The heart is the first organ to function in vertebrate embryos. The human heart, for example, starts its contractile activity around the 21st day of embryonic development (Britten et al.,1994; Wisser and Dirschedl,1994). At this developmental stage, embryonic vertebrate hearts are seen as relatively simple tubular blood vessels built up by an inner endocardial tube in contact with the blood, a middle layer of a cell-free extracellular matrix called the cardiac jelly (Davis,1924), and an outer myocardial tube in direct contact with the pericardial fluid (Fig. 1). The pumping action of this multilayered tube differs considerably from that of mature vertebrate hearts because it generates hemodynamically effective unidirectional blood flow in the absence of valves. This fact poses the question how the early embryonic heart works. Visual observations of pulsating embryonic heart tubes show that their pumping action is characterized by the cyclic generation of traveling mechanical waves sweeping from its venous to its arterial end. These traveling waves have traditionally been interpreted as myocardial peristaltic waves (e.g., Fano and Badano,1890; Patten and Kramer,1933; Goss,1942; Xavier-Neto et al.,2007). It was consequently thought that the early embryonic heart might propel fluid like the gut or like a technical peristaltic pump. During the past 20 years, however, hemodynamic data have accumulated that do not comply with this view (e.g., Hu and Clark,1989; Hu et al.,1991; Forouhar et al.,2006; Butcher et al.,2007). Recent hemodynamic data from zebrafish embryos suggest that the tubular hearts of vertebrate embryos are dynamic suction pumps that work on the basis of the so-called Liebau effect (Forouhar et al.,2006). This effect is a nonperistaltic valveless pumping phenomenon that was discovered in physical models of valveless tubular pumps—so-called “Liebau pumps”—in the 1950s (Liebau,1954a,b,1955a,b). The Liebau effect is relatively unknown among biologists and physicians. The idea that the embryonic heart tube works as a Liebau pump thus may appear as a new theory to many people working in the field of cardiovascular development. It is a fact however that, at the present time, this theory is more than 50 years old but obviously did not receive much recognition up to the publication of the above-mentioned suction pump data (Forouhar et al.,2006) in Science.
Understanding the pump action of early embryonic hearts may be interesting and relevant to investigators of cardiac development and disease because an abnormal pumping function of the tubular embryonic heart could lead to dire consequences at later stages of development. Thus, in the present article a review is given on our current knowledge about the pumping mechanism of the tubular embryonic heart. Our review starts with a description of the historical evolution of the two above-mentioned theories of early cardiac pumping mechanics (peristaltic pump theory vs. Liebau pump theory). This is because the two theories have evolved from different scientific backgrounds that do not use the same language. We then discuss pros and cons for both of these theories. We show that the currently available data suggest that the tubular embryonic heart of higher vertebrates works neither as a technical peristaltic pump nor as a classic Liebau pump. We hope that our article will stimulate interdisciplinary discussions and future studies on this exciting topic.
Historical Evolution of the Peristaltic Pump Theory
The theory of peristaltic heart motion evolved primarily on the basis of direct visual observations of beating hearts in living animals. Its historical roots may be traced back up to Aristotle (ca. 384–322 B.C.), who made the first documented observations of pulsating embryonic heart tubes. Aristotle observed 3-day-old chick embryos through windows in the eggshell and described the beating heart tube as a moving or jumping point (Aristotle: Historia Animalium, Book VI 3, translated by Peck,1970). Since that time, generations of natural scientists were fascinated by the visual experience of the jumping point on top of the egg's yolk (Fig. 2). However, it was not until the 18th century when this motion phenomenon was correctly interpreted as a sign for the mechanical pumping action of the embryonic heart. The foundation for this interpretation were laid down by William Harvey (1578–1657) and other natural scientists of the 17th century who discovered (1) the blood circulation, (2) the pumping function of the heart, and (3) the muscular nature of the movements of the mature four-chambered heart (e.g., Harvey,1628; Lower,1669).
