Structural identification of thrombosthenin in rat megakaryocytes

Cultured megakaryocytes: changes in the cytoskeleton after ADP-induced spreading

Megakaryocytes from guinea pig bone marrow were isolated and maintained in liquid culture and were treated with ADP, thrombin, arachidonic acid, or collagen. Megakaryocytes spread with an active ruffled membrane in response to ADP (1-100 microM), thrombin (1.0 U/ml), and arachidonic acid (50 microM) but responded to collagen surfaces only if fibronectin was added to the cultures. Spreading could be blocked completely by dibutyryl cyclic AMP (dibutyryl cAMP) or isobutylmethylxanthine at 1 mM, as well as by cytochalasin D (2 microgram/ml), but not by colchicine up to 1 mg/ml. The distribution of contractile proteins was examined by immunofluorescence. In untreated, spherical cells, staining with antimyosin, antifilamin, anti-alpha-actinin, or with fluorescein-labeled subfragment 1 (FITC-S1) was diffuse and unpatterned. With antitubulin antibody, however, microtubules were seen in a dense array throughout the unspread cells. In actively ruffling spreading cells, myosin, filamin, and actin were visualized in the region of the ruffled membrane while alpha-actinin was seen most prominently in a band located proximal to the inner part of the ruffle. In fully spread cells, actin, myosin, filamin, and alpha-actinin were seen in filaments that filled the cytoplasm. Antimyosin and anti-alpha-actinin staining of the filaments was periodic with approximately 1 micrometer center-to-center spacing. Actin, filamin, and alpha-actinin were also identified in punctate spots throughout the spread cytoplasm. Microtubules were absent from the ruffle but filled the cytoplasm of fully spread cells. Rings, 1.5-2.5 micrometer in diameter, were seen with antitubulin in 13% of the spread cells. Our results show that megakaryocytes respond to platelet agonists, but typically by spreading, rather than extending, filopodia. From the changes in localization of contractile proteins and from time-lapse cinematography, we propose a model for cell spreading.

Aspects of platelet formation and release

The surface architecture and location of megakaryocytes in the extravascular compartment of mouse bone marrow are considered. The finger-like cytoplasmic processes that extend into sinuses pass through apertures, approximately 3.0 millimicron in diameter, occurring singly or in groups. The cytoplasmic processes enlarge on first entering a sinus and form villi that seem to anchor them to endothelium. The villi consist largely of microfilaments. The cytoplasmic processes extend, attenuate, and undergo irregular constriction along their lengths. The considerations demarcate modicums of cytoplasm of platelet size one from another that are released when they break. Extravascular platelet release may also occur, and evidence is presented that points to the likelihood of the platelet's being engulfed and phagocytosed by macrophages before reaching the microcirculation.

The transmural passage of blood cells into myeloid sinusoids and the entry of platelets into the sinusoidal circulation; a scanning electron microscopic investigation

Scanning electron microscopic observations of rat bone marrow reveal that the sinusoidal wall is continuous and has no permanent patent apertures allowing free communication between the extravascular and intravascular myeloid compartments. Blood cells migrate into the sinusoids by perforating the endothelial cell body. Platelets are derived from long intrasinusoidal "proplatelet" processes which originate from the cell body of extravascularly located megakaryocytes. Proplatelet processes frequently occur in clusters, with the probability that all processes in a cluster arise from a single megakaryocyte. The release of platelets into the circulation may be initiated by local constriction along these processes, at which places either individual platelets or larger segments of proplatelet cytoplasm are pinched off. The larger segments may subsequently undergo further fragmentation into individual platelets.

Cytochemical localization of lysosomal enzymes in rat megakaryocytes and platelets

Platelets secrete lysosmal enzymes during the "platelet release reaction" early in clot formation. This study was undertaken to identify primary lysosomes of platelets and to detemine their origin in megakaryocytes. Using electron microscopy and cytochemistry, we localized two lysosomal enzymes, arylsulfatase and acid phosphatase, in megakaryocytes and platelets of normal and thrombocytopenic rats. In platelets and mature megakaryocytes, reaction product for both enzymes is confined to vesicles measuring 175-250 nm. These vesicles, which are primary lysosmes, first appear in the earliest recognizable megakaryocytes and increase in number during cellular maturation. In immature and maturing megakaryocytes, arylsulfatase and acid phosphatase can also be demonstrated in an organell similar to GERL (Golgi-endoplasmic reticulumlysosome), i.e., single smooth-surfaced cisternal with associated vesicles near the stacked Golgi cisternae. Scant reaction product for acid phosphatase is also sometimes seen in Golgi cisternae and endoplasmic reticulum. No reaction product was found in alpha-granules at any stage of megakaryocyte maturation, nor in alpha- or serotonin granules of platelets. Thus, our findings indicate that the primay lysosomes of megakaryocytes and platelets are small vesicles derived from GERL early in megakaryocyte differentiation. They can be indentified only after cytochemical staining and are distinct from both alpha- and serotonin granules.

Intracytoplasmic filaments in pulmonary lymphatic endothelial cells. Fine structure and reaction after heavy meromyosin incubation

The cytochemistry and ultrastructure of intracytoplasmic filaments of pulmonary lymphatic endothelial cells of neonatal rabbits were studied by comparison with myofilaments of the peribronchial and pulmonary vascular smooth muscle cells. Two types of endothelial filaments were observed: thin filaments (diameter: 50 A) which lie close to the abluminal cell membrane; and thick filaments (diameter: 90 A) which are dispersed throughout the cell cytoplasm. Following heavy meromyosin (HMM) treatment, characteristic arrowhead complexes formed in the thin lymphatic endothelial filaments as well as in the actin filaments of the smooth muscle cells. There was no detectable reaction of HMM with the thick filaments. After incubation with EDTA, the thin filaments were labile, and the thick filaments became the major filamentous component in the endothelial cells. In smooth muscle cells, the actin myofilaments were also labile while the 100 A filaments were stable. These observations support the hypothesis that the actin-like thin endothelial lymphatic filaments form part of a contractile system, while the thick filaments constitute a plastic cell skeleton. The significance of the contractile system in lymphatic endothelial cells might lie in a mechanism for the active regulation of the endothelial intercellular junction and gaps and hence the permeability of the lymphatic endothelial cell lining.