Boundaries in development: Formation and function

Developing organisms may contain billions of cells destined to differentiat e in numerous different ways. One strategy organisms use to simplify the or chestration of development is the separation of cell populations into disti nct functional units. Our expanding knowledge of boundary formation and fun ction in different systems is beginning to reveal general principles of thi s process. Fields of cells are subdivided by the interpretation of morphoge n gradients, and these subdivisions are then maintained and refined by loca l cell-cell interactions. Sharp and stable separation between cell populati ons requires special mechanisms to keep cells segregated, which in many cas es appear to involve the regulation of cell affinity. Once cell populations become distinct, specialized cells are often induced along the borders bet ween them. These boundary cells can then influence the patterning of surrou nding cells, which can result in progressively finer subdivisions of a tiss ue. Much has been learned about the signaling pathways that establish bound aries, but a key challenge for the future remains to elucidate the cellular and molecular mechanisms that actually keep cell populations separated.