Direct Differentiation of Human Embryonic Stem Cells toward Osteoblasts and Chondrocytes through an Intermediate Mesenchyme Progenitor Lineage

Abstract

The recent establishment of in vitro cultures of human embryonic stem cells (hESC) from preimplantation embryos has provided a biologically relevant in vitro model for studying early human embryonic development and the signals involved in early stages of cellular commitment to different lineages. In addition, hESC represent a good source of an unlimited supply of cells that can generate clinical-grade, transplantable, lineage-specific cells to treat a large number of skeletal diseases including bone and cartilage diseases (e.g. localized bone and cartilage defects, non-healing fractures and systemic age-related degenerative conditions such as osteoporosis and osteoarthritis) (for review, [1], [2]). Typically, the basic in vitro methods for differentiating hESC into specific cell lineages are based on three procedures; 1) direct differentiation as a monolayer on extracellular matrix substrates, 2) differentiation in co-culture with primary tissue derived cells, and 3) the formation of 3-D spherical structures in suspension culture, termed embryoid bodies (EBs) [3-6]. However, developing clinically relevant protocols for directing the differentiation of hESC into definitive, homogenous populations of osteogenic or chondrogenic cells still faces several challenges due to the complexity of pathways that are conserved between embryonic and hESC differentiation for all cell types of the three germ layers. Our group, and others, have employed several strategies to first, generate mesenchymal progenitor cells (termed MSC) from hESCs demonstrating the characteristic phenotype of adult bone marrow-derived mesenchymal stem cells (BM-MSC), and second differentiate these cells into homogenous populations of osteoblast or chondrocytes by using protocols established for adult MSC. Thus, our objective in this chapter is to provide an overview on the majority of published protocols for derivation of osteoblasts and chondrocytes from hESC through an intermediate mesenchymal progenitor lineage.