Structural and elemental characterization of heart cells grown in a collagen matrix.

A novel preparation of spontaneously contracting heart cells embedded in a collagen strand provides an ideal experimental system for correlative structure-function experiments that utilize the techniques of electron microscopy, quantitative electron probe x-ray microanalysis (EPXMA) and imaging. Heart cells grown within the strand for 1 day possess the subcellular content and distribution of physiologically relevant elements--Na, Mg, P, S, Cl, K, and Ca--found in intact heart cell preparations. The presence of junctional specializations between, and organized myofibrils within, the majority of cells after 1 day in culture also establishes that the collagen matrix promotes vigorous cell development as well as maintains physiological integrity. EPXMA, combined with ultrastructural analyses, provides elemental content data on a cell-by-cell basis. In studies presented here, viable cells, comprising over 80% of the strand cell population, could be distinguished easily from those which had been functionally compromised, not only by aberrant structure but also by altered subcellular compartmentation of Na, K, Cl, and Ca. Within individual viable cells, compartmental differences in element content were notable especially between mitochondria and cytoplasm. However, nuclear euchromatin, but not heterochromatin, appeared approximately identical to cytoplasm in elemental and water content. In such cells, the cytoplasmic K:Na ratio was maintained at a high level (approximately 15:1). The results with respect to K, Na, and other elements demonstrated the integrity of membrane transport mechanisms regulating the movement and distribution of ions and the maintenance of ionic homeostasis in cells of the strand preparation.

The sarcoplasmic reticulum of mouse heart: its divisions, configurations, and distribution.

The sarcoplasmic reticulum (SR) is a prominent, highly ramified component of mouse myocardial cells. The use of ferrocyanide-reduced osmium tetroxide (OsFeCN) as a postfixative solution facilitates appreciation of both its extent and three-dimensional architecture. We have found that the individual volume fractions (Vv) of myofibrils, mitochondria, and SR are similar in cells of the right and left ventricular walls. Vv(total SR) is approximately 7%, a value considerably larger than previously reported. We attribute this disparity in large part to the recognition factor which comes into play with OsFeCN-treated tissue. Previous observations pertaining to the stereology of myocardial SR have likely substantially underestimated both volume fraction and surface density of this membrane system, since none to this point has utilized specific staining such as that conferred by the OsFeCN regimen. Our stereological measurements of different depths of the ventricular cell indicate that although considerable differences are found between SR configuration at peripheral and deep cell levels, no significant difference exists between the volume fractions of either the total SR or its individual constituents. Two different stereologic regimens gave close agreement on volume fractions of the various SR segments; the majority (approximately 92%) of the total SR is network SR, whereas the remainder is composed of the various categories of junctional SR (peripheral, apposed to the surface sarcolemma; interior, complexed with the transverse-axial tubular system; corbular, existing free of sarcolemmal contact). In the adult mouse, interior junctional SR greatly preponderates the other types of junctional SR; corbular SR is qualitively assessed to be a far more common component of atrial cells than of ventricular cardiomyocytes.

The transverse-axial tubular system (TATS) of mouse myocardium: its morphology in the developing and adult animal.

Invaginations of the sarcolemma that generate the transverse-axial tubular system (TATS) of the ventricular myocardial cells have begun to develop in the mouse by the time of birth. The formation of the TATS appears to be derived from the repetitive generation of caveolae, which forms "beaded tubules". Beaded tubules are retained in the adult, in which they frequently present a spiraled topography. Development of the TATS progresses so rapidly that complex systems are already present in the cardiac muscle cells of young mice; by 10-14 days of age, the ultrastructure is essentially identical to that of the adult. The mouse myocardial TATS is composed of anastomosed elements that are directed transversely and axially (longitudinally). Many tubules have an oblique orientation, however, and most elements of the TATS are highly pleiomorphic. In this respect the TATS of the mouse heart is relatively primitive in appearance in comparison with the more ordered TATS latticeworks typical of the ventricular cells of other mammals. Stereological analysis of the mouse TATS indicates that the volume fraction (VV) and surface density (SV) are considerably greater than previously reported (3.24% and 0.5028 micron-1, respectively). The most complex ramifications of the TATS are embodied in the subsarcolemmal caveolar system and the deeper tubulovesicular "labyrinths", both of which can be found in early postnatal and adult ventricular cells. In atrial cells, TATS development is initiated several days later than in the ventricular cells. The TATS of adult atrial myocardial cells is less prominent than the ventricular TATS and consists largely of axial elements; the incidence of the TATS, furthermore, is more pronounced in the left than in the right atrium.