Extracellular matrix environment influences chondrogenic pattern formation in limb bud micromass culture: experimental verification of theoretical models

Various theoretical models have been proposed to explain the periodicity in the pattern of limb chondrogenesis, but experimental comparison of these models have seldom been performed properly. In the present study, micromass culture of limb bud mesenchyme cells was undertaken to test the validity of three theoretical models: the reaction-diffusion model, the cell sorting model, and the mechanochemical model. Computer simulations were undertaken to predict the factors that can affect the coarseness of the chondrogenic pattern. According to the predictions, we performed micromass culture of limb mesenchyme in collagen and agarose gel. Then we carried out time-lapse observation to analyze the cell movement during pattern formation. From computer simulations it was theoretically predicted that changes in the surrounding extracellular matrix should alter the periodicity of the chondrogenic pattern in vitro, and we found that pattern changes actually occurred under different culture conditions. When compared with the culture in a liquid medium, the chondrogenic pattern became less coarse when the cells were cultured in collagen or agarose gel, and the pattern change appeared to be independent of the cell differentiation. Time-lapse observation revealed a decrease in cell motility when the cells were cultured in gel. It was found that both the reaction-diffusion and cell sorting models fit the pattern change produced and that the mechanochemical model is not primarily important in the chondrogenic pattern formation in vitro.     

TGFbeta2 acts as an "activator" molecule in reaction-diffusion model and is involved in cell sorting phenomenon in mouse limb micromass culture.

It was previously speculated that TGFbeta acts as an "activator"-molecule in chondrogenic pattern formation in the limb micromass culture system, but its precise role and relationship with the cell sorting phenomenon have not been properly studied. In the present study, we examined whether the TGFbeta2 molecule satisfies the necessary conditions for an "activator"-molecule in the reaction-diffusion model. Firstly, we showed that TGFbeta2 became localized at chondrogenic sites during the establishment of a chondrogenic pattern, and exogenous TGFbeta2 promoted chondrogenesis when added in the culture medium. Secondly, TGFbeta2 protein was shown to promote the production of its own mRNA after 3 hr, indicating that a positive feedback mechanism exists which may be responsible for the emergence of the chondrogenic pattern. We then found that when locally applied with beads, TGFbeta2 suppressed chondrogenesis around the beads, indicating it induces the lateral inhibitory mechanism, which is a key element for the formation of the periodic pattern. We also examined the possible effects of TGFbeta2 on the cell sorting phenomenon and found that TGFbeta2 exerts differential chemotactic activity on proximal and distal mesenchyme cells of the limb bud, and at very early phases of differentiation TGFbeta2 promotes the expression of N-cadherin protein which is known to be involved in pattern formation in this culture system. These findings suggest that TGFbeta2 acts as an "activator"-like molecule in chondrogenic pattern formation in vitro, and is possibly responsible for the cell sorting phenomenon.     

Cadherin-1, -2, and -11 expression and cadherin-2 function in the pectoral limb bud and fin of the developing zebrafish.

Cadherins are cell adhesion molecules that play important roles in development of a variety of organs, including the vertebrate limb. In this study, we analyze cadherin expression patterns in the embryonic zebrafish pectoral limb buds and larval pectoral fins by using both in situ hybridization and immunocytochemical methods. cadherin-1 is detected in the epidermis of the embryonic limb buds and the larval pectoral fins. Cadherin-2 is expressed in the pectoral limb bud mesenchyme and chondrogenic condensation. As development proceeds, cadherin-2 expression is detected in newly differentiated pectoral fin endoskeleton, but its expression is greatly down-regulated in the fin endoskeleton of larval zebrafish. cadherin-11 is found in the basal region of the embryonic limb buds and in the proximal endoskeleton of the larval pectoral fins. Interfering with cadherin-2 function using two specific antisense morpholino oligonucleotides disrupts formation of the chondrogenic condensation/endoskeleton, suggesting that cadherin-2 is crucial for the normal development of the zebrafish pectoral fins.     

