Embryonic stem cells (ES cells) are obtained by extracting cells from very early embryos — at the blastocyst (hollow ball) stage — and growing them in laboratory dishes. Human ES cells are generated mainly from blastocysts that are the result of in vitro fertilization for assisted reproduction, but are not needed for implantation into the mother. In some countries, such as Britain and the United States, the parents can donate these 'spare' blastocysts for medical research.

Adult stem cells are also called tissue-specific stem cells because each type of adult stem cell produces only a limited set of specialized cells characteristic of a particular tissue — epidermis, blood, and so on. In adults, tissue-specific stem cells are located throughout the body. The so-called hematopoietic stem cells in bone marrow and umbilical cord blood, which make all the different types of blood cells, are the easiest to isolate, and have been used in therapy for decades — as bone marrow transplants for diseases such as leukemia, where the normal development of blood cells has gone awry.

Other types of tissue-specific stem cells are usually found deep within tissues and are harder to get at and harder to study, especially in humans. Familiar examples are the epidermal stem cells which continually renew the outer layer of the skin as it gets worn away, and the epithelial stem cells in the gut, that are similarly continually replacing the gut lining. More recent discoveries are of bronchoalveolar stem cells from the lungs of adult humans, which are thought to renew the lining of parts of the lungs, for example. And adult stem cells have been found in the inner ear in mice that could be involved in renewing cells involved in balance sensing. But even when cells are discovered that appear to behave like stem cells when grown in the laboratory, it's hard to know whether they act like stem cells in the body, because their natural behavior is hard to observe.

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