Magnetic superlattices with variable interlayer exchange coupling: a new approach for the investigation of low-dimensional magnetism.

We have investigated the magnetic order in an [Fe(2)/(VHx)(13)] x 200 superlattice as a function of temperature and hydrogen content in the vanadium layers. A J(radially)-T magnetic phase diagram was established where J(radially) denotes the interlayer exchange coupling between adjacent Fe planes. We propose that Fe/V superlattices, in which the ratio of interlayer to intralayer coupling can be tuned continuously and reversibly via hydrogen in the nonmagnetic vanadium, offer a new approach for the study of low-dimensional magnetism and crossover effects near the transition from ferromagnetic to antiferromagnetic order.

A forkhead gene, FoxE3, is essential for lens epithelial proliferation and closure of the lens vesicle.

In the mouse mutant dysgenetic lens (dyl) the lens vesicle fails to separate from the ectoderm, causing a fusion between the lens and the cornea. Lack of a proliferating anterior lens epithelium leads to absence of secondary lens fibers and a dysplastic, cataractic lens. We report the cloning of a gene, FoxE3, encoding a forkhead/winged helix transcription factor, which is expressed in the developing lens from the start of lens placode induction and becomes restricted to the anterior proliferating cells when lens fiber differentiation begins. We show that FoxE3 is colocalized with dyl in the mouse genome, that dyl mice have mutations in the part of FoxE3 encoding the DNA-binding domain, and that these mutations cosegregate with the dyl phenotype. During embryonic development, the primordial lens epithelium is formed in an apparently normal way in dyl mutants. However, instead of the proliferation characteristic of a normal lens epithelium, the posterior of these cells fail to divide and show signs of premature differentiation, whereas the most anterior cells are eliminated by apoptosis. This implies that FoxE3 is essential for closure of the lens vesicle and is a factor that promotes survival and proliferation, while preventing differentiation, in the lens epithelium.

The two-exon gene of the human forkhead transcription factor FREAC-2 (FKHL6) is located at 6p25.3.

The gene for the human transcription factor forkhead related activator 2 (FREAC-2; HGMW-approved symbol FKHL6) has been characterized and found to consist of two exons separated by an intron of 3.6 kb. The first exon encodes the forkhead DNA-binding domain and one of the transcriptional activation domains, AD2. The second exon contains the coding sequence corresponding to the C-terminal activation domain AD1. The full-length FREAC-2 protein is predicted to be 444 amino acids, which adds 39 amino acids to the previously published partial cDNA sequence. A 2-kb CG island is centered around the 5' end of the FREAC-2 gene. Fluorescence in situ hybridization was used to localize the human FREAC-2 gene to chromosomal position 6p24-p25, and the localization was further refined by radiation hybrid mapping to 6p25.3.

The human forkhead protein FREAC-2 contains two functionally redundant activation domains and interacts with TBP and TFIIB.

Forkhead-related activator 2 (FREAC-2) is a human transcription factor expressed in lung and placenta that binds to cis-elements in several lung-specific genes. We have identified the parts of FREAC-2 responsible for trans-activation and found two functionally redundant activation domains on the C-terminal side of the DNA binding forkhead domain. Activation domain 1 consists of the most C-terminal 23 amino acids of FREAC-2 and contains a sequence motif conserved in an activation domain of another forkhead protein, FREAC-1. Activation domain 2 is built up by three synergistic subdomains in the central part of the FREAC-2 protein. FREAC-2 was shown to interact in vitro with TBP and TFIIB. The target site for FREAC-2 on TBP was localized to the N-terminal repeat in the core domain of TBP. TFIIB binds FREAC-2 close to the cleft between its two globular domains. The part of FREAC-2 that binds TBP was mapped to 21 amino acids in the C-terminal end of the forkhead domain. This sequence is well conserved among forkhead proteins, raising the possibility that interaction with TBP may be a general characteristic of this family of transcription factors. Overexpression of TFIIB potentiates activation by FREAC-2 in a manner dependent on the FREAC-2 activation domains. Nuclear localization of FREAC-2 was found to depend on sequences from both ends of the forkhead domain.

Differences in lodgement of tumour cells in muscle and liver.

Differences in the lodgement of circulating tumour cells in various organs are considered an important factor in metastatic organ selection. The present vital microscopic studies show that the pattern of intravascular arrest of tumour cells in muscle after intra-arterial injection is similar to that observed earlier, in the liver, after intraportal injection. However, parallel isotope studies on the lodgement process (at 5 min and 3 h after injection) showed that the tumour cells trapped in the muscle microvasculature were destroyed at a higher rate than in the liver. Tumour cells kept in test tubes, and thus not being subjected to the shearing forces of the circulation, had a higher survival rate than cells trapped in the muscle. The results indicate that stronger retardation forces acting on the tumour cells in muscle (arterial dissemination) than in the liver (venous dissemination) may be one mechanism behind the increased tumour cell destruction in muscle.

Vital microscopic studies on the capillary distribution of leukocytes in the rat cremaster muscle.

Leukocyte capillary distribution was studied with vital fluorescence microscopy in the rat cremaster muscle. Ten terminal arterioles, each in a different animal, with 2 to 7 capillaries and with a total leukocyte flux ranging from 97 to 571 cells, were analysed. Nonparametric statistics was used for the analyses of the experimental data. In order to characterize the pathways for leukocyte flux in the networks, the leukocyte concentration for each capillary was calculated in relation to the network inflow leukocyte concentration and to the bifurcation inflow leukocyte concentration. The study shows a strong positive correlation between the relative capillary leukocyte concentration and the capillary volume flow and the position of the capillary in the network, respectively. There was no correlation between the relative capillary leukocyte concentration and capillary branching angle. A relative leukocyte concentration above unity, indicating an enrichment of leukocytes, was found in capillaries with a high volume flow and in capillaries located at the downstream end of the arterioles. In addition, the analyses show a very strong correlation between capillary volume flow and the position of the capillary along the arteriole, the capillaries located at the downstream end having the highest volume flows. In conclusion, the present study provides evidence for the existence of volume flow dependent preferential pathways for leukocyte passage across the capillary network in the rat cremaster muscle.

