Embryonic stem cells can turn into virtually any type of cell in the body. But controlling this process, known as differentiation, is tricky. If embryonic stem cells are left to grow in a tissue-culture dish, they will differentiate more or less at random, into a mixture of different types of cells. The M.I.T. group, led by Ali Khademhosseini, an assistant professor in the Harvard-M.I.T. division of Health Sciences and Technology and a recipient of a TR35 award in 2007, put embryonic stem cells into “building blocks” containing gel that encouraged the cells to turn into certain types of cell.