Stem cell proliferation versus differentiation and development. Stem cells must strike a balance between self-renewal and differentiation. (a) Some possible cell division patterns that allow for replenishment of stem cells and production of some differentiated cells. Each cell may produce one stem cell and one differentiated cell, or two differentiated cells or two stem cells in defined parts of the tissue or culture. Alternatively, a gradient of growth conditions can be established, with cell fates differing from one end of the gradient to the other. (b) Establishing a developmental niche through stem cell contact with a cell or group of cells. Molecular signals (dashed arrows) provided by niche cells (in this case, for plants, a distal tip cell) help orient the mitotic spindle for stem cell division and ensure that one daughter cell retains stem cell properties.