Physiological Changes: Stem Cells

The mesodermal layer in the embryo is composed of cells that will divide many times as an organism grows, and the cellular descendants will eventually differentiate into unique cell types that fabricate very diverse mesenchymal tissues. Thus, from a uniform population of mesenchymal cells, unique differentiation pathways give rise to mesenchymal tissues which differ greatly in morphology and function. In addition, these tissues have unique shapes and unique chemistries. The important aspects of the complex molecular signaling that determines the shapes and sizes of various tissues will not be considered here, but the concept of a multichoice differentiation pathway—for example, bone versus cartilage—is essential to understand with respect to neonatal and adult life.

In the embryo, as in the adult, a common mesenchymal progenitor cell gives rise to either bone or cartilage. This cell could also become any other differentiated mesenchymal cells, and is thus called a mesenchymal stem cell. The local molecular microenvironment controls whether mesenchymal stem cells divide, whether they go down one differentiation pathway or another (e.g., bone versus cartilage), whether they are quiescent, or whether they expire. This global scheme has been called mesengenesis, and it is pictured in Figure 1, with the mesenchymal stem cell at the top and each differentiation pathway separated and depicted in an over-simplified sequence of lineage steps. The molecules and microenvironments that control these events in the embryo and in the adult are not fully known.