体视学对于睾丸生物学研究的贡献

男性和女性的生育控制问题是本世纪一个重要的生物学研究领域。女性生育的有效控制方法已存在好多年了，然而，用于男性的有效的方法还没有找到。近年来，人们试图确定荷尔蒙对精子产生的调节作用，期望找到与女性避孕同样有效并可接受的方法。睾丸包括生精管和裂隙腔，生精管中容纳的是滋养幼芽上皮细胞的Sertoli细胞，裂隙腔中容纳的是产生睾丸激素的Leydig细胞。在精子产生过程中，精原细胞通过有丝分裂繁殖，其下一代经过减数分裂产生单倍体圆形精子细胞，接着发生一系列复杂的形态变化成为成熟的长形精子细胞，成熟后的精子就具有受精能力。 维持幼芽上皮细胞的Sertoli细胞在人的一生中不发生大的变化。Sertoli细胞维持幼雅细胞的能力是有限的，但在不同人之间其能力有一个较大的变化范围。因而，Sertoli细胞的数目决定了睾丸产生精子的最大能力。对于精子产生的调节涉及到内分泌、副分泌和自分泌机制。体视学和各种形态测定工具已成为实验条件和自然条件下睾丸生物学研究的一把钥匙，这些研究包括了解Sertoli细胞的繁殖以及调节精原细胞转变为成熟细胞的效率的因素。从而提出精子发育的规律性，由荷尔蒙对精子产生调节作用的模型。     

The Greenwell Springs LL4 Chondrite: A New Fall from Louisiana,USA

Abstract— A 664 gm stone fell into suburban Baton Rouge in November 1987. The stone has a nearly complete fusion crust and was undamaged upon impact. The ordinary chondrite contains equilibrated olivine, Fa₂₈, and largely equilibrated bronzite, Fs₂₃. Some bronzite is still compositionally zoned and polysynthetically twinned, thus requiring petrologic classification of the meteorite as a type 4. A major element analysis of the meteorite falls exclusively into the field for the LL compositional group.     

Turbulent magnetic fields in the Sun

Nigel Weiss recounts his Presidential Address 2001, given to the RAS A&G Ordinary Meeting on 9 February 2001.
Recent high-resolution observations, from the ground and from space, have revealed the fine structure of magnetic features at the surface of the Sun. At the same time, advances in computing power have at last made it possible to develop models of turbulent magnetoconvection that can be related to these observations. The key features of flux emergence and annihilation, as observed by the MDI experiment on SOHO, are reproduced in kinematic calculations, while three-dimensional numerical experiments reveal the dynamical processes that are involved. The pattern of convection depends on the strength of the magnetic field: as the mean field decreases, slender rising plumes give way to a regime where magnetic flux is separated from the motion and then to one where locally intense magnetic fields nestle between broad and vigorously convecting plumes. Moreover, turbulent convection is itself able to act as a small-scale dynamo, generating disordered fields near the solar surface.     

Limiting apparent-resistivity values for dipping-bed earth models

Consideration is given to some special features of normalized apparent-resistivity (NAR) curves, resulting from the deployment of Wenner electrode arrays on the surface above dipping earth structures. Limiting values of the potential are derived when a direct-current source is located at points on the surface above a simple two-region dipping-bed earth model and their influence on corresponding NAR curve characteristics is investigated. Particular attention is given to the exploitation of such features to provide a new and direct approach to model parameter estimation, either as an alternative to traditional curve-matching techniques or as a source of supporting information when other earth model characterization methods have been employed. Throughout, the emphasis is on the single-dipping-bed model, but application to more complex structures is discussed, including examples of two dipping beds, dipping dikes and more general tilted unconformities.     

Climatic change over the Lowveld of South Africa

There has been a 38% decrease in expected annual rainfall totals over the Lowveld, in the eastern part of South Africa, during the last two decades. The downward trend in mean annual rainfall is not replicated in the rest of the summer rainfall region above the escarpment. Rainfall variability over the Lowveld has been increasing since about the 1950s, although the increase in variability appears to have been slowing down in more recent years. Changes in the frequency and intensity of El Niño/Southern Oscillation extreme events are only partly responsible for the observed desiccation and increase in rainfall variability. The CSIRO 9-level general circulation model simulates, for 2 × CO₂ conditions, an insignificant decrease of 10% in the annual mean and a slight increase in the inter-annual variability of rainfall over the Lowveld. Other general circulation models likewise simulate only small changes in annual mean rainfall over the region. However, the simulated increase in rainfall variability by the CSIRO 9-level model is likely to be conservative since the model, being linked to a slab ocean, is unable to represent important features of ocean-atmosphere coupling in the region. Significant changes in the frequencies of extreme drought events and of heavy rains in the Lowveld are likely to occur even with only small changes in the rainfall climatology of the region.     

