Tissue formation during embryogenesis

The formation of functional tissue and organs during embryonic development is a complex process involving multiple cell types derived from ectoderm, mesoderm, endoderm, and the germ line. Direct cell-cell contacts and paracrine signaling by morphogenic gradients of growth factors and growth factor antagonists are a few of the mechanisms driving cell specification. Cell behavior during organogenesis is dynamic and is dependent on cell movements of subsets of cells. Other cells revert their phenotype by a process called epithelial-to-mesenchyme transition and may become migratory. To obtain functional organs, cells respond to environmental signals like pulsatile blood flow which specifies blood vessels in arteria or veins. The timely, proper dosing and sequential integration of all these elements during organogenesis specifies cell types and shapes the organ. These processes can to a very large extent be mimicked in vitro by sequential treatment of stem cells to obtain functional tissue-specific cells. Indeed, modern tissue engineering relies on reiteration of these developmental processes in combination with typical engineering disciplines to generate functional substitutes to replace lost or worn-out tissue.