Morphogenesis of precursor subpopulations of chicken limb mesenchyme in three dimensional collagen gel culture

Although homogeneous in appearance, several lines of evidence suggest early (stage 17-19) limb mesenchymal cells are committed to particular cell lineages, e.g., myogenic or chondrogenic. However, subsequent expression of cell or tissue phenotype in the developing limb does not occur in a randomized process but rather in a spatially specific pattern. The potential regulatory mechanisms controlling the "patterned" expression of tissue phenotype in the limb have not been resolved. The purpose of this study was to determine if, prior to the formation of an apical ectodermal ridge, nondissociated limb mesenchyme has inherent morphogenetic potential to form nonrandomized patterns of tissue organization. The hypotheses to be tested were that, if provided a spatially permissive culture environment, 1) mesenchymal cells committed to a particular lineage would segregate into precursor (sub)populations prior to overt expression of phenotype and 2) the ultimate expression of a tissue phenotype may be regulated, in part, by histogenic interactions between the precursor cell groups. For these studies, mesoblasts (intact mesenchyme minus ectoderm) from stage 17-19 hindlimb buds were explanted intact to the surface of a 1-3 mm thick hydrated lattice of repolymerized type I collagen and incubated for 2-11 days. Examination of cultures at variable intervals revealed three distinct temporal sequences (periods) which were arbitrarily termed early morphogenesis (0-3 days), cytodifferentiation (3-5.5 days), and primitive tissue formation (5.5-11 days) based on similarities to in situ limb development. By the end of the first period, the mesenchymal cells had sorted into three distinct precursor populations: 1) an epithelial-like outgrowth of premyogenic and prefibrogenic cells at the surface of the gel lattice (termed the "surface subset") which circumscribed, 2) a centrally positioned prechondrogenic condensate ("central subset"), and overlaid 3) a dispersed, population of free cells that invaded the collagen lattice ("seeded subset"). Subsequent cytodifferentiation led to the appearance of multinucleated myotubes within the surface subset and chondrification of the central subset. Cells of the seeded subset remained dispersed within the collagen lattice. Primitive histogenic events were initiated during the final period of development including 1) at sites where surface cells established boundaries with the central subset, collectives or "bundles" of variable sized myotubes were formed which became partially ensheathed by the attenuated processes of fibroblastlike cells; and 2) a secondary site of chondrogenic activity was initiated within the gel lattice at the boundary between the central and seeded cell populations. Transformation of seeded fibroblasts into chondroblasts accompanied expansion of the secondary chondrogenic element within the gel lattice.(ABSTRACT TRUNCATED AT 400 WORDS)     

Differences in the temporal dynamics of the visual ON and OFF pathways

The temporal structure of spike trains recorded from optic fibers and single units of the lateral geniculate nucleus (LGN) and primary visual cortex of the cat was studied with a novel method of inter-spike interval analysis. ON type relay cells of the LGN exhibited a multimodal interval distribution preferring a distinct interval (fundamental interval) and its multiples during the sustained light response, whereas most OFF cells showed a broad, unimodal distribution. The general pattern of the interval distribution was relatively independent of stimulus size and contrast and the degree of light adaptation. Simultaneously recorded S-potentials originating from the retinal input generally produced only a single peak at the fundamental interval length. Therefore, the multimodal interval distribution of LGN cells seems to be a result of intra-geniculate inhibition. Cortical cells also showed a weak tendency to fire with spike intervals similar to LGN cells. Therefore, the regular firing pattern observed at peripheral stages of the visual pathway can persist at higher levels and might promote the occurrence of oscillatory activity.     

Frizzled-7 and limb mesenchymal chondrogenesis: effect of misexpression and involvement of N-cadherin.

