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 (V_v) and surface area per unit cell volume (S_v) of the interior junctional sarcoplasmic reticulum (IJSR), total JSR and extended JSR were greater in the left atrium than in right. The V_v and S_v 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 S_v and V_v 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 S_v of the plasmalemma of the atria and of the S_v 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 (V_v) and surface density (S_v) 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 V_v and mitochondrial V_v and S_v were less. The S_v of the free SR, total SR, and the V_v and S_v 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.     

The atrial myocardial cells of mouse heart: a structural and stereological study

Structural and stereological studies of mouse atrial myocardial cells, carried out in the same fashion as our previous investigations on mouse ventricle, demonstrate an extremely well-developed sarcoplasmic reticulum (SR) in atrial cells. The volume fraction (Vv) of the SR exceeds 12% in mouse atrial cells; perimyofibrillar network SR constitutes the major portion. We have confirmed the findings of Bossen et al. (1981, Tissue Cell 13, 71-77) of a difference between atria in terms of coupling density, the right atrium having a significantly lower incidence of interior junctional SR than the left. The SR of mouse atrium comprises a rich variety of specialized segments, including the IJSR, peripheral junctional SR, corbular SR, cisternal SR (including regions similar to fenestrated collars of striated skeletal muscle SR), as well as a peculiar form of extended junctional SR (EJSR). Although less frequent in occurrence than corbular SR, the EJSR seems closely related, since it occurs in multiple clusters at or near the Z-line regions, contains internal granular densities, and bears surface-connected structures resembling junctional processes. Seen in thin sections, mouse atrial EJSR elements are more complex than corbular SR, being larger in diameter and frequently circular in profile. Thick-section and serial-section analyses reveal that bodies of EJSR are in fact hollow spheroids. The transverse-axial tubular system of mouse atrium is rather poorly developed in comparison to its ventricular counterpart. The Golgi apparatus and associated specific atrial granules are prominent cell components. "Focal ellipsoidal deposits" (FEDs) previously described by Page and co-workers (1986, Amer. J. Physiol.) are consistently located adjacent to the Golgi region, but immunocytochemical staining for two different segments of atrial natriuretic peptide reveals no specific reaction in FEDs, whereas the SAGs are densely labeled for both antibodies.     

Axial Tubules of Rat Ventricular Myocytes Form Multiple Junctions with the Sarcoplasmic Reticulum

Ryanodine receptors (RyRs) are located primarily on the junctional sarcoplasmic reticulum (SR), adjacent to the transverse tubules and on the cell surface near the Z-lines, but some RyRs are on junctional SR adjacent to axial tubules. Neither the size of the axial junctions nor the numbers of RyRs that they contain have been determined. RyRs may also be located on the corbular SR and on the free or network SR. Because determining and quantifying the distribution of RyRs is critical for both understanding and modeling calcium dynamics, we investigated the distribution of RyRs in healthy adult rat ventricular myocytes, using electron microscopy, electron tomography, and immunofluorescence. We found RyRs in only three regions: in couplons on the surface and on transverse tubules, both of which are near the Z-line, and in junctions on most of the axial tubules—axial junctions. The axial junctions averaged 510 nm in length, but they occasionally spanned an entire sarcomere. Numerical analysis showed that they contain as much as 19% of a cell's RyRs. Tomographic analysis confirmed the axial junction's architecture, which is indistinguishable from junctions on transverse tubules or on the surface, and revealed a complexly structured tubule whose lumen was only 26 nm at its narrowest point. RyRs on axial junctions colocalize with Ca_v1.2, suggesting that they play a role in excitation-contraction coupling.     

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.     