Having recognized the muscular nature of cardiac motions, the attention of physiologists became focused on the question about the origin of the rhythmic action of the heart muscle. Two alternative views—an extrinsic and an intrinsic theory of heart beat—were proposed by physiologists of the 17th and 18th centuries (for review see Fye,1987). The extrinsic theory, which developed later into the neurogenic theory of heart beat, saw the generation of stimuli for myocardial contractions outside of the heart within the central nervous system. Stimuli generated in the cerebellum or in the spinal cord were thought to reach the heart by means of motoric nerves. The intrinsic theory, which developed later into the myogenic theory of heart beat, favored the effect of local stimulation in causing cardiac contractions. Albrecht von Haller (1708–1777) noted that each cardiac contraction cycle started at the venous end of the heart (venae cavae segment) from where it progressed in proximodistal order first to the atria and finally to the ventricles. Furthermore, he noted that the contraction of each heart cavity (venae cavae segment, atria, ventricles) was preceded by distension of its wall and, therefore, concluded that distension of the heart wall, caused by the inflow of blood, was the local stimulus for the initiation of myocardial contractions (von Haller,1757). Such a description of the normal sequence of contractions of the heart cavities resembles descriptions of propulsive peristalsis of the gut or ureter (Biedermann,1904,1906). It is no wonder then that proponents of the intrinsic (myogenic) theory of heart beat characterized the normal pumping action of the four-chambered heart as peristaltic heart motion (Gaskell,1883). Thus, during the 18th and 19th century, peristaltic motion was obviously defined in a much broader sense than today and encompassed any spatially and temporally ordered sequence of muscle contractions progressing in a unidirectional manner along the length of hollow organs. There were no specific requirements for the dynamics of the muscle (e.g., fast/slow) or fluid flow (e.g., continuous/pulsatile) and for the absence of valves.
The scientific dispute between the proponents of the extrinsic (neurogenic) and intrinsic (myogenic) theories of heart beat dominated the research on the physiology of the postnatal heart until the end of the 19th century and boosted the interest in physiological studies on the embryonic heart. Due to the absence of nervous elements in the early embryonic heart, proponents of the intrinsic (myogenic) theory became interested in studying its pumping action under normal and various experimental conditions (for reviews, see Pickering,1893; His,1949; Kamino,1991). Using the easily accessible chick embryo for such in vivo examinations, it was quickly realized that the pumping action of the embryonic heart tube was characterized by the cyclic generation of traveling mechanical waves sweeping from its venous to its arterial end. This motion phenomenon was generally interpreted as propulsive myocardial peristalsis (Wernicke,1876; Preyer,1885; Fano and Badano,1890; Gaskell,1883; Pickering,1893). Thus, the theory of peristaltic heart motion, which had evolved from physiological studies on the mature four-chambered heart during the 18th century, now was also applied to the tubular embryonic heart.
During the 19th century, the general perception of propulsive peristalsis became more and more confined to hollow, preferably tubular organs lacking valves and showing slowly progressing contraction waves such as the gastrointestinal tract and the ureter (Biedermann,1904). Due to this shift in the general perception of peristalsis and as a result of the discovery of the cardiac conduction system, the sequential contractions of the mature heart chambers were rarely characterized as peristaltic motion since the beginning of the 20th century. The pulsations of the valveless embryonic heart tube, on the other hand, were still regarded as propulsive myocardial peristalsis (e.g., Fano and Badano,1890; Patten and Kramer,1933; Goss,1942; Xavier-Neto et al.,2007).
During the 20th century, mathematicians, physicists, and engineers became interested in peristaltic fluid transport. Thereby, the perception of propulsive peristalsis underwent further evolution that now went in two directions. Biomedical sciences still preferred to define propulsive peristalsis on the basis of “biological standards” such as the gut or ureter. Technological sciences, on the other hand, preferred to define peristalsis on the basis of “technical standards” such as roller pumps (Fig. 3). The use of the gut as a “biological standard” for peristaltic movements did not really change the traditional theory of peristaltic movement of the tubular embryonic heart. It was only suggested that the pulsations of the embryonic heart tube should be called “peristaltoid” rather than peristaltic because of the absence in the heart tube of definite longitudinal and circular muscle layers (Patten and Kramer,1933). The consequences of technology-based definitions of peristalsis had by far more impact on the perception of the pumping mechanism of the embryonic heart tube. By using roller pumps as a “technical standard” for peristaltic pumps, engineers have noted that the pumping performance of the embryonic zebrafish heart seems to differ in several respects from that of a peristaltic pump (Forouhar et al.,2006): (1) The peak flow velocity (mm/sec) generated by a roller pump corresponds to the speed of the compression wave. In the embryonic zebrafish heart, however, the peak blood flow velocity recorded in the ventricular inflow exceeds the speed of the contraction wave traveling along the wall of the inflow region. (2) Roller pumps show a linear relationship between the compression wave frequency and the flow rate (ml/sec). In the zebrafish embryo, however, the relationship between cardiac contraction frequency and the flow rate is nonlinear and exceeds the maximum flow rate possible for a roller pump. (3) Peristaltic pumps have nonstationary sites of active compression that move in a specific direction along the length of a flexible tube. These unidirectionally propagating compression waves propel the fluid content of the tube in the direction of wave propagation. The tubular embryonic zebrafish heart, however, seems to possess only a single, stationary center of active myocardial contractions at its venous pole, which generates bidirectional, as opposed to expected unidirectional, mechanical waves traveling along the endocardial layer. (4) Technical peristaltic pumps are positive displacement pumps that built up positive pressure to push the fluid forward. The tubular embryonic zebrafish heart, on the other hand, seems to work as a solid-dynamic suction pump. The main conclusion drawn from these findings is that tubular embryonic hearts do not work as peristaltic pumps (Forouhar et al.,2006).
