Morphological study of the transverse-axial tubular system (TAxTS) in rat heart using ferrocyanide-osmium method and thick sectioning.

The structure of the transverse-axial tubular system (TAxTS) in ventricular and atrial myocytes of the rat heart was investigated by examining thick (as much as 0.2 micron) and thin sections stained by the ferrocyanide-osmium method. Ventricular myocytes contain longitudinally and transversely oriented tubules which represent invaginations of the plasma membranes and form orderly latticeworks. These tubules are irregular in diameter and course tortuously. Dilated, triangular cisterns are often seen at the points of interconnections between longitudinal and transverse elements. The junctional sarcoplasmic reticulum forms 'couplings' with both transversely and longitudinally oriented TAxTS tubules, and a large proportion of the surface area of the TAxTS tubules is involved in such couplings. In atrial myocytes, the TAxTS tubules are poorly developed and pleomorphic, often with a beaded appearance, and show an irregular distribution from one cell to another and from one area to another within a given cell. The osmium-ferrocyanide stain reveals the TAxTS to be much more extensive than is evident from examination of routinely stained preparations.