|MadSci Network: Anatomy|
Hi! Thanks for your question!
During embryonic development, the heart is one of the few organs that must function almost as soon as it is formed. Indeed, the fact that the heart is one of the earliest organs to develop speaks to its importance.
The human heart begins to beat and pump blood through the embryo around day 22 of gestation. The electric stimulus that triggers the muscular portion of the heart, known as the myocardium, to contract is myogenic. This means that the contractions arise spontaneously within the myocardium itself, and propagate from cell to cell. Input from the central nervous system can modify the heart rate (the frequency of heart beats), but it does not initiate beats.
The ability of cardiac myocytes (the cells that comprise the myocardium) to beat is an intrinsic property of these cells. In fact, myocytes removed from the early heart and grown in culture will beat sporadically, and if they become connected to each other, will then begin to beat rhythmically, in unison. As a functional organ, the heart begins to beat very early, even before it has assumed its final form. Interestingly, the heart begins to beat even before structures such as valves and septa (singular: septum; the muscular walls that divide the chambers) have formed! The initial contractions are peristaltic--that is, they proceed in a wave-like fashion along the length of the heart. Later, once the heart has matured and the conduction system has developed, the contractions proceed in an orderly, timed sequence through the different chambers.
Because beating is an intrinsic property of the cardiac myocytes, exactly what causes the heart to begin contracting is essentially a cell biological question. Unfortunately, there is not yet a definite answer to this question. We are beginning to understand how cardiac myocytes are specified very early in development, long before the heart has even formed. And we are learning more about their differentiation--that is, how these cells, which initially are not any different from cells that will give rise to the stomach or to muscles in the arm, develop the characteristics that allow them to contribute to the beating heart. From research in these areas, it is clear that certain things are required in order for the heart to beat. For instance, cardiac myocytes must have contractile proteins, such as actin and myosin, that are properly assembled into a scaffold (known as a sarcom ere) that allows contraction to take place. In addition, these cells must have specialized structures called gap junctions, which allow them to communicate so they can beat together in a synchronized fashion. Thus, while it is clear that certain elements must be in place for the heart to beat, it is not yet clear what stimuli are responsible for initiating contraction.
I hope this information is useful! Please feel free to contact me if you have any further questions. I have listed below a couple review articles that discuss some aspects of early heart development. They are relatively detailed, but unfortunately, do not discuss much about the heartbeat per se. There is very little, if any, available literature which specifically addresses the onset of cardiac contractility.
Fishman, M.C. and Chien, K.R. (1997) Fashioning the vertebrate heart: earliest embryonic decisions. Development 124:2099-2117.
Mohun, T and Sparrow, D. (1997) Early steps in vertebrate cardiogenesis. Current Opinion in Genetics & Development 7:628-633.
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