Historical Evolution of the Liebau Pump Theory
The Liebau pump theory evolved primarily on the basis of observations made in physical models of valveless tubular pumps. Compared with the peristaltic pump theory, the Liebau pump theory has only a short history. It was introduced in the 1950s by the German cardiologist Gerhart Liebau (1955a). Liebau became interested in valveless pumping phenomena as a result of his own observations on the hemodynamics of the healthy and failing human cardiovascular system. He constructed and tested physical models for the peripheral vascular bed and for the embryonic and mature human heart and, thereby, discovered a valveless pumping phenomenon that was unknown at his time (Liebau,1954a,b,1955a,b,1968,1970). The simplest physical model in which he observed this pumping phenomenon—later called the “Liebau effect” (Moser et al.,1998; Kenner,2004)—was an open tube system consisting of a water-filled flexible tube of finite length placed horizontally in a water-filled bath (Liebau,1955a). He found that rhythmic compression of this tube at asymmetric positions along its length generated unidirectional fluid flow, while compression at the point of symmetry (middle) did not generate unidirectional flow (Fig. 4). Corresponding observations were also made in a closed tube system in which the flexible tube was connected to a rigid tube (Fig. 5; Liebau,1955b). Based on these findings and on his textbook knowledge of the anatomy of the early embryonic heart, he speculated that the pumping function of the embryonic heart tube might be based on his newly discovered valveless pumping phenomenon (Liebau,1955a,b,1968,1970).
Apart from his speculations on the pumping action of tubular hearts, Liebau used his observations on physical models primarily to explain some physiological as well as some clinically important pathophysiological phenomena of the mature human cardiovascular system (Liebau,1954a,b,1955a,b,1968,1970). It thus might be surprising to note that the Liebau effect has remained unnoticed by the majority of physicians and biologists for a long time. This might be explained by the fact that, despite 20 years worth of research on valveless pumping, Liebau could not offer sufficient information about the fluid and structural dynamics of Liebau pumps that could be used to properly identify Liebau effect-driven flow in living organisms. Moreover, he also failed to provide an easily comprehensible physical explanation for his valveless pumping phenomenon that could be used to properly simulate the behavior of Liebau pumps. The mystery of the physics behind the Liebau effect might have hampered its perception by physicians and biologists. On the other hand, however, it has attracted the attention of a few physicists, mathematicians, and engineers. Thus, in the years following on Liebau's initial experiments, the interest in the Liebau phenomenon of valveless pumping shifted away from the field of biomedical sciences into the field of technology-related sciences.
Several experimental studies have been conducted to properly characterize the pumping behavior of Liebau pumps in closed as well as open systems (Bredow,1968; Ottesen,2003; Hickerson et al.,2005; Hickerson and Gharib,2006; Bringley et al.,2008). These studies have uncovered several unique characteristics of such pumps, which might be used to identify Liebau effect-driven flow in living organisms (Fig. 6): (1) The pumping tube must have a flexible wall and a finite length. (2) The two ends of the tube's wall must be bordered by materials whose impedance differs from that of the tube. (3) Only a small section of the pumping tube must be actively compressed. (4) This active compression site must be at asymmetric positions (not in the middle) along the length of the tube (Figs. 3, 4). (5) The flow generated by Liebau pumps is typically pulsatile. (6) There is no structurally fixed direction of the net flow. The direction of the net flow sensitively depends on the frequency of compression and therefore shows flow reversals at certain frequencies. (7) The rhythmic compressions of the active site generate wall motions seen as bidirectional elastic waves traveling along the passive upstream and downstream sections of the tube. (8) The speed of these traveling waves does not necessarily have the same velocity as the fluid. (9) There is a nonlinear relationship between the contraction frequency and flow rate.
Up to the end of the 20th century, attempts to explain and simulate the complex behavior of Liebau pumps have primarily focused on first principles of solid and fluid mechanics. The Liebau effect was, for example, attributed to inertia effects and impedance differences between the two flow pathways flanking the active compression site (Thomann,1978; Moser et al.,1998; Kenner,2004). At the beginning of the 21st century, the wave dynamics were introduced into simulation models for Liebau pumps by Jung and Peskin (2001). By focusing on wave dynamics, a group of engineers from the California Institute of Technology has developed a theory, which explains the Liebau effect entirely by wave dynamics (Hickerson et al.,2005; Hickerson and Gharib,2006; Avrahami and Gharib,2008). The periodic compressions of the asymmetrically positioned active site are said to generate pairs of elastic waves, which travel along the passive tube sections toward the ends of the tube where they are partially reflected. The sum interaction of these waves is said to cause pulsatile net flow in a specific direction.