Products of the Frizzled family of tissue polarity genes have been identified as putative receptors for the Wnt family of signaling molecules. Wnt-signaling is implicated in the regulation of limb mesenchymal chondrogenesis, and our recent study indicates that N-cadherin and related activities are functionally involved in Wnt-7a-mediated inhibition of chondrogenesis. By using an in vitro high-density micromass culture system of chick limb mesenchymal cells, we have analyzed the spatiotemporal expression patterns and the effects on chondrogenesis of RCAS retroviral-mediated misexpression of Chfz-1 and Chfz-7, two Frizzled genes implicated in chondrogenic regulation. Chfz-1 expression was localized at areas surrounding the cartilaginous nodules at all time points examined, whereas Chfz-7 expression was limited to cellular aggregates during initial mesenchymal condensation, and subsequently was down-regulated from the centers toward the periphery of cartilage nodules at the time of chondrogenic differentiation, resembling the pattern of N-cadherin expression. Chondrogenesis in vitro was inhibited and limited to a smaller area of the culture upon misexpression of Chfz-7, but not affected by Chfz-1 misexpression. Analyses of cellular condensation and chondrogenic differentiation showed that the inhibitory action of Chfz-7 is unlikely to be at the chondrogenic differentiation step, but instead affects the earlier precartilage aggregate formation event. At 24 hr, expression of N-cadherin, a key component of the cellular condensation phase of chondrogenesis, was delayed/suppressed in Chfz-7 misexpressing cultures, and was limited to a significantly smaller cellular condensation area within the entire culture at 48 hr, when compared with control cultures. Chfz-1 misexpressing cultures appeared similar to control cultures at all time points. However, neither Chfz-1 nor Chfz-7 misexpression affected mesenchymal cell proliferation in vitro. These results suggest that Chfz-7 is active in regulating N-cadherin expression during the process of limb mesenchymal chondrogenesis and that Chfz-1 and Chfz-7 are involved in different Wnt-signaling pathways.     

Leg bud mesoderm retains morphogenetic potential to express limb-like characteristics ("limbness") in collagen gel culture.

Recent in situ hybridization studies have correlated expression of potential regulatory genes with pattern formation in limb bud mesoderm (Tabin: Cell 66:199-217, 1991); however, the mechanism(s) controlling their expression in mesoderm and their relevance to the establishment of a limb morphogenetic pattern remain unknown. One likely candidate for regulating patterning events in limb mesoderm is the apical ectodermal ridge, as its removal in ovo results in a graded truncation of limb skeletal elements in the proximal-distal axis dependent upon the time of excision (Rowe and Fallon: J Embryol Exp Morph 68:1-7, 1982). In the present study, we investigate whether the hypothetical imprint of ridge ectoderm is retained in cultured mesoderm. Specifically, we sought to determine if a subpopulation of limb mesoderm that forms in collagen gel culture (Markwald et al: Anat Rec 226:91-107, 1990), retains any expression of "limbness" in the absence of limb ectoderm as characterized by the formation of a predictable number and distribution of limb-like chondrogenic elements in comparison to the temporal and spatial relationships of the in situ proximal, hindlimb skeletal structures. Accordingly, explants of undissociated mesoderm from stage 18-22 chicken leg buds were cultured without ectoderm on collagen gel lattices and the central subpopulation of mesoderm was examined morphologically. We show that this central subset of mesoderm will form chondrogenic cells which were not expressed uniformly throughout the subset, but rather distinct nodules or elements of cartilage were elaborated. Moreover, the number of elements expressed by the central subset increased with the age of the mesoderm at the time of explantation; spatially and temporally, the sequence of elements that formed always proceeded from the proximal, anterior margin of the subset to its distal, posterior border. The shapes of the initial elements (designated I and II) resembled the forms of in situ proximal skeletal structures (girdle and femur-like), whereas more distal elements (III-V) were often fused and without structural similarity to in situ skeletal structures. When cultures were established from the posterior mesoderm of stage 19/20 or 21 mesoblasts, the frequency of element I formation was reduced approximately one-half, whereas formation of more distal elements was unaffected. Conversely, element formation from the central subset established from isolated anterior mesoderm was virtually identical to intact mesoblasts, indicating a capacity to regulate for the loss of mesoderm as occurs in situ (Hampé: Archs Anat Microsc Morph Exp 48:345-378, 1959).(ABSTRACT TRUNCATED AT 400 WORDS)     

