The avian embryo offers a number of distinct advantages for embryonic investigations. The legacy of its long history of descriptive studies coupled with physiological and biochemical studies of the 19th century and the experimental embryology of the 20th century is an enormous bibliography concerning system and organ development. It is readily accessible between laying (blastoderm stage) and hatching and the comparatively large size of the embryo, even at gastrulation stages, greatly facilitates microsurgical manipulation. Accessibility can be further improved by in vitro culture methods pioneered by New (2; see Chapter 15) that allow early embryos to be maintained for 3 d on culture rings. Older embryos can be cultured to term by other methods. Birds, like mammals, are amniotes, but are much cheaper to purchase and generally have no associated animal housing costs. Microsurgical manipulations are readily performed until about embryonic d 8, by which time all of the major patterning events are complete and all of the organ systems are established. Tissue grafting has facilitated the determination of instructive and permissive tissue interactions important in sculpting the embryo and facilitated studies of position effects, lineage, commitment, and determination (see, e.g., Chapters 16-19, and 22). Many lineage-tracing studies have been undertaken in the chick embryo using engrafted quail tissue, which is distinguished cytologically or immunochemically (see Chapter 22), or by application of fluorescent tracers (see Chapter 23). The development of avian retroviral vectors has facilitated the ectopic expression of genes (see Chapter 35) and the exciting new development of hammerhead ribozyme vectors (3,4) offers the prospect of being able to inhibit expression of any gene in this embryo. The field owes a considerable debt to Hamburger and Hamilton (5), who provided a detailed staging regime using definitive criteria for the chick embryo; a similarly detailed staged series is not available for the mouse embryo. For stages between fertilization and gastrulation, Eyal-Giladi and Kochav (6) provide a further staging regime. The recently published Atlas of Chick Development (7) is recommended to those undertaking histological studies and also provides a good first reference source for descriptions of early development and organogenesis.
There are, however, some areas in which other vertebrate embryos offer advantages over avian embryos. First, the chicken (or any other bird) has little genetics, and it is doubtful whether this will change greatly in the foreseeable future. A linkage map is being constructed, but the chicken has about 80 chromosomes; many of them are microchromosomes and are difficult to distinguish one from another. Unfortunately, it seems as if most of the transcribed genes reside on these microchromosomes. It is now clear that transgenic chickens can be generated, but the methods are more laborious than for mice, and the costs of maintaining transgenic flocks will largely preclude this technology except for agricultural purposes. Finally, the very earliest stages of development, from fertilization to formation of the blastoderm, occur prior to laying and are therefore not readily accessible.
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