[Frontiers in Bioscience 14, 210-224, January 1, 2009] 210 Stromal cells Muriel Vayssade, Marie-Danielle Nagel Domaine Biomateriaux-Biocompatibilite, CNRS UMR 6600, Universite de Technologie de Compiegne, BP 20529 60205 Compiegne Cedex, France TABLE OF CONTENT 1. Abstract 2. Introduction 3. Interactions between stromal cells and their environment, and cell-cell interactions in the real in vivo environment 3.1. Stromal cells 3.1.1. Denominations used to refer to stromal cells 3.1.2. Sources of stromal cells 3.1.3. Characterization of stromal cells 3.2. Differentiation pathways 3.3. In vivo functions of stromal cells 3.3.1. HSC differentiation 3.3.2. Host immune response 3.3.3. Homing mechanisms 4. Interactions between stromal cells and materials : biohybrid systems 4.1. Materials 4.1.1. Natural materials 4.1.1.1. Titanium 4.1.1.2. Ceramics 4.1.1.2.1. Hydroxyapatite 4.1.1.2.2. Tricalcium phosphate 4.1.1.2.3. Animal bone or skeleton 4.1.1.3. Biomolecules 4.1.1.3.1. Hyaluronic acid 4.1.1.3.2. Matrigel 4.1.1.3.3. Collagen 4.1.1.3.4. Silk 4.1.1.3.5. Alginate 4.1.1.3.6. Chitosan 4.1.2. Synthetic materials 4.2. Bioreactors used to culture stromal cells 5. Conclusions 6. References 1. ABSTRACT Stromal cells, or mesenchymal stem cells, are adherent clonogenic cells that can form colonies. They are mainly isolated from bone marrow but can also be found in umbilical cord blood, adipose tissues and amniotic fluids. These stem cells are easy to culture in vitro, and can differentiate into osteoblasts, chondrocytes, or adipocytes when stimulated appropriately. When seeded on a natural (titanium, ceramics, collagen fibers, silk, etc.) or synthetic (PLLA, PLGA, etc.) biomaterial scaffold, they adhere and differentiate to form a new tissue. Many studies have also explored stromal cell differentiation in bioreactors to form a 3-dimensional culture. This review focuses on the biomaterials used for tissue engineering with stromal cells. 2. INTRODUCTION Mammalian bone marrow contains two types of adult stem cells. The first, the hematopoietic stem cells (HSC), are responsible for producing blood cells throughout life, and gives rise to different lineages. The second type are the stromal or mesenchymal stem cells. Although less well characterized than HSC, stromal cells were first studied because of their important role in formating the hematopoietic microenvironment (1). We now know that stromal cells have the potential to differentiate to form skeletal tissues, neural tissues, muscles, etc. Recent findings indicate that stromal cells seeded on appropriate scaffolds can be used to regenerate certain tissues. The natural or synthetic scaffold provides the