Structures located at the levels of the Z bands in mouse ventricular myocardial cells

Within ventricular myocardial cells of the mouse, the myoplasmic regions located immediately adjacent to the Z lines of the sarcomeres contain a variety of structures. These include: (1) transversely oriented 10 nm (‘intermediate’) filaments that apparently contribute to the cytoskeleton of the myocardial cell; (2) the majority of the transverse elements of the T-axial tubular system; (3) specialized segments of the sarcoplasmic reticulum (SR) that are closely apposed to the sarcolemma or T-axial tubules (junctional SR); (4) ‘extended junctional SR’ (‘corbular SR’) that exists free of association with the cell membrane; (5) ‘Z tubules’ of SR that are intimately apposed to the Z line substance; and (6) leptofibrils. In addition, fasciae adherentes supplant Z lines where myofibrils insert into the transverse borders (intercalated discs) of the cells. The concentration of these myocardial components at the level of the Z lines suggests that a particular specialization of structural and physiological activities exists in the Z-level regions of the myoplasm. In particular, it appears that the combination of intermediate filaments, T tubules, and Z-level SR elements forms a series of parallel planar bodies that extend across each myocardial cell to impart transverse rigidity. The movement and compartmentation of calcium ion (Ca²⁺) would seem especially active near the Z lines of the myofibrils, in view of the preferential location there of Ca²⁺-sequestering myocardial structures such as T tubules, junctional SR, extended junctional SR and Z tubules.     

Ultrastructure of the heart of the marine mussel,Geukensia demissa

The structure of the heart of Geukensia demissa, a common object of physiological and biochemical investigation, is described by scanning, transmission and freeze-fracture electron microscopy. A single-cell epithelial layer covers the ventricle, but an endothelium is lacking. Myofibers are small (6–7 μm diam.), mononucleate, and tapered. Glycogen is concentrated peripherally. Mitochondria are particularly concentrated under the sarcolemma, near the ends of the nucleus, and in rows between bundles of myofilaments. The myofilaments (6–8nm thin, 30–35 nm thick filament diam.) are loosely arranged into sarcomeres (2–4 μm) by Z bodies. Many of these Z bodies interconnect, and some anchor to the sarcolemma forming attachment plaques. Cells are joined by intercalated discs consisting of fascia adherentes, spot desmosomes, and gap junctions. The gap junctions include intramembrane particles. T tubules are absent. The sarcolemma is coupled to the junctional sarcoplasmic reticulum (JSR) over 357ndash;40% of the cell surface. Tubules extend from the JSR deep into and throughout the cell as an irregularly dispersed network. The SR occupies 1% of the cell volume. A few, small (0.1–1.0 μm) unmyelinated nerves are present, but no neuromuscular junctions were seen. The auricles have fewer and smaller myocytes than the ventricle. The auricles also contain podocytes with pedicels having 20–35 nm slits and containing sieve-like projections. The morphology of the Geukensia heart is similar to that of other bivalves.     

CARDIAC MUSCLE : Its Ultrastructure in the Finch and Hummingbird with Special Reference to the Sarcoplasmic Reticulum

Cardiac muscle fibers of the hummingbird and finch have no transverse tubules and are smaller in diameter than those of mammalian hearts. The fibers are connected by intercalated discs which are composed of desmosomes and f. adherentes; small nexuses are often interspersed. As in cardiac muscle of several other animals, the junctional SR of the couplings is highly structured in these two birds but, in addition, and after having lost sarcolemmal contact, the junctional SR continues beyond the coupling to extend deep into the interior of the cells and to form belts around the Z-I regions of the sarcomeres. This portion of the sarcoplasmic reticulum, which we have named "extended junctional SR," and which is so prominent and invariant a feature of cardiac cells of hummingbirds and finches, has not been observed in chicken cardiac cells. The morphological differences between these species of birds may be related to respective differences in heart rates characteristic for these birds.     

