The mouse hematopoietic system was subjected to repeated depletion and regeneration either by serial transfer of bone marrow cells through lethally irradiated recipients or by repeated treatment with the cycle-active drug hydroxyurea (HU). The capacity of surviving stem cells to proliferate and self-renew was assayed at intervals by two methods: (a) the spleen colony method; and (b) competitive repopulation of irradiated recipients using chromosome markers, with normal bone marrow cells as an internal control. The progressive decline in stem cell function that occurred during serial transfer of bone marrow and that had already begun after a single transfer was not seen during HU treatment; up to 25 pairs of HU injections given over more than 1 yr had no discernible effect on the number of stem cells present 3 wk after the final injection or on their capacity to self-renew. Within 2 d after exposure to HU, the average self-renewal capacity of surviving stem cells was enhanced. This implies that the drug selectively eliminates poorly self-renewing stem cells and hence that these enter cycle more readily than stem cells with a high self-replicative potential. However, the fact of being in cycle at the time of injection did not of itself affect self-renewal. The results show that serial transfer of bone marrow is not a valid method for studying clonal aging phenomena because it does not fulfill the assumptions on which such studies are based. No evidence was obtained for any intrinsic limitation in the capacity of bone marrow populations for repeated regeneration after HU-induced depletion. However, this does not necessarily imply that individual hematopoietic clones are capable of indefinite expansion because hematopoiesis may (as suggested by the relative resistance of highly self-replicative stem cells to mitogenic signals) proceed on the basis of clonal succession.

MR imaging depicts implant integrity more accurately than US; neither method reliably depicts minimal leakage with shell collapse. Mammography is useful in screening bilumen implant integrity.

The appearance of free silicone at mammography, ultrasonography (US), and magnetic resonance (MR) imaging is variable. The classic appearance is dense areas of opacity on mammograms, a highly echogenic pattern with or without hypoechoic masses on US images, and foci of low signal intensity on fat-suppressed T1-weighted MR images or high signal intensity on water-suppressed T2-weighted MR images. Mammography is a reliable, cost-effective, and readily available means of demonstrating silicone. The major disadvantage of US is that its accuracy depends on the capability of the operator to recognize the abnormality. Although MR imaging outperforms US or mammography in detection of implant rupture, it is not clear that MR imaging is superior in detection of free or residual silicone. The sequelae of noncontained silicone include granuloma formation, fibrosis, and migration. After extrusion from an implant, silicone migrates primarily to local sites, such as the ipsilateral chest wall and axillary nodes. Migration of silicone into the axilla can involve the brachial plexus, resulting in neuropathy. Silicone can also migrate into more distal regions, including the arm and subcutaneous tissues of the abdominal wall. Whatever the source, silicone in breast tissue interferes with the interpretation of mammographic findings.

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Human plasma contains chemoattractant activity for cultured cells from the mouse thymic lymphoma 6C3HED and also for lymphoblasts from concanavalin A-stimulated mouse spleen cells. A major portion of the attractant activity for both cell types could be attributed to plasma lysophosphatidylcholine. Studies on synthetic lysophosphatides showed that polar head group structure, acyl chain length, and stereochemical configuration are important determinants for attractant activity.Substances that cause the accumulation of lymphoid cells in sites of inflammation or control the migration of these cells through the tissues of lymphoid organs may play important roles in immune responses (1). Oriented migration (chemotaxis), increased random motility (chemokinesis), or other cellular mechanisms may cause cells to migrate toward the source of a chemoattractant gradient. t Many different substances have been reported to be chemoattractants for lymphoid cells. Among these are plasma and altered components of plasma (2), a proteolytic fragment of IgG (3), lymphokine-containing supernates from mixed lymphocyte cultures (4), lectins that are mitogenic for lymphocytes (5), a 10,500-dalton factor from immune calf thymus (6), specific antigens (2), arachidonic acid and its metabolites (7,8), and bacterial components, including staphylococcal protein A (5).In the course of studies on the attractant activity of plasma, we discovered that plasma could attract not only concanavalin A (Con A)-stimulated spleen lymphocytes but also neoplastic cells from the cultured thymic lymphoma 6C3HED. Both cell types responded similarly to plasma lipid extract and lipid fractions. Isolation and characterization of the active principle led us to conclude that much of the attractant activity in plasma could be accounted for by lysophosphatidylcholine (LPtdCho). To study the mechanism by which LPtdCho exerts its effect on lymphoid cells, we set out to define the structural requirements for molecules with attractant activity.MATERIALS AND METHODS Solvents. Organic solvents purchased from Burdick-Jackson Laboratories (Muskegon, MI) were used without further purification.Media. Cell cultures were maintained in medium A [RPMI-1640 medium supplemented with 10% (vol/vol) heat-inactivated fetal calfserum, 2 mM L-glutamine, and 100 units ofpenicillin, and 100 Mug ofstreptomycin per ml]. Medium B [minimal essential medium with Earle's salts supplemented with 0.5% bovine serum albumin (Cohn fraction V; Sigma), 70 mM sodium Hepes (pH 7.5), and 100 units ofpenicillin, and 100 Ag ofstreptomycin per ml] was used for migration assays.Migration Assay. Cell migration under agarose gels was assayed essentially as described by Nelson et al. (9). Six round glass coverslips were placed in hexagonal array in 60 X 15 mm plastic culture dishes. Four milliliters of a mixture of 5 vol of medium B and 1 vol of 80 mM sodium Hepes, pH 7.5/5% Litex LSA agarose (Accurate, Hicksville, NY) was added over the coverslips in each dish. Three wells were cut in the gel over each c...