Leptomeric fibrils and T-tubule desmosomes in the Z-band region of the mouse heart papillary muscle.

The papillary muscle of the heart of adult white mice is investigated. Intrafibrillary located leptomeric fibrils, frequently encountered in the Z-band region of the myofibrils. The leptomeric fibrils are always running in a transverse direction and often in close proximity to the transverse tubules (which are also located at this level). There seems to be a close connection between the dense striae of the leptofibrils and the Z-bands of ordinary myofibrils. The leptomeric fibrils are spindle-shaped and have a length varying between 0.6 and 1.2 microgrometer. The banding periodicity of the fibrils is approximately 0.16 micrometer. Occasionally desmosomes are observed in the T-tubule system.