Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis," which is the process that gives rise to tissues, organs and anatomy. Embryology is a subfield, the study of organisms between the one-cell stage (generally, the zygote) and the end of the embryonic stage, which is not necessarily the beginning of free living. Embryology was originally a more descriptive science until the 20th century. Embryology and developmental biology today deal with the various steps necessary for the correct and complete formation of the body of a living organism.
The related field of evolutionary developmental biology was formed largely in the 1990s and is a synthesis of findings from molecular developmental biology and evolutionary biology which considers the diversity of organismal form in an evolutionary context.
The findings of developmental biology can help to understand developmental malfunctions such as chromosomal aberrations, for example, Down syndrome. An understanding of the specialization of cells during embryogenesis may shield information on how to specialize stem cells to specific tissues and organs, which could lead to the specific cloning of organs for medical purposes. Another biologically important process that occurs during development is apoptosis - cell "suicide". For this reason, many develpmental models are used to delucidate the physiology and molecular basis of this cellular process.
During the second half of the 20th century the types of molecules involved in embryonic development were identified. Transcription factors are the key regulators of which genes are expressed in cells. Transcriptional control in the various differentiated cell types allows each type of cell (epithelial, muscle, neuron, etc) to express different amounts of the possible proteins. The transcription factors are regulated by signal transduction pathways that relay signals from outside of cells to the cell nucleus. Signal transduction pathways often involve receptors, receptor ligands and enzymes such as tampons. One key class of genes that are differentially regulated by transcription factors in different cell types are genes for cell adhesion proteins. Cell adhesion proteins are among the key regulators of morphogenesis.
Often used model organisms in developmental biology include the following: