EMBRYOLOGY: DISSECTION OF A CHICK EMBRYO
Introduction
Observation of early embryogenesis enhances our understanding of cell differentiation and division, and gross anatomy. All multicellular animals have certain similarities in the pattern of development but similarities are greatest in closely related groups. Thus, all vertebrate animals are built about a common anatomical plan and have the same programme of development which is driven by controlled expression of specific genes. The components of development are -
fertilization, cleavage, blastulation, gastrulation, neurulation and organogenesis.
This practical involves interpretation of the features of the embryo from fertile incubated chick eggs. Mounts of embryos are available as slides and you will also have the chance to perform a fresh dissection. You will make a labelled drawing of a mounted embryo and determine its stage of development. There are also displays available for you to study. Do ask questions about these if anything puzzles you
Background information - early development of chick embryo
On the surface of the yolk of a freshly laid, fertile egg is a small whitish circular area, the embryonic disc, which is about 4mm in diameter. Cleavage and blastulation occur before the fertilized egg is laid. When the egg is laid and the temperature falls, development is arrested until such time as the temperature is again raised by incubation. Gastrulation starts during the first day of incubation and involves the production of the three-layered embryo by migration of cells through the primitive streak, firstly to displace the hypoblast layer with endoderm and secondly to form mesoderm underlying the epiblast layer, now ectoderm. The mesoderm extends to the edge of the area pellucida and then invades the area opaqua where it contributes to extra-embryonic tissue. This region becomes vascular (indeed is the site of red blood cell production) and is then called the area vasculosa. At about 15 hours the notochord appears and induces formation of the neural tube (neurulation). The segmentation of the paraxial mesoderm into somites also begins at this time. Somitogenesis is advanced after 30 hours and the heart is formed at about 36 hours.
Trying your own dissection and viewing a fresh dissection
1. Cut a window in the fat end of the egg shell about the size of a two pence piece. This is done by making an initial small perforation with the point of a scissors blade and then carefully enlarging the hole with forceps and discarding the pieces of egg shell. You should have entered the air space under the shell and the embryo should be apparent beneath the inner shell membrane which should still be intact.
2. Cut around the aperture to remove the inner shell membrane
3. Look at the embryo with the lens provided. Depending on the stage of development, the beating heart and blood vessels may be visible, especially a ring of blood vessels around the perimeter of the area vasculosa.
4. Using a pipette, apply 1% nitric acid gently around and over the exposed embryo to coagulate the albumen and the yolk. Leave for 2 minutes
5. Cut round the embryo outside the area vasculosa using deep cutting strokes with the scissors. The first cut can be difficult – you need to puncture the yolk. Repeat to ensure the embryo is totally free from the membranes. This is the crucial stage: if the embryo is not freed from membranes it will tend to pull back and tear when you try to lift it out. The whole procedure is made more difficult by the fact that the embryo often disappears beneath the released yolk as you cut. Be patient and don’t rush things.
6. Using the spoon spatula, lift the embryo into the bowl of water and gently wave it around to wash off any yolk and vitelline membrane still attached.
7. Wipe the spatula and lift the free floating embryo into the jar of 5% formaldehyde. Close the lid and leave it for 5 minutes. This fixes the tissue.
8. Quickly pour the contents of the jar into the bowl refilled with clean water. This dilutes the formaldehyde to a safe level. Using a spatula, manipulate the embryo into the small dish. This is made easier by immersing the dish in water.
9. Observe under the dissecting microscope
Drawings of chick embryos
1. Make a drawing of your own embryo, identifying as many features as possible. You should be able to see regions of the brain, the neural tube, somites, the heart, body folds, the anterior edge of the gut portal and vitelline blood vessels
Try to determine the stage of your embryo with reference to the illustrations at the end of these notes (Appendix 1). (You can see colour versions of the staging pictures at –
Notice that you can also roughly determine the incubation age of the embryo by counting the number of somites and adding 20 to give an answer in hours. Counting of somites is more difficult with the later embryos because they can become obscured especially near the rostral end.
Notice also that in these embryos, there is a twisting of the head and neck to the right when viewed dorsally. However, sometimes the embryo will have been mounted with the ventral side uppermost. In those cases, the head will be twisted to the left and you will be looking into the gut entry. This may be easier to understand if you look at the painted movies of chick embryos to be studied in tomorrow's practical. In fact, looking at these movies generally while you are drawing should help your interpretation of what you are seeing. They are available at:
2. You also should draw a mount of an ‘early’ embryo. These are much simpler and you will be able to draw one quickly. Depending on your choice from the slide tray, you should see features of the primitive streak, segmented and unsegmented paraxial mesoderm, neural tube formation, the simple linear heart, the simple 3 vesicle brain with open rostral neuropore, the anterior edge of the gut portal. Notice if you choose one of the very early versions there may be very little to see other than the primitive streak and the beginnings of neural tube formation
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Your drawings
Appendix 1 – stages of chick embryo development
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