Formation of the arterial media during vascular development.

We are still in the earliest stages of studying the molecular biology of vascular development. Key questions, even simple questions, such as the origin of endothelial precursors from the epiblast or the mesoderm, remain largely unanswered. For the smooth muscle cell, we do not even have a satisfactory molecular definition of cell type, because the known cell type-specific markers generally disappear when these cells are placed in vitro, and even in vivo smooth muscle cells identified by location can be very undifferentiated. A few bright spots illuminate this cloudy prospect. We do have endothelial lineage markers, and, given the powerful tools of promoting analysis, it seems likely that we will soon known a lot more about the identification of the endothelial lineage. It is hoped that this will help us understand how patterns of development of the vessel wall are controlled. Similarly, the failure of our early studies to identify molecules responsible for investment by smooth muscle can be seen as an exciting finding. If platelet-derived growth factor, fibroblast growth factor, and transforming growth factor beta are not expressed until after smooth muscle investment, still other as yet unidentified factors must be present to account for this stage of development of the vessel wall.