In collaboration with biologists, the same group has recently tested whether the embryonic zebrafish heart might work as a Liebau pump (Farouhar et al., 2006) and, thereby, went back to the biomedical roots of the Liebau pump theory. This first attempt to identify Liebau effect-driven flow in a living organism has indeed uncovered several characteristics of the pumping behavior of the embryonic zebrafish heart that, on one hand, differ from those of a technical peristaltic pump (see above) and, on the other hand, correspond to those of a Liebau pump. Such characteristics are for example (1) a nonlinear relationship between the contraction frequency and the flow rate with characteristic peaks that can exceed the maximum flow rate possible for a technical peristaltic pump, and (2) existence of only a single center of active myocardial contractions at an asymmetric position (venous pole) along the length of the heart tube, which generates bidirectional waves traveling along the endocardial layer. It has, therefore, been concluded that tubular embryonic hearts might work as Liebau pumps rather than peristaltic pumps.
To summarize the historical evolution of our present views of the pumping action of the tubular embryonic heart, we can state that both the tubular embryonic heart and the mature four-chambered heart were originally regarded as peristaltic pumps. This was because propulsive peristalsis was originally defined in a much broader sense than today. This definition encompassed any kind of unidirectional progressing sequences of active contractions of the muscular wall of hollow organs that generated unidirectional fluid flow. There were no specific requirements for the dynamics of the muscle (e.g., fast/slow) or fluid flow (e.g., continuous/pulsatile) and for the absence of valves. During the 19th and 20th century, however, the definition of propulsive peristalsis underwent some changes. Biomedical sciences tended to define propulsive peristalsis on the basis of “biological standards” such as the gut or ureter. Technological sciences, on the other hand, tended to define peristalsis on the basis of “technical standards” such as roller pumps. Both tendencies had influence on the perception of the pumping action of the embryonic and mature heart. The sequential contractions of the mature heart chambers were no longer characterized as peristaltic motion and the pulsations of the embryonic heart tube were called peristaltoid rather than peristaltic because of the differences in muscle arrangement between the embryonic heart tube and the gut tube. Moreover, based on the discovery of striking differences in the pumping performance between embryonic heart tubes and technical peristaltic pumps, engineers have recently concluded that the tubular embryonic heart does not work as a peristaltic pump. The fluid and structural dynamics of the embryonic heart tube rather seem to suggest that it acts as a Liebau pump.
PROS AND CONS
In view of the above-mentioned data, two intimately related questions that arise are as follows: (1) Are the arguments put forward against a peristaltic pumping action of the tubular embryonic heart strong enough to justify abandonment of the traditional peristaltic pump theory? (2) Is the evidence that is said to suggest a Liebau effect-based pumping function of the tubular embryonic heart strong enough to justify the full acceptance of this theory? Finding answers to these questions requires careful evaluation of the currently available data. In the following section, we will discuss pros and cons for both of the competing theories. This discussion will focus on three aspects of valveless pumping: (1) the fluid dynamics, (2) the direction of net flow, and (3) the solid (morphological) dynamics.
The arguments put forward against a peristaltic pumping action of the early embryonic heart are mainly of hemodynamic nature and derive from three observations: (1) the blood flow generated by the tubular embryonic heart is pulsatile rather than peristaltic (Hu and Clark,1989). (2) The peak blood flow velocity (mm/sec) recorded in the ventricular inflow exceeds the speed of the corresponding contraction wave traveling along the wall of the inflow region (Farouhar et al., 2006; Butcher et al.,2007). (3) The relationship between the contraction frequency and the blood flow rate (ml/sec) is nonlinear and exceeds the maximum flow rate possible for a technical peristaltic pump (Farouhar et al., 2006).