First measurements from a deep-tow transient electromagnetic sounding system

An electromagnetic sounding system has been developed to map the shallow electrical conductivity structure of the deep sea floor. The instrument consists of a magnetic source and several colinear magnetic receivers forming an array which is towed along the seafloor. The source generates a time varying magnetic field; the shape of the resulting magnetic field waveform at the receivers depends on the electrical conductivity below the seafloor between the receivers and the source. The instrument can be towed systematically over a study area under acoustic transponder or GPS navigation to construct a map of the electrical conductivity. Towing speeds of greater than 1 m s^–1 (2 knots) can be achieved without adversely effecting data quality. The instrument is sufficiently robust to survive continual contact with thinly sedimented, abrasive basalt. We present the first results from a deployment in August, 1990 near the Cleft Segment of the Juan de Fuca Ridge along an 8 km track to the west of the spreading center. Unforeseen problems with the instrument restricted the utility of the measurements for constructing detailed vertical conductivity profiles, but the measurements were adequate to determine an average conductivity in the upper 25 m, at more than 70 stations. The conductivity was found to vary from 0.1 to 0.4 S/m along the track.     

Metal donation and apo-metalloenzyme activation by stable isotopically labeled metallothionein

Coupled HPLC-ICP-MS has been used to quantitatively study the effects of GSSG and GSH on the ability of metallothionein (MTII) to donate essential and non-essential metals to apo-carbonic anhydrase. Stable isotopically labeled (67)Zn(3)Cd(4) MTII was used to enable Zn donated from MTII to be differentiated from extraneous sources of Zn. Transfer of both (67)Zn and Cd from MTII to apo-carbonic anhydrase was noted in the absence of either GSSG or GSH. GSSG increased the initial transfer of both Zn and Cd. Thereafter, a gradual increase in the (67)Zn content at the expense of Cd was noted over 24-h indicating continued interaction and exchange between MTII and the enzyme commensurate with the relative preferences shown by the proteins for these two metals. Although GSH also increased transfer of (67)Zn from MT it reduced the simultaneous transfer of Cd to the enzyme thereby conferring protection against Cd induced activation.     

The onset of separation in neutral,turbulent flow over hills

The onset of separation in turbulent, neutrally stratified, boundary-layer flow over hills is considered. Since the flows are fully turbulent, the occurrence of intermittent separation, in the sense of any reversal of near surface flow, will depend strongly on the detailed structure and behaviour of the turbulent eddies. Very little is known about such intermittent separation and the phenomenon cannot be studied with numerical models employing standard turbulence closures; eddy-resolving models are required. Therefore, here, as elsewhere in the literature, the arguably less physically significant process of mean flow separation is studied. Numerical simulations of flow over idealised two- and three-dimensional hills are examined in detail to determine the lowest slope, _crit, for which the mean flow separates.Previous work has identified this critical slope as that required to produce a zero surface stress somewhere over the hill. This criterion, when a mixing-length turbulence closure is applied, reduces to requiring the near-surface vertical velocity shear to vanish at some point on the hill's surface. By applying results from a recent linear analysis for the flow perturbations to this condition, a new expression for _crit is obtained. The expression is approximate but its relative simplicity makes it practically applicable without the need for use of a computer or for detailed mapping of the hill. The approach suggested differs from previous ones in that it applies linear results to a non-linear expression for the surface stress. In the past, a linear expression for the surface stress has been used. The proposed expression for _crit leads to critical angles that are about twice previous predictions. It is shown that the present expression gives good agreement with the numerical results presented here, as well as with other numerical and experimental results. It is also consistent with atmospheric observations.     

Accretion disc evolution in cataclysmic variables (the 1988 international time project in La Palma

A summary of data collected during a sixty night international campaign devoted to cataclysmic variables is presented.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.