CARDIOGENESIS

Tavishi

Embryonic development is so cool. I always imagine it in those cool timelapse videos of like butterflies emerging from chrysalises. Unfortunately, not many of those out there, so I generally have to draw it out to visualize it when I'm studying.

My favorite organ system development, however, is the cardiovascular system. The heart is the first functioning organ in the human embryo, and its development begins around week 3 of existence (note: this is after implantation).

At this point, the developing fetus looks like this:

Neurulation is still occurring at this point, but it has certainly begun.

Earlier in development, the primitive streak forms on the blastocyst, where the cells move in towards, differentiating into different cellular layers: endoderm, mesoderm, and ectoderm. This is the process of gastrulation.

The heart is primarily composed of mesodermal cells, and at the primitive streak, precardiac cells form, later migrating to form the primitive heart tubes. At 18 days, the embryo looks approximately like this:

The pink horseshoe shape you see above are the cardiac progenitor cells, and the area that I've done in pink is called the cardiogenic region. Over the course of a few days, the cells more concretely form two tubes on either side of the neural tube. These are called the endocardial tubes, which will then form the primitive heart tube.

The primitive heart tube is about two big steps away from turning into a real heart. The primitive heart tube just lacks division. It has no boundaries, because that's kind of the process of septation. Either way, it has a lot to work through.

It is still separated vaguely into different parts that turn into other parts. For instance, the concept of a primitive atrium and ventricle. Again, very primitive, as there's no left or right side of the heart. Below is a diagram of a bird's eye view of the primitive heart tube and its layers:

In a fully grown heart, there is a right ventricle, right atrium, left ventricle, and left atrium. Deoxygenated blood enters through the right atrium, is pumped into the right ventricle, and shuttled off to the lungs via the pulmonary artery. Oxygenated blood returns to the heart via the pulmonary vein and is deposited into the left atrium, and is then pumped through the left ventricle and into the aorta, where it is carried to the whole body.

In a primitive heart tube, blood enters through the primitive atrium, deposited by the sinus venosus. The sinus venosus receives its blood from the vitelline vein, umbilical vein, and cardinal vein. The sinus venosus will later develop into the sinus venarum, coronary sinus and the sinoatrial node. The sinus venarum is part of the right atrium that is formed from smooth tissue rather than rough from the ridges on the inside of the heart (pectinate muscles)!

The primitive atrium isn't separated from the primitive ventricle, and when blood comes into it, it is pushed into the ventricle by muscles. The primitive ventricle sends blood into the bulbus cordis, which is the intermediate structure between the primitive ventricle and truncus arteriosus. The bulbus cordis and the primitive ventricle go on to form the ventricles in a mature heart. The truncus arteriosus will later form the atria and the pulmonary trunk.

In the process of cardiac looping, the tube starts to shape itself into more of a heart. The atria move to their own side, preparing for the separation of cavities via septa! Additionally, we just sorta make a chonkier ventricle. Below is the process drawn out by me! The aorta and pulmonary trunk start to more cohesively form, too.

After this, all the separation and division occurs in the process of septation. The heart puts up a lot of walls. For one, a wall divides the right and left side of the heart, consisting of an interatrial septum and interventricular septum. Then, there's walls between each atria and ventricle, or the atrioventricular canal septation (valves here!). Finally, we have the division between the pulmonary trunk (takes blood to lungs and back) and the aorta (takes blood to body). The aorta and pulmonary trunk stem from the truncus arteriosus, whereas the vena cavae stem from the sinus venosus. Those are the blood vessels that bring blood to the right atrium. The right part of the SV turns into the vena cavae, and the left turns into part of the sinus venarum.

The interatrial septum forms from the roof of the primitive atrium. This forms the flawed septum primum, which has a hole called the ostium primum. Later, the endocardial cushions close up the ostium primum, but they still have a hole called the ostium secundum. And what's left but to patch a broken patch? Another patch, or the septum secundum. However, a hole called the foramen ovale is left.

All the other septation goes similarly: add tissue from endocardial cushions to patch holes, keep blood corralled, let it flow freely if necessary, though.

More importantly, I hope my illustrations were a fun addition to the post. It's not something I usually do, but I might do it more. Along with this change in the format of posts, another big change is occurring- I'm moving to London for college! Wahoo!!! I am very excited, needless to say, but that does mean I might need to take a few weeks off of writing Kradigan posts! So. Until next time?