An important question is whether these data really speak against a peristaltic pumping action of the early embryonic heart. The answer must be yes, if we use the fluid dynamics of a roller pump as defining criteria (Farouhar et al., 2006). This is because the pumping behavior of roller pumps is characterized by (1) the generation of continuous rather than pulsatile flow, (2) a peak flow velocity that does not exceed the speed of the compression wave, and (3) a linear relationship between the compression frequency and the flow rate (Fig. 3). If we go into biological systems, however, we will find at least one example of peristalsis, which shows flow characteristics that are qualitatively similar to that recorded in embryonic heart tubes. This example is gastric emptying. Gastric emptying of liquid meals is accomplished by two mechanisms: (1) a pressure pump, and (2) propulsive peristalsis (Kunz et al.,1998). Peristaltic emptying of the stomach generates a pulsatile transpyloric flow (Anvari et al.,1995) whose peak flow velocity is approximately 60 cm/sec (Hausken et al.,1991), and thereby exceeds the propagation speed of the propulsive peristaltic waves in the antrum, which is approximately 3 mm/sec (Kunz et al.,1998). Biological peristaltic pumps obviously seem able to generate a wider range of fluid dynamical features than technical peristaltic pumps. With respect to the embryonic heart tube, this idea is supported by recent data from a computational model, which show that peristaltic heart tubes can generate pulsatile blood flow as a consequence of the presence of endocardial cushions at the inflow and outflow of the ventricular loop (Taber et al.,2007). Considering all of the above-mentioned fluid dynamic data, we do not think that it is justified, at the present time, to conclude that the early embryonic heart does not work as a peristaltic pump. The data rather show that it does not work according to a technical standard for peristalsis (roller pumps). On the other hand, however, we must emphasize that this fact does not justify exclusion of the possibility that the tubular embryonic heart may work as a Liebau pump.
Direction of Net Flow
While going through the literature about valveless pumping, we became aware of one previously neglected aspect of fluid flow that may cause problems in considering the Liebau effect as a good candidate to explain unidirectional blood flow in the valveless embryonic heart. This aspect is the direction of the net flow generated by valveless pumps. Due to the absence of valves, the direction of peristaltic as well as Liebau effect-driven net flow is not a structurally fixed quality so that both types of valveless pumps can generate anterograde as well as retrograde net flow. The direction of peristaltic net flow corresponds to the direction of the waves of compression traveling along the length of a peristaltic pump. Thus, if the early embryonic heart should work as a peristaltic pump, we must expect that pulsating embryonic hearts regularly show a positive correlation between the direction of the traveling contraction waves and the direction of the net flow. This seems indeed to be the case. We do not know of any observation in embryonic hearts in which the normal anterograde sequence of contractions (inflow → ventricular loop → outflow) was associated with retrograde net flow. We have observed, however, situations in chick and mouse embryonic hearts in which we found a reverse contraction sequence that was associated with retrograde net flow from the ventricular loop toward the systemic venous sinus (data not shown). Thus, with respect to the direction of the net flow, the observations made in living embryos are in full accord with the peristaltic pump theory.
We cannot say that the same holds true for the Liebau pump theory. In fact, the picture obtained from the data reported in the literature is an ambiguous one. Liebau's initial observations in physical models suggested that the direction of net flow generated by his valveless pumps depended primarily on the position of the active site of compression with respect to the ends of the flexible tube (Liebau,1954a,b,1955a,b,1968,1970). The net flow was shown to run from the longer toward the shorter passive portion of the flexible tube (Figs. 3, 4). Corresponding observations were reported by several other researchers (Moser et al.,1998; Kilner,2005; Vogel,2007; Bringley et al.,2008). If we assume that this relationship is characteristic for Liebau pumps, we should be surprised to note that Forouhar and coworkers (2006) see the tubular embryonic heart as a Liebau pump whose active compression site lies at the inflow region, because such a configuration would be expected to generate retrograde instead of anterograde net flow (Fig. 7). It should be noted here, however, that the situation is not as clear as it may appear on the first view. This is because we also found several papers in which the net flow was described as running from the shorter toward the longer passive portion of a Liebau pump (Ottesen,2003; Kenner,2004; Hickerson et al.,2005; Hickerson and Gharib,2006), which is a relationship that would fit with the idea that the early embryonic heart may work as a Liebau pump (Fig. 8). How can this discrepancy be explained? Physical experiments as well as mathematical simulations have shown that the direction of Liebau effect-driven net flow depends not only on the position of the active compression site but additionally depends on several other factors of which the compression frequency seems to be the most important one (Bredow,1968; Ottesen,2003; Kenner,2004; Kilner,2005; Hickerson et al.,2005; Hickerson and Gharib,2006; Jung,2007; Timmermann and Ottesen,2009). In fact, changing the frequency of compression can reverse the direction of Liebau effect-driven net flow in a seemingly unpredictable manner and such reversals have been observed even at frequencies that were in the range of the steadily increasing beating rate of the tubular embryonic heart. It thus might be possible that the above-mentioned differences in the direction of Liebau effect-driven flow may reflect differences in the experimental settings. It is the unstable and seemingly unpredictable behavior of Liebau pumps that makes it difficult to consider the Liebau effect as a good candidate to explain unidirectional blood flow in the valveless embryonic heart.
