A new class of filaments intermediate in diameter between actin and myosin filaments has been demonstrated in skeletal muscle cells cultured from chick embryos. These filaments, which account for the majority of free filaments, average 100 A in diameter. They may run for more than 2 in a single section and can be distinguished in size and appearance from the thick and thin filaments assembled into myofibrils. The 100-A filaments are seen scattered throughout the sarcoplasm at all stages of development and show no obvious association with the myofibrils. The 100-A filaments are particularly conspicuous in myotubes fragmented by the mitotic inhibitors, colchicine and Colcemid. In addition, filaments similar in size and appearance to those found in myotubes are present in fibroblasts, chondrocytes, and proliferating mononucleated myoblasts. The 100-A filaments are present in cells arrested in metaphase by mitotic inhibitors. Definitive thick (about 150 A) or thin (about 60 A) myofilaments are not found in skeletal myogenic cells arrested in metaphase. Myogenic cells arrested in metaphase do not bind fluorescein-labeled antibody directed against myosin or actin. For these reasons, it is concluded that not all "thin" filaments in myogenic cells are uniquely associated with myogenesis.

The parasitic protozoan Toxoplasma gondii has been examined with the electron microscope in order to study the fine structure and the formation of the membranes surrounding the cell . The study of the ultrastructure of the membranes covering the parasite shows the existence of a three-membraned complex . Only the outer membrane is considered to be the plasma membrane ; the two membranes below it form an inseparable whole of changeable molecular architecture (modifications in appearance depending on the methods of fixation, local differentiation) . During reproduction, which takes place by fission or more often by endogeny, the membranes of the daughter individuals are formed from the membranes of the parent . At first the middle and inner membranes of the parent extend, separating the cytoplasm of the daughter cells from that of the parent . The three-membrane complex of the endozoites is completed at the time of their liberation ; the external membrane of the parent covers the leaving endozoites ; thus, the plasma membrane of the daughter cells derives also from that of the parent . These findings on the origin and role of limiting membranes during reproduction differ entirely from those described so far for other cells .

To identify the structures to be rapidly transported through the axons, we developed a new method to permit local cooling of mouse saphenous nerves in situ without exposing them. By this method, both anterograde and retrograde transport were successfully interrupted, while the structural integrity of the nerves was well preserved. Using radioactive tracers, anterogradely transported proteins were shown to accumulate just proximal to the cooled site, and retrogradely transported proteins just distal to the cooled site . Where the anterogradely transported proteins accumulated, the vesiculotubular membranous structures increased in amount inside both myelinated and unmyelinated axons. Such accumulated membranous structures showed a relatively uniform diameter of 50-80 nm, and some of them seemed to be continuous with the axonal smooth endoplasmic reticulum (SER) . Thick sections of nerves selectively stained for the axonal membranous structures revealed that the network of the axonal SER was also packed inside axons proximal to the cooled site . In contrast, large membranous bodies of varying sizes accumulated inside axons just distal to the cooled site, where the retrogradely transported proteins accumulated. These bodies were composed mainly of multivesicular bodies and lamellated membranous structures . When horseradish peroxidase was administered in the distal end of the nerve, membranous bodies showing this activity accumulated, together with unstained membranous bodies . Hence, we are led to propose that, besides mitochondria, the membranous components in the axon can be classified into two systems from the viewpoint of axonal transport: "axonal SER and vesiculotubular structures" in the anterograde direction and "large membranous bodies" in the retrograde direction.KEY WORDS axonal transport " membrane transport -local cooling -saphenous nerve electron microscopy -radioactive tracerThe axonal transport system comprises fast anterograde, fast retrograde, and slow anterograde transport (for reviews, see 30,37,43,50). Of these three types of axonal transport, slow anterograde transport has been recognized as a bulk movement of the axoplasm (31,37,46). Through the axoplasm, various materials are rapidly transported both anterogradely and retrogradely . This fast ax-J.

When Asterias or Thyone sperm come in contact with egg jelly, a long process which in Thyone measures up to 90 µm in length is formed from the acrosomal region . This process can be generated in less than 30 s . Within this process is a bundle of microfilaments . Water extracts prepared from acetone powders of Asterias sperm contain a protein which binds rabbit skeletal muscle myosin forming a complex whose viscosity is reduced by ATP . Within this extract is a protein with the same molecular weight as muscle actin . It can be purified either by collecting the pellet produced after the addition of Mg++ or by reextracting an acetone powder of actomyosin prepared by the addition of highly purified muscle myosin to the extract . The sperm actin can be polymerized and by electron microscopy the polymer is indistinguishable from muscle F-actin . The sperm actin was shown to be localized in the microfilaments in the acrosomal processes by : (a) heavy meromyosin binding in situ, (b) sodium dodecyl sulfate (SDS) gel electrophoresis of the isolated acrosomal processes and a comparison to gels of flagella which contain no band corresponding to the molecular weight of actin, and (c) SDS gel electrophoresis of the extract from isolated acrosomal caps . Since the precursor for the microfilaments in the unreacted sperm appears amorphous, we suspected that the force for the generation of the acrosomal process is brought about by the polymerization of the sperm actin . This supposition was confirmed, for when unreacted sperm were lysed with the detergent Triton X-100 and the state of the actin in the sperm extract was analyzed by centrifugation, we determined that at least 80% of the actin in the unreacted sperm was in the monomeric state .