Three-dimensional reconstruction of the myofiber pattern in the fetal and neonatal mouse heart

Echocardiographic tissue characterization demonstrates differences in the left and right sides of the ventricular septum

The left and right ventricular function of the heart are influenced by the complex structure of the ventricular septum. The cyclic variation of ultrasonic backscatter over the cardiac cycle is known to be sensitive to both structural and functional characteristics of the myocardium. The objective of this study was to investigate differences in the measured magnitude and normalized delay of cyclic variation between the left and right sides of the ventricular septum in normal adult subjects (N = 31). The measured mean magnitudes of cyclic variation were found to be 4.9 ± 0.4 dB and 2.4 ± 0.3 dB (mean ± SE; p < 0.0001) and the corresponding normalized delay values were found to be 0.94 ± 0.05 and 1.59 ± 0.12 (mean ± SE; p < 0.0001) for the left and right sides, respectively. These results show significant differences in the measured magnitude and normalized delay of cyclic variation between the left and right sides of the ventricular septum in normal subjects that appear consistent with predictions based on previously described models of cyclic variation of backscatter and reported measurements of transmural differences in strain properties of the septum.

How are the myocytes aggregated so as to make up the ventricular mass?

Of late, it has become fashionable in the surgical literature to describe the ventricular mass as though arranged in the form of a continuous myocardial band, which starts at the aorta and ends at the pulmonary trunk. On the basis of this concept, its supporters have produced revisionist accounts of cardiac development and ventricular function, as well as using it as the basis for proposed surgical maneuvers. They seem unaware, however, that the original concept itself has never been supported by independent anatomic studies, while, to the best of our knowledge, they have not themselves performed anatomic investigations to prove its substance. Furthermore, the current proponents of the "unique myocardial band" ignore a large body of previous anatomic study which showed that the ventricular mass is arranged in the form of a modified blood vessel, with each myocyte anchored to its neighbor within a 3-dimensional myocardial mesh, rather than being arranged in a fashion analogous to skeletal muscles, with discrete origins and insertions of myocardial bands or tracts. In this review, we summarize the evidence showing that there are no anatomic structures within the ventricular myocardium that permit it to be unraveled in systematic fashion so as to produce the purported myocardial band. We also re-visit our own previous investigations, which supported the conventional approach, namely that the myocytes are aggregated together within a supporting fibrous matrix in the form of a 3-dimensional meshwork.

New aspects of the ventricular septum and its function: an echocardiographic study

OBJECTIVES

To examine whether the line dividing the septum into two layers is found consistently by conventional echocardiography and to evaluate functional differences in the right and left side of the septum in terms of wall thickening, strain rate, and strain imaging.

DESIGN

In a systematic study in 30 normal subjects, M mode and Doppler myocardial imaging data from the interventricular septum (IVS) were recorded. Velocity curves, regional strain rate, and strain profiles were obtained. Systolic deformation (wall thickening, radial and longitudinal strain rate, and strain) of both sides were assessed. Furthermore, three patients with one sided abnormalities were studied.

RESULTS

A bright echo consistently segmented the IVS into a left and right part. In this normal population radial deformation was different for the left and right side of the septum (mean (SD) wall thickening on the left, 49 (46)%, and on the right, 17 (38)%; strain rate on the left, 3.8 (0.6) 1/s, and on the right, 2.1 (1.9) 1/s; strain on the left, 41 (17)%, and on the right, 22 (14)%), whereas longitudinal deformation was found to be similar (strain rate on the left, -2.2 (0.7) 1/s, and on the right, -2.0 (0.6) 1/s; strain on the left, -28 (12)%, and on the right, -25 (12)%). The presented clinical examples show that abnormalities can be strictly limited to one layer.

CONCLUSIONS

Differential radial deformation and knowledge of fibre architecture showing an abrupt change in the middle of the septum, together with the clinical cases, suggest the septum to be a morphologically and functionally bilayered structure potentially supplied by different coronary arteries.

Developmental changes in contractility and sarcomeric proteins from the early embryonic to the adult stage in the mouse heart

