Generation of keratinocyte stem-like cells from human fibroblasts via a direct reprogramming approach

Skin repair and reconstruction are important after severe wound and trauma. Keratinocyte stem cells (KSCs) in the basal layer of the epidermis can regrow the stratified epidermis but are almost depleted after skin injury. Thus, generating enough KSCs is indispensable for skin regeneration. Pluripotent stem cells such as ESC and iPSC can differentiate into KSCs, but their applications are challenged by ethical issues and risks of tumor formation. Lineage reprogramming from one cell type into another one makes it feasible to generate the desired cell type. Here, we develop a method to convert human fibroblasts into induced keratinocyte stem-like cells (iKSC) by coupling transient expression of reprogramming factors with a chemically defined culture medium, without the formation of iPSC. iKSC resemble normal KSC in the morphological and phenotypic features and can differentiate in vitro and regenerate stratified epidermis after transplantation in vivo. Therefore, iKSC may provide abundant cellular sources for skin repair and regeneration.

Model on cell movement, growth, differentiation and de-differentiation: reaction-diffusion equation and wave propagation

We construct a model for cell proliferation with differentiation into different cell types, allowing backward de-differentiation and cell movement. With different cell types labeled by state variables, the model can be formulated in terms of the associated transition probabilities between various states. The cell population densities can be described by coupled reaction-diffusion partial differential equations, allowing steady wavefront propagation solutions. The wavefront profile is calculated analytically for the simple pure growth case (2-states), and analytic expressions for the steady wavefront propagating speeds and population growth rates are obtained for the simpler cases of 2-, 3- and 4-states systems. These analytic results are verified by direct numerical solutions of the reaction-diffusion PDEs. Furthermore, in the absence of de-differentiation, it is found that, as the mobility and/or self-proliferation rate of the down-lineage descendant cells become sufficiently large, the propagation dynamics can switch from a steady propagating wavefront to the interesting situation of propagation of a faster wavefront with a slower waveback. For the case of a non-vanishing de-differentiation probability, the cell growth rate and wavefront propagation speed are both enhanced, and the wavefront speeds can be obtained analytically and confirmed by numerical solution of the reaction-diffusion equations.

Uncontrolled growth resulting from dedifferentiation in a skin cell proliferation model

By introducing a small backward dedifferentiation probability of postmitotic cells to progenitor cells in a recently proposed skin cell proliferation model, the homeostasis of the system can be disrupted resulting in uncontrolled growth. It is found that when the dedifferentiation probability exceeds a small critical value, the stable fixed point of the system vanishes leading to unlimited cell growth resembling scenarios in carcinogenesis. Explicit expression for the critical dedifferentiation probability and phase diagram are calculated analytically and the associated nonlinear dynamics is analyzed. In the presence of stochastic fluctuations, our model predicts that the escape rate from homeostatic growth to uncontrolled growth is greatly enhanced by a small but finite dedifferentiation probability. These results are verified by numerical solutions of the dynamical system and chemical Langevin equations.