Solid (Morphological) Dynamics
Peristaltic and Liebau pumps show striking differences in the dynamics of its wall motion that might be used to identify the nature of early cardiac pumping function. The action of peristaltic pumps is characterized by the generation of nonstationary areas of active compression traveling along the length of the tubular pump. In biological peristaltic pumps, these traveling compression waves result from the sequential action of multiple segments of active muscular contractions. Classic Liebau pumps, on the other hand, are flexible tubes that possess only a single, stationary center of active compressions. This active motor center lies at an asymmetric position along its length (Figs. 4, 5) and is said to generate bidirectional elastic waves traveling along the passive upstream and downstream sections of the pump. Thus, if the tubular embryonic heart were a classic Liebau pump, we expect to find only a single center of active myocardial contractions at an asymmetric position along its length. The rest of the heart tube should consist of passive segments. If, on the other hand, the tubular embryonic heart were a biological peristaltic pump, we expect to find multiple segments of active myocardial contractions along the whole length of the organ that normally act in proximodistal order (inflow → ventricular loop → outflow) to generate unidirectionally propagating waves of active contractions.
Based on high-resolution in vivo imaging data, Forouhar et al. (2006) have calculated that embryonic zebrafish heart tubes seem to possess only a single center of active myocardial contractions at its inflow (see Fig. 2 in Forouhar et al.,2006) and regard this finding as a supporting evidence for their theory that the embryonic vertebrate heart works as a Liebau pump. A possibly corresponding observation has, subsequently been made in the tubular heart of bird embryos by another research group (Jenkins et al.,2007). This group analyzed the morphological dynamics of pulsating embryonic quail hearts using an ultrahigh-speed optical coherence tomography imaging system, which facilitated visualization of morphological dynamics that could not be achieved at low temporal resolution. The study uncovered striking differences in the dynamics of wall motion between the proximal (inflow) and distal (outflow) portions of the heart tube. The cardiac inflow was identified as a region of fast contraction and fast propagation of the contractile waves (“kickstart” center), whereas the portions downstream to the inflow were identified as regions of decreasing contraction speed and decreasing wave propagation speed. These data were interpreted as suggesting “… active contraction at the beginning of the tube initiating a wave which then propagate more passively through the rest of the tube” (Jenkins et al.,2007). Although this study has not definitively determined the presence of only a single site of active myocardial contraction in the tubular embryonic quail heart, it is tempting to speculate that the presence of a leading kickstart center might support the Liebau pump theory of early cardiac pumping function.
On the other hand, however, we have to consider that regional differences in the wall motion of tubular embryonic hearts have previously been documented by several other researchers and, thus are not really new findings (Boucek et al.,1959; Castenholz and Flórez-Cossio,1972; Steding and Seidl,1990; Sarre et al.,2006; Vennemann et al.,2006). Furthermore, the presence of a leading kickstart region at the cardiac inflow must not necessarily mean that the tubular embryonic heart owns only a single segment of active myocardial contractions. In fact, there is ample evidence (1) that each segment of the tubular heart of higher vertebrate embryos (inflow, ventricular loop, outflow) is capable of active myocardial contraction; and (2) that the mechanical waves traveling along the wall of the pulsating heart tube of higher vertebrate embryos result from active, sequential contractions of the embryonic cardiac segments. The historically oldest evidence comes from in vitro experiments. If an explanted heart tube from a chick embryo is dissected into multiple pieces, each of these physically isolated pieces will undergo active contractions in a segment-specific frequency, which is high in pieces derived from proximal (inflow) and low in pieces derived from distal (outflow) heart segments (Fano and Badano,1890; Johnstone,1924; Barry,1942). Corresponding observations were made in intact embryonic chick heart tubes in which the segments were electrically isolated from each other by the placement of ligatures around the tube (Pickering,1893; Johnstone,1924). Optical mapping of the electrical activity of the myocardium of the tubular embryonic chick heart has uncovered spatiotemporal sequences of periodic electrical activation that correspond to the propagation pattern of its traveling contractile waves (Chuck et al.,2004; Sedmera et al.,2004). In view of the fact that myocardial contraction is coupled to electric activation, this observation may be regarded as a strong evidence for an active nature of the traveling contractile waves. One might reply that electric activation of an embryonic myocardial segment downstream to the cardiac inflow region must not necessarily mean that it will respond with active contraction, because the segment might be in an immature, precontractile state of electromechanical uncoupling (Hirota et al.,1983; Kamino,1991). However, why should such a segment then start active contraction if it is experimentally isolated from the rest of the heart (see above)? Moreover, during the formation of the embryonic heart tube, the inflow segment becomes added to the venous pole of the heart only after the ventricular segment has already started rhythmic beating (Patten,1949), what makes it difficult to believe that the cardiac segments downstream to the inflow are in a passive state in the fully formed embryonic heart tube. A final evidence for the active nature of the traveling contractile waves is the observation that fully formed embryonic heart tubes can develop partial AV-blocks similar to those found in mature four-chambered hearts (Pickering,1893; Lewis,1923; Paff et al.,1964). If the traveling waves were passively propagating elastic waves initiated by active myocardial contractions of the cardiac inflow segment, we would expect that every contraction of the inflow segment should initiate an elastic wave traveling along the passive downstream segments of the heart. Such a 1:1 relationship is found in the normally beating heart tube. In heart tubes with partial AV-block, however, the relationship between atrial (inflow) and subsequent ventricular contractions can be 2:1 or 3:1 (Pickering,1893; Lewis,1923; Paff et al.,1964). This finding conflicts with the idea of passively propagating elastic waves but can be easily explained by conduction disturbances in the electric activation of the myocardium.
