Many of the signals required to induce formation of specialized adult cells must be present in these tumors, but unlike embryos, tumors generate adult cell types in a hopelessly undirected manner.
If a developing embryo is not to end up a mass of disorganized tissues, it must do more than generate adult cell types. Embryos must orchestrate and choreograph an elaborate stage production that gives rise to a functional organism. They must direct intricate cell movements that bring together populations of cells only to separate them again, mold and shape organs through the birth of some cells and the death of others, and build ever more elaborate interacting systems while destroying others that serve only transient, embryonic functions. Throughout the ceaseless building, moving, and remodeling of embryonic development, new cells with unique characteristics are constantly being generated and integrated into the overall structure of the developing embryo. Science has only the most rudimentary understanding of the nature of the blueprint that orders embryonic development. Yet, recent research has begun to illuminate both how specific adult cells are made as well as the central role of stem cells in this process.
The term "stem cell" is a general one for any cell that has the ability to divide, generating two progeny (or "daughter cells"), one of which is destined to become something new and one of which replaces the original stem cell. In this sense, the term "stem" identifies these cells as the source or origin of other, more specialized cells. There are many stem cell populations in the body at different stages of development. For example, all of the cells of the brain arise from a neural stem cell population in which each cell produces one brain cell and another copy of itself every time it divides. The very earliest stem cells, the immediate descendants of the fertilized egg, are termed embryonic stem cells, to distinguish them from populations that arise later and can be found in specific tissues (such as neural stem cells). These early embryonic stem cells give rise to all the tissues in the body, and are therefore considered "totipotent" or capable of generating all things.
Review of the Research
While the existence of early embryonic stem cells has been appreciated for some time, the potential medical applications of these cells have only recently become apparent. More than a dozen years ago, scientists discovered that if the normal connections between the early cellular progeny of the fertilized egg were disrupted, the cells would fall apart into a single cell suspension that could be maintained in culture. These dissociated cells (or embryonic stem cell "lines") continue to divide indefinitely in culture. A single stem cell line can produce enormous numbers of cells very rapidly. For example, one small flask of cells that is maximally expanded will generate a quantity of stem cells roughly equivalent in weight to the entire human population of the earth in less than sixty days. Yet despite their rapid proliferation, embryonic stem cells in culture lose the coordinated activity that distinguishes embryonic development from the growth of a teratoma. In fact,