Mechanisms of Development 121 (2004) 1011–1013 www.elsevier.com/locate/modo
The chick embryo – past, present and future as a model system in developmental biology The embryo of the domestic fowl (Gallus gallus) holds the record as the animal with the longest continuous history as an experimental model for studies in developmental biology, spanning more than 2 Millenia. Throughout this time, it attracted great naturalists, artists, philosophers, and pioneers of biology and stimulated them to think about the most fundamental questions on generation and life like no other organism has ever done, except the human. The ancient Egyptians are documented as having opened hens’ eggs at different periods during incubation to observe the progress of embryo development, and by around 300 BC Aristotle undertook careful studies of the morphology of the embryo (as much as he could without the aid of magnifying devices); this can be considered as the ﬁrst ‘scientiﬁc’ study of embryo development and his work referred to by his followers right up to the 19th Century (Needham, 1959). After the mediaeval ‘Dark Ages’, the resurgence of an interest in Anatomy and embryo development in the Renaissance attracted ﬁgures including Leonardo Da Vinci (1452–1519), Ulisse Aldrovandi (1522–1605) and Hieronymus Fabricius ab Aquapendente (1537–1619) to return to the study of the embryo within the egg. At this time the debate between Preformation and Epigenesis was starting to gather momentum and the chick embryo was often a reference to which subscribers to either camp resorted to support their favourite theory. As an attempt to contribute to the debate, William Harvey (1578–1657) observed chick embryos at early stages of development and concluded that the heart was the ﬁrst functioning organ to develop in the embryo. By observing the motion of the blood through the heart and early vessels, he discovered the circulation of the blood and understood the function of arteries and veins. The ensuing 200 years or so were accompanied by an increase in the number and the detail of anatomical studies of the embryo, each time enriched by a new technical advance. The introduction of histological sectioning and of selective staining methods allowed Pander and von Baer to start to understand the signiﬁcance of germ layers in development. These pioneers also started to ask questions 0925-4773/$ - see front matter q 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.mod.2004.06.009
about causality in development—what mechanisms are responsible for such stereotyped development? Further and increasingly careful histological studies followed throughout the 19th Century, with the most important contributions being made by Rauber and Hensen and culminating in a beautiful histological atlas by Mathias Duval (1889). By the end of the 19th Century, experimental embryology (Entwicklungsmechanik) started to replace simple histological observation as it became clear that principles could only emerge from experimental manipulation of the embryo, but the initial advances were mainly made through work in other organisms (sea urchins and amphibians for ‘embryoembryonic regulation’ and induction, marine invertebrates for lineage studies, Drosophila for developmental genetics) and the chick was a little slower in catching up. But there were some salient chick studies at this time, including Graeper’s spectacular 3-d stereo time-lapse movies of embryos labelled with spots of vital dyes to follow cell movements (made in 1926, published in 1929 and unrivalled to the present day), which revealed the cell movements preceding and during gastrulation and Waddington’s cross-species transplants of primitive streak and node and his hypoblast rotations which led to the ﬁrst evidence that extraembryonic endoderm (hypoblast) plays a role in positioning the embryonic axis (Waddington, 1932, 1933), as well as consolidating the concept that Hensen’s node is a source of...
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