Fibroblasts are mesenchymal cells which come from the embryonic mesoderm. They are useful for a wide range of cellular and molecular studies as they are one of the easiest types of cells to grow in culture. Their durability also makes them amenable to a wide variety of manipulations ranging from studies employing gene transfection to microinjection. Fibroblasts secrete a non-rigid extracellular matrix which is rich in type I and/or type III collagen. Fibroblasts in various parts of the body are intrinsically different and display different regulatory functions on surrounding cells. For example, splenic fibroblasts regulates the development of NK cells from blood progenitors through the expression of bioactive IL-15. Human Splenic Fibroblasts are also useful tools to stablish in vitro disease models for High Throughput and High Content Screening.