Development of the Heart

A List of Derivatives. Unless you have a great memory, keeping track of the terminology associated with heart development can be a hassle. I can't make it easy, but at least I can give you a straightforward list of embryonic structures and their adult derivatives.

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 do very little except grow and differentiate—the brain, lungs, liver, digestive system, gonads, and kidneys all fall into this 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 very metabolically active cells. A circulatory system is needed to keep these cells from dying, and it needs to do it by the middle of the fourth week of gestation (circa embryonic day 24). And this system has to work continuously while undergoing some serious remodeling.

The CV system at 24 days as drawn by S. Gilbert from his Pictorial Human Embryology, © Univ. Washington Press.

The cardiovascular system is radically remodeled several times during development.

The system is revamped at least four times.

1. It starts out as an approximately bilateral system of contractile blood vessels. Parts coalesce at midline to form a single channel or 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 (see below).
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.

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 in converting a simple one-chamber pump into a complex two-chambered heart with sophisticated valves. They may be called "neural" crest, but these cells have an almost miraculous ability to make different types of tissues (cranial skeleton, adrenals, neurons, glia, even muscles in the ciliary body).

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

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.

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. It lost that function when placental mammals invented our placentas. But, the vitelline system is still associated with the gut. In fact, the portal vein is one important adult derivative of a plexus vitelline veins that surround the duodenum (by the way, the duo+denum is two+ten fingers in length, and "vitelline" means glassy, and gets this name from the shiny appearance of the membrane.)
2. The umbilical system: Originally, there are two umbilical veins that return recharged 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 useless after birth. See the List of Derivatives if you want the details.
3. The cardinal system: It is messy. Here is the main story—The anterior cardinals (or precardinals) drain the brain. The blood gets dumped into the common cardinal veins, and these common cardinals empty into the sinus venosus. If you had to guess at this point, you would probably guess that these guys turn into parts of the internal jugular veins and the superior vena cava of adults. You would be right. The lower part of the left anterior cardinal vein actually regresses, but before it does that, a new conduit is needed to carry blood from the left side of our embryo's cranium back to the heart. That is where the left brachiocephalic vein comes in. It is a left-to-right venous shunt that only develops secondarily. Now for the posterior cardinals: These veins service the "mesonephros." Never heard of it. That's because it regresses pretty quickly (except for a few leftovers in the testes). So when the mesonephros regresses so do most parts of the posterior cardinal veins. The rostral-most part on the right side turns into the root of the azygos 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 2nd and 3rd generation systems are called the sub- and supra-cardinals. The subcardinals drain the kidneys and gonads, and 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, however. 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 the adult compartments by six growing walls of tissue (septae).

• Septum primum. This first septum grows down the middle of the common atrium and eventually fuses with tissues that surround the narrow lumen between the atrium and ventricle (there is only one atrium and one ventricle at this point in the 5th week of gestation).
• Septum secundum. This slow growing second atrial septum grows parallel to the 1st septum, but is delayed by a couple of weeks. It is a more robust wall. Both the 1st and 2nd septums (septae) have big holes in them. To make it rough on you the hole in the first septum is called the second hole, or for formal types, the foramen secundum. The hole in the second septum is called the foramen ovale. I'll come back to the function role of these holes.
• AV septum/endocardial cushion: The endocardial cushions give rise to most of the cardiac skeleton that splits the atrial and ventricular sides of the heart. The cushions also build mitral and tricuspid valves.
• Interventricular septum, muscular part: A septum can also form when a central region grows particularly slowly. The surrounding tissue can bulge outward and fuse. This may be the mode by which the interventricular septum forms.
• Interventricular septum, membranous part: Maybe you have felt this part of the septum in the Gross Lab, tucked out of sight in the aortic vestibule, maybe not. When this membrane forms from a motley crew of cells from the endocardial cushions and the bulbar ridges, the heart is finally four-chambered.
• Aortico-pulmonary septa, bulbar ridges. There is no pulmonary trunk at early stages, just one big common arterial outflow, called the truncus arteriosus, that delivers all blood to an aortic sac. So how does this truncus arteriosus get split into an ascending aorta and a pulmonary trunk? That is the job of the bulbar ridges. These ridges grow into the truncus from either side and fuse in the middle. They spiral neatly down the truncus until they reach the ventricle(s). Magic happens, and the aortic half hooks up to the left ventricle; the pulmonary trunk hooks up to the right ventricle. This last step occurs at the same time that the membranous part of the interventricular foramen is forming. A process as complex as this can screw up. Imagine if that partitioning of the truncus into the two divisions isn't 50-50 but say, 20-80. The bore of the pulmonary trunk may be too small and that of the aorta too wide If that happens you often end up with a constellation of four (or five) related abnormalities in a neonate:
  1. stenosus (turbulent and noisy flow of blood in a narrow pulmonary trunk) often with cyanosis
  2. a failure of the membranous part of the ventricular septum to form fully—in other words, a ventricular septal defect
  3. the opening of the aorta straddles both ventricles (overriding aorta)
  4. an enlarged right ventricle (it is working harder to eject blood into the narrow lumen of the pulmonary trunk).

References in Embryology

Pictorial Human Embryolgy by S.G. Gilbert

This is a stunning piece of art that every pediatrician; no, make that every physican, should have. It is about $20 from the University of Washington Press. ISBN 0295-96631-9 hardcover and ISBN 0-295-96632-7 in paperback. It doesn't cover abnormalities, but for a concise description, both visual and verbal, of normal human development, this book cannot be beat.

The Developing Human, Clinically Oriented Embryology, 5th ed

This book by K.L. Moore and T. Persaud serves the market. All you need, and more, to pass the boards. If you want to know why developmental biology is intellectually exciting, you will have to search elsewhere.

Ronan O'Rahilly's book