Xenopus oocytes, which are arrested in G2 of meiosis I, contain complexes of cyclin B-cdc2 (M phase-promoting factor) that are kept repressed by inhibitory phosphorylations on cdc2 at Thr-14 and Tyr-15. Progesterone induces a cytoplasmic signaling pathway that leads to activation of cdc25, the phosphatase that removes these phosphorylations, catalyzing entry into M phase. It has been known for 25 years that high levels of cAMP and protein kinase A (PKA) are required to maintain the G 2 arrest and that a drop in PKA activity is required for M phase-promoting factor activation, but no physiological targets of PKA have been identified. We present evidence that cdc25 is a critical target of PKA. (i) In vitro, cdc25 Ser-287 serves as a major site of phosphorylation by PKA, resulting in sequestration by 14-3-3. (ii) Endogenous cdc25 is phosphorylated on Ser-287 in oocytes and dephosphorylated in response to progesterone just before cdc2 dephosphorylation and M-phase entry. (iii) High PKA activity maintains phosphorylation of Ser-287 in vivo, whereas inhibition of PKA by its heat-stable inhibitor (PKI) induces dephosphorylation of Ser-287. (iv) Overexpression of mutant cdc25 (S287A) bypasses the ability of PKA to maintain oocytes in G 2 arrest. These findings argue that cdc25 is a physiologically relevant target of PKA in oocytes. In the early embryonic cell cycles, Ser-287 is phosphorylated during interphase and dephosphorylated just before cdc2 activation and mitotic entry. Thus, in addition to its role in checkpoint arrest, cdc25 Ser-287 serves as a site for regulation during normal, unperturbed cell cycles.
A pressure-induced phase transformation in NaCl which occurs rapidly and reversibly at approx 300 kbar and room temperature has been observed in a diamond-anvil high-pressure cell. X-ray diffraction data indicate that the high-pressure polymorph has the cesium chloride (B2) structure. The lattice parameters of the low-(B1) and high-(B2) pressure phases at the transformation pressure are, respectively, 4.872±O.004 A and 2.997 ±O.OO4 A, and the volume change for the transformation is -1.00±O.05 cm 3 mole-I. The entropy change for the phase transform ation has been calculated from the volume change and from the high-temperature-pressure data obtained y the shock experiments of Fritz et al. and found to be 1.5±O.3 cal deg-1 mole-I. Comparison with otb r alkali chlorides indicates that a linear relationship exists between the entropy change and the volume chango, tor the Bl-B2 phase transformation. A thermodynamic equation accounting for this relationship has been derived under the assumption that the Griineisen parameter is proportional to the A th power of the volume. An equation which relates this factor A to the adiabatic bulk modulus and its pressure and temperature derivatives has also been derived.
Analysis of post-mortem buoyancy loss in Nautilus shells suggests that extensive nekroplanktonic drifting occurs infrequently. Most shells do not reach the surface but settle to the sea floor, after a short period of ascent. This occurs because the rate of water influx into the phragmocone due to ambient hydrostatic pressure is sufficiently rapid in most cases to overcome positive buoyancy before the shell reaches the surface. The resulting geographic distribution of Nautilus shells would therefore mirror the distribution of the live animals. Thus, post-mortem drift in Nautilus cannot be used as a basis for questioning the validity of cephalopod paleobiogeography. Estimate of influx rates in ammonoid siphuncles indicates that many, if not most, ammonoid shells also would not become nekroplanktonic. This is especially true for small (<5 cm diameter) shells. Cephalopod paleobiogeographic investigation appears less subject to criticism stemming from the supposed obfuscating effects of post-mortem drift than previously thought.
In this article, we discuss these positions and illustrate them with drawings, anatomic slices or dissection, and sonograms. Positions studied include those for best imaging of the anterior tibiotalar joint, anterior tibiofibular ligament, anterior talofibular ligament, calcaneofibular ligament, peroneal tendons, Achilles tendon, flexor hallucis longus, posterior deltoid ligament, anterior deltoid ligament, and posterior medial tendons.
Sonographic imaging longitudinal to the pectoralis muscle fibers showed fiber disruption, retraction, and possible hypoechoic or anechoic hematoma, most commonly involving the musculotendinous junction of the sternal head. Distal tendon assessment is important to evaluate for a full-thickness pectoralis major tear.
Consideration of the physics of sinking of hollow, rigid bodies leads to equations of motion for sinking cephalopod shells. We have derived equations of motion for three post-mortem sinking situations: sinking with a fixed amount of water in the phragmocone; rapid phragmocone filling (no siphuncular tube); and slow phragmocone filling (siphuncular tube intact). In all three cases sinking speed can be closely approximated by the terminal velocity calculated from the total weight, buoyancy, and drag parameters of the shell.Experiments on modern Nautilus shells yield sinking velocities in agreement with calculated values. The experiments also show that orientation of a sinking Nautilus shell varies as the phragmocone fills with water. With small negative buoyancy the shell sinks with its plane of symmetry upright, but as it fills, it begins to rock from side to side and leans over and sinks with its plane of symmetry horizontal when the camerae are about 55% full.The maximum sinking speed of upright adult Nautilus shells is approximately 30 cm/sec, which appears to be too small for embedding in the bottom upon impact. The maximum depth to which Nautilus sinks in the upright position ranges from about 7 m for rapidly filling shells to as much as 600 m for slowly filling shells. In the latter case, the shell will continue to fill after coming to rest on the bottom, and the stability of the vertical orientation will be removed within 1 or 2 days. Thus, primary vertical preservation of cephalopod shells indicates water depths less than about 10 m.
The effect of pressure on the molar volumes of wüstite (Fe0.924O) and three spinel phases of Fe2SiO4, Fe1.8Mg0.2SiO4 and Fe1.6Mg0.4SiO4 has been determined by means of the X‐ray diffraction method at pressures up to 255 kb at room temperatures. The isothermal bulk moduli for these minerals have been evaluated at zero pressure from the volume data by a least‐squares fit to the Birch equation. The value for wüstite thus determined is 1.42±0.10 Mb. The values for the spinel phases of olivines range between 1.96 and 2.12±0.10 Mb and are indistinguishable from each other. The mean volume thermal expansivity for the spinel phase of Fe2SiO4 has been determined to be (23±1)×10−6 °C−1 at a pressure of 1 bar and a temperature range of 8°–396°C by means of the X‐ray diffraction technique. On the basis of these data, the stability boundary for an assumed reaction, Fe2SiO4 (spinel) = 2 FeO(wüstite) + SiO2(stishovite), has been estimated to be P (kb) = 227.5 ‐ 0.022 T(°C).
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