List of abbreviations
of micros-
specialist terms
explained in
English +

Every attempt was made to provide correct information and labelling, however any liability for eventual errors or incompleteness is rejected!

dieser Seite

Dr. med.
H. Jastrow

of use
Overview heart muscle (Textus muscularis striatus cardiacus):
Pages with explanations are linked to the text below the images if available! (Labelling is in German)
heart muscle cell +
capillary (rat)
intercalated disk (rat) T- tubules, L tubules + myofibrils (rat) intercalated disk, beta-
glycogen granules (monkey)
overview of an intercalated
disk (monkey)
detail of an intercalated
disk (monkey)
muscle filaments in longi-
tudinal section (monkey)
 cross-section of an inter-
calatd disk (monkey)
detail of similar view
cross-section of muscle
filaments 1 (monkey)
cross-section of muscle
filaments 2 (monkey) 
cross-sectioned overview
with capillaries (monkey)
oblique section
exact longitudinal section
myofibrils and
mitochondria (monkey)
cell organelles including
multivesicular body (rat)
caveolae, basement-
membrane (rat)

Cardiac muscle cells (Terminologia histologica: Cardiomyocyti, Myocyti cardiaci) are about 50-150 µm long and 10 to 20 µm thick and have a cylindrical shape. They show typically only 1 and in some cases 2 long, central nuclei of a cigar shape about 12 µm in length which get indented in case of contraction. The nuclei are surrounded by less electron-dense cytoplasm (light court; image) since myofibrils are replaced here by the following organelles: small Golgi-apparatuses, lysosomes, lipofuscin vesicles, and a few RER. The cells show large amounts of mitochondria of the crista-type with a relatively electron-dense matrix and usually over 50 to far above 100 crestsThe arrangement of these mitochondria is characteristic, they form large rows paralleling the myofibrils and here are called interfibrillar mitochondria. Further, only in normal cardiomyocytesmany are located in groups of great number directly beneath the 9 nm thick cell membrane called sarcolemma (Terminologia histologica: Sarcolemma). Here they are termed subsarcolemmal mitochondria. However, the latter are in direct continuity to the interfibrillary subpopulation on many places. The shape of mitochondria adapts to available local space. Three-dimensionally reconstructed mitochondria are many micrometres long, some hundred nanometres in diameter and multiple branched tubular structures in most cases. However, close to the nuclei they generally are more spherical and have smaller individual volumes (perinuclear mitochondria). One should also keep in mind that mitochondria lack a stable inner skeleton and thus also change in shape during the muscle contraction process. The sarcolemma shows several micropinocytotic vesicles increasing in number close to capillariesT-tubules (Terminologia histologica: Tubuli transversi) are very deep tubular invaginations of the cell membrane in regions of Z-discs as can be seen here. They always run along the outer edge of each Z-disc in a distance of only a few nanometres surround the discs and then continue to Z-disc of the neighbouring parallel running myofibril. Thus the T-tubules are in direct continuity with the extracellular space and every sarcomere is supplied by only one such tubule whereby neighbouring tubules are interconnected by longitudinal tubules. In comparison to the skeletal muscle cardiac T-tubules are a little wider (diameter 100 - 300 nm in man; considerably thinner in mouse and rat). Thus the basal lamina also enters the T-tubules as intratubular basal lamina (Terminologia histologica: Lamina basalis intratubularis). The latter has a little thinner lamina densa which in parts seems to be interrupted. It is anchored to the cell membrane by alpha5-beta1-integrins. The latter is stabilised by cytoskeletal elements like actin, vinculin and spectrin and has many integrated voltage-dependent channels for calcium ions (Ca++, DHP-receptors). This is of functional importance since thus extracellular Ca++ may easily and quickly penetrate deeply into the cell via the T-tubules.
The smooth endoplasmic reticulum of cardiac muscle cells is termed sarcoplasmic reticulum (SER; Terminologia histologica: Reticulum sarcoplasmicum). In the heart it is not as prominent as in skeletal muscle and forms a network of multiply interconnected, longitudinally oriented to the orientation of the myofibrils, therefore the term L-tubules. The tubules partly have tubular elements (Terminologia histologica: Elementa tubularia) which are connected to reticular elements (Terminologia histologica: Elementa reticularia), i.e., net-like interconnected tubules. At the level of the M-line the network of L-tubules is denser and it stretches towards the T-tubules up to a distance of only few nanometres showing terminal cisterns (junctional SER; Terminologia histologica: Cisternae terminales) which are local dilatations about 70 nm in diameter. In the sections in which one such cistern is facing a T-tubule for several hundred nanometres the resulting complex is termed a diad (Terminologia histologica: Dias) since when this is cross-sectioned one will see two parallel lying tubes. These diads are only seen in heart- but NOT in skeletal muscleHowever, often two terminal cisterns of the SER also accompany one T-tubule, which then is in the centre between the two cisterns which results in formation of a triad  (Terminologia histologica: Trias) with 3 parallel tubes. On some places the SER nearly attaches to the sarcolemma while corbules, i.e. vesicular protrusions (corbular SER; missing in Terminologia histologica; proposal: Corbulae). The SER is the most important short term storage for calcium ions (Ca++), whereby especially the terminal cisterns and the corbular SER with the Ca++-binding protein calsequestrin and the receptors present in the SER membranes (ryanodin receptor in the terminal cisterns or a IP3-regulated receptor in the corbular SER, respectively) are of importance. In order to realise a quick relaxation of the heart muscle after contraction which is important for refill of the ventricles during the diastole ATP-consuming calcium ion pumps are located in the membranes of the SER. These pumps are regulated by the cardiac protein phospholamban and rapidly pump the Ca++ back into the lumen of the SER. Further calcium ion pumps regulated by Calmodulin are present in the cell membrane and in the membrane of the T-tubules in order to quickly pump back Ca++.
The complex intercellular junctions (Terminologia histologica: Junctiones intercellulares speciales) connecting heart muscle cells consist of interdigitations plus adhesive strips (Fasciae adhaerentes), spot desmosomes (Terminologia histologica: Maculae adhaerentes) and gap-junctions (Terminologia histologica: Maculae communicantes, Nexus).

--> Differential diagnosis of muscle tissues, skeletal muscle, cell-to-cell junctions, caveolae, L-tubules, actin filaments
--> Electron microscopic atlas Overview
--> Homepage of the workshop

Some images were kindly provided by Prof. H. Wartenberg; other images, page & copyright H. Jastrow.