Developmental changes in force-generating capacity and Ca2+ sensitivity of contraction in murine hearts were correlated with changes in myosin heavy chain (MHC) and troponin (Tn) isoform expression, using Triton-skinned fibres. The maximum Ca2+-activated isometric force normalized to the cross-sectional area (FCSA) increased mainly during embryogenesis and continued to increase at a slower rate until adulthood. During prenatal development, FCSA increased about 5-fold from embryonic day (E)10.5 to E19.5, while the amount of MHC normalized to the amount of total protein remained constant (from E13.5 to E19.5). This suggests that the development of structural organization of the myofilaments during the embryonic and the fetal period may play an important role for the improvement of force generation. There was an overall decrease of 0.5 pCa units in the Ca2+ sensitivity of force generation from E13.5 to the adult, of which the main decrease (0.3 pCa units) occurred within a short time interval, between E19.5 and 7 days after birth (7 days pn). Densitometric analysis of SDS-PAGE and Western blots revealed that the major switches between troponin T (TnT) isoforms occur before E16.5, whereas the transition points of slow skeletal troponin I (ssTnI) to cardiac TnI (cTnI) and of beta-MHC to alpha-MHC both occur around birth, in temporal correlation with the main decrease in Ca2+ sensitivity. To test whether the changes in Ca2+ sensitivity are solely based on Tn, the native Tn complex was replaced in fibres from E19.5 and adult hearts with fast skeletal Tn complex (fsTn) purified from rabbit skeletal muscle. The difference in pre-replacement values of pCa50 (-log([Ca2+] M-1)) required for half-maximum force development) between E19.5 (6.05 +/- 0.01) and adult fibres (5.64 +/- 0.04) was fully abolished after replacement with the exogenous skeletal Tn complex (pCa50 = 6.12 +/- 0.05 for both stages). This suggests that the major developmental changes in Ca2+ sensitivity of skinned murine myocardium originate primarily from the switch of ssTnI to cTnI.

Developmental patterning of the myocardium

The heart in higher vertebrates develops from a simple tube into a complex organ with four chambers specialized for efficient pumping at pressure. During this period, there is a concomitant change in the level of myocardial organization. One important event is the emergence of trabeculations in the luminal layers of the ventricles, a feature which enables the myocardium to increase its mass in the absence of any discrete coronary circulation. In subsequent development, this trabecular layer becomes solidified in its deeper part, thus increasing the compact component of the ventricular myocardium. The remaining layer adjacent to the ventricular lumen retains its trabeculations, with patterns which are both ventricle- and species-specific. During ontogenesis, the compact layer is initially only a few cells thick, but gradually develops a multilayered spiral architecture. A similar process can be charted in the atrial myocardium, where the luminal trabeculations become the pectinate muscles. Their extent then provides the best guide for distinguishing intrinsically the morphologically right from the left atrium. We review the variations of these processes during the development of the human heart and hearts from commonly used laboratory species (chick, mouse, and rat). Comparison with hearts from lower vertebrates is also provided. Despite some variations, such as the final pattern of papillary or pectinate muscles, the hearts observe the same biomechanical rules, and thus share many common points. The functional importance of myocardial organization is demonstrated by lethality of mouse mutants with perturbed myocardial architecture. We conclude that experimental studies uncovering the rules of myocardial assembly are relevant for the full understanding of development of the human heart.

Local and regional variations in myofibrillar patterns in looping rat hearts

BACKGROUND

In chickens, cytodifferentiation, right side dominance in myofibril development, and variations in myofibrillar patterns in different areas and layers of the myocardial wall exist which have been implicated in the process of heart looping. Little comparable information is available for developing myofibrillar patterns in the early development of mammalian hearts.

METHODS

We have used transmission electron microscopy (TEM), confocal scanning laser microscopy (CSLM), and 3-D reconstruction techniques also present in the looping hearts of embryonic day (ED) 9.5 to 11.5 rat hearts.

RESULTS

Local and regional variations and right side dominance in myofibrillar patterns were shown during looping in 9.5 through 11.5 days of development in embryonic rat heart. At 9.5 days of development, myofibrils near the lumen of the myocardial wall were primarily in circumferential bands while near the pericardial surface they were primarily in longitudinal bands. In older embryos, regional variations in myofibrillar organization was found in areas associated with the cardiac cushions, trabeculae, and myocardial wall of the developing heart chambers. Based on sarcomeric structure, myofibrils in the ventricle and outflow tract were more advanced than those found in the atrial wall.

CONCLUSIONS

The local and regional patterns of myofibrils in looping rat hearts are similar to those which have been found in developing chicken hearts. This study and others indicate cytodifferentiation and development of the contractile apparatus has a crucial role in the process of heart looping.

Determination of relative fiber orientation in heart muscle: methodological problems

Knowledge of the muscle pattern in the heart is important to understanding cardiac contraction and propagation of the electrical stimulus. Most work on this pattern has been carried out by blunt gross dissection, whereby fiber bundles are easily visible on the peeled heart wall. However, it has never been shown, to our knowledge, that the orientation of macroscopic fiber bundles seen in a peeled heart corresponds to that of the constituent myofibers (muscle cells). For this purpose, one needs to carry out a three-dimensional microscopic reconstruction within a documented macroscopic reference frame. To draw valid conclusions in such a coordinated macroscopic and microscopic study, one must estimate the (slice) angle between the long axis of a muscle cell and the plane of section. Otherwise any alleged differences between the macroscopic and microscopic orientations may be just an artifact of sectioning. In this study we have shown that, provided the images of the myofibers meet simple criteria, one can be reasonably confident that the potential error incurred by sectioning is small. On this basis, we demonstrated that while there is a general correspondence between the macroscopic fiber and the microscopic myofiber orientations, there are significant differences in detail.