Versican Knock‐Down Compromises Chondrogenesis in the Embryonic Chick Limb

Mesenchymal cell aggregation is critical for cartilage formation in the vertebrate limb. The extracellular matrix (ECM) plays a critical role in governing cell behavior and cell phenotype in this tissue, and the hyalectin versican is highly expressed in the ECM of precartilage mesenchymal cells and developing synovial joints. Although several in vitro studies have been conducted in an attempt to address versican's role during limb mesenchymal condensation, factors such as differences in cell density in culture, variations between chondrogenic cell lines, and the inability to prolong the viability of limb explants have led to conflicting data, mandating an in vivo analysis. By using a morpholino directed strategy in ovo, we performed knock‐down of versican expression in the presumptive ulnar region of the developing chick wing at time points critical to skeletogenesis. These data indicate that in ovo misexpression of versican compromised mesenchymal condensation with resulting ulnar cartilages reduced in length distally by an average of 53% relative to contralateral control limbs. In select versican morphants the olecranon process was also reduced in size proximally and failed to cup the humerus, likely impairing joint morphogenesis. This study represents the first report assessing the role of versican in the developing chick limb in ovo, further demonstrating the importance of versican proteoglycan expression during chondrogenesis and extending previous findings to suggest a role for versican during synovial joint development. Anat Rec, 291:19–27, 2007. © 2007 Wiley‐Liss, Inc.     

Cholinesterases and peanut agglutinin binding related to cell proliferation and axonal growth in embryonic chick limbs

Embryonic cholinesterases are assigned important functions during morphogenesis. Here we describe the expression of butyrylcholinesterase and acetylcholinesterase, and the binding of peanut agglutinin, and relate the results to mitotic activity in chick wing and leg buds from embryonic day 4 to embryonic day 9. During early stages, butyrylcholinesterase is elevated in cells under the apical ectodermal ridge and around invading motoraxons, while acetylcholinesterase is found in the chondrogenic core, on motoraxons and along the ectoderm. Peanut agglutinin binds to the apical ectodermal ridge and most prominently to the chondrogenic core. Measurements of thymidine incorporation and enzyme activities were consistent with our histological findings. Butyrylcholinesterase is concentrated near proliferative zones and periods, while acetylcholinesterase is associated with low proliferative activity. At late stages of limb development, acetylcholinesterase is concentrated in muscles and nonexistent within bones, while butyrylcholinesterase shows an inverse pattern. Thus, as in other systems, in limb formation butyrylcholinesterase is a transmitotic marker preceding differentiation, acetylcholinesterase is found on navigating axons, while peanut agglutinin appears in non-invaded regions. These data suggest roles for cholinesterases as positive regulators and peanut-agglutinin-binding proteins as negative regulators of neural differentiation.     

Beat-to-beat modulation of atrioventricular conduction during dynamic exercise in humans

A complex balance between extrinsic neural and intrinsic mechanisms is responsible for regulating atrioventricular (AV) conduction. We hypothesized that atrial excitation interval is shortened during dynamic exercise by extrinsic cardiac autonomic activity and that if AV conduction time responds inversely to fluctuation in atrial rhythm, ventricular excitation interval will be maintained at the predetermined cardiac cycle length. To examine such inverse relationship between PP interval and the subsequent change in PR interval (DeltaPR), we analyzed the beat-to-beat changes in PP, PR, and RR intervals during stair-stepping exercise for 10 min in 11 sedentary and 9 trained subjects. In the sedentary group, the average PR interval significantly shortened during exercise, in parallel with the reduction in the average PP and RR intervals. The variance of PP and RR intervals was also significantly decreased during exercise. The reduction in the variance of RR interval was, however, much greater than that of PP interval, implying that AV conduction time changes inversely to fluctuation in atrial excitation rhythm. Indeed, the variance of PR interval was augmented during exercise and there was a clear inverse relationship between PP and DeltaPR intervals. Although trained subjects were characterized by their lower heart rate response during dynamic exercise, the responses in the variability of PP, PR, and RR intervals were fundamentally identical with those in sedentary subjects. We conclude that the AV nodal mechanism that operates at a higher level of heart rate during dynamic exercise may cancel fluctuation in atrial excitation interval and keep ventricular excitation rhythm at the predetermined cardiac cycle length.     

