166 research outputs found
Acetylcholinesterase-Positive Langerhans Cells in the Epidermis and Wool Follicles of the Sheep
Dendritic acetycholinesterase-positive cells in the basal layer of the epidermis and in wool follicles of the sheep have been identified ultrastructurally as Langerhans cells using a copper ferrocyanide technique to demonstrate acetycholinesterase. This enzyme is located on the outer surface of the plasma membranes of the Langerhans cells and in the intercellular spaces between these cells and the adjacent epithelial cells. No enzyme activity has been detected within Langerhans cells.Langerhans cells in fetal and adult sheep are morphologically similar and no developmental stages have been detected. Ultrastructurally these cells are similar to those described in other mammals. However, in sheep epidermis they are restricted to the basal layer and are less frequent than in man or guinea pig where they are mainly found in the spinous layer. Langerhans cells in the sheep, which form a system in contact with all keratinocytes in the basal layer, may have a significant influence on the function of the epidermis
Loan products to manage liquidity stress when broad-based black empowerment enterprises invest in productive assets
Investments in productive assets by broad-based black economic empowerment (BEE) enterprises in South Africa (SA) during the 1990s have been constrained, in part, by a lack of access to capital. Even if capital can be sourced, BEE businesses often face a liquidity problem, as conventional, equally amortized loan repayment plans do not take into account the size and timing of investment returns, or there are lags in the adjustment of management to such new investments. This paper describes five alternative loan products to the conventional equally amortized loan: the single payment non-amortized loan; the decreasing payment loan; the partial payment loan; the graduated payment loan; and the deferred payment loan. Recent SA experience with the graduated payment loan and the deferred payment loan suggests that there is scope to alleviate the liquidity problem if a wholesaler of funds can offer such terms to private banks and venture capital investors who then on-lend to finance BEE asset investments that are otherwise considered relatively high credit risks. This would shift the liquidity problem away from the client to the wholesaler of the funds, but requires access to capital at favourable interest rates. Such capital could be sourced from empowerment funds earmarked by the private sector, donors and government.Financial Economics,
Determinants of the demand for regular farm labour in South Africa, 1960-2002
This paper estimates long-run price (wage) elasticities of demand for regular farm labour in South Africa using both Ordinary Least Squares (OLS) regression and a Two-stage Least Squares (2SLS) simultaneous-equation model for the period 1960-2002. Both models include a piecewise interactive slope dummy variable with 1991 as the threshold year to reflect South African (SA) commercial farmersâ expectations that farm labour costs would increase as new labour legislation was introduced from the early 1990s onwards. The long-run price (wage) elasticity of demand for regular farm labour in South Africa during 1960-1990 was estimated as -0.25 for OLS and -0.23 for 2SLS regression, respectively. For the period 1991-2002, this elasticity estimate rose to -1.32 and -1.34 for OLS and 2SLS regression, respectively. These results suggest that a marked structural decline in the demand for regular labour has occurred since 1991 that raises questions about the appropriateness of labour laws and minimum wage legislation that have increased the cost of regular farm labour in South Africa.Regular farm labour, SA agriculture, price (wage) elasticities of demand, Farm Management, Labor and Human Capital,
Two decades of pulsar timing of Vela
Pulsar timing at the Mt Pleasant observatory has focused on Vela, which can
be tracked for 18 hours of the day. These nearly continuous timing records
extend over 24 years allowing a greater insight into details of timing noise,
micro glitches and other more exotic effects. In particular we report the
glitch parameters of the 2004 event, along with the reconfirmation that the
spin up for the Vela pulsar occurs instantaneously to the accuracy of the data.