To summarize the above-mentioned data on the myocardial dynamics of early embryonic hearts, we can state that there is ample evidence that the traveling mechanical waves seen in the beating embryonic heart tube of higher vertebrates result from the sequential action of multiple segments of active myocardial contractions along the whole length of the organ. This fact is in accord with the traditional theory of a peristaltic embryonic heart action but strongly speaks against the possibility that the early embryonic heart works as a classic Liebau pump, because such a pump has only a single center of active compressions at an asymmetric position along its length. We should note here, however, that the above-mentioned evidence against the Liebau pump theory comes from higher vertebrate (chick) embryos. Thus, we cannot completely exclude the possibility that the tubular embryonic hearts of lower vertebrates such as the zebrafish may work as classic Liebau pumps as suggested by Forouhar and coworkers (2006). Furthermore, we should note that Liebau's research did not exclusively focus on physical pump models with only a single center of active compressions. He also reported on a few experiments in physical models with two, sequentially acting compression sites suggesting that the tubular hearts of higher vertebrate embryos may work as “bi-” or “multi-segmental” Liebau pumps (Liebau,1955a,1970). For unknown reasons, however, research on the Liebau effect has exclusively focused on mono-segmental pump models, while the bi-segmental models did not receive much recognition during the past 50 years. We, therefore, have no data about the characteristics of such pumps that might be used to identify them in biological systems.
Our evaluation of the functional significance of the dynamics of early cardiac wall motion should not be confined to myocardial dynamics, only. This is because the wall structure of tubular embryonic hearts differs considerably from that of the simple technical tubes traditionally used in physical models of Liebau pumps. The latter have only a single-layered flexible wall while the former have a multilayered wall of which the myocardium forms only the relatively thin outer layer. The mass of the multilayered embryonic heart wall consists of a thick layer of extracellular matrix—the cardiac jelly (Davis,1924)—interposed between the outer myocardium and inner endocardium (Fig. 1). Several data suggest that the cardiac jelly may play important roles in early cardiac pumping function: (1) The systolic narrowing of tubular embryonic hearts ends with complete occlusion of the lumen of the contracting heart segment (Patten et al.,1948; McQuinn et al.,2007; Männer et al.,2008,2009). Using geometrical analyses, Barry (1948) has shown that, under physiological conditions (e.g., ∼20% systolic shortening in circumferential length of the myocardium), the lumen of a tubular embryonic heart cannot be occluded at end-systole without the presence of a layer of cardiac jelly whose thickness must be ∼45% of the radius of the end-diastolic lumen. Because lumen occlusion is regarded as a requirement for effective peristalsis, he concluded that the presence of a thick cardiac jelly layer is a requisite for effective pumping function if the tubular embryonic heart is to propel the blood with a peristaltic-like pumping mechanism. (2) The cardiac jelly shows an uneven spatial distribution, which forces the open lumen of the heart tube to acquire an elliptic cross-section (McQuinn et al.,2007; Männer et al.,2008). This finding might be interesting because mathematical analyses have shown that tubes of elliptic cross-section have a higher mechanical efficiency of peristaltic pumping compared with tubes of circular cross-section (Usha and Rao,1995; Taber and Perruchio,2000). (3) There is evidence that the cardiac jelly is a resilient component of the embryonic heart wall, which becomes deformed during systole and springs back to its original shape during diastolic relaxation and, thereby, might suck the blood into the inflow segment of the heart tube (Barry,1948; Männer et al.,2009). In view of all these data, it may appear that the wall structure of early embryonic hearts is optimized for peristaltic-like blood transport and that the presence of cardiac jelly may be seen as a supporting evidence for the peristaltic pump theory. However, recent studies on Liebau pumps have shown that this must not be the case. Inspired by the multilayered structure of embryonic heart tubes, engineers have constructed a multilayered Liebau pump in which a thick gelatinous layer acts as cardiac jelly (Loumes et al.,2008). The thick gelatinous layer was found to amplify the elastic waves responsible for Liebau effect-driven pumping, so that only small excitations of the active compression side (10% external radius) were needed to produce significant flow (Loumes et al.,2008). The presence of a thick layer of cardiac jelly obviously seems to be advantageous for the pumping function of both kinds of tubular hearts; those working as peristaltic pumps (Barry,1948) as well as those working as Liebau pumps (Loumes et al.,2008). Therefore, the presence of a thick layer of cardiac jelly cannot be regarded as supporting evidence for either of the two competing theories of early cardiac pumping mechanism.