Pattern of pulses that maximize force output from single human thenar motor units

We assessed the sequence of nerve impulses that maximize force output from individual human thenar motor units. When these motor units were stimulated intraneurally by a variable sequence of seven pulses, the pattern of pulses that elicited maximum force always started with a short (5-15 ms) interpulse interval termed a "doublet. " The twitch force summation caused by this "doublet" elicited, on average, 48 +/- 13% (SD) of the maximum tetanic force. The peak amplitude of "doublet" forces was 3.5 times that of the initial twitches, and twitch potentiation appeared to have little influence on twitch force summation elicited by the "doublets." For some units, the second optimal interpulse interval was also short. Peak forces elicited by the third to sixth interpulse intervals did not change substantially when the last interpulse interval was varied between 5 to 55 ms, so maximum force could not be attributed to any unique interpulse interval. Each successive pulse contributed a smaller force increment. When five to seven pulses were delivered in an optimal sequence, the evoked force was close to that recorded during maximal tetanic stimulation. In contrast, maximal force-time integral was evoked with one short interpulse interval (5-15 ms) then substantially longer interpulse intervals (>100 ms). Maximum force and force-time integrals were therefore elicited by different patterns of stimuli. We conclude that a brief initial interpulse interval (5-15 ms) is required to elicit maximum "doublet" force from human thenar motor units and that near-maximal tetanic forces can be elicited by only five or six additional post-"doublet" pulses if appropriately spaced in time. However, the rate at which these post-"doublet" stimuli must be provided is fairly uncritical. In contrast, maximum post-"doublet" force-time integrals were obtained at intervals corresponding to motoneuronal firing rates of approximately 7 Hz, rates close to that typically used to recruit motor units and to maintain weak voluntary contractions.     

Epidermis, basement membrane, and connective-tissue healing after amputation of mouse digits: implications for mammalian appendage regeneration

Soft tissues from amputation sites of mice were examined at both light and electron microscope levels to determine whether features of growth buds (blastemas), which are necessary for amphibian limb regeneration, exist in nonregenerating mice. Several such features were found. A small area of the wound bed was covered by wound epithelium which, as in regenerating newt limbs, initially lacked an underlying basement membrane. Serially sectioned digits revealed blastemalike growth in the subdermal layer surrounding periosteal chondrogenic cells. Mesenchymelike cells were seen among the fibroblasts and leucocytes within the proliferating tissues. However, no evidence of dedifferentiation was seen in the dermis, which persisted as an apparent intact obstruction to growth bud formation. Existence of the essential ingredients of growth buds and soft-tissue proliferation adjacent to chondrogenic cells proximally suggest that the tissues of mammalian healing may differ quantitatively rather than qualitatively from tissues of appendage regeneration. This premise is encouraging for efforts at growth enhancement in mammals.     

A quantitative study of the pulsatile parameters of CRH-41 secretion in unanesthetized free-moving rats

Summary Having recently improved the sensitivity of the RIA CRH-41 measurements in perfusates from push-pull cannulas implanted in the rat median eminence, we explored quantitatively the pulsatile parameters of the CRH-41 measured with this technique at 2.5 or 5 min intervals in a series of unanesthetized male rats under basal conditions. The data were analysed by computer using 4 algorithms, i.e. Santen & Bardin, Ultra, Pulsar and Periodogram. Under a basal mean secretion rate of 15.5 pg CRH-41/15 min, the pulsatile pattern of CRH-41 release was statistically assessed with the 4 methods, with a mean pulse frequency of 3.1 c/h, pulse length of 11.6 min and a peak amplitude above mean levels of 4 pg. The respective advantages of the 4 algorithms and the physiological relevance of the neurohormone's pulsatility are discussed.     