Cytological changes in palisade cells of developing sunflower leaves

Summary Changes in the relative volume of palisade cells (V_v) allocated to various organelle compartments during postemergent leaf development was measured using stereological techniques. The surface to volume ratios (S_v) of the chloroplast and mitochondrial membranes were also measured at each stage. Three leaf stages were sampled, each was defined based on lamina length (10, 45, and 150 mm). The last stage represented a fully expanded leaf. Chloroplast and nuclear compartment V_v values changed significantly in the early stages when cells were actively dividing. Mitochondrial and vacuolar compartment V_v values showed significant changes in the latter stages during cell expansion. The oil vesicle and microbody compartments showed no significant change in V_v value during the developmental process. The surface to volume ratios of the chloroplast membranes increased significantly throughout all stages of the leaf development while mitochondrial cristae S_v values did not change. Organelle replication rates appeared to be independent of changes in cell volume with each organelle exhibiting a specific replication activity pattern. The results of this study suggest two possible mechanisms for the control of cell structural development involving both intrinsic and extrinsic factors.     

CHANGES IN THE SARCOPLASMIC RETICULUM AND TRANSVERSE TUBULAR SYSTEM OF FAST AND SLOW SKELETAL MUSCLES OF THE MOUSE DURING POSTNATAL DEVELOPMENT

The sarcoplasmic reticulum (SR) and transverse tubular system (TTS) of a fast-twitch muscle (extensor digitorum longus-EDL) and a slow-twitch muscle (soleus-SOL) of the mouse were examined during postnatal development. Muscles of animals newborn to 60 days old were fixed in glutaraldehyde and osmium tetroxide and examined with an electron microscope. At birth the few T tubules were often oriented longitudinally, but at the age of 10 days most of them had a transverse orientation. In the EDL, the estimated volume of the TTS increased from 0.08% at birth to 0.4% in the adult; corresponding values for the SOL were 0.04% at birth and 0.22% in the adult. A similar relative change was observed in surface area of the TTS during development. Calculated on the basis of a 30 µm diameter fiber, the surface area of the TTS in the EDL increased from 0.60 cm² TTS/cm² fiber surface in the newborn to 3.1 cm²/cm² in the adult, compared with 0.15 cm²/cm² at birth to 1.80 cm²/cm² in the adult for the SOL. The SR in the newborn muscles occurred as a loose network of tubules that developed rapidly within the subsequent 20 days, especially at the I band level. The volume of the SR increased in the EDL from 1.1% of fiber volume at birth to 5.5% in the adult. In the SOL the change was from 1.7% to 2.9%. The SOL approached the adult values more rapidly than the EDL, although the EDL had more SR and T tubules. Fibers of both EDL and SOL muscles showed variation in Z line thickness, mitochondrial content, and diameter, but over-all differences between the two muscles in amount of SR and TTS were significant. It is considered that the differing amounts of SR and TTS are closely related to the differing speeds of contraction that have been demonstrated for these two muscles.     

EFFECT OF SHORT-TERM SHADING ON THE CYTOLOGY OF PALISADE TISSUE IN MATURE LEAVES OF THE SUNFLOWER HELIANTHUS ANNUUS

Mature sunflower leaves were exposed to partial shading (35 or 14% of normal sun) or darkness (0% of normal sun) for approximately 8 hr. During this period one-half of each test leaf was shaded; the other half was used as a normal sun control. Palisade cell structure from both halves of each leaf was compared. Shading of leaves had little effect on organelle percent volume values (V_v) with exception of the starch compartment which decreased as shading increased. The surface to volume ratio (S_v) of the chloroplast thylakoids increased while the S_v of the mitochondrial membranes decreased as shading increased. Palisade cell volume did not change in shaded portions of the leaf, except in the fully shaded (dark) tissues where cell volume decreased. Changes in the actual volume of organelle compartments were strongly correlated with changes in cell volume. Thus a general osmotic response may account for some of the volume changes associated with differences in light intensity. Shading increased thylakoid surface areas 10–30% over the full sun controls. The ratio of stromal to granal thylakoid surface area remained constant in both the control and partially shaded samples. However, in darkened samples this ratio decreased as stromal membranes increased more than granal membranes. Changes observed in thylakoid surface areas associated with shading did not support thylakoid models which propose the interconversion of granal membranes to stromal membranes and vice versa.     

