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.