Developmental biology or embryology (Greekεμβρυολογία) 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 sometimes reserved to refer more specifically to the study of organisms between the one-cell stage (generally, the zygote) and the beginning of free living. Embryology was originally a more descriptive science until the 20th century. Embryology or developmental biology today now deals with the various steps necessary for the correct and complete formation of the body of a living organism. In the United Kingdom and other countries it sometimes reserved specifically to refer to the study of human embryos.
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.
Biological warfare, also known as germ warfare, is the use of any organism (bacteria, virus or other disease-causing organism) or toxin found in nature, as a weapon of war.
As a strategic weapon, biological warfare is again militarily problematic, because unless it is used to poison enemy civilian towns, it is difficult to prevent the attack from spreading, either to allies or to the attacker, and a biological warfare attack invites immediate massive retaliation, usually in the same form.
The primary civil defense against biological weaponry is to wash one's hands whenever one moves to a different building or set of people, and avoid touching door knobs, walls, the ground and one's mouth and nose.
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