How studies on the avian embryo have opened new avenues in the understanding of development: A view about the neural and hematopoietic systems

Abstract

The chick embryo is as ancient a source of knowledge on animal development as the very beginning of embryology. Already, at the time of Caspar Friedrich Wolff, contemplating the strikingly beautiful scenario of the germ deploying on the yellow background of the yolk inspired and supported the tenants of epigenesis at the expense of the preformation theory. In this article, we shall mention some of the many problems of developmental biology that were successfully clarified by research on chick embryos. Two topics, the development of the neural system and that of blood and blood vessels, familiar to the authors, will be discussed in more detail. Model systems in developmental biologyAt the dawn of studies on metazoan development, the goal was to observe and describe a large variety of species in order to account as much as possible for the biodiversity of the animals living on the planet. As a consequence, studies of development were performed on a wide variety of species. From the end of the nineteenth century, biologists switched their interests from a descriptive to an analytic trend. They started trying to decipher the mechanisms underlying the developmental processes rather than merely describing them. From that time, experimental approaches became focused on a limited number of animal species, in order to find out the mechanisms through which a single cell, the egg, gives rise to an organism. These species can be considered as model systems for unraveling the processes involved in this extraordinary achievement of nature.At the turn of the twentieth century, sea urchins and frogs were the main animals used for these purposes. The reasons for this choice pertain to the fact that their embryos develop in water and are easy to obtain. In the case of the frog, the embryo is large and robust enough to be manipulated even up to relatively late stages. Among vertebrates, mammalian embryos were less accessible, albeit those whose development arose the most interest and curiosity. Avian embryos appeared as a particularly attractive model for several reasons: they develop very much like their mammalian counterparts (except at their earliest stages), are easy to obtain and are directly accessible to manipulation in the egg. They can even continue to develop normally when explanted in vitro. Thus, for the study of vertebrate development, frog and chick have been for quite a while the two main model systems, not only for descriptive, but also for experimental studies in embryology.Following the advent of developmental genetics, the avian model suffered from the poor knowledge available about chick developmental genetics. In contrast a great deal was already known in this respect about the mouse. All through the first two-thirds of the twentieth century, many mutations were identified in this species; the mutants have been conserved and made available to the researchers. Moreover, the rapid development of genetic engineering in the 1970s and 1980s made it possible to alter the genome of the mouse by transgenesis and even ...

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