Derived from a group of cells called inner cell mass, which is part of the early (4-5 day) embryo called blastocyst.The blastocyst is the stage of embryonic development prior to implantation in the uterine wall. At this stage , the embryo is made up to 150 cells and consists of a sphere made up of an outer layer of cells (called trophectoderm), a fluid filled cavity (called blastocoel) and a cluster of cells on the interior part ( called inner cell mass). Embryonic-like stem cells called embryonic germ cells can also be derived from primordial germ cells of the developing fetus. Human embryonic germ cells are derived from primordial germ cells , which occur in a specific part of the fetus called gonadal ridge

A 5 day blastocysts are used to derive embryonic stem cells. For deriving ES cell cultures, the outer layer is removed, and inner cell mass is obtained.When ES cells cultured ,pluripotent stem cells grow.A pluripotent stem cell is a kind of cell that is capable of self renewal and of differantiating into most of the cells of the body.Human ES cells and human EG cells are capable of long term self renewal, while retaining its properties.
Human embryonic germ cells almost have similar properties with embryonic stem cells but differ.In both cases, the cells replicate for an extended period of time,show no chromosomal abnormalities , generate both XX and XY cultures and express a set of markers regarded as charecteristic of pluripotent cells. They both spontaneously differentiate into tissues ,derivates of all three primary germ layers,endoderm, mesoderm and ectoderm when culture conditions are adjusted.

However, the ES cells ,derived from human blastocycts and EG cells, derived from embryonic germ cells vary with respect to their growth charecteristics in vitro,and their behaviour in vivo. Human ES cells also have been propogated for approximately two years in vitro,for several hundred population doublings,whereas human embryoid body-derived cells from cultures of embryonic germ cells have been maintained for only70 to 80 population doublings .Also human ES cells will generate teratomas containing differantiated cell types ,
if injected into immunocompromised mice colonies,while human EG cells will not.

The potential Use of Embryonic Stem Cells: Disesases that might be treated by transplanting human ES-derived cells include Parkinson’s disease, diabetes, traumatic spinal cord injury, Purkinje cell degeneration, Duchenne’s muscular distrophy,heart failure ,and osteogenesis imperfecta. However, for treatments,ES cells must be directed to differantiate into specific cell types prior to transplant. Additionally, at this stage, any therapies based on the use of human ES cells are stil hypothetical and highly experimental.