Definitions stem cell

In vivo (normal reproduction) or in vitro fertilization (IVF) of ova (female germ cells) and spermatozoa (male germ cells) forms zygotes which contain the total genetic materials, one half from the male DNA and one half from the female DNA. In favourable condition, the zygote  divides and  forms the  blastomere  (8  cells),  and then the blastocyst (120-150 cells) around day five. Blastocysts consist of stem cells. At this stage, division of a blastocyst may produce  two  or  more   normal  human  embryos.  During  the  third  week  of  human development, the primitive streak, which is the primitive central nervous system, appears. At this  stage, the embryo is a unique entity which is no longer twinnable. Some  scientists consider this point as the moment when human life begins as such (Balint, 2001).

Stem cells in blastocysts are capable of differentiating along each of the germ layers of the
ectoderm (skin, nerves, brain), the mesoderm (bone, muscle), and the endoderm  (lungs, digestive system) (Hyun, 2008). After this stage of human foetus  development, stem cells can be also found in different tissues but their capability is limited. For instance, as Lewis (2009) stated, while mesenchymal stem cells are able to produce bone, cartilage, and muscle, bone marrow stem cells can give rise only to white blood  cells. The following part sheds more light on the issue.