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PubMed1999

Postnatal differentiation of the ductus deferens, tail of the epididymis, and distal body of the epididymis in goats occurs independently of rete testis fluid.

Observations from extratesticular rete-ligated, mature goats indicated that epithelial morphology in the tail of the epididymis can be maintained without any input from testicular fluid (Goyal et al., Acta Anat., 1994;150: 127-135). Hence, the objective of this study was to determine whether the tail of the epididymis and/or other regions of the male excurrent ducts can differentiate prior to the appearance of lumen in the seminiferous tubules, which is an indicator for the onset of seminiferous tubular fluid secretion. Based on age and scrotal circumference (SC), 20 male goats were divided into four groups of five animals each: 1-4 weeks (SC, 6.5-7.5 cm), 7-10 weeks (SC, 8.5-11.0 cm), 12-15 weeks (SC, 11.0-14.0 cm), and 15-25 weeks (SC, 16.0-19.0 cm). Tissues were collected from the testis, six regions of the epididymis (proximal, middle and distal head; proximal and distal body; and tail), and the ductus deferens, and were processed for light and electron microscopic examination. Changes in epithelial height and cytological features associated with absorption (microvilli, pinocytotic and coated vesicles) and protein secretion (RER, Golgi body) were used as markers for differentiation. Differentiation of all of these features was comparable to that observed in the 15-25-week-old animals in the ductus deferens by > or = 1 week, in the tail of the epididymis by > or = 7 weeks, in the distal body of the epididymis by > or = 12 weeks, and in the proximal body of the epididymis and all three regions of the head of the epididymis by > or = 15 weeks. Seminiferous tubules developed lumens between 12 and 15 weeks. In conclusion, epithelial differentiation in the ductus deferens, tail of the epididymis, and distal body of the epididymis follows a time-dependent, spatial, ascending order and is achieved before lumen formation in the seminiferous tubules. Conversely, epithelial differentiation in all three regions of the head and the proximal body of the epididymis occurs simultaneously and after lumen formation in the seminiferous tubules.

Age Factors↗

PubMed2000

MMP-2 expression during early avian cardiac and neural crest morphogenesis.

Matrix metalloproteinase-type 2 (MMP-2) degrades extracellular matrix, mediates cell migration and tissue remodeling, and is implicated in mediating neural crest (NC) and cardiac development. However, there is little information regarding the expression and distribution of MMP-2 during cardiogenesis and NC morphogenesis. To elucidate the role of MMP-2, we performed a comprehensive study on the temporal and spatial distribution of MMP-2 mRNA and protein during critical stages of early avian NC and cardiac development. We found that ectodermally derived NC cells did not express MMP-2 mRNA during their initial formation and early emigration but encountered MMP-2 protein in basement membranes deposited by mesodermal cells. While NC cells did not synthesize MMP-2 mRNA early in migration, MMP-2 expression was seen in NC cells within the cranial paraxial and pharyngeal arch mesenchyme at later stages but was never detected in NC-derived neural structures. This suggested NC MMP-2 expression was temporally and spatially dependent on tissue interactions or differed within the various NC subpopulations. MMP-2 was first expressed within cardiogenic splanchnic mesoderm before and during the formation of the early heart tube, at sites of active pharyngeal arch and cardiac remodeling, and during cardiac cushion cell migration. Collectively, these results support the postulate that MMP-2 has an important functional role in early cardiogenesis, NC cell and cardiac cushion migration, and remodeling of the pharyngeal arches and cardiac heart tube.

Animals↗

PubMed1999

In search of spatial extremes.

In environmental studies attention increasingly focuses on identification of spatial extremes: locations with observations that are apparently higher than either a preset background threshold or neighbouring observations. We consider various procedures for identifying values and locations of these extremes: extreme value theory, conditionally simulated fields and disjunctive kriging. In a recent research project we studied the distribution of nine environmental pollutants (heavy metals, polyaromatic hydrocarbons and mineral oil) in a large industrial estate in the southern Netherlands. This area is interesting as it is influenced by three contaminating processes: inundation by the river Meuse, anthropogenic heightening with contaminated materials and local industrial activities. Concentrations are measured. In this paper the peaks over threshold method was useful for finding extremes for spatially uncorrelated variables, whereas conditional simulations applied in particular to areas with spatial dependence.

