Embryo Circulation




(1)
Professor of Anesthesiology, Albany Medical College, Albany, NY, USA

 



Keywords

CardiogenesisCardiovascular lineageHeart progenitorsHeart loopingCardiac jellySeptationSinus venosusCardiac tube


1.1 Introduction


Over the past several decades, the search for the unifying paradigm between the form and function of the early vertebrate embryo heart has focused on genetic patterns [1–3] as the blueprints for early heart formation, enhanced by phylogenetic and morphologic observations [4–7]. More recently, however, there has been a resurgence of interest in epigenetic factors such as intracardiac flow patterns and fluid forces as significant factors in early embryo cardiogenesis [8, 9] and vascular formation [10–12]. The availability of new techniques such as confocal microscopy, phase contrast magnetic resonant imaging, digital particle velocimetry, and high-frequency ultrasonographic imaging, used for in vivo observation of embryonic flow dynamics, have yielded new insights into the early embryo hemodynamics [13].


While it has been commonly assumed that the early vertebrate embryo heart works as a peristaltic pump this view has been contested on the grounds of newly acquired imaging and hemodynamic data. The existing evidence no longer supports the accepted mode of heart’s peristaltic blood propulsion and has called for radical re-evaluation of the traditionally accepted model of early circulation [14–18]. In the light of new findings, Forouhar et al. proposed that the early embryo heart works as a dynamic suction pump (vide infra) [14]. The existing evidence presented in this paper together with the evidence reviewed by Männer [15] suggests that the heart works neither as a peristaltic, nor as a dynamic suction pump, which leaves the question of early embryonic blood propulsion essentially unanswered.


Nearly a hundred years ago, Austrian philosopher and educator R. Steiner maintained that the blood in the organism possesses its own motive force and that the heart rather than being the organ of propulsion, dams-up the flow of the blood in order to create pressure. Steiner further suggested that observation of the early embryonic circulation offers the best proof of this phenomenon [19, 20]. Despite the fact that over the years, several publications have appeared in support of this theory, only a few deal specifically with early embryo circulation [21–25].


1.2 Morphologic Features


The heart and the system of vessels are the first functional organs to develop in the vertebrate embryos. Although species specific at the sub-cellular level, the early functional and morphological features are nearly identical among all vertebrates [1, 3, 6]. The embryo circulatory system is a functional unit, consisting of the extra-embryonic yolk sac circulation and of the circulation belonging to embryo proper.


The yolk sac (vitelline) vascular formation is the first to form in the mammalian embryo and consists of mesodermally derived endothelial and erythroid (red blood cell) precursors. They share a common progenitor, the hemangioblast, which differentiates already at the pre-gastrulating stage and migrates into the region of the yolk sac. The erythroid cells amass in a narrow circumferential band at the proximal end of the yolk sac. At this stage, the so-called blood island contains only a few endothelial precursors. The majority of the endothelial cell elements, however, are assembled into a loose vascular network, the primary capillary plexus, just distally to the blood island. During subsequent development, the endothelial cells partition the erythroid precursors into smaller channels. Finally, the cell-filled vascular bed is formed and is joined with the primary capillary plexus just prior to the onset of circulation. The vitelline circulation supports the nutritive and respiratory functions of the embryo [26, 27].


The first inception of the vertebrate heart (mammalian and avian) arises from the presomitic cranial mesoderm (cardiogenic plate) during early gastrulation. The progenitors of bilateral cardiac fields merge at the anterior margin to form the “cardiac crescent.” These fields contain precursors for myocardial and endocardial (endothelial) cells (Fig. 1.1). Specification into the cardiomyocytes and endocardial cells occurs just before formation of the cardiac crescent. The endocardial cells assemble into loose vascular plexus adjacent to developing cardiomyocytes and coalesce into a single, capillary-size endocardial tube which is the first vascular structure of the vertebrate embryo. The resulting tubular heart consists of an external myocardial and of an inner endocardial layer [1, 27, 28]. There is evidence that a common cardiovascular progenitor exists, with a potential to become a cardiomyocyte, an endothelial cell or a vascular smooth muscle cell, which suggests that the heart, the vessels and their content, the blood, share the same origin [29–32] (Fig. 1.2).

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Fig. 1.1

Cardiogenesis in mouse at embryonic day (E) 7.5, 8, and 8.5: The primary heart forming field is shown in red, and the secondary in blue. Heart shading corresponds to approximate heart-field contributions to future heart regions. AS atria and sinus venosa, CT conotruncus, RV right ventricle, LV left ventricle, AHF anterior (secondary) heart field. (Adapted from ref. [28], used with permission of Wolters Kluwer Health)


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Fig. 1.2

The origin of cardiovascular lineage. Mesoderm-derived cardiovascular progenitor cells serve as precursor of the red blood cell, heart, endothelial and vascular smooth muscle cell lineages. (Adapted from ref. [29], used with permission of Wolters Kluwer Health)


The tubular heart is formed by progressive fusion of the paired primordia in the caudal direction. At its upper pole, the tube consists of the inception of the bulbar sac and of the apical portions of the ventricles. Caudally, it divides into the paired venous limbs, the future sinus venosus, riding over the anterior intestinal portal [33]. The myocardium first invests the endocardial primordia at the bulbar end and then progressively in the caudal direction, as the fusion of the endocardial primordia progresses [34].


Further development of the tubular heart is a sequential process of lengthening and bending, as it frees itself from its attachment to the dorsal mesocardium, forming a dorsally oriented C-loop. The subsequent looping of the heart tube is marked by pole reversal and torsion, in which the venous pole of the tube heart shifts upward and the arterial pole moves downward, forming a doubly bent, S-shaped organ (Figs. 1.3 and 1.4). The loop heart consists of four distinct parts which follow each other serially in the direction of the flowing blood (caudo-cranially):


  1. 1.

    The sinus venosus, located at the junction of vitelline veins, receives the blood returning from the yolk sac and the venous blood from the embryo


     

  2. 2.

    The early atrium as the first dilation of the heart tube


     

  3. 3.

    The ventricle formed by the bent mid-portion of the original cardiac tube


     

  4. 4.

    The aortic bulb which connects the ventricle with the ventral aortic roots


     

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Fig. 1.3

Linear heart tube of HH stage 9 chick embryo (a). The vertical portion of the heart consists of tissue mass belonging to the future right ventricle, RV; of the aortic sac, A; and of paired venous limbs, RVL and LVL; riding on anterior intestinal portal, AIP. (b) Early torsion and looping of the heart in HH stage 12 chick embryo and (c) completed right-handed loop in HH stage 17/18 embryo. PO proximal outflow tract, RV right ventricle, LV left ventricle, AV atrio-ventricular canal, LA left atrium, RA right atrium. (d) Schematic representation of looping. Note torsion and pole reversal in which the caudal, venous end of the heart (RV) shifts upwards, and the arterial pole (LV) moves downward, an early morphological gesture indicating that the heart is primarily an organ of impedance rather than propulsion. RV right ventricle, LV left ventricle, A atrium, O outflow tract. (Reproduced from ref. [33], used with permission of John Wiley and Sons)

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May 1, 2020 | Posted by in CARDIOLOGY | Comments Off on Embryo Circulation

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