Periodic changes in diameter of the lumen of valveless tubular pumps are final consequences of pumping-related wall motions and the patterns of these changes show striking differences between peristaltic and Liebau pumps. The lumen of Liebau pumps, for example, becomes occluded only at the site(s) of active compressions (Hickerson et al.,2005; Hickerson and Gharib,2006), whereas ideal peristaltic pumps show traveling waves of lumen occlusion caused by the unidirectional propagating waves of compression. Due to the presence of cardiac jelly, the systolic narrowing of a given segment of the tubular embryonic chick heart ends with complete occlusion of its lumen (Patten et al.,1948; Barry,1948; McQuinn et al.,2007; Männer et al.,2008,2009). The traveling contraction waves of the tubular embryonic chick heart thus are accompanied by corresponding waves of lumen occlusion during the initial phase of its pumping activity when all embryonic cardiac segments downstream to the primitive atrium have a thick layer of cardiac jelly (Fig. 9). This picture fits very well with the pattern expected for a peristaltic pump but differs from the pattern expected for a classic mono-segmental Liebau pump.
The peristaltic-like pattern of lumen occlusion is found until developmental stages when the embryonic ventricles start formation of a trabeculated muscle layer (Männer et al.,2009). The formation of myocardial trabeculations is accompanied by a remarkable thinning of the cardiac jelly in the apical portions of the embryonic ventricles, so that their lumen now is no longer occluded at end-systole. The ventricular inflow (atrioventricular region) and the outflow segment of the heart tube, on the other hand, retain a thick layer of cardiac jelly and, therefore, still show lumen occlusion during contraction of their myocardial mantles. These embryonic cardiac segments now act as functional heart valves because they prevent backflow of blood during the cardiac cycle (Patten et al.,1948). Their cardiac jelly layers soon become populated with endocardium-derived mesenchyme and undergo remodeling to form so-called endocardial cushions, which contribute to the formation of the definitive heart valves. In view of the valvular action of its atrioventricular and outflow segments, the question arises whether the tubular embryonic heart should be regarded as a valveless pump during advanced developmental stages.
CONCLUSIONS AND PERSPECTIVES
Considering the pros and cons for both of the two competing theories of early cardiac pumping mechanism, we can state that the currently available data suggest that the tubular hearts of higher vertebrate embryos work neither as technical peristaltic pumps nor as classic Liebau pumps. The fact that the fluid dynamics of early embryonic hearts differ from those of a technical standard for peristalsis (roller pump) does not necessarily mean that we should abandon the possibility that they pump blood by active, sequential contractions of their primitive segments (inflow → ventricle → outflow), which is a behavior that may be regarded as peristalsis from a biological viewpoint. In fact, there is ample evidence for the existence of multiple, sequentially acting segments of active myocardial contractions along the whole length of the heart tubes of higher vertebrate embryos. This fact, on the other hand, does not necessarily exclude the possibility that Liebau effect-driven flow might contribute to the pumping function of embryonic vertebrate hearts. We cannot exclude, for example, the possibility that the tubular hearts of lower vertebrate embryos (e.g., zebrafish) might work as classic mono-segmental Liebau pumps, while those of higher vertebrate embryos (e.g., chick, mouse, human) might work as biological peristaltic pumps. Moreover, the possibility that the embryonic heart tube of higher vertebrates might work as a multi-segmental Liebau pump has never been tested so far. Aristotle's jumping point obviously has saved some of its mysteries into the 21st century. Uncovering the physical mechanisms generating unidirectional fluid flow in valveless embryonic heart tubes may help to understand valveless pumping phenomena in diverse biological systems. Moreover, valveless pumping may be of interest not only for biologists but also for physicians. The Liebau effect, for example, may help to explain some aspects of the function and dysfunction of the so-called Fontan circulation (Kilner,2005). Tubular embryonic hearts, on the other hand, might be good models for the construction of valveless artificial pumps suited for biomedical applications (Loumes et al.,2008). We hope that our article will stimulate interdisciplinary discussions and future studies on the exciting topic of valveless pumping in the developing and mature cardiovascular system.
The authors thank Mrs. Kirsten Falk-Stietenroth and Mr. Hans-Georg Sydow for technical and photographical assistance. We also thank Lars Thrane (DTU Fotonik, Denmark) and Jesper Heebøll (IMFUFA, Roskilde, Denmark) for critical reading of the manuscript.