Aggregation of rat platelets was induced in vitro by homologous rat Walker 256 carcinosarcoma cells and the extent of tumor cell-platelet interactions examined ultrastructurally. By 30s there was surface contact between unstimulated platelets and tumor cell microvilli. By midphase aggregation (2-3 min) tumor cells became enmeshed within expanding platelet aggregates. Tumor cell microvilli and platelet pseudopodia interdigitated as aggregation progressed. During the later stages of aggregation (6-10 min) tumor cells formed large processes which penetrated deep into the aggregate. Platelet activation (i.e. degranulation) occurred in gradient fashion and was concentrated near tumor cell membrane sites involved in process formation. At these later stages tumor cells near the aggregate periphery were found to have engulfed platelets or platelet fragments. Tumor cell-platelet interactions in the pulmonary microvasculature were also studied in vivo following injection of murine Lewis lung carcinoma, 16C mammary adenocarcinoma, and B16 amelanotic melanoma tumor cells into the tail vein. Platelets demonstrated a biphasic association with arrested tumor cells with peak interactions occurring at 10-30 min and 4-24 h. Ultrastructurally, tumor cells exhibited newly formed processes which interdigitated with the platelet aggregate. Such processes formed only in areas of contact with platelets and not in areas of contact with endothelial cells or other blood elements (i.e. erythrocytes, polymorphonuclear leukocytes). Numerous tumor cell mitochondria were concentrated in the areas of greatest platelet-tumor cell process activity. At early time intervals (2-10 min), intravascular platelet degranulation was observed primarily in platelets associated with tumor cell processes. Tumor cells also were found to have engulfed platelet fragments in vivo.
Fish chromatophores serve as excellent study models for cytoskeleton-dependent organelle translocations because the distribution of pigmentary organelles can be observed against a time frame by microscopy. In this study the distribution of microfilaments along with microtubules in cultured melanophores of the killifish (Fundulus heteroclitus Linneaus) are examined using whole-cell transmission electron microscopy (WCTEM), fluorescence, and laser scanning confocal microscopy. Dispersing, dispersed, aggregating and aggregated states of pigment are induced by adding either caffeine (for dispersion) or epinephrine (for aggregation) to the cells in a standard culture medium. The cells that exhibited a random melanosome distribution in the standard culture media without these two reagents, served as the control. The results indicate that: (i) a structure considered to be the actin-filament organizing center (AFOC) is in close proximity to the microtubule-organizing center (MTOC); (ii) the radial layout of microfilaments remains similar over four physiological states of pigmentary response with the exception of epinephrine-aggregated pigment, in which the aggregate blocks the viewing of the AFOC and central microfilament rays, yet radial microfilaments, whether central and/or peripheral, are apparent in all physiological states of distribution; and (iii) microfilaments serve, together with microtubules, as scaffolding for melanosomes which migrate in bi-directional rows on cross-bridges, thus shedding light on the mechanisms for orderly melanosome translocations in a structural continuum.
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