Membrane Systems of Crab Fibers

An electron microscopic study of internal and surface-connectedmembrane systems of leg muscle of the crab shows that thereare three kinds of surface-connected membrane systems in additionto an intracellular sarcoplasmic reticulum (SR). One is a systemof large infoldings of the sarcolemma referred to as clefts.These are longitudinally-oriented, flattened infoldings of boththe plasma membrane and the fibrous sheath of the fiber, andwere probably seen earlier with the light microscope. Extendinginto the fiber both from these clefts and from the free fibersurface are two systems of tubules of much smaller caliber,the Z tubules and the A tubules. The Z tubules are located,as their name indicates, near the Z lines of the myofibrils,and are thought to be attached to them mechanically. The A tubulesare found in pairs, near the ends of each A band, and are closelybound to the SR in two-part structures called dyads. Local-activationexperiments, like those done earlier by Huxley and Taylor, suggestthat the A tubules are involved in excitation-contraction coupling;no such experimental suggestion of function exists for the Ztubules.     

Thin sections. I. A study of section thickness and physical distortion produced during microtomy

Knowledge of the thickness of sections is important for proper interpretation of electron micrographs. Therefore, the thicknesses of sections of n-butyl methacrylate polymer were determined by ellipsometry, and correlated with the color shown in reflected light. The results are: gray, thinner than 60 mmicro; silver, 60 to 90 mmicro; gold, 90 to 150 mmicro; purple, 150 to 190 mmicro; blue, 190 to 240 mmicro; green, 240 to 280 mmicro; and yellow, 280 to 320 mmicro. These results agree well with optical theory and with previous published data for thin films. Sections, after cutting, are 30 to 40 per cent shorter than the face of the block from which they were cut. Only a small improvement results from allowing the sections to remain in the collecting trough at room temperature. Heating above room temperature, however, reduces this shortening, with a corresponding improvement in dimensions and spatial relationships in the sections. When the thickness of the section is considered in interpreting electron micrographs instead of considering the section to be two-dimensional, a more accurate interpretation is possible. The consideration of electron micrographs as arising from projections of many profiles from throughout the whole thickness of the section explains the apparent lack of continuity often observed in serial sections. It is believed that serial sections are actually continuous, but that the change in size of structure through the thickness of one section and the consideration of only the largest profile shown in the micrograph can account for the lack of continuity previously observed.