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--------------------------- Stem cell: An ancestral cell, the most primitive type of cell. Stem cells are relatively unspecialized (undifferentiated) cells that are characteristically of the same family type (lineage). They retain the ability to divide throughout life and give rise to cells that can become highly specialized and take the place of cells that die or are lost. Stem cells thus contribute to the body's ability to renew and repair its tissues. Unlike mature cells, which are permanently committed to their fate, stem cells can both renew themselves as well as create new cells of whatever tissue they belong to (and other tissues). Bone marrow stem cells, for example, are the most primitive cells in the marrow. From them all the various types of blood cells are descended. Bone marrow stem-cell transfusions (or transplants) were originally given to replace various types of blood cells. Stem cells from bone marrow can also, quite remarkably, give rise to non-marrow cells. In a 1999 report in the journal Nature, scientists from Boston led by Dr. Louis M. Kunkel reported that they gave bone marrow transplants from normal mice to dystrophic mice. Some 12 weeks later about 10% of the muscle fibers in the diseased animals were making the correct form of dystrophin, the protein that is defective in Duchenne muscular dystrophy. This work suggests that bone marrow stem cells may offer new ways of treating muscular dystrophy (and other non-blood diseases). --------------------- Totipotent: capable of developing into a complete organism or differentiating into any of its cells or tissues <totipotent blastomeres> --------------------- Pluripotent: not fixed as to developmental potentialities : having developmental plasticity <pluripotent stem cell> -------------------- Gamete:The sperm or egg. In humans, the gametes normally have 23 chromosomes. ------------------- Sperm: A sperm is the male "gamete" or sex cell. It combines with the female "gamete," called an ovum, to form a zygote. The formation process is called "fertilization." (see ovum, zygote). ------------------- Ovum: An ovum is an egg that exists in the ovary of the female. This egg is called the female "gamete" or sex cell. It combines with the male gamete, called a sperm, to form a zygote. This formation process is called "fertilization." (see sperm, zygote). ---------------------- Blastula: an early metazoan embryo typically having the form of a hollow fluid-filled rounded cavity bounded by a single layer of cells --------------------- Zygote: The cell formed by the union of a male sex cell (sperm) and a female sex cell (an ovum). The zygote develops into the embryo as instructed by the genetic material within the unified cell. The unification of a sperm and an ovum is called fertilization. --------------------- Developmental Biology Embryo • 1. the stage of a multicellular organism that develops from a zygote before it becomes free-living. 2. specifically, in vertebrates, the period from after the long axis appears until all major structures are represented. In humans, this is from about two weeks after fertilization to the end of the seventh or eighth week. --------------------------- Embryo: The organism from fertilization to, in humans, the beginning of the third month of pregnancy. After that point in time, it is termed a fetus. ---------------------------- Fetus: The unborn offspring from the end of the 8th week after conception (when the major structures have formed) until birth. Up until the eighth week, the developing offspring is called an embryo. ---------------------------- Bob Martone [log in to unmask] http://www.samlink.com/~bmartone