Leukocyte effects on the vascular resistance and glomerular filtration of the isolated rat kidney at normal and low flow states.

Because of their large volume and high internal viscosity, leukocytes may obstruct capillary blood flow, particularly in low flow states. In the present study rat kidneys were isolated and perfused with a cell-free colloid solution. The urine production was continuously measured and samples were taken for determination of inulin clearance. In this way changes in glomerular filtration rate (GFR) and tubular reabsorption (TR) could be followed before, during, and after a bolus injection of leukocytes separated from whole blood. The experiments showed that leukocyte infusion caused a sustained increase of the renal vascular resistance, which at low flows ranged from 5 to 20%, whereas at normal flow rates only small and transient increases occurred. The passage of a leukocyte bolus caused, in addition, a decrease in GFR and to a small degree also in TR. The study indicates that leukocytes may disturb the perfusion of glomeruli and peritubular vessels in low flow states.

The effect of hypoperfusion on the capillary distribution of leukocytes in cross-striated muscle.

In order to analyse whether microvascular redistribution of the leukocytes has any influence on the development of leukocyte capillary plugging at low flow states, the arteriole to capillary distribution of leukocytes was studied with vital microscopy in the rat cremaster muscle at normal perfusion pressure and after blood pressure reduction by 50% through aortic clamping. The reduction of the blood pressure led to a redistribution of the leukocytes, characterized by a shift of the leukocyte flux from capillaries at the downstream end of the arterioles towards more proximal branches in 8 of 19 networks studied, but flow cessation due to permanent leukocyte plugging was observed in only 2 of 94 capillaries observed. The arterioles of the 8 networks showing redistribution of the leukocytes had a significantly larger diameter than the rest of the arterioles. A possible explanation to the redistribution of leukocytes in 8 of the networks might be that their wider arterioles allowed a peripheral displacement and a skimming off of the leukocytes upon pressure reduction. The results from this investigation do not indicate that redistribution of leukocytes in the capillary networks of muscle should be significantly involved in the development of permanent leukocyte capillary plugging at low flow states.

Microscopic studies on the influence of erythrocyte concentration on the post-junctional radial distribution of leukocytes at small venular junctions.

The influence of flow patterns and erythrocyte concentration on the post-junctional radial position of leukocytes was studied in microvascular junctions in an isolated mesocaecum preparation and in vivo in the rat cremaster muscle. Main branch and side branch dimensions (mesentery) were approximately 60 micrograms and approximately 20 micrograms, respectively. For the analysis the main branch was divided into three equally wide sectors. Firstly, the observations indicate that the main branch and side branch flows remain separated downstream of a junction and, secondly, that the blood cells mainly leave the junction within their original streams. At zero hematocrit 71-87.5% of the leukocytes from the side branch flowed in the closest main branch sector; 12.5-29% reached the central sector-the lowest numbers were seen in the central sector when the flow velocity ratio, main branch-side branch was 2:1. No cells became marginated. At 20% and 40% hematocrit, all leukocytes from the side branch flowed in the closest main branch sector. Marginated leukocytes in the side branch or upstream in the main branch remained marginated only if passing the junction on the uninterrupted wall. In the cremaster muscle, 3-capillary confluences were studied. The blood from the capillaries seemed to flow in three separate streams in the venule. Since these streams were narrower than those observed in the mesentery, the leukocytes in the capillaries producing the peripheral streams continued to flow in contact with the vascular wall, i.e. they became marginated in the venules without any lateral movement. On the basis of this study and previous work it is proposed that the radial position of leukocytes downstream of a junction is determined not only by leukocyte-erythrocyte interactions but also by the specific flow patterns that arise in microvascular confluences.

The initiation of post-capillary margination of leukocytes: studies in vitro on the influence of erythrocyte concentration and flow velocity.

Leukocyte margination is an important rheological phenomenon, being a prerequisite for leukocyte adhesion and invasion of the tissues in inflammation. In the present study the influence on leukocyte radial distribution of erythrocyte concentration and flow velocity was studied with fluorescence microscopy in a glass capillary model. This consisted of a narrow (10 microns) short stenosis expanding into a 50 microns straight tube. The blood cells were brought to the centre of the stream by the stenosis whereas the radial position of the leukocytes was determined in the straight post-stenotic segment of the glass tube. The analyses showed that a 0 hematocrit 86% (flow velocity 0.2 mm/s) to 99% (flow velocity 1.2 mm/s) of the leukocytes stayed in the centre of the tube after leaving the stenosis. No leukocytes were observed at the capillary wall. At 10% hematocrit and low flow velocity, 0.1 and 0.3 mm/s, 36 and 34%, respectively, of the leukocytes were displaced to a marginal position. At 1.0 mm/s only 10% of the leukocytes were displaced from the central stream. At 40% hematocrit, 45, 47 and 40% of the leukocytes were found in a wall near position at 0.1, 0.4 and 0.7 mm/s flow velocity, respectively. Although the results obtained in the present study are valid only for the special geometry of the in vitro model they clearly indicate the importance of leukocyte-erythrocyte interactions for the initiation of leukocyte margination in post-capillary vessels.