Relative impact of uniaxial alignment vs. form-induced stress on differentiation of human adipose derived stem cells

ADSCs are a great cell source for tissue engineering and regenerative medicine. However, the development of methods to appropriately manipulate these cells in vitro remains a challenge. Here the proliferation and differentiation of ADSCs on microfabricated surfaces with varying geometries were investigated. To create the patterned substrates, a maskless biofabrication method was developed based on dynamic optical projection stereolithography. Proliferation and early differentiation of ADSCs were compared across three distinct multicellular patterns, namely stripes (ST), symmetric fork (SF), and asymmetric fork (AF). The ST pattern was designed for uniaxial cell alignment while the SF and AF pattern were designed with altered cell directionality to different extents. The SF and AF patterns generated similar levels of regional peak stress, which were both significantly higher than those within the ST pattern. No significant difference in ADSC proliferation was observed among the three patterns. In comparison to the ST pattern, higher peak stress levels of the SF and AF patterns were associated with up-regulation of the chondrogenic and osteogenic markers SOX9 and RUNX2. Interestingly, uniaxial cell alignment in the ST pattern seemed to increase the expression of SM22α and smooth muscle α-actin, suggesting an early smooth muscle lineage progression. These results indicate that geometric cues that promote uniaxial alignment might be more potent for myogenesis than those with increased peak stress. Overall, the use of these patterned geometric cues for modulating cell alignment and form-induced stress can serve as a powerful and versatile technique towards controlling differentiation in ADSCs.     

Stimulation pattern that maximizes force in paralyzed and control whole thenar muscles

The pattern of seven pulses that elicited maximal thenar force was determined for control muscles and those that have been paralyzed chronically by spinal cord injury. For each subject group (n = 6), the peak force evoked by two pulses occurred at a short interval (5-15 ms; a "doublet"), but higher mean relative forces were achieved in paralyzed versus control muscles (41.4 +/- 3.9% vs. 22.7 +/- 2.0% maximal). Thereafter, longer intervals evoked peak force in each type of muscle (mean: 35 +/- 1 ms, 36 +/- 2 ms, respectively). With seven pulses, paralyzed and control muscles reached 76.4 +/- 5.6% and 57.0 +/- 2.6% maximal force, respectively. These force differences resulted from significantly greater doublet/twitch and doublet/tetanic force ratios in paralyzed (2.73 +/- 0.08, 0.35 +/- 0.03) compared with control muscles (2.07 +/- 0.07, 0.25 +/- 0.01). The greater force enhancement produced in paralyzed muscles with two closely spaced pulses may relate to changes in muscle stiffness and calcium metabolism. Peak force-time integrals were also achieved with an initial short interpulse interval, followed by longer intervals. The postdoublet intervals that produced peak force-time integrals in paralyzed and control muscles were longer than those for peak force, however (77 +/- 3 ms, 95 +/- 4 ms, respectively). These data show that the pulse patterns that maximize force and force-time integral in paralyzed muscles are similar to those that maximize these parameters in single motor units and various whole muscles across species. Thus the changes in neuromuscular properties that occur with chronic paralysis do not strongly influence the pulse pattern that optimizes muscle force or force-time integral.     

Kinetics of tamoxifen-regulated Cre activity in mice using a cartilage-specific CreER(T) to assay temporal activity windows along the proximodistal limb skeleton.