Animals↗

PubMed2003

Dopaminergic amacrine cells in the inner nuclear layer and ganglion cell layer comprise a single functional retinal mosaic.

Many types of retinal neuron are distributed in an orderly manner across the surface of the retina. Indeed, the existence of such regularity amongst a population of neurons, termed a retinal mosaic, may be a defining feature of functionally independent types of retinal neuron. We have examined the spatial distribution of dopaminergic amacrine cells in the ferret retina both in the inner nuclear layer (INL) and in the ganglion cell layer (GCL) to determine whether the cells in each layer form an independent retinal mosaic as evidence of whether they should be considered as two separate types. Ferret retinas contain approximately 1,900 dopaminergic amacrine cells, of which 27% are located in the GCL, and the rest in the INL. Based on analysis of their Voronoi domains as well as autocorrelation analysis and tests for complete spatial randomness, we found that the distribution of INL cells was statistically regular, while that of the GCL cells was not. However, by using cross-correlation analysis, these two groups of cells were found to be spatially dependent: an exclusion zone was detected in the cross-correlogram of roughly the same size as that found in the autocorrelograms of both INL and GCL cells. Such a pattern would be expected if dopaminergic amacrine cells in the INL and GCL were members of a single regular population differing only in their somatic depth. By using computer simulations, we tested this hypothesis directly, confirming that a random assignment of 27% from the total population produces cross-correlograms that are indistinguishable from those of the biological mosaics. We conclude, therefore, that the cells in the two layers form a single functional population; those in the GCL appear to be misplaced. Somatic positioning with respect to depth within the retina is not, by itself, a reliable guide for functional classification.

Amacrine Cells↗

PubMed2006

Tandem-pore domain potassium channels are functionally expressed in retinal (Müller) glial cells.

Tandem-pore domain (2P-domain) K+-channels regulate neuronal excitability, but their function in glia, particularly, in retinal glial cells, is unclear. We have previously demonstrated the immunocytochemical localization of the 2P-domain K+ channels TASK-1 and TASK-2 in retinal Müller glial cells of amphibians. The purpose of the present study was to determine whether these channels were functional, by employing whole-cell recording from frog and mammalian (guinea pig, rat and mouse) Müller cells and confocal microscopy to monitor swelling in rat Müller cells. TASK-like immunolabel was localized in these cells. The currents mediated by 2P-domain channels were studied in isolation after blocking Kir, K(A), K(D), and BK channels. The remaining cell conductance was mostly outward and was depressed by acid pH, bupivacaine, methanandamide, quinine, and clofilium, and activated by alkaline pH in a manner consistent with that described for TASK channels. Arachidonic acid (an activator of TREK channels) had no effect on this conductance. Blockade of the conductance with bupivacaine depolarized the Müller cell membrane potential by about 50%. In slices of the rat retina, adenosine inhibited osmotic glial cell swelling via activation of A1 receptors and subsequent opening of 2P-domain K+ channels. The swelling was strongly increased by clofilium and quinine (inhibitors of 2P-domain K+ channels). These data suggest that 2P-domain K+ channels are involved in homeostasis of glial cell volume, in activity-dependent spatial K+ buffering and may play a role in maintenance of a hyperpolarized membrane potential especially in conditions where Kir channels are blocked or downregulated.

Animals↗

PubMed2003

Examination of membrane rupture as a mechanism for mammalian cell detachment from fibronectin-coated biomaterials.

