To date, the cell and molecular mechanisms regulating tendon healing are poorly understood. Here, we establish a novel model of tendon regeneration using neonatal mice and show that neonates heal via formation of a ‘neo-tendon’ that differentiates along the tendon specific lineage with functional restoration of gait and mechanical properties. In contrast, adults heal via fibrovascular scar, aberrant differentiation toward cartilage and bone, with persistently impaired function. Lineage tracing identified intrinsic recruitment of Scx-lineage cells as a key cellular mechanism of neonatal healing that is absent in adults. Instead, adult Scx-lineage tenocytes are not recruited into the defect but transdifferentiate into ectopic cartilage; in the absence of tenogenic cells, extrinsic αSMA-expressing cells persist to form a permanent scar. Collectively, these results establish an exciting model of tendon regeneration and uncover a novel cellular mechanism underlying regenerative vs non-regenerative tendon healing.

Mesenchymal stem cells (MSCs) are an attractive cell source for cartilage tissue engineering given their ability to undergo chondrogenesis in 3D culture systems. Mechanical forces play an important role in regulating both cartilage development and MSC chondrogenic gene expression, however, mechanical stimulation has yet to enhance the mechanical properties of engineered constructs. In this study, we applied long-term dynamic compression to MSC-seeded constructs and assessed whether varying pre-culture duration, loading regimens and inclusion of TGF-β3 during loading would influence functional outcomes and these phenotypic transitions. Loading initiated before chondrogenesis decreased functional maturation, although chondrogenic gene expression increased. In contrast, loading initiated after chondrogenesis and matrix elaboration further improved the mechanical properties of MSC-based constructs, but only when TGF-β3 levels were maintained and under specific loading parameters. Although matrix quantity was not affected by dynamic compression, matrix distribution, assessed histologically and by FT-IRIS analysis, was significantly improved on the micro-(pericellular) and macro-(construct expanse) scales. Further, whole genome expression profiling revealed marked shifts in the molecular topography with dynamic loading. These results demonstrate, for the first time, that dynamic compressive loading initiated after a sufficient period of chondroinduction and with sustained TGF-β exposure enhances matrix distribution and the mechanical properties of MSCseeded constructs.

A link is found between T wave alternans and vulnerability to ventricular fibrillation, and a new approach is provided for quantification of susceptibility to malignant arrhythmias. Complex demodulation reveals that alternation of the electrocardiogram is concentrated during the first half of the T wave, coinciding with the vulnerable period of the cardiac cycle. During myocardial ischemia and reperfusion, there are marked increases in the degree of T wave alternans that parallel the established time course of changes in vulnerability. The influence of the sympathetic nervous system in arrhythmogenesis is also accurately detected. Ultimately, complex demodulation of the electrocardiogram could provide a technique for identification and management of individuals at risk for sudden cardiac death.

In small saline-perfused rabbit mesenteric arteries (diam 221 +/- 4 microns, means +/- SE; n = 48) in situ, the interactions of endothelium-derived relaxing factor (EDRF)-mediated flow-dependent dilation and myogenic constriction were studied. When pump flow was increased two- to fivefold (2.8 +/- 0.1-fold), input perfusion pressure rose by 133 +/- 17%. Vessel diameter first increased passively by 9 +/- 1% and then decreased to or below control values reflecting the vascular myogenic activity. This was followed by a 16 +/- 3% increase in diameter, which was flow dependent, because nonperfused vessels exposed to the same intravascular pressures did not dilate. When the perfusate viscosity was increased with dextran solutions, both the basal diameters and the flow-induced dilator responses were significantly augmented, indicating that the increase in shear stress was the stimulus. The flow-dependent dilation was abolished by inhibition of EDRF with either hemoglobin (10 microM) or NG-nitro-L-arginine (0.3 mM) and also after preincubation with neuraminidase (0.2 U/ml, 30 min), which removes part of the membrane glycocalyx. Thus, myogenic responses in small mesenteric arteries can be effectively opposed by shear-induced release of EDRF. This might be a major mechanism for maintaining adequate tissue perfusion when pressure and shear stress increase simultaneously (e.g., exercise hyperemia, autoregulation) and otherwise myogenic activity would reduce vascular conductivity.

The hypothesis that a-adrenergic coronary vasoconstriction helps maintain a uniform transmural distribution of myocardial blood flow during exercise was tested in dogs. Carotid artery loops were surgically constructed and a splenectomy performed three weeks prior to study. On the day of study, the dog was anesthetized briefly (fentanyl and nitrous oxide) for percutaneous catheterization, and a-receptors in one myocardial region were blocked with phenoxybenzamine (0.25 mg/kg) infused selectively into the left circumflex coronary artery. Recirculation of phenoxybenzamine was minimized by drainage of coronary sinus outflow during the infusion. After the dog recovered from the anesthesia, regional blood flow was measured at rest and during graded treadmill exercise with the microsphere technique calibrated by reference blood samples. Average transmural flow was limited by avasoconstriction and was less in the region where a-receptors were intact than in the region where they were blocked, as has been described by others. The ratio of inner layer myocardial blood flow to outer layer flow was better maintained in the region with a-receptors intact than in the region with a-receptors blocked when myocardial oxygen consumption was 150 /tl/min/g or greater (p<0.001). Even though average transmural flow was limited by a-receptor activation, inner layer myocardial blood flow was greater in the region with a-receptors intact than in the region with a-receptors blocked when myocardial oxygen consumption was 500 fxVm\n/g or more (p<0.05). In conclusion, adrenergic coronary vasoconstriction mediated by a-receptors helps to maintain a uniform transmural distribution of myocardial blood flow during exercise in spite of limiting average transmural flow. (Circulation Research 1988;62:286-298) W hen the coronary vessels are dilated pharmacologically with adenosine and the heart is electrically paced from 100 to 250 beats/ min, the ratio of inner myocardial blood flow to outer myocardial blood flow (inner/outer flow ratio) falls dramatically from 1.0 to 0.4.' This is consistent with the concept that myocardial compression impedes blood flow to the inner layers of the myocardium more than it does to the outer layers. Because a larger proportion of each cardiac cycle is spent in systole as heart rate increases, tachycardia tends to exaggerate the effects of this transmural gradient of compression.In contrast to the situation with adenosine and pacing, when coronary vasodilation and tachycardia develop in response to exercise, the transmural distribution of coronary blood flow changes little, and the inner/outer flow ratio remains at or above l.O. Received February 10, 1987; accepted August 7, 1987. ergic coronary vasoconstriction mediated by a-receptors is part of the physiological response to exercise. 5 " 9The resulting restriction of functional hyperemia seems paradoxical because the metabolic demand for myocardial blood flow is greatly elevated during exercise. However, the adrenergic vasoconstriction during exercise might i...