Abstract
Avian embryos have been used for centuries to study development due to the ease of access. Because the embryos are sheltered inside the eggshell, a small window in the shell is ideal for visualizing the embryos and performing different interventions. The window can then be covered, and the embryo returned to the incubator for the desired amount of time, and observed during further development. Up to about 4 days of chicken development (out of 21 days of incubation), when the egg is opened the embryo is on top of the yolk, and its heart is on top of its body. This allows easy imaging of heart formation and heart development using non-invasive techniques, including regular optical microscopy. After day 4, the embryo starts sinking into the yolk, but still imaging technologies, such as ultrasound, can tomographically image the embryo and its heart in vivo. Importantly, because like the human heart the avian heart develops into a four-chambered heart with valves, heart malformations and pathologies that human babies suffer can be replicated in avian embryos, allowing a unique developmental window into human congenital heart disease. Here, we review avian heart formation and provide comparisons to the mammalian heart.
Highlights
Birds have been diversifying to inhabit nearly every conceivable habitat on the earth’s surface since the Cretaceous period
Cell trajectories can be followed in vivo, to observe how the heart and other organs are formed and how progenitor cells continue to contribute to organ formation
New technologies, such as CRISPR-Cas9, and TALEN-mediated gene inactivation, are further allowing researchers to genetically modify the avian embryos and test the effect of gene disruptions, which for a long time could only be done in mice and zebrafish embryos through gene knockout or knockin lines
Summary
Birds have been diversifying to inhabit nearly every conceivable habitat on the earth’s surface since the Cretaceous period. High aerobic activities (e.g., flying) in endotherms (e.g., birds and mammals) require an efficient CVS that is afforded by four-chambered hearts, high systolic pressure and high resting metabolism [3,5] Both the avian and human heart are located along the midline of the anterior part of the thoracic cavity. The oxygenated blood moves to the left ventricle, where it is pressurized for systemic circulation Both avian and mammalian hearts are surrounded by a thin, fibrous pericardial sac that is filled with serous fluid that lubricates the motions resulting from cardiac contractions and confines the heart so it does not rattle around the thoracic cavity or overfill with blood. The left ventricle in a bird’s heart is by far the largest heart chamber, powered by a thick cardiac muscle to pressurize the blood for transport throughout the body, and must work especially hard in birds during flapping flight. The blood vessels to the forelimbs (subclavian arteries) develop from the dorsal aorta and left systemic arch
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