Shape and position of the sarcoplasmic reticulum and the Golgi apparatus in the sole plate and remaining subsarcolemmal muscle region of the mouse using imidazole-osmium staining

Intracellular Membrane Contact Sites in Skeletal Muscle Cells

Intracellular organelles are common to eukaryotic cells and provide physical support for the assembly of specialized compartments. In skeletal muscle fibers, the largest intracellular organelle is the sarcoplasmic reticulum, a specialized form of the endoplasmic reticulum primarily devoted to Ca2+ storage and release for muscle contraction. Occupying about 10% of the total cell volume, the sarcoplasmic reticulum forms multiple membrane contact sites, some of which are unique to skeletal muscle. These contact sites primarily involve the plasma membrane; among these, specialized membrane contact sites between the transverse tubules and the terminal cisternae of the sarcoplasmic reticulum form triads. Triads are skeletal muscle-specific contact sites where Ca2+ channels and regulatory proteins assemble to form the so-called calcium release complex. Additionally, the sarcoplasmic reticulum contacts mitochondria to enable a more precise regulation of Ca2+ homeostasis and energy metabolism. The sarcoplasmic reticulum and the plasma membrane also undergo dynamic remodeling to allow Ca2+ entry from the extracellular space and replenish the stores. This process involves the formation of dynamic membrane contact sites called Ca2+ Entry Units. This review explores the key processes in biogenesis and assembly of intracellular membrane contact sites as well as the membrane remodeling that occurs in response to muscle fatigue.

The transillumination possibility of imidazole-osmium postfixed tissue and its consequences for the handling of tissue samples

Osmium postfixation is established as a routine procedure for transmission electron microscopy (TEM). On the one hand, this routine procedure leads to good results for TEM, but on the other hand results in blackened tissue samples that do not allow examination of any structures within the embedded tissue sample by a light microscope. Equivalent fixation results for TEM are achieved with imidazole-osmium postfixation, and with this postfixation method tissue is not blackened and can be transilluminated with point light sources. This allows easier recognition of histological details within tissue samples and makes it possible to screen embedded samples for appropriate ultrastructural processing. Jejunum is used to demonstrate the method.

About the morphological relationships of the sarcoplasmic reticulum in the sole plate area of the frog

In the present investigation the sole plate area of motor end plates of the frog is ultrastructurally examined with different postfixation methods. We concentrated in this case on the proof of the smooth and rough sarcoplasmic reticulum of the sole plate. The relations of the smooth and rough sarcoplasmic reticulum to subsynaptic folds and the local T-system and its connections to diads and triads in the sole plate area are represented. The morphological differences between mammal and frog are pointed out. The possible functions of the sarcoplasmic reticulum in the myofibril-free sarcoplasm are discussed.

The transillumination possibility of imidazole-osmium postfixed muscle tissue and its consequences for the handling of muscle tissue samples

The osmium postfixation of tissue leads to good results for transmission electron microscopy, but also produces completely blackened tissue samples that do not allow the recognition of internal structures. With imidazole-osmium postfixation, one achieves comparable results in high electron microscopic resolution as with routine osmium postfixation. But the tissue samples are not blackened and can, therefore, be transilluminated with point light sources. The new postfixation technique makes it possible to recognize histological details such as vessels, nerve fibers, and the cross-banding pattern in an untrimmed block. This makes it possible to screen-embedded tissue samples for appropriate ultrastructural processing.