an egg and sperm cell fuse to form a zygote, a single new cell which divides and becomes a hollow ball of cells- embryo. inner cells of this ball are embryonic stem cells that differentiate to form all of specialised cells of body. an adult stem cell is undifferentiated cell of organism that can give rise to many more cells of same type- bone marrow is good source of it.
many differentiated cells can divide to replace themselves but some tissues cannot do this and stem cells can stay in these tissues for years only needed if cells are injured or affected by disease then start dividing to replace different types of damaged cell
people with type 1 diabetes have to inject themselves with insulin every day because specialised cells in pancreas do not work
treatment with stem cells may be able to help conditions such as paralysis and diabetes - embryonic stem cells may be be able to grow into almost any different type of cell needed in body
stem cells from plant meristems can be used to make clones of mature parent plant very quickly and economically- gives us way of producing large numbers of rare plants reliable and safely. plant cloning also gives way of producing large populations of identical plants for research to observe effects of changing variables on genetically identical individuals
cloning large numbers of identical plants from stem cells in meristems is also widely used in horticulture and in agriculture it is used to produce large numbers of identical crop plants with special features like disease resistance
