Passive material properties of intact ventricular myocardium determined from a cylindrical model

The equatorial region of the canine left ventricle was modeled as a thick-walled cylinder consisting of an incompressible hyperelastic material with homogeneous exponential properties. The anisotropic properties of the passive myocardium were assumed to be locally transversely isotropic with respect to a fiber axis whose orientation varied linearly across the wall. Simultaneous inflation, extension, and torsion were applied to the cylinder to produce epicardial strains that were measured previously in the potassium-arrested dog heart. Residual stress in the unloaded state was included by considering the stress-free configuration to be a warped cylindrical arc. In the special case of isotropic material properties, torsion and residual stress both significantly reduced the high circumferential stress peaks predicted at the endocardium by previous models. However, a resultant axial force and moment were necessary to cause the observed epicardial deformations. Therefore, the anisotropic material parameters were found that minimized these resultants and allowed the prescribed displacements to occur subject to the known ventricular pressure loads. The global minimum solution of this parameter optimization problem indicated that the stiffness of passive myocardium (defined for a 20 percent equibiaxial extension) would be 2.4 to 6.6 times greater in the fiber direction than in the transverse plane for a broad range of assumed fiber angle distributions and residual stresses. This agrees with the results of biaxial tissue testing. The predicted transmural distributions of fiber stress were relatively flat with slight peaks in the subepicardium, and the fiber strain profiles agreed closely with experimentally observed sarcomere length distributions. The results indicate that torsion, residual stress and material anisotropy associated with the fiber architecture all can act to reduce endocardial stress gradients in the passive left ventricle.     

Deregulation of Collagen Phagocytosis in Aging Human Fibroblasts: Effects of Integrin Expression and Cell Cycle

Intracellular degradation of collagen by phagocytosis in fibroblasts is essential for physiological remodeling of the extracellular matrix in a wide variety of connective tissues but imbalances between degradation and synthesis can lead to loss of tissue collagen. As aging is associated with loss of dermal and periodontal collagen and with increased lysomomal enzyme content in fibroblasts, we examined the regulation of collagen phagocytosis by integrin expression and the cell cycle in anin vitrofibroblast aging model. Two different fibroblast lines (CL1; CL2) at the fourth subculture were passaged up to replicative senescence to model aging processesin vitro.Cells were incubated with collagen-coated or BSA-coated green fluorescent beads for 3 h to assess α₂β₁-integrin-mediated or nonspecific phagocytosis, respectively. Single-cell suspensions were stained with DAPI and sulforhodamine 101 to separate cycling G₁and noncycling G₀cells. Staining for α₂-integrin, bead internalization, and bivariate analyses of DNA/protein content were measured by three-color flow cytometry. Serum deprivation was used to induce increases in the proportion of G₀cells. For G₁cells, the proportion of collagen phagocytic cells was >50% for all passages and collagen beads were internalized >5-fold more frequently than BSA beads. In contrast, G₀cells with diploid DNA content but low protein content exhibited greatly reduced phagocytic capacity (<10% of cells internalized collagen or BSA beads), the number of beads per cells was 4-fold less, and α₂integrin expression was very low compared to G₁cells. The proportion of collagen phagocytic cells and the proportion of α₂-integrin-positive cells increased with transit through the cell cycle. At higher passage numbers mean cell volume and cytoplasmic granularity were reduced 30% but at replicative senescence cells with large surface area and subdiploid DNA predominated. The proportion of collagen and BSA phagocytic G₁cells increased 1.5- and 5-fold, respectively, and the number of beads per cell increased <3-fold. However, surface α₂-integrin staining remained unchanged. These data indicate that the collagen and nonspecific internalization pathways were greatly upregulated, independent of cell cycle phase, and that cellular agingin vitrostrongly influences the specificity and rate of phagocytic processes in fibroblasts. We suggest that age-related loss of collagen in connective tissues undergoing turnover may be a manifestation of a deregulated increase of collagen phagocytosis in which the net loss of degraded collagen exceeds new synthesis.     

Gene conversions mediating antigenic variation in Trypanosoma brucei can occur in variant surface glycoprotein expression sites lacking 70-base-pair repeat sequences.

African trypanosomes undergo antigenic variation of their variant surface glycoprotein (VSG) coat to avoid immune system-mediated killing by their mammalian host. An important mechanism for switching the expressed VSG gene is the duplicative transposition of a silent VSG gene into one of the telomeric VSG expression sites of the trypanosome, resulting in the replacement of the previously expressed VSG gene. This process appears to be a gene conversion reaction, and it has been postulated that sequences within the expression site may act to initiate and direct the reaction. All bloodstream form expression sites contain huge arrays (many kilobase pairs) of 70-bp repeat sequences that act as the 5' boundary of gene conversion reactions involving most silent VSG genes. For this reason, the 70-bp repeats seemed a likely candidate to be involved in the initiation of switching. Here, we show that deletion of the 70-bp repeats from the active expression site does not affect duplicative transposition of VSG genes from silent expression sites. We conclude that the 70-bp repeats do not appear to function as indispensable initiation sites for duplicative transposition and are unlikely to be the recognition sequence for a sequence-specific enzyme which initiates recombination-based VSG switching.     