Cartilage differentiation occurs over a broad time range from early embryonic development, when the mesenchymal condensations that give rise to cartilage models for future bone first appear, and continuing through adult life, when there is ongoing maintenance of articular joint surfaces and re-activation of cartilage formation after fracture. The chondrogenic response also figures in the pathogenesis of degenerative and inflammatory joint diseases. We have generated a transgenic line expressing tamoxifen-dependent Cre recombinase that gives efficient recombination in the chondrogenic lineage, both during embryogenesis and postnatally, and provides a valuable tool for analysis of gene function selectively in chondrogenic cells using conditional genetic approaches. Because the cartilage model of the limb skeleton forms progressively in a proximodistal order during discrete, well-defined time periods, evaluation of the spatial extent of tamoxifen-induced recombination along the limb axis during these time windows has also enabled us to examine the pharmacokinetics of single-dose tamoxifen injections during pregnancy.     

The spectrum of symptoms and QT intervals in carriers of the gene for the long-QT syndrome.

BACKGROUND. The familial long-QT syndrome is characterized by a prolonged QT interval on the electrocardiogram, ventricular arrhythmias, and sudden death. It is not certain, however, that the length of the QT interval is a sensitive or a specific diagnostic criterion. Recently, we identified genetic markers on chromosome 11 that distinguished between carriers and noncarriers of the gene for the long-QT syndrome in three families. In this study, we compared the clinical features of carriers and noncarriers and assessed the diagnostic accuracy of the QT interval. METHODS. We obtained medical histories and electrocardiograms from 199 family members. QT intervals corrected for heart rate (QTc) were determined independently by two blinded investigators. Carriers of the long-QT gene (83 subjects) and noncarriers (116 subjects) were distinguished by genetic-linkage analysis. RESULTS. Fifty-two of the carriers of the long-QT gene (63 percent) had a history of syncope, whereas four (5 percent) had a history of aborted sudden death. The QTc intervals of the gene carriers ranged from 0.41 to 0.59 second (mean, 0.49). By contrast, the QTc intervals of the noncarriers ranged from 0.38 to 0.47 second (mean, 0.42). On average, carriers of the gene for the long-QT syndrome had longer QTc intervals than noncarriers, but there was substantial overlap (in 126 of the 199 subjects, or 63 percent). The use of a QTc interval above 0.44 second as a diagnostic criterion resulted in 22 misclassifications among the 199 family members (11 percent). QTc intervals of 0.47 second or longer in males and 0.48 second or longer in females were completely predictive but resulted in false negative diagnoses in 40 percent of the males and 20 percent of the females. CONCLUSIONS. In families affected by the long-QT syndrome, measurement of the QTc interval may not permit an accurate diagnosis. DNA markers make it possible to make a genetic diagnosis in some families, but not all gene carriers have symptoms.     

踝关节外骨骼助行模式对下肢肌肉激活与协调模式的影响EI北大核心CSCD

踝关节外骨骼可用来提高人的行走效率,辅助老年人、运动功能障碍患者等进行日常活动或康复训练,但外骨骼的助行模式会对穿戴者的下肢肌肉激活与协调模式产生影响。本文利用一款绳驱动踝关节外骨骼,设计了不同助力时机与助力大小组合的助行模式,采集了7名穿戴者在跑步机上以1.25 m/s速度水平行走时的下肢表面肌电信号,研究不同助行模式对穿戴者下肢肌肉激活与协调模式的影响。实验结果表明,比目鱼肌激活程度在踝关节外骨骼助力时有明显降低,在助力时机为步态周期49%、助力大小为0.7 N·m/kg时,最高可降低(38.5±10.8)%。并且,相对于助力时机,助力大小对比目鱼肌激活程度影响更为显著。踝关节外骨骼不同模式助行时,所测量下肢肌肉可分解为5个基本协同模式,且合适的助力时机与助力大小条件下,下肢肌肉协调模式和正常行走相比改变较小。此外,比目鱼肌-胫骨前肌、股直肌-半腱肌的协同收缩度在外骨骼助力时比正常行走均有升高。本研究有助于理解健康穿戴者如何调整自身的神经肌肉控制机制来适应不同外骨骼助力,并为选择合适的助行模式以及合理利用外骨骼提高行走效率提供依据。