was created in the stratum corneum, the simulation was continued for another year. (c, d) The evolution of active (c) and quiescent (d) colonies is shown in Regions 1 and 2. The blue line shows the total number of cells along the secondary y-axis. The response of the active and quiescent lineages post-wounding is shown in Region 3. Post-wounding activities: (c) Three of the originally active colonies (pink, red and brown) entered quiescent state post-wounding, one was lost, and the rest remained active. (d) Several quiescent colonies were reactivated. Two remained active (pink and yellow) after one year and one (green) was lost.

Figure 7. Flowchart of the model algorithm.

The user defines the basement membrane area and the number of cells introduced. Each cell contains variables (shown in the black box), which include an identification number, location (randomly generated), cycle position (randomly generated) and cell type (user defined). Each cell goes through a set of rules for division and differentiation. The physical solver then resolves the forces between neighbouring cells.