Uveitis in Children

Endogenous uveitis in 117 children aged 15 years or under was investigated at The Hospital for Sick Children, Toronto, in a 12-year period from 1953 to 1964. This group included 55 children with anterior uveitis, 59 with posterior uveitis, and three with diffuse uveitis. An etiologic diagnosis could be made or the uveitis recognized as part of a definite clinical syndrome in approximately 47% of the 117 children. The commonest cause of posterior uveitis was toxoplasmosis and the commonest associated finding in anterior uveitis was juvenile rheumatoid arthritis. Chronic cyclitis of unknown etiology was a relatively common disease.     

Density gradient centrifugation of hemopoietic colony-forming cells

Buoyant density distributions of hemopoietic colony-forming units (CFU) from normal mouse marrow were determined by equilibrium density gradient centrifugation in bovine serum albumin (BSA) gradients. The distributions were compared with those obtained for the total population of nucleated cells from normal mouse marrow. The buoyant density distribution for CFU was found to differ from the density distribution for the total nucleated cell population, and the portion of the total cell population with densities much less than the mean value was found to contain up to a 30-fold greater proportion of CFU than an uncentrifuged control. These results provide a preliminary approach to the purification and characterization of normal hemopoietic colony-forming stem cells.     

Stimulation of uptake of tritiated thymidine into mouse marrow cells in culture by a factor from L-cell conditioned medium

Uptake of ³H-thymidine into suspension cultures of mouse marrow cells was stimulated by the addition of serum-free conditioned medium harvested from cultures of mouse L-cells. Characterization of the “conditioning factor activity” (CFA) by gel filtration, ion exchange chromatography, velocity sedimentation, polyacrylamide gel electrophoresis and susceptibility to trypsin digestion indicated that the CFA detected by stimulation of ³H-thymidine uptake is the same as the CFA detected previously by its ability to stimulate colony formation by marrow cells in vitro. The ³H-thymidine uptake assay was used to investigate the kinetics of disappearance of CFA as a function of time in the presence of mouse marrow cells. The CFA recoverable from the cultures decreased rapidly during the first day, and approached background levels by the fifth day. There was no evidence of inhibitory substances in the depleted media. Even if the cultures continued to receive fresh conditioned medium at daily intervals, ³H-thymidine uptake decreased sharply after the fifth day, indicating that the marrow cells had lost their capacity to respond to CFA.     

Cellular morphology of human malignant melanoma in primary culture

Summary Early monolayer outgrowths of cells from human cutaneous malignant melanomas mostly derived from metastatic lesions were examined microscopically. Cells resembling the two dendritic types of melanoma previously described in the established lines could readily be recognized. Of 22 specimens, 14 consisted of cells with a triangular dendritic morphology, four had both triangular and elongated dendritic morphology, and one had a cuboidal morphology. The remaining three specimens showed only fibroblastic outgrowths. It is concluded that cells with a triangular dendritic morphology are either the most common type of the secondary cutaneous melanomas, or alternately the most adaptable to the present culture conditions. An association of a more favorable prognosis with the homogeneous triangular dendritic cell type is noted. This study was supported in part by grants from The Medical Research Council of Canada and The Ontario Cancer Treatment and Research Foundation.     

Characterization of human malignant melanoma cell lines. III. Membrane immunofluorescence reactivity with sera from patients with melanoma

Summary Seven well-characterized human malignant melanoma cell lines have been evaluated in terms of their reactivity in membrane immunofluorescence tests with sera from 48 patients with melanoma, 23 patients with other forms of cancer and 28 normal controls. There was a significantly greater degree of reactivity of melanoma sera (33.7%) than of sera of normal controls (22.2%) or of sera from patients with other forms of cancer (24.2%). The incidence of strong reactors among the melanoma patients was found to be inversely proportional to the extent of disease in the melanoma patients: Stage I, 54.5%, Stage III, 36.8% and Stage IV, 29.4%. Reactivity against non-melanoma cell lines was similar in the three subject groups and was unaffected by stage of disease in the melanoma patients. No single cell line showed preferential reactivity with melanoma sera. There was an increased overall incidence of reactivity of all three subject groups against non-pigmented cell lines.A-B-0 antigens and heterophile antigens were excluded as a cause of seropositivity. The antigen(s) was trypsin-sensitive and neuraminidase-resistant.These data suggest that long term cultures of human melanoma may contain melanoma-associated antigens which may be useful in the further study and search for melanoma-specific antigens.     

Regulation of blood cell diferentiation.

The hemopoietic (blood forming) system contains pluripotent stem cells able to give rise to a variety of differentiated progeny, including erythrocytes, granulocytes, megakaryocytes, monocytes, macrophages, and possible other cell types. Although a good deal is known about cell lineage relationships in the hemopoietic system, only limited information is available about the mechanisms regulating the proliferation and differentiation of the stem cells and their progeny. An approach to this latter problem has been provided by the develoment of new techniques for the cultivation of hemopoietic cells in short-term cultures. In such cultures, the proliferation and differentiation of hemopoietic cells can be studied under controlled conditions. Two areas of investigation show particular promise: elucidation of the role of the cell surface membrane in regulation; and the possible development, through a detailed investigation of the properties of leukoviruses, of new methods for the genetic analysis of hemopoietic cells.