Synthetic biomaterials intended for the reconstruction of tissues and organs must be capable of sustaining adhesive contact with adjacent cells and tissues under mechanical and hydrodynamic stresses. To facilitate this adhesion, extracellular matrix proteins or peptide sequences are frequently immobilized to the biomaterial interface. These ligands enhance cell attachment by raising the number of cell receptor/ligand interactions, but consequently they may alter the mechanism of cell detachment. In particular, as the cell membrane is more strongly immobilized to the substratum, the tendency for cell detachment to involve membrane rupture may increase. To test this hypothesis, cells were fluorescent stained with a membrane dye, allowed to attach to fibronectin-coated model substrates for 30 min, and then subjected to a spatially dependent range of shear stress for 5 min (28-220 dyn/cm2) using a radial-flow chamber. Phase-contrast and fluorescent images were analyzed to determine the probability for cell detachment and the area of fluorescent debris left by detaching cells as a function of fibronectin concentration, magnitude of shear stress, and time. It was found at all concentrations of fibronectin that the majority of detaching cells left membrane fragments, the mean size of these fragments was independent of shear stress, and the shape independent of the direction of flow. However, mean fragment area increased with concentration of fibronectin and decreased with duration of shearing flow. We postulate that the area of debris reflects the extent of cell attachment prior to the application of shear and that adhesive complexes can disassemble at the onset of flow.

3T3 Cells↗

PubMed2004

1H-MRSI of radiation effects in normal-appearing white matter: dose-dependence and impact on automated spectral classification.

PURPOSE: To identify radiation-induced changes in healthy white-matter spectra in the first six months following radiotherapy, and assess the impact of these changes on an automated algorithm for detecting spectral abnormalities. MATERIALS AND METHODS: 1H-MRSI was performed on 10 patients with grade IV gliomas who were to undergo radiation therapy. Choline (Cho), creatine (Cr), and N-acetylaspartate (NAA) ratios were studied as a function of dose and time. The impact of these spectral changes on a spectral analysis algorithm was evaluated. RESULTS: The Cho/NAA ratios rose to values of 0.66 +/- 0.15, 0.75 +/- 0.21, and 0.73 +/- 0.15 two months after therapy, compared to immediate post-therapy values of 0.56 +/- 0.15, 0.60 +/- 0.16, and 0.61 +/- 0.15 for the < 25, 25-50, and > 50 Gy dose groups, respectively. These maxima were followed by a dose-dependent recovery. A similar trend was found in the Cho/Cr ratio. The automated spectral analysis system incorporated the changing Cho/NAA ratio into a global redefinition of healthy tissue, but did not account for dose-dependent spatial variations in Cho/NAA ratios. CONCLUSION: Radiation significantly alters the spectra of healthy tissues in the first six months after radiotherapy. This suggests that the radiation dose distribution should be considered during analysis of post-therapy spectra.

Aspartic Acid↗

PubMed2006

Nicotine reverses adult-onset hypothyroidism-induced impairment of learning and memory: Behavioral and electrophysiological studies.

Nicotine alleviates cognitive impairment associated with a variety of health conditions. We examined the effect of chronic nicotine treatment on adult-onset hypothyroidism-induced impairment of learning and memory in rats. Hypothyroidism was induced by surgical removal of thyroid glands (thyroidectomy). One month later, chronic nicotine treatment (1 mg/kg sc, twice/day) was instituted for 4-6 weeks. Test of hippocampus-dependent spatial learning and memory in the radial arm water maze showed that hypothyroidism impaired learning as well as short-term and long-term memory retention. Chronic nicotine treatment reversed the hypothyroidism-induced learning and memory impairment. In normal rats, chronic nicotine treatment had no effect on learning and memory. Extracellular recordings from the CA1 region of anesthetized hypothyroid rats showed severe reduction of both early-phase and late-phase long-term potentiation (LTP) magnitude, which was reversed in nicotine-treated hypothyroid rats. These results show that chronic nicotine treatment prevents hypothyroidism-induced impairment of spatial cognition and LTP.

Action Potentials↗

PubMed1998

Effects of injurious compression on matrix turnover around individual cells in calf articular cartilage explants.