Quantitation of 'junctional feet' content in two types of muscle fiber from hind limb muscles of the rat

1. Transverse tubules in fibers from rat soleus and extensor digitorum longus (EDL) muscles of the rat were infiltrated with silver dichromate (black reaction of Golgi). This provides a faithful, high-contrast outline of the tubules, which allows distinction between segments involved in junction formation with the sarcoplasmic reticulum and segments that are free. 2. Electron micrographs of semithin transverse sections were used to quantitate T tubule parameters and to measure cross-sectional area and perimeter of individual fibers. Thin sections and data from the literature were used to obtain the contribution of caveolae to external surface area and the frequency of junctional feet along the junctional T tubule membrane. 3. From the above data we calculate the ratio of number of feet to total external surface area for a given fiber segment. The ratio is compared with data in the literature on the total amount of 'charge movement' (in nC/uF of total external surface area). 4. The average feet/surface area ratio is twice as large in EDL than in soleus fibers, while the charge movement is up to five-fold larger. Probably some of the total charge movement is not directly associated with events related to the turning on of the SR permeability to calcium.     

The membrane systems of the cardiac muscle cell of Meganychtiphanes norvegica (M. Sars), euphauciacea,crustacea

Summary The membrane systems of cardiac muscle cells of the euphausiacean Meganychtiphanes norvegica are described. Transverse tubules are found both at the Z-band level (T_z-tubules) and at the H-band level (T_h-tubules). Within the sarcomere narrow longitudinal tubules branch off from the T_z-tubules. At the H-band level these tubules expand forming flattened cisternae in dyadic and triadic couplings with the sarcoplasmic reticulum (SR). Adjacent myofibrils are separated by a well developed SR. Modifications of the SR are seen at the H-band level where junctional cisternae are formed.     

Laser Confocal scanning microscopy of the surface membrane/T-tubular system and the sarcoplasmic reticulum in insect striated muscle stained with DiIC18(3)

The structure of the surface membrane/transverse tubular (T-tubular) system and of the sarcoplasmic reticular (SR) of the labial adductor muscle of the honey bee (Apis mellifera) was examined by laser confocal scanning microscopy, after staining with the fluorescent membrane probe DiIC₁₈(3). The following components of the surface membrane/T-tubular system were visualized: transverse tubular networks that are located in the A-band close to the A–I junction and form dyads with the SR, longitudinal tubules that link the T-tubular networks within and between sarcomeres, and surface invaginations of larger diameter that contain tracheoles. The well developed SR forms a dense network of branching and anastomosing tubules in the A-band. A few tubular elements in the interfibrillar space in the 1-band link the SR of adjacent sarcomeres. This study demonstrates the advantages of the laser confocal microscope and lipophilic fluorescent dyes for studying the 3-D structure of cellular membrane systems.     

A functional identification of cardiac junctional sarcoplasmic reticulum

Junctional sarcoplasmic reticulum (SR) has been identified in microsomes from canine ventricular muscle by the presence of calsequestrin and ryanodine-sensitive Ca2+ release channels. These properties, however, are not common to cardiac cells from all species. Seiler et al (1) have recently described a high Mr polypeptide in canine junctional SR similar to the spanning protein subunits of skeletal muscle triads. We now report the existence of a polypeptide with the same mobility in SR from rabbit ventricular muscle and show that those cardiac membranes can associate with transverse (T-) tubules from rabbit skeletal muscle in K cacodylate medium. We propose that this polypeptide and the reaction with T-tubules be considered as criteria for the identification of cardiac junctional SR.     