Clonal analysis of cardiac morphogenesis in the chicken embryo using a replication-defective retrovirus: I. Formation of the ventricular myocardium

Cells of the precardiac mesoderm (stages 4-6) and dividing myocytes of early hearts (stages 10-15) were tagged with a replication-incompetent retrovirus (CXL) (Mikawa et al., 1991b) encoding bacterial beta-galactosidase (beta-gal). Two protocols were used to infect the cardiogenic cells. (1) Small blocks (approximately 50 micron 2) of anterolateral mesoderm were dissected from gastrula-stage embryos (stages 4-6) and incubated in liquid medium containing the retrovirus. After removal of CXL, the tissues were dispersed into single-cell suspensions and pressure injected into the precardiac areas of recipient embryos (stages 4-6). Such embryos were then incubated in vitro at 37 degrees C for 2 days (New, 1968), and those embryos with beating hearts were fixed for X-gal histochemistry and paraffin serial sectioning. (2) CXL was pressure injected in ovo (embryonic stages 4-15) into cardiogenic tissues and the eggs subsequently returned to an incubator. At selected stages of development embryos or whole hearts were fixed, stained with X-gal, and serially sectioned after paraffin embedding. The first method showed that (1) cells of the precardiac mesoderm could be infected with the retrovirus, (2) the transplanted cells would differentiate into beating myocytes, and (3) beta-gal expression was sufficiently high to be detected histochemically. With the second procedure we could show that (1) beta-gal-tagged cells formed colonies in the myocardium, (2) the labeled cells were exclusively myocytes, (3) the number of cells per colony increased with increasing age of embryonic development, (4) the size of colonies was larger in the left than the right ventricle, (5) many of the colonies were transmural, i.e., they extended from epicardial to endocardial layers of the myocardium and generally exhibited a cone or funnel-shape with the base of the cone nearest the epicardium, (6) the orientation of myocytes within each colony changed at different layers of the myocardium, and (7) the cones contained both beta-gal+ and beta-gal- myocytes. DNA labeling studies with [3H]thymidine indicated that cardiogenic cells divided every 16-18 hr during the first week of development and that the CXL-labeled cells divided indistinguishably from unlabeled myocytes. Based on these observations a model for the growth of the myocardium is presented.

Myofiber orientation in the weanling mouse heart

This study provides a quantitative description at the cellular level of myofiber orientation throughout the ventricles of the mouse heart. We employed computer-based methods of three-dimensional reconstruction from 3 microns plastic-embedded serial sections. Registration marks were introduced by drilling minute holes into each plastic block. Subfields of selected sections were photographed at 20 x magnification, using a computer-controlled microscope. The 35-mm film frames were projected onto a digitizer tablet and the epi- and endocardial boundaries were digitized manually. The "heads" and "tails" of linear segments of a representative myofiber sample present in each projected image were digitized in point mode. The many x-, y-, z-coordinate tables generated by digitization were reassembled automatically, giving a numerical description of the myofiber pattern. This pattern was studied interactively on a high-performance graphics workstation. We find that the heart wall is, to a first approximation, a "sandwich," in which the myofibers in the middle layer run mainly circumferentially, whereas those in the inner and outer layers run parallel or oblique to the apical-basal axis, a variant of the classical model of the myofiber pattern. We observed a "sleeve" in the interventricular septum, formed by longitudinal and oblique myofibers, a feature which apparently has not been described previously. Myofibers not running parallel to the transverse or longitudinal planes were not resolved in this study. We conclude that three-dimensional reconstruction of the cardiac myofiber pattern at the light-microscopic level, while laborious, is technically feasible and scientifically worthwhile.

Fiducial points for three-dimensional computer-assisted reconstruction of serial light microscopic sections of umbilical cord

Fascicles of human sural (peripheral sensory) nerve were used as external fiducial points (FP) for accurate registration of serial light microscopic sections in three-dimensional reconstruction of human umbilical cord. This paper describes a method for embedding the FPs within the paraffin wax block simultaneously with the specimen to be sectioned. Using a new design of embedding box, the FPs are embedded close to the specimen and are transferred to the slides as part of the tissue sections. Three to four FPs were used to align and scale the serial tissue sections for digitization and computerized reconstruction, using a commercially available software program on IBM-compatible 80386 hardware. A three-dimensional solid surface graphic model of a segment of human umbilical cord was generated.