The effects of mechanical injury on the metabolism of cartilage matrix are of interest for understanding the pathogenesis of osteoarthrosis and the development of strategies for cartilage repair. The purpose of the present study was to examine the effects of injury on matrix turnover in a calf articular cartilage explant system for which the effects of mechanical loading on cell activity and the cell-mediated pathways of matrix metabolism are already well characterized. New methods of quantitative autoradiography were used in combination with established biochemical and biomechanical techniques for the analysis of cell and matrix responses to acute mechanical injury, with particular attention to the processes of localized matrix turnover in the cell-associated matrices of individual chondrocytes. Matrix deposition and turnover around cells in control explants was spatially dependent, with the highest rates of proteoglycan deposition and turnover and the lowest rates of collagen deposition (as indicated by [3H]proline autoradiography) occurring in the pericellular matrix. Injurious compression was associated with (a) an abrupt decrease in the tensile load-carrying capacity of the collagen matrix, apparently associated with mechanical failure of the tissue, (b) a considerable but subtotal decrease in cell viability, marked by the emergence of an apparently inactive cell population interspersed within catabolically active but abnormally large cells, and (c) sustained, elevated rates of proteoglycan turnover, particularly in the cell-associated matrices of apparently viable cells, which involved the increased release of aggregating species in addition to a spectrum of degradation fragments that were also in controls. These results may represent an in vitro model for the responses of chondrocytes and the cartilage extracellular matrix to mechanical injury.

Animals↗

PubMed2003

Analysis and generalized correction of the effect of spatial gradient field distortions in diffusion-weighted imaging.

Nonuniformities of magnetic field gradients can cause serious artifacts in diffusion imaging. While it is well known that nonlinearities of the imaging gradients lead to image warping, those imperfections can also cause spatially dependent errors in the direction and magnitude of the diffusion encoding. This study shows that the potential errors in diffusion imaging are considerable. Further, we show that retrospective corrections can be applied to reduce these errors. A general mathematical framework was formulated to characterize the contribution of gradient nonuniformities to diffusion experiments. The gradient field was approximated using spherical harmonic expansion, and this approximation was employed (after geometric distortions were eliminated) to predict and correct the errors in diffusion encoding. Before the corrections were made, the experiments clearly revealed marked deviations of the calculated diffusivity for fields of view (FOVs) generally used in diffusion experiments. These deviations were most significant farther away from the magnet's isocenter. For an FOV of 25 cm, the resultant errors in absolute diffusivity ranged from approximately -10% to +20%. Within the same FOV, the diffusion-encoding direction and the orientation of the calculated eigenvectors can be significantly altered if the perturbations by the gradient nonuniformities are not considered. With the proposed correction scheme, most of the errors introduced by gradient nonuniformities can be removed.

Anisotropy↗

PubMed1989

High-flux signals and spatial localization in high-resolution 1H spectroscopy with surface coils.

To perform in vivo localized proton spectroscopy with water suppression, spin-echo sequences, made of binomial pulses, are commonly used with surface coils. The frequency selective response to such a sequence is also-spatially dependent, that is dependent on the sample shape and on the pulse angle adjustment. It is consequently pointed out in this paper that quantitative analysis for relative peak intensities may be strongly affected by the contribution of the high-flux regions. In vivo proton spectroscopy of rat brain exemplifies this difficulty. It is shown that the use of selective prepulses to suppress high-flux signals may be of poor efficiency depending on chemical shift, while the use of hard nonselective prepulses works for any chemical shift.

Animals↗

PubMed1992

Intensity artifacts in MRI caused by gradient switching in an animal-size NMR magnet.

The switching of magnetic field gradients in MRI gives rise to eddy currents in the structural components of superconducting magnet systems. The associated magnetic fields cause intensity artifacts which are particularly severe in some animal-size systems. We treat theoretically three mechanisms which cause intensity artifacts in one-dimensional projection images obtained by a spin-echo technique. The first is an off-resonance effect, caused by applying the refocusing pulse before the read compensation gradient pulse has decayed sufficiently. The other two mechanisms are caused by a spatial dependence of the phase accumulated by the spins at the time of formation of the echo, as a result of the eddy current fields. First, interference causes a loss of transverse magnetization because of a variation in the phase of spins which lie on the same isochromat during the read gradient pulse. Second, a variation of the phase of the spins in a direction orthogonal to the isochromats causes spins throughout the sample to refocus at different times. These two mechanisms are fundamentally different, since interference can occur even if the main magnetic field is homogeneous, whereas improper refocusing does not. It is shown that there is no loss of intensity by the interference mechanism if phase encoding is used to form two-dimensional images. This may well be a major reason why images obtained by 2DFT have been found to be generally superior to those obtained by projection reconstruction. Experimentally, the distribution of intensity in one-dimensional projection images of a square slice phantom is compared with theoretical intensities, estimated using eddy current field reported in the preceding paper.

