The chick embryo rules (still)!

Consider the avian egg. Humans have used it as a tasty and nutritious food source since before recorded history. Likewise, the first recorded description of a developing warm-blooded animal was made on the avian embryo. Since the time of Aristotle, who opened chicken eggs daily to examine progressive stages of embryogenesis, the chick embryo has been a favored model system for studies in developmental biology. Generations of students have been indoctrinated into developmental biology by early encounters with the chick embryo—either alive, freshly removed from the egg, or processed as the whole-mount and serial section slides often used in undergraduate developmental biology laboratories. The chick embryo has several attributes that make it a superb model system. These features include the practical issue that chicken eggs are readily available throughout the year and throughout the world in large numbers and at relatively low cost. Collectively, probably more is known about the descriptive embryology, teratology, and experimental embryology of the chick embryo than of any other model organism, so a vast database exists in the literature and now on-line. The chick, as a warm-blooded higher vertebrate, undergoes true growth during its morphogenesis in which the embryo and its developing organs increase dramatically in size, similar to the situation in the human embryo. Therefore, studies of the chick are highly relevant for understanding human birth defects. But it is as an experimental organism that the chick embryo truly shines. Schools of chick experimental embryology were established early in the 20th century in many countries, including Belgium, France, Japan, Germany, the United Kingdom, and the United States. Techniques for manipulating chick embryos, especially microsurgically, were derived largely from those established earlier for amphibian embryos. In the ensuing years, cut and paste experimental embryology was refined to an art form in the developing chick embryo, with its easy accessibility either by means of windows cut in the shell or in ex ovo whole-embryo culture. With the advent of the modern molecular era, there may have been those who thought that the chick embryo might be left behind. Yet today, the chick embryo remains one of the best model embryos for numerous in vivo manipulations: for example, overexpressing secreted proteins, such as growth factors or their naturally occurring inhibitors, by implanting beads coated with particular factors, using transfected cells expressing such factors, or viral vectors expressing the factors. The chick embryo offers the major advantage that such procedures can be done quickly and easily in a highly precise temporal and spatial manner. 2004 is the year of the chick genome, with sixfold pass coverage recently completed. The availability of this resource combined with the use of recently developed techniques for gene misexpression in chick embryos, using electroporation, lipofection, or viral transfection, combined with the use of knockdown approaches such as RNAi, morpholinos, and conventional antisense, ensure that the chick embryo will remain at the forefront of research in developmental biology. It is with such confidence in the continued perdurance of the chick as a valuable experimental model, and the goal of placing a large body of information before the community at one time, that we have assembled this special issue on the chick embryo as a model system. We hope that you will enjoy reading this issue, which marks the infancy of the chick model as a molecular genetic system.