Basic - Primitive Heart Tube
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The heart is the first organ to function within an embryo. It starts to function at the beginning of the fourth week when the nutritional and oxygen requirements of the growing embryo can no longer be met by diffusion from the placenta. The heart initially forms from two tubes located bilaterally (on either side) of the trilaminar embryo in the cranial (head) region. The image on the right shows these primitive tubes developing in an embryo approximately 18 days after conception.
When looking down at this early embryo you can see multiple blood islands dispersed throughout the embryo. These will form the early blood vessels. At the most cranial end of the embryonic disc these blood islands are actually the primitive heart tube. From the side you can see one of the heart tubes and heart cavity developing in this position.
The disc-like embryo then undergoes a process of folding, in which both the cranial and lateral parts of the embryo fold ventrally (forwards). This brings the heart-forming region to a ventral (frontal) position. The following animation shows the development of the heart tubes and how embryonic folding brings them to fuse in the midline. (Click image to play on current page or Play video on new page)
<html5media height="720" width="560">File:Heart_folding_002.mp4</html5media>
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Blood Islands: The initial small patches which form within mesoderm that differentiate into both the blood vessel wall and blood cells. These islands enlarge and connect together to form the initial vascular beds.
Caudal: Anatomical term referring to structures that are more towards the tail.
Cranial: Anatomical term referring to structures that are more towards the head.
Dorsal: Anatomical term referring to structures that are more towards the back.
Embryonic disc: A flat two- or three-layered area; the first traces of the human embryo.
Heart cavity: (Also pericardial cavity). The space in which the heart lies. In the adult heart this space diminishes so that the heart wall (visceral pericardium) and cavity wall (parietal pericardium) are continuous.
Lateral: Anatomical term referring to structures that are away from the midline.
Placenta: The developmental organ formed from maternal and fetal contributions in animals with placental development. In human, the placenta at term is a discoid shape "flat cake" shape; 20 cm diameter, 3 cm thick and weighs 500-600 gm. The placenta has many different functions including metabolism, transport and endocrine.
Primitive heart tube: Initial, straight, tube-like structure of the embryonic heart.
Trilaminar embryo: Term used to describe the early three (3) layered embryo following gastrulation when it now has a structure consisting of the 3 germ cell layers: ectoderm, mesoderm, endoderm. In humans, this developmental stage occurs during week 3. Do not confuse the term "germ cell layers" with "germ cells", which refer to the egg and sperm.
Ventral: Anatomical term referring to structures that are more towards the front.