Rheological inheritance controls the formation of segmented rifted margins in cratonic lithosphere.

Observations from rifted margins reveal that significant structural and crustal variability develops through the process of continental extension and breakup. While a clear link exists between distinct margin structural domains and specific phases of rifting, the origin of strong segmentation along the length of margins remains relatively ambiguous and may reflect multiple competing factors. Given that rifting frequently initiates on heterogenous basements with a complex tectonic history, the role of structural inheritance and shear zone reactivation is frequently examined. However, the link between large-scale variations in lithospheric structure and rheology and 3-D rifted margin geometries remains relatively unconstrained. Here, we use 3-D thermo-mechanical simulations of continental rifting, constrained by observations from the Labrador Sea, to unravel the effects of inherited variable lithospheric properties on margin segmentation. The modelling results demonstrate that variations in the initial crustal and lithospheric thickness, composition, and rheology produce sharp gradients in rifted margin width, the timing of breakup and its magmatic budget, leading to strong margin segmentation.

Diverticulitis mimicking a tuboovarian abscess. Report of a case in a young woman.

BACKGROUND: Diverticulitis is an uncommon condition in young women. When it occurs, it is often not recognized until complications such as perforation or fistulization occur. There has been no recent discussion in the gynecologic literature of diverticulitis in a young woman presenting as gynecologic disease. CASE: A 31-year-old woman with a long history of "irritable bowel syndrome" developed a 4-5-cm left adnexal mass associated with mild discomfort and dysparunia. Acute worsening of her pain led to abdominal exploration and left salpingo-oophorectomy for an unruptured tuboovarian abscess. She initially improved but then developed a recurrent pelvic abscess. Workup revealed extensive diverticulosis with probable sigmoid diverticulitis. Reexploration, drainage of the abscess and fecal diversion were required. CONCLUSION: Because of the proximity of the left ovary to the sigmoid colon, it is possible for diverticulitis to perforate into the ovary, producing a tuboovarian abscess indistinguishable from that due to other more common causes. A high index of suspicion is required to make the diagnosis, especially in young women. Failure to treat underlying diverticulitis can lead to persistent or recurrent pelvic infection.

Frank-Starling relationship as an important determinant of fetal cardiac output.

The importance of the Frank-Starling mechanism was evaluated in seven chronically instrumented fetal lambs (128-141 days gestation). Continuous determinations of left ventricular (LV) internal dimensions and pressures were obtained while LV end-diastolic diameter (LVEDD) was reduced by superior vena cava occlusion and increased by infusion of fetal blood into left atrium. A highly significant relationship was found to exist between stroke volume and LV extent of shortening (delta D) (r = + 0.99, P less than 0.001). Altering LVEDD from 10,5 to 13mm or LV end-diastolic pressure from 2.5 to 8 mmHg resulted in a 68% augmentation, in delta D. Spontaneous respiratory efforts resulted in frequent beat-to-beat variations in LVEDD and delta D, which maintained cardiac output constant over a wide range of respiratory rates. Moreover, LV output determined by indicator-dilution techniques remained unchanged over a wide range of spontaneous heart rates (114-180 beats/min) as a result of changes in delta D appropriate to alterations in LVEDD. Thus, changes in resting myocardial fiber length are of fundamental importance in fetal cardiovascular homeostasis and, within physiologic limits, it is quite clear that the Frank-Starling mechanism is operative and effective in the fetal lamb;

Influence of poststimulation potentiation and heart rate on the fetal lamb heart.

Isolated cardiac muscle techniques and studies of the chronically instrumented fetal lamb heart were employed to evaluate the ability of fetal myocardium to exhibit poststimulation potentiation. Isometric tension development and the response to paired electrical stimulation were significantly reduced in isolated fetal ventricular myocardium when compared to the adult (P less than 0.001). As in the adult, increasing stimulation frequency raised fetal isometric tension via an increase in the rate of rise of tension development in the presence of reduction in time-to-peak tension. In seven fetal lambs the left ventricle was chronically instrumented with endocardial ultrasonic crystals and a high-fidelity micromanometer. After a 2-wk recovery period, heart rate was increased by atrial pacing from an average control level of 150 to 300 beats/min. Left ventricular dP/dt increased progressively and then fell beyond a heart rate of 270/min. When comparable pre- and immediate postpacing beats were analyzed, a step-wise increase in the velocity of left ventricular shortening and the mean rate of circumferential fiber shortening was observed in association with an increase in the extent of shortening. Thus, increases in the frequency of contraction exert a significant positive inotropic effect on the fetal heart.

Conditional mutants in Chlamydomonas reinhardtii blocked in the vegetative cell cycle. I. An analysis of cell cycle block points.

Conditional "cycle-blocked" (cb) mutants of Chlamydomonas reinhardtii have been detected and isolated. These mutants exhibit normal vegetative growth at permissive temperature but are unable to complete a cell cycle (or a specified number of cell cycles) at restrictive temperature. A simple technique has been devised to determine the cell cycle stage in each mutant when the defective gene product, which ultimately affects cell division, completes its function. This stage is called the "block point", and is determined by scoring the residual cell division in an exponentially growing population after shift to temperature restrictive conditions. In the cb mutants isolated so far, block points representing many stages throughout the cell cycle have been found. Two categories of cb mutants are described here: one set which prevents the subsequent cell division when the cell encounters the block point after a shift to restrictive temperature, and another set which permits an additional round of cell division after the block point is encountered. The general applicability of block point analysis to other cell systems is presented.