Preparation and morphology of sarcoplasmic reticulum terminal cisternae from rabbit skeletal muscle

We have developed a procedure to isolate, from skeletal muscle, enriched terminal cisternae of sarcoplasmic reticulum (SR), which retain morphologically intact junctional "feet" structures similar to those observed in situ. The fraction is largely devoid of transverse tubule, plasma membrane, mitochondria, triads (transverse tubules junctionally associated with terminal cisternae), and longitudinal cisternae, as shown by thin-section electron microscopy of representative samples. The terminal cisternae vesicles have distinctive morphological characteristics that differ from the isolated longitudinal cisternae (light SR) obtained from the same gradient. The terminal cisternae consist of two distinct types of membranes, i.e., the junctional face membrane and the Ca2+ pump protein-containing membrane, whereas the longitudinal cisternae contain only the Ca2+ pump protein-containing membrane. The junctional face membrane of the terminal cisternae contains feet structures that extend approximately 12 nm from the membrane surface and can be clearly visualized in thin section through using tannic acid enhancement, by negative staining and by freeze-fracture electron microscopy. Sections of the terminal cisternae, cut tangential to and intersecting the plane of the junctional face, reveal a checkerboardlike lattice of alternating, square-shaped feet structures and spaces each 20 nm square. Structures characteristic of the Ca2+ pump protein are not observed between the feet at the junctional face membrane, either in thin section or by negative staining, even though the Ca2+ pump protein is observed in the nonjunctional membrane on the remainder of the same vesicle. Likewise, freeze-fracture replicas reveal regions of the P face containing ropelike strands instead of the high density of the 7-8-nm particles referable to the Ca2+ pump protein. The intravesicular content of the terminal cisternae, mostly Ca2+-binding protein (calsequestrin), is organized in the form of strands, sometimes appearing paracrystalline, and attached to the inner face of the membrane in the vicinity of the junctional feet. The terminal cisternae preparation is distinct from previously described heavy SR fractions in that it contains the highest percentage of junctional face membrane with morphologically well-preserved junctional feet structures.     

Immunolocalization of sarcolemmal dihydropyridine receptor and sarcoplasmic reticular triadin and ryanodine receptor in rabbit ventricle and atrium

The subcellular distribution of sarcolemmal dihydropyridine receptor (DHPR) and sarcoplasmic reticular triadin and Ca2+ release channel/ryanodine receptor (RyR) was determined in adult rabbit ventricle and atrium by double labeling immunofluorescence and laser scanning confocal microscopy. In ventricular muscle cells the immunostaining was observed primarily as transversely oriented punctate bands spaced at approximately 2-micron intervals along the whole length of the muscle fibers. Image analysis demonstrated a virtually complete overlap of the staining patterns of the three proteins, suggesting their close association at or near dyadic couplings that are formed where the sarcoplasmic reticulum (SR) is apposed to the surface membrane or its infoldings, the transverse (T-) tubules. In rabbit atrial cells, which lack an extensive T-tubular system, DHPR-specific staining was observed to form discrete spots along the sarcolemma but was absent from the interior of the fibers. In atrium, punctate triadin- and RyR-specific staining was also observed as spots at the cell periphery and image analysis indicated that the three proteins were co- localized at, or just below, the sarcolemma. In addition, in the atrial cells triadin- and RyR-specific staining was observed to form transverse bands in the interior cytoplasm at regularly spaced intervals of approximately 2 micron. Electron microscopy suggested that this cytoplasmic staining was occurring in regions where substantial amounts of extended junctional SR were present. These data indicate that the DHPR codistributes with triadin and the RyR in rabbit ventricle and atrium, and furthermore suggest that some of the SR Ca2+ release channels in atrium may be activated in the absence of a close association with the DHPR.     

