An Introduction to the Chick: Gallus Gallus Domesticus

The chicken embryo (Gallus gallus domesticus) is an extremely valuable model organism for research in developmental biology, in part because most of their development takes place within an egg that is incubated outside of the mother. As a result, early developmental stages can be accessed, visualized and manipulated by simply creating a small hole in the eggshell. Since billions of chickens are raised worldwide for meat and egg production, scientists can easily and economically acquire large numbers of fertilized eggs throughout the year. Furthermore, chickens share significant genetic conservation with humans, so the genetic mechanisms that have been found to regulate chicken development are also relevant to our own biology.

This video focuses on introducing the domesticated chicken as a scientific model. The discussion begins with a review of chicken phylogeny, revealing the features that make them amniotes, like other birds, reptiles, and mammals. Highlights from the millennia of chicken research will be presented, ranging from Aristotle’s postulates about the function of extra-embryonic membranes to more recent, Nobel-prize winning discoveries in neuroscience. Additionally, some current examples of studies performed in chicken embryos will be provided, such as in vivo tracking of cell movements during development and the recruitment of blood vessels to developing tumors (a process known as angiogenesis).

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Basic Chick Care and Maintenance
Chicks (Gallus gallus domesticus) are a valuable research tool, not only for studying important concepts in vertebrate development, neuroscience, and tumor biology, but also as an efficient system in which to propagate viruses. Although eggs can be purchased from external suppliers and working with chicks requires very little specialized equipment, an understanding of proper handling procedures is required for normal embryo development.

This video will provide an overview of egg handling principles, including an explanation of the incubation parameters that can profoundly impact development: temperature, humidity, and egg rotation. Most experiments that use chicken eggs require access to the embryo within the shell, which is achieved by cutting a small, resealable hole, or “window.” This process is described in step-by-step detail, along with several other techniques essential for working with chicks, such as candling and India ink injection. Finally, the video will review some practical applications of these basic techniques in advanced scientific research.

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Development of the Chick
The chicken embryo (Gallus gallus domesticus) provides an economical and accessible model for developmental biology research. Chicks develop rapidly and are amenable to genetic and physiological manipulations, allowing researchers to investigate developmental pathways down to the cell and molecular levels.

This video review of chick development begins by describing the process of egg fertilization and formation within the chicken reproductive tract. Next, the most commonly used chick staging nomenclature, the Hamburger Hamilton staging series, is introduced. Major events in chick development are then outlined, including the dramatic cellular movements known as gastrulation that form the three major cell layers: The ectoderm, mesoderm, and endoderm. Cells from these layers go on to generate all the tissues within the organism, as well as extraembryonic membranes, which are necessary for the transport of gases, nutrients, and wastes within the eggshell. To conclude the discussion, some exciting techniques will be presented as strategies for studying chick development in greater detail.

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In ovo Electroporation of Chicken Embryos
Electroporation is a technique used in biomedical research that allows for the manipulation of gene expression via the delivery of foreign genetic material into cells. More specifically, in ovo electroporation is performed on early developing chicks (Gallus gallus domesticus) contained within their eggshells. In this procedure, DNA or knockdown constructs are first injected into a target tissue. However, the genetic material is unable to penetrate the plasma membrane to carry out its function within the cell. To solve this problem, an electrical field is applied, causing temporary disruptions to membrane stability. This electric field also causes the negatively charged nucleic acids to migrate toward the positively charged electrode through the holes in the plasma membrane, thus effectively driving the DNA or knockdown construct into the cell. The major advantage of this technique is that the delivery of genetic material can be localized to isolated cell types at specific developmental time points. As a result, the genetic mechanisms that govern individual developmental events can be examined.

This video provides an overview of the principles behind in ovo electroporation and introduces the tools required for the technique, including capillary needles, electrodes, and an electroporator. A step-by-step protocol for carrying out the procedure is also presented prior to discussion of a few fascinating examples of how the technique is used to perform a variety of genetic manipulations in chicken embryos.

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Chick ex ovo Culture
One strength of the chicken (Gallus gallus domesticus) as a model organism for developmental biology is that the embryo develops outside the female and is easily accessible for experimental manipulation. Many techniques allow scientists to examine chicken embryos inside the eggshell (in ovo), but embryonic access can be limited at later stages of development. Fortunately, chicks can also be cultured ex ovo, or outside of the eggshell. The major advantage to ex ovo culture is greater access to tissues that might otherwise be obstructed by the shell or the orientation of the chick within the egg, especially for embryos in later stages of development.

There are two principle strategies to ex ovo culture: whole yolk culture and explant culture. During whole yolk culture, the eggshell is cracked and the contents are transferred to a simple housing vessel. However, in explant culture methods, the embryo is excised from the yolk and mounted in the housing vessel to maintain membrane tension, which is important for normal development.

Basic protocols for whole-yolk and explant techniques will be provided in this video, along with a discussion of the pros and cons of culturing chicks outside of the shell. Finally, experimental applications of ex ovo culture will be discussed, demonstrating how this approach is used to improve access to the embryo for microscopy and genetic manipulation of late stage embryos.

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