Related facts that were new to me:<p>The cells of the placenta correspond genetically to the fetus, not the mother (although it meshes with maternal cells, so that the physical organ contains a layer of maternal cells that intermingle with fetal cells).<p><a href="http://www.nsta.org/publications/news/story.aspx?id=53497" rel="nofollow">http://www.nsta.org/publications/news/story.aspx?id=53497</a><p>The division between placental and non-placental cells is, rather sensibly, the first step of specialization in the developing fetus.<p><a href="https://en.wikipedia.org/wiki/Cell_potency" rel="nofollow">https://en.wikipedia.org/wiki/Cell_potency</a><p>The chain of increasing specializatio can be broken up like this:<p>> <i>Totipotent</i> (a.k.a. omnipotent) stem cells can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable organism.[4] These cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg are also totipotent.[5]<p>> <i>Pluripotent</i> stem cells are the descendants of totipotent cells and can differentiate into nearly all cells,[4] i.e. cells derived from any of the three germ layers.[6]<p>> <i>Multipotent</i> stem cells can differentiate into a number of cell types, but only those of a closely related family of cells.[4]<p>> <i>Oligopotent</i> stem cells can differentiate into only a few cell types, such as lymphoid or myeloid stem cells.[4]<p>> <i>Unipotent</i> cells can produce only one cell type, their own,[4] but have the property of self-renewal, which distinguishes them from non-stem cells (e.g. progenitor cells, muscle stem cells).<p><a href="https://en.wikipedia.org/wiki/Stem_cell#Potency_definition" rel="nofollow">https://en.wikipedia.org/wiki/Stem_cell#Potency_definition</a>