DYNAMICS OF ARBUSCULE DEVELOPMENT AND DEGENERATION IN A ZEA MAYS MYCORRHIZA

A quantitative light and electron microscope study of developing and degenerating mycorrhizal arbuscules of Glomus fasciculatum in Zea mays was carried out in order to estimate three parameters during the colonization cycle. These were: 1) V_v(f,c), the fraction of the host cell volume occupied by a volume of fungus; 2) V_v(cy,c), the fraction of the host cell volume occupied by host cytoplasm; 3) S_v(pr,c), the surface-area-to-volume ratio of the host protoplast to the whole host cell. Uninfected cortical cells had an S_v(pr,c) of 0.13 μm²/μm³. As the fungus penetrates the cell wall, the protoplast invaginates, causing a decrease in protoplast volume and an increase in protoplast S_v. The S_v(pr,c) of a cell containing a mature arbuscule is 1.275 μm²/μm³. Because of the shrinkage of the protoplast, the S_v of the protoplast to its own volume rather than the original cell volume is 2.55 μm²/μm³, or almost a 20-fold increase. Total cell size is unaffected. When the arbuscule is mature, the fungus occupies 42% of the cell, with 24% as 1-μm-diam branches, and 18% as trunk. Arbuscular branch formation progresses at a linear rate and is the most important factor in causing the increased host S_v. The correlation coefficient for V_v(br,c) the volume fraction for arbuscular branches, vs. Sv(pr,c) is r = 0.932 (P < 0.001). Degeneration of the arbuscule is marked by a rapid decrease in branches, host S_v, and host cytoplasm. The trunk develops and degenerates at a slower rate than the branches.     

Detection and localization of triadin in rat ventricular muscle

Summary Dyads (transverse tubule—junctional sarcoplasmic reticulum complexes) were enriched from rat ventricle microsomes by continuous sucrose gradients. The major vesicle peak at 36% sucrose contained up to 90% of those membranes which possessed dihydropyridine (DHP) binding sites (markers for transverse tubules) and all membranes which possessed ryanodine receptors and the putative junctional foot protein (markers for junctional sarcoplasmic reticulum). In addition, the 36% sucrose peak contained half of the vesicles with muscarine receptors. Vesicles derived from the nonjunctional plasma membrane as defined by a low content of dihydropyridine binding sites per muscarine receptor and from the free sarcoplasmic reticulum as defined by the M_r 102K Ca²⁺ ATPase were associated with a diffuse protein band (22–30% sucrose) in the lighter region of the gradient. These organelles were recovered in low yield. Putative dyads were not broken by French press treatment at 8,000 psi and only partially disrupted at 14,000 psi. The monoclonal antibody GE4.90 against skeletal muscle triadin, a protein which links the DHP receptor to the junctional foot protein in skeletal muscle triad junctions, cross-reacted with a protein in rat dyads of the same M_r as triadin. Western blots of muscle microsomes from preparations which had been treated with 100mm iodoacetamide throughout the isolation procedure showed that cardiac triadin consisted predominantly of a band of Mr 95 kD. Higher molecular weight polymers were detectable but low in content, in contrast with the ladder of oligomeric forms in rat psoas muscle microsomes. Cardiac triadin was not dissolved from the microsomes by hypertonic salt or Triton X-100, indicating that it, as well as skeletal muscle triadin, was an integral protein of the junctional SR. The cardiac epitope was localized to the junctional SR by comparison of its distribution with that of organelle markers in both total microsome and in French press disrupted dyad preparations. Immunofluorescence localization of triadin using mAb GE4.90 revealed that intact rat ventricular muscle tissue was stained following a well-defined pattern of bands every sarcomere. This spacing of bands was consistent with the interpretation that triadin was present in the dyadic junctional regions.     

The T-tubule system in the myocardia of the sand rat and mouse as demonstrated by horseradish peroxidase

Summary The transverse tubule (T-tubule) system in papillary muscles of the sand rat and the mouse were studied with the aid of a diffusion tracer (horseradish peroxidase). The T-tubule system in the sand rat showed a typical mammalian pattern with sarcolemmal tubules invaginating at the Z-band level of the sarcomere. These tubules follow a transverse direction in the cell with frequent longitudinal side-branches which connect tubules at different Z-band levels. In the mouse myocardium, the T-tubules also start as lateral invaginations from the sarcolemma at the Z-band levels. In the cell interior, however, the tubules ramify and brake up into a complicated system of spirally running tubules. These spirals, of relative small diameter (400Å–700Å), frequently expand and form lobulated cisternae at the Z-band levels.