Structure and nucleotide-dependent changes of thick filaments in relaxed and rigor plaice fin muscle.

The myosin crossbridge array, positions of non-crossbridge densities on the backbone, and the A-band "end filaments" have been compared in chemically skinned, unfixed, uncryoprotected relaxed, and rigor plaice fin muscles using the freeze-fracture, deep-etch, rotary-shadowing technique. The images provide a direct demonstration of the helical packing of the myosin heads in situ in relaxed muscle and show rearrangements of the myosin heads, and possibly of other myosin filament proteins, when the heads lose ATP on going into rigor. In the H-zone these changes are consistent with crossbridge changes previously shown by others using freeze-substitution. In addition, new evidence is presented of protein rearrangements in the M-region (bare zone), associated with the transition from the relaxed to the rigor state, including a 27-nm increase in the apparent width of the M-region. This is interpreted as being mostly due to loss or rearrangement of a nonmyosin (M9) protein component at the M-region edge. The structure and titin periodicity of the end-filaments are described, as are suggestions of titin structure on the myosin filament backbone.

Packing of alpha-helical coiled-coil myosin rods in vertebrate muscle thick filaments.

Muscle myosin filament backbones are aggregates of long coiled-coil alpha-helical myosin rods, with the myosin heads arranged approximately helically on the filament surface, but the details of the rod packing are not known. Computed Fourier transforms of plausible molecular packing models for the vertebrate striated muscle myosin filament have been compared with observed high-angle X-ray diffraction patterns from plaice fin muscle. Models considered include those in which the coiled-coil rod parts of myosin are packed into various kinds of subfilaments or into a curved molecular crystalline layer. A general conclusion is that if the myosin rods are tilted by less than about 1 degree or more than about 3 degrees from the filament long axis, very poor agreement is obtained between the computed and observed high-angle diffraction patterns. Qualitative comparison of calculated Fourier transforms, taken together with electron micrograph information, shows that the curved molecular crystal model and a model with hexagonally close-packed 4-nm subfilaments appear to explain the whole set of observations more satisfactorily than the alternatives. It is argued on other grounds that of these two possibilities the curved molecular crystal model is the more plausible.

Crossbridge states in isometrically contracting fish muscle: evidence for swinging of myosin heads on actin.

We have previously shown that time-resolved X-ray diffraction studies of the 2-D pattern from isometrically contracting flatfish (turbot) fin muscle have considerable advantages over similar studies of other vertebrate muscles due to the simple lattice and the better long-range order in these muscles (5, 24). Here we show not only that two structurally different myosin head to actin attached states must exist in the crossbridge cycle but that we are also able to define the likely crossbridge configurations in these states. A non-force producing "weak-binding" state is evident from the lead of the (11) equatorial reflections (and actin mass) time-course relative to that of tension (and the (10) equatorial reflection decrease) by about 30 msec. The first myosin layer line at 429 A has a weakened but altered intensity distribution, with no change in axial spacing, in patterns from active muscle. We show this to be consistent with myosin heads binding in the non-specific manner envisaged for the "weak-binding" state. Evidence for the second force-producing attached state, or series of states, comes from the observation of only a small increase in the intensity of the 360 A actin layer line between resting and active muscle patterns. It might be thought that a substantial increase in this layer line would be expected if myosin heads were even transiently attached to the thin filaments in a force-producing state. However, this is not so because internal changes in the structure of the thin filaments in active muscle have the opposite effect of causing this layer line to decrease in intensity. Observation of a small net intensity increase is therefore evidence for myosin head attachment with the symmetry of the actin helix. From the equatorial diffraction pattern, Fourier synthesis maps were computed at 10 msec intervals throughout the isometric tetanus, enabling changes in the mass distribution to be visualised between the force- and non-force producing populations of crossbridges. This difference map shows that in the force-producing state myosin heads have their centres of mass on average at a smaller radius from the thin filament axis compared to the case for non-force producing myosin heads. Since there is good evidence that there is no substantial change in myosin head shape during contraction (30) these observations are consistent with myosin heads swinging on actin as fairly rigid structures.

Antigen-induced arthritis in the rabbit: ultrastructural changes at the chondrosynovial junction.

Structural changes at the chondrosynovial junction of the lateral border of the lateral femoral condyle have been studied by electron microscopy in rabbits with antigen-induced arthritis of 6 h-27 days duration. Rapid changes in the collagen fibrils of the extracellular matrix in the synovial lining and articular cartilage were noted. Collagen fibrils with unusually large diameters were observed. Overgrowth of cartilage by inflamed synovium was seen within 3-6 days of induction of arthritis and by day 12 the interface between these two tissues was largely indistinguishable. The synovial pannus at this time was fibrotic and infiltrated with plasma cells, lymphocytes and macrophages. Few polymorphonuclear leucocytes were found in the developing pannus. Macrophages were found with extended processes which enveloped neighbouring cells. Some blood vessels had thickened endothelial cells though lymphocytes were not observed in their vicinity. This study reveals the rapidity with which synovial pannus can develop and suggests that there are a number of mechanisms operating to cause cartilage breakdown in antigen-induced arthritis.

Taenia crassiceps: ultrastructural observations on the oncosphere and associated structures.

Electron microscopic observations were made on unhatched eggs of Taenia crassiceps in utero. The outermost envelope consists of a thin, relatively smooth capsule over a highly convoluted outer envelope which contains a highly granulated cytoplasm and numerous mitochondria. The inner envelope, consisting of a thick embryophore and the cytoplasmic component of the embryophore cell, resembles that found in most other taeniids. Three epithelial layers separate the oncosphere from the embryophore. While these layers are narrow and difficult to distinguish, the 'oncospheral membrane' is distinct, darkly stained and relatively thick. The oncospheral hook lies within the oncoblast with its blade portion held by cytoplasmic folds within a modified 'sheath' in the 'basal epithelial layer'. Round, dense bodies are concentrated at the epithelial membranes around the 'sheaths'. Hook muscles insert on the basal lamina at the 'collar' region of the hooks. Penetration gland cells are packed with numerous dense disc-shaped secretory bodies.