Joachim R. Sommer

Comparative ultrastructure of cardiac cell membrane specializations. A review

Abstract Among animal hearts, there are many differences in the anatomic features of various myocardial cells, the occurrence of junctional complexes joining adjacent cells, the geometry of apposition of cells forming bundles of fibers, and the form and extent of specializations of intracellular membrane systems. The working fibers of chicken and frog hearts, which have no transverse tubules, are considerably smaller in diameter than those of their mammalian counterparts, which do have such tubules. Purkinje fibers in the hearts of small mammals differ in structure and appositional geometry from those of the larger mammals, and again from those of the chicken. All Purkinje fibers lack transverse tubules. Focal specializations of the sarcoplasmic reticulum, the junctional sarcoplasmic reticulum, are prominent in chicken and mammalian, but not in frog cardiac muscle. Gap junctions are frequent in mammalian hearts, but seem to be rare, small or absent altogether in both frog and chicken hearts.

Comparative stereology of the mouse and finch left ventricle

The volume fractions and surface per unit cell volume of some subcellular components of the left ventricles of the finch and mouse were quantitated by stereologic techniques. These species were chosen for study because they have similar heart rates but differ morphologically in some respects: fiber diameter is larger in the mouse; the mouse has transverse tubules while the finch does not; and the finch has a form of junctional sarcoplasmic reticulum (JSR), extended JSR (EJSR), located in the cell interior with no direct plasmalemmal contact, while the mouse interior JSR (IJSR) abuts on transverse tubules. Our data show that the volume fraction (Vv) and surface area per unit cell volume (Sv) of total SR, and free SR (FSR) are similar. The volume fractions of mitochondria, myofibrils, and total junctional SR were also similar. The Sv of the cell surface of the finch was similar to the Sv of the cell surface of the mouse (Sv-plasmalemma plus Sv of the transverse tubules). The principal difference was in the distribution of JSR; the mouse peripheral JSR (PJSR) represents only 9% of the total JSR, while the finch PJSR accounts for 24% of the birds JSR. The similar volume fractions of total junctional SR (PJSR + EJSR in the finch; PJSR + IJSR in the mouse) suggest that the EJSR is not an embryologic remnant, and raises the possibility that some function of JSR is independent of plasmalemmal contact.

Real-time quantitative elemental analysis and mapping: microchemical imaging in cell physiology.

Recent advances in widely available microcomputers have made the acquisition and processing of digital quantitative X‐ray maps of one to several cells readily feasible. Here we describe a system which uses a graphics‐based microcomputer to acquire spectrally filtered X‐ray elemental image maps that are fitted to standards, to display the image in real time, and to correct the post‐acquisition image map with regard to specimen drift. Both high‐resolution quantitative energy‐dispersive X‐ray images of freeze‐dried cyrosections and low‐dose quantitative bright‐field images of frozen‐hydrated sections can be acquired to obtain element and water content from the same intracellular regions. The software programs developed, together with the associated hardware, also allow static probe acquisition of data from selected cell regions with spectral processing and quantification performed on‐line in real time. In addition, the unified design of the software program provides for off‐line processing and analysing by several investigators at microcomputers remote from the microscope. The overall experimental strategy employs computer‐aided imaging, combined with static probes, as an essential interactive tool of investigation for biological analysis. This type of microchemical microscopy facilitates studies in cell physiology and pathophysiology which focus on mechanisms of ionic (elemental) compartmentation, i.e. structure‐function correlation at cellular and subcellular levels; it allows investigation of intracellular concentration gradients, of the heterogeneity of cell responses to stimuli, of certain fast physiological events in vivo at ultrastructural resolution, and of events occurring with low incidence or involving cell‐to‐cell interactions.

Corbular sarcoplasmic reticulum of rabbit cardiac muscle

The structure of corbular sarcoplasmic reticulum as part of the sarcoplasmic reticulum (SR) in perfusion-fixed rabbit cardiac muscle was studied by thin sections and freeze fracture. In thin sections, processes on the surface of corbular SR have all the anatomical features of junctional processes of junctional SR. By freeze fracture, the E face of corbular SR was particle poor and showed deep pits; the P face was particle rich. The demonstrated structural homology of corbular SR to all forms of junctional SR justifies its inclusion in that group.

