[Ultrastructural characteristics of the pentalaminar contact and its role in myoblast fusion].

Two types of the pentalaminar structure were found in developing skeletal muscles. One of them is characterized by three electron-dense lines 23-25 nm in size. The other one, 11 nm in size, has only one electron-dense central line and forms membrane invaginations (blebs), 100-250 nm in diameter. The transformation of the "bridge" contact into the 2nd type pentalaminar structure and electron-dense bodies--"lenses" (60-65 nm) was established. The "lenses" contain an electron-dense material, similar to that of the "bridges". Neuraminidase hydrolysis shows that the "bridges" consist of glycoproteins. The muscle cells were extracted with 0.5% triton X-100. The detergent removes most of phospholipids. The first type of the pentalaminar structure remains stable to detergent treatment, whereas the second type may be dissolved. Besides, a partial destruction of surface membranes--"breaks"--are observed in the regions of membrane transmission to the pentalaminar structure. The detergent appears to act on those sites of the surface membrane which are instable and ready to fuse, especially on the bases of invaginations.