Artifacts↗

PubMed1993

In vivo proton spectroscopy and spectroscopic imaging of [1-13C]-glucose and its metabolic products.

Metabolism of [1-13C]-glucose was studied in situ in cat brain using gradient-enhanced proton-detected heteronuclear spectroscopy. Proton detection of [1-13C]-glucose, [3-13C]-lactate, 4-[13C]-glutamine, 4-[13C]-glutamate and the combined signals 2-[13C]-glutamate/glutamine and 3-[13C]-glutamate/glutamine was achieved, despite the fact that some of the associated proton resonances are close to the water signal. Two-dimensional [1H-13C]-spectra demonstrate the possibility of in situ spectral assignment with 1H sensitivity and 13C resolution. Spectroscopic images of glucose and its metabolic products were also acquired, showing the possibility to study spatial dependence of metabolism.

Animals↗

PubMed1994

Quantitative characterization of the eddy current fields in a 40-cm bore superconducting magnet.

The temporal and spatial dependence of the eddy current fields, generated by switching off x, y, and z gradients in a 40-cm bore Bruker superconducting magnet, have been studied by measuring the offset frequency of the proton FID obtained from a small spherical sample. The measurements were made with the pre-emphasis unit deactivated. The data obtained at each location were well fitted to a sum of four exponentially decaying components. The shortest decay is, in each case, associated with the decay of the current in the gradient coil itself, while the other decays are associated with three eddy currents with decay times ranging from 13 to 480 ms. Each of the three eddy currents generates an essentially uniform gradient for at least 4 cm on either side of the isocenter. However, when the eddy currents are generated by either the vertical transverse gradient or the longitudinal gradient, the eddy current gradients are also accompanied by a field shift.

Magnetic Resonance Imaging↗

PubMed1994

Surface coil cardiac tagging and 31P spectroscopic localization with B1-insensitive adiabatic pulses.

A technique is presented for MRI tagging in the presence of inhomogeneous B1 fields. A rectangular tagging grid is produced with B1-insensitive adiabatic pulses in a magnetization preparation period that precedes image acquisition. Phantom results demonstrate that the method is well-suited to surface coil experiments. The technique is applied to a canine model of myocardial ischemia to track the spatially dependent wall motion of the left ventricle during the cardiac cycle. Transmural 31P spectra are acquired from the same double-tuned surface coil, with tagging and spectroscopy performed for the first time, during normal, ischemic, and recovery conditions for the same animal.

Adenosine Triphosphate↗

PubMed1995

Calculation of sensitivity correction factors for surface coil MRS.

Quantification of MRS signals obtained with surface coils is difficult due to the inhomogeneous response of these coils. This inhomogeneity results in the measured signal from a defined volume of interest (VOI) being spatially dependent. To account for the sensitivity variation with position from the surface coil, we have developed a method of calculating correction factors for defined VOIs based on an experimentally obtained 3D sensitivity coil map. These factors may then be applied to spectra obtained from these VOIs to accurately take into consideration the varying coil sensitivity resulting in a reduction of measured signal. This method is demonstrated here to be able to correct for the inhomogeneity of surface coils over a range of two coil radii to within 4% accuracy.

Humans↗

PubMed1997

Theory of heterogeneous relaxation in compartmentalized tissues.

A new model of compartmentalized relaxation--that which occurs for spins (protons) exchanging between compartments of different relaxation rates--is presented. This model generalizes previous ones by allowing spatially dependent relaxation within compartments. Solutions for the diffusion-Bloch equations are found via an efficient numerical technique known as the generalized moment expansion, and they agree well with the solutions to the standard two-site exchange equations (TSEE) for many typical situations. Specific models are developed for liposomes, red blood cells, capillaries, and arteries with respect to applied contrast agents. A parameter derived from tissue characteristics is introduced to predict the nature of the solutions. A new method is proposed for using contrast agents to detect capillaries, which exploits their high surface-to-volume ratio relative to the other elements of the vasculature.

Arteries↗

PubMed1998

Concomitant gradient terms in phase contrast MR: analysis and correction.

Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.

Artifacts↗