This places a lower limit of about 30 seconds for the acceleration of the
pulsar to the new rotational frequency. We also confirm of the low braking
index for Vela, and the continued fall in the DM for this pulsar.Comment: Isolated Neutron Stars conference, London, April 24-28 200
Contribution of pulsars to the gamma-ray background and their observation with the space telescopes GLAST and AGILE
Luminosities and uxes of the expected population of galactic gamma-ray
pulsars become foreseeable if physical distributions at birth and evolutive
history are assigned. In this work we estimate the contribution of pulsar uxes
to the gamma-ray background, which has been measured by the EGRET experiment on
board of the CGRO. For pulsar luminosities we select some of the most important
gamma-ray emission models, taking into account both polar cap and outer gap
scenarios. We nd that this contribution strongly depends upon controversial
neutron star birth properties. A comparison between our simulation results and
EGRET data is presented for each model, nding an average contribution of about
10%. In addition, we perform the calculation of the number of new gamma-ray
pulsars detectable by GLAST and AGILE, showing a remarkable di erence between
the two classes of models. Finally, we suggest some improvements in the
numerical code, including more sophisticated galactic m odels and di erent
populations of pulsars like binaries, milliseconds, anomalous pulsars and
magnetars.Comment: 6 pages, 6 figures, to be published in the Proceedings of the 6th
International Symposium ''Frontiers of Fundamental and Computational
Physics'' (FFP6), Udine (Italy), Sep. 26-29, 200
Recent glitches detected in the Crab pulsar
From 2000 to 2010, monitoring of radio emission from the Crab pulsar at
Xinjiang Observatory detected a total of nine glitches. The occurrence of
glitches appears to be a random process as described by previous researches. A
persistent change in pulse frequency and pulse frequency derivative after each
glitch was found. There is no obvious correlation between glitch sizes and the
time since last glitch. For these glitches and
span two orders of magnitude. The pulsar suffered the
largest frequency jump ever seen on MJD 53067.1. The size of the glitch is
6.8 Hz, 3.5 times that of the glitch occured in
1989 glitch, with a very large permanent changes in frequency and pulse
frequency derivative and followed by a decay with time constant 21 days.
The braking index presents significant changes. We attribute this variation to
a varying particle wind strength which may be caused by glitch activities. We
discuss the properties of detected glitches in Crab pulsar and compare them
with glitches in the Vela pulsar.Comment: Accepted for publication in Astrophysics & Space Scienc
Pulsar Timing at Urumqi Astronomical Observatory: Observing System and Results
A pulsar timing system has been operating in the 18-cm band at the Urumqi
Astronomical Observatory 25-m telescope since mid-1999. Frequency resolution
allowing dedispersion of the pulsar signals is provided by a 2X128X2.5 MHz
filterbank/digitiser system. Observations of 74 pulsars over more than 12
months have resulted in updated pulsar periods and period derivatives, as well
as improved positions. Comparison with previous measurements showed that the
changes in period and period derivative tend to have the same sign and to be
correlated in amplitude. A model based on unseen glitches gives a good
explanation of the observed changes, suggesting that long-term fluctuations in
period and period derivatives are dominated by glitches. In 2000 July, we
detected a glitch of relative amplitude Delta_nu/nu~24X1e-9 in the Crab pulsar.
The post-glitch decay appears similar to other large Crab glitches.Comment: 11 pages, 9 figures, 4 tables, accepted by MNRA
Influences of neutron star parameters on evolutions of different types of pulsar; evolutions of anomalous X-ray pulsars, soft gamma repeaters and dim isolated thermal neutron stars on the P-\.{P} diagram
Influences of the mass, moment of inertia, rotation, absence of stability in
the atmosphere and some other parameters of neutron stars on the evolution of
pulsars are examined. It is shown that the locations and evolutions of soft
gamma repeaters, anomalous X-ray pulsars and other types of pulsar on the
period versus period derivative diagram can be explained adopting values of
B G for these objects. This approach gives the possibility to explain
many properties of different types of pulsar.Comment: 18 pages, 1 figur
Precision Timing Measurements of PSR J1012+5307
We present results and applications of high precision timing measurements of
the millisecond pulsar J1012+5307. Combining our radio observations with
results based on optical observations, we derive complete 3-D velocity
information for this system. Correcting for Doppler effects, we derive the
intrinsic spin parameters of this pulsar and a characteristic age of 8.6 +/-
1.9 Gyr. Our upper limit for the orbital eccentricity of only 8 * 10^-7 (68%
C.L.) is the smallest ever measured for a binary system. We demonstrate that
this makes the pulsar an ideal laboratory to test certain aspects of
alternative theories of gravitation. Our precise measurements suggest
deviations from a simple pulsar spin-down timing model, which are consistent
with timing