Cardiac muscle: An attempt to relate structure to function☆☆☆

Abstract Two structure-function hypotheses were tested in this paper: Are the physiological functions—the force-frequency relationship and/or the response to low sodium media—related to the degree of differentiation of the coupling [a specialized close association of sarcolemma and sarcoplasmic reticulum]? A comparative study of the ultrastructure of representatives of several vertebrate classes [mammalia, aves, reptilia, amphibia, pisces and chondrichthyes] revealed that the coupling rather than other structural differences was the best candidate for a structure-function study. A wide variation in the degree of differentiation of the coupling and the associated sarcoplasmic reticulum was found throughout the range of animal classes. Well-formed couplings occurred in all hearts except that of the frog and mudpuppy where they were sparsely distributed and poorly differentiated. Parallel comparative function studies—the force-frequency or interval-strength relationship and the response to low-sodium media—were performed on hearts from animals from the same classes. Although all hearts developed steady tensions when exposed to low-sodium media, there were microscopic differences: In the frog the sarcomeres shortened to a steady value, whereas in hearts from other animals, groups of sarcomeres twitched repeatedly and without synchrony among groups (vermiculation). The structure-function hypothesis implied by this functional difference was disproven: the sarcomeres in the chicken embryo hearts, both with and without identifiable couplings, vermiculated; sarcomeres in hearts of the turtle and salamander, with well-formed couplings, sometimes vermiculated and sometimes shortened like those of the frog. Using the maximum rate of rise of tension in a contraction as the measure of contractility, the hearts of all animals tested fell into two classes according to the characteristics of their force-frequency relationships. In one class, which included all but amphibian hearts, contractility increased between contractions, whereas in the other class, contractility declined or did not change between contractions. Hearts without well-differentiated couplings were never found in the first class, whereas hearts with or without couplings were found in the second class.

Comparative stereology of mouse atria

The left and right atria of the mouse were compared to each other and to the mouse left ventricle using stereologic techniques. The volume fraction (Vv) and surface area per unit cell volume (Sv) of the interior junctional sarcoplasmic reticulum (IJSR), total JSR and extended JSR were greater in the left atrium than in right. The Vv and Sv of the free SR, transverse tubules, and mitochondria were similar in the two atria. It is suggested that the differences in junctional sarcoplasmic reticulum between the atria can be accounted for by a difference in distribution of two types of cells whose anatomy is analogous to working and conducting fibers in the ventricle. The Sv and Vv of the transverse tubules, mitochondria, and all the components of the sarcoplasmic reticulum except for the free SR were greater in the left ventricle than in either atrium. The greater calcium content and sensitivity to extracellular calcium of the atria may explain the greater volume of free SR in the atria as compared to the left ventricle. The Sv of the plasmalemma of the atria and of the Sv of the plasmalemma of the transverse tubules of the left ventricles supports the suggestion of others that there is a constant ratio of surface area to cell volume in cardiac cells.

Comparative stereology of the lizard and frog myocardium.

The atria and ventricles of the frog and lizard were quantitated using stereologic techniques. The volume fraction (Vv) and surface density (Sv) of the free, junctional and total sarcoplasmic reticulum and mitochondria of the lizard atrium and ventricle were greater than in the corresponding chambers in the frog. Myofibrillar volume fraction and plasmalemmal surface density did not differ between the two species. The volume fraction and surface density of the free and total SR, and myocardial granules were greater in the lizard atrium than ventricle but the myofibrillar Vv and mitochondrial Vv and Sv were less. The Sv of the free SR, total SR, and the Vv and Sv of myocardial granules of the frog atrium were greater than in the frog ventricle. There were no differences between myofibrils and mitochondria in the frog atrium and ventricle.

Implications of structure and geometry on cardiac electrical activity

Electrical activity in the heart is dependent on the structure of the cellular components and their appositional geometry. The cells of the conduction system in mammals have a structure favoring faster conduction vis a vis the common working cells of the ventricles, which is further enhanced in large mammals and, significantly, in birds by an increase in cell diameters and tight packing of the component cells into large bundles. Tight packing also generates very narrow intercellular clefts that because of accumulation and depletion phenomena may contribute significantly to the measured electrical activity. Conduction cells may exist in mammalian atria analogous to their presence in bird atria, their electrical activity being possibly influenced by their diffuse anatomical integration with the common atrial working cells. Cell and bundle connections appear to be frequent within one length constant.

Filipin—cholesterol complexes in the sarcoplasmic reticulum of frog skeletal muscle

The polyene antibiotic filipin forms electron microscopically visible complexes with cholesterol. In skeletal muscle fibers the filipin complexes are found mainly in the surface plasmalemma and transverse tubules, as closely packed single units. In the sarcoplasmic reticulum the filipin complexes are much less common, occur as clusters, and have a pronounced predilection for the intermediate cisterna. The incidence and distribution of the filipin complexes in situ and in SR vesicles compares well with the quantitative chemical analysis of the cholesterol content in comparable subcellular fractions of skeletal muscle reported in the literature.

The intermediate cisterna of the sarcoplasmic reticulum of skeletal muscle.

The intermediate cisterna is a portion of junctional SR. Its anatomical delineation depends on the geometry of the free SR, which itself may vary between a cisternal and the conventional tubular form. The characteristic narrowness of the intermediate cisterna may reflect a functional state in which the cisternal envelope collapses. Tubular SR resists such a collapse because it is near the minimum radius of curvature of its membranes. Total collapse, induced by cations, occurs only in cisternal SR. Freeze-fracture showed identical collapse in quick-frozen muscle fibers not exposed to fixation or cryoprotectants. Total collapse of the SR envelope was also observed in isolated SR vesicles, provided their diameters had previously been considerably enlarged by fusion of several vesicles. The SR may oscillate between different geometries in vivo , perhaps as a result of cation fluctuations during the contraction—relaxation cycle.