Fetal stem cells: betwixt and between.

Fetal stem cells can be isolated not only from fetal blood and hemopoietic organs in early pregnancy, but from a variety of somatic organs as well as amniotic fluid and placenta throughout gestation. Fetal blood is a rich source of hemopoietic stem cells, which proliferate more rapidly than those in cord blood or adult bone marrow. First-trimester fetal blood, liver, and bone marrow also contain a population of mesenchymal stem cells, which appear to be more primitive with greater multipotentiality than their adult counterparts. Fetal stem cells may thus represent an intermediate cell type in the current debate focusing on dichotomized adult versus embryonic stem cells, and thus prove advantageous as a source for downstream cell therapy applications. They have also been implicated in fetomaternal trafficking in pregnancy, and in long-term microchimerism in postreproductive women.