Development of the Heart

The Main Points

The cardiovascular system is functionally important in development.

This statement may seem like a "no-brainer", but it isn't. Most cells and organs in an embryo don't have a lot to do except grow and differentiate—brain, lungs, liver, digestive system, gonads, kidneys, fall into that category. But the cardiovascular system has to do real work early on. The reason is that once the embryo has gotten thicker than 200 to 400 microns, the diffusion of nutrients, oxygen, and carbon dioxide is inadequate to service an inner mass of metabolically very active cells. A circulatory system is needed to keep these cells from dying, and it needs to do it by the end of the third week of gestation (circa embryonic day 18).

The cardiovascular system is not just made up of splanchnic mesoderm.

The heart also gets a critical influx of cells from the neural crest. These crest cells contribute to the formation of the endocardial cushions, which in turn are critical to converting a simple one-chamber pump into a complex two-chambered heart with fancy valves. They may be called "neural" crest, but these cells have an almost miraculous ability to make different types of tissue (cranial skeleton, adrenals, neurons, glia, even muscles in the iris).

The cells that give rise to the heart (and liver) are initially in front of the neural plate.

There is a big block of splanchnic mesoderm in front of the CNS and in front of the oropharyngeal opening (or membrane). The CNS grows forward over this mesenchymal mesoderm, and the mesoderm rotates down toward the yolk sac. The end result is that the heart, which is part of this mesoderm, gets tucked into the embryo's "neck." Just behind the heart is more of this big block of mesoderm, now called the transverse septum. It becomes the liver and the diaphragm.

The cardiovascular system is radically remodeled several times during development.

There system is revamped at least four times.

1. It starts out as an approximately bilateral system of simple contractile blood vessels. Parts coalesce at midline to form a single channel, central pump.
2. Lots of stuff starts to regress in the embryo, including some early renal structures (the mesonephros). As a result entire systems of veins (and some embryonic arteries) also regress. Virtually the entire posterior cardinal venous system gets resorbed.
3. The heart gets split into two pumps. The right side basically pumps blood straight into the descending aorta (through the ductus arteriosus shunt). The left side pumps blood into the rapidly growing cranium and brain.
4. At birth the whole pattern of circulation has to be modified radically in a matter of minutes. The main objectives are to shut down the chorionic/placental circulation and open up the pulmonary circulation. There are several subsidiary changes.

Three systems of veins disgorge blood into the sinus venosus.

1. The vitelline system from the yolk sac: This system is a nursery for blood cells. You think of a yolk sac as being part of a feeding system for an embryo. Clearly, it lost that function when we placental mammals invented our placentae. But, the vitelline system is still associated with the gut. In fact, the portal vein is one important adult derivative of the vitelline veins. By the way, "vitelline" means glassy or shiny.
2. The umbilical system: Originally, there are two umbilical veins that return refreshed blood from the chorionic plexi of the placenta. We owe our oversized human brains to these veins (and the placenta). Most of the right umbilical vein regresses. The left umbilical vein takes a short cut through the liver (the ductus venosus), and the oxygenated blood is delivered into the inferior vena cava and then into the sinus venosus/right atrium. As you would expect, most of this umbilical system is pretty much useless after birth.
3. The cardinal system: This one can give you the shakes if you try to master every little detail. Here is the main story—The anterior cardinal (or precardinals) drain the brain. The blood gets dumped into a common cardinal vein, and then into the sinus venosus. The lower part of the left anterior cardinal vein decides to regress, but before it does that, we need a new channel to carry blood from that side of our cranium back to the heart. That is where the left brachiocephalic vein comes in. It is a left-to-right venous shunt. Ok, now for the posterior cardinals. These veins service the "mesonephros." Never heard of it. That's because it regresses pretty quick (except for a few gonadal remnants). So when the mesonephros regresses so do most parts of the posterior cardinal veins. A tad of a remnant turns into the root of the azygous vein (the part that's attached to the superior vena cava). The inferior vena cava comes from bits and pieces of the 2nd and 3rd generation versions of the cardinal system. These replacements are called the sub- and supra-cardinals. The subcardinals drain the kidneys and gonads, whereas the more dorsally situated supracardinals drain the body wall.

The heart starts out with its venous side (sinus venous and atrium) located caudally.

From an adult perspective we think of the atria as being "on top." It doesn't start that way. The venous side of the heart is initially situated next to the transverse septum. The three systems of veins, reviewed above, penetrate the septum transversum to enter the sinus venosus. But over a 3-4 day period the heart rotates rolls in the sagittal plane. The result is that the sinus venous and common atrium are now located dorsal (deep) to the common ventricle.

The heart and arterial trunk are split into parts by six tissue walls (septae).

• Septum primum
• Septum secundum
• AV septum/endocardial cushion
• Interventricular septum, muscular part
• Interventricular septum, membranous part
• Aoartico-pulmonary septa, bulbar ridges