stem cells, unspecialized human or animal cells that can produce mature specialized body cells and at the same time replicate themselves. Embryonic stem cells are derived from a blastocyst (the blastula typical of placental mammals; see embryo), which is very young embryo that contains 200 to 250 cells and is shaped like a hollow sphere. The stem cells themselves are the cells in the blastocyst that ultimately would develop into a person or animal. "Adult" stem cells are derived from the umbilical cord and placenta or from blood, bone marrow, skin, and other tissues. The similar embryonic germ line cells come from a fetus that is 5 to 9 weeks old and are derived from tissue that would have developed into the ovaries or testes.

Medical researchers are interested in using stem cells to repair or replace damaged body tissues because stem cells are less likely than other foreign cells to be rejected by the immune system when they are implanted in the body. Embryonic stem cells have the capacity to develop into every type of tissue found in an adult; germ line cells and adult stem cells are less versatile. The processes that control such development, however, are not understood at present. Stem cells have been used experimentally to form the hematopoietic (blood-making) cells of the bone marrow; heart, blood vessel, muscle, and insulin-producing tissue; and sperm cells. Embryonic germ line cells have been used to help paralyzed mice regain some of the ability to move. Since the 1990s umbilical cord blood stem cells have sometimes been used to treat heart and other defects in children who have rare metabolic diseases and to treat children with certain anemias and leukemias. It has been shown that stem cells from this blood can migrate to damaged tissues and repair them.

Human stem cells have typically been extracted from surplus fertilized embryos produced during in vitro fertilization procedures. Some experimenters, however, have used embryos that were fertilized especially to produce stem cells. In so-called therapeutic cloning a nucleus from a patient's body cell, such as a skin cell, would be inserted into an egg that has had its nucleus removed to produce a blastocyst whose stem cells could be used to create tissue that would be compatible with that of the patient. Such a procedure was reported in 2005 to have been successfully undertaken in part by South Korean researchers who produced stem cell lines using genetic material from patients, but the data was subsequently shown to have been fabricated. (It was later determined, however, that the laboratory had produced stem cells using an egg that had developed through parthenogenesis, which does not involve fertilization or result in a viable human embryo.) Because extraction of embryonic stem cells destroys the embryo, the use of embryonic stem cells has been opposed by opponents of abortion. Japanese researchers used viruses in 2007 to transfer genes to human skin cells and induce those cells to become stem cells, and U.S. researchers have used similar induced stem cells in mice experimentally to treat sickle-cell disease.

The first embryonic stem cells to be isolated were extracted by British researchers from mouse blastocysts; the first human stem cells isolated and cultured were extracted by American scientists in 1998. In 1994 a National Institutes of Health (NIH) panel argued that creating human embryos for use in certain experiments might be justified, but Congress subsequently enacted (1995) a ban on federal financing for research involving human embryos in reaction to that report. The Dept. of Health and Human Services ruled in 1999, however, that that ban did not apply to financing work with stem cells, and guidelines for financing such research were issued by NIH the next year.

President George W. Bush, who had campaigned against financing embryonic stem cell research, announced in Aug., 2001, that he would support federal funding of research with embryonic stem cells, but only with the estimated 60 stem cell lines then existing. Some scientists challenged the assumption that these 60 stem cell lines would be sufficient for experimental and therapeutic needs, while others said the figure included some stem cell lines that had not yet been determined to be viable. In fact, in 2004, there were only 15 approved stem cell lines available to researchers funded by the U.S. government. The restrictions have not prevented other researchers, in the United States and elsewhere, from developing new embryonic stem cell lines and undertaking research with them using private funding, and California voted (2004) to create a $3 billion fund to underwrite embryonic stem cell research. A federal legislation that would have expanded the number of stem cell lines available for federally funded research was vetoed by the President Bush in July, 2006.

See also fetal tissue implant.

stem, supporting structure of a plant, serving also to conduct and to store food materials. The stems of herbaceous and of woody plants differ: those of herbaceous plants are usually green and pliant and are covered by a thin epidermis instead of by the bark of woody plants. There is relatively more pith in herbaceous stems, and the cambium, which increases the diameter of woody stems, is usually almost inactive; it is therefore characteristic of herbaceous stems that, although they increase in height, their increase in diameter is small. Most herbaceous plants are annuals; some have specialized underground stems (see bulb, corm, rhizome, and tuber) that store food and enable the plant to survive unfavorable growing conditions. Aerial stems may be specialized as tendrils, thorns, or runners (stolons); another specialization is the fleshy, moisture-retaining stem of many arid-land plants (such as most cacti and other succulents). Aerial stems are usually erect; however, in the climbing plants they require support and in others (e.g., melons) they are prostrate. The vascular system in the stem consists chiefly of xylem (upward-conducting) and phloem (downward-conducting) tissue, usually in vascular bundles arranged concentrically on either side of the cambium—the xylem (wood) inside, the phloem outside. In monocotyledonous plants, which generally lack cambium, the bundles are scattered throughout the stem tissue. The sap ducts are formed of elongated cells joined end to end; in the xylem the cell ends dissolve away completely to form continuous tubes and in the phloem they develop perforations and are called sieve plates. Herbaceous stems are marked externally by leaf and bud nodes; woody stems also bear lenticels (pores for transpiration), scars where leaves, twigs, and fruits have dropped off, and bud scars. The annual extension growth of a woody stem develops from a terminal bud usually protected by bud scales or stipules; when the scales fall away, a characteristic bud scar remains. The sap of certain stems contain gums, latexes, and resins used commercially; many are the source of wood of great economic importance.

brain stem, lower part of the brain, adjoining and structurally continuous with the spinal cord. The upper segment of the human brain stem, the pons, contains nerve fibers that connect the two halves of the cerebellum. It is vital in coordinating movements involving right and left sides of the body. Below the pons and continuous with the spinal cord is the medulla, which transmits ascending and descending nerve fibers between the spinal cord and the brain. The medulla also directly controls many involuntary muscular and glandular activities, including breathing, heart contraction, artery dilation, salivation, vomiting, and probably laughing. The nuclei of some of the nerves that originate in the brain are also located in the brain stem. Nerve fibers in the brain stem do not readily regenerate, hence injury may result in permanent loss of function. See also nervous system.

In living organisms, an undifferentiated cell that can produce other cells that eventually make up specialized tissues and organs. There are two major types of stem cells, embryonic and adult. Embryonic stem cells are located in the inner mass of a blastocyst (an embryo at a very early stage of development), and they eventually give rise to every cell type of the adult organism. Adult stem cells are found in some tissues in the adult body, such as the epidermis of the skin, the lining of the small intestine, and the bone marrow, where they serve in the regeneration of old or worn tissue. In cancer treatment, blood-forming adult stem cells are routinely harvested from bone marrow, stored, and then reinfused into patients to replace blood cells destroyed by chemotherapy or radiation therapy. This potential for replacing damaged tissues has aroused great interest in using embryonic stem cells to treat a number of other conditions, such as Parkinson disease, severe burns, and damage to the spinal cord. Mouse embryonic stem cells are widely used to create genetically modified mice that serve as models for investigating human disease. However, the use of human embryonic stem cells, which requires destroying the blastocysts from which they are obtained, has raised objections by those who feel blastocyst-stage embryos are human beings. The first human stem cell line was created in 1998, using cells harvested from embryos produced through in vitro fertilization. The use of human embryonic stem cells is allowed in some countries and prohibited or restricted in others.

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Plant axis that emerges from the roots, supports the branches, bears buds and shoots with leaves, and contains the vascular (conducting) tissues (xylem and phloem) that transport water, minerals, and food to other parts of the plant. The pith (a central core of spongy tissue) is surrounded by strands (in dicots; see cotyledon) or bundles (in monocots) of conducting xylem and phloem, then by the cortex and outermost epidermis, or bark. The cambium (an area of actively dividing cells) lies just below the bark. Lateral buds and leaves grow out of the stem at intervals called nodes; the intervals on the stem between the nodes are called internodes. In flowering plants, various stem modifications (rhizome, corm, tuber, bulb, stolon) let the plant survive dormantly for years, store food, or sprout asexually. All green stems perform photosynthesis, as do leaves; in plants such as the cacti (see cactus) and asparagus, the stem is the chief site of photosynthesis.

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