Figure 3.

Processes affecting the distribution of ALP activity n(a, t) in the population of hMSCs. (A) When a cell with ALP activity a + a′ divides, it is replaced by two new cells with ALP activity a and a′. The colored circles represent the level of the ALP activity of the cell, according to the color scale. High level of the ALP activity is represented by red color, whereas no ALP activity is represented by black color. Such divisions occur at the rate $k_d(a,a^{\prime })$ and result in the stem cell proliferation. (B) A cell can instantaneously lose detectable ALP activity because of terminal osteogenic differentiation into an osteoblast. Such processes occur at the rate k_f(a). (C) ALP activity in a cell can increase due to intracellular ALP synthesis, denoted by influx s_i(a). (D) ALP activity in a cell can decrease due to intracellular ALP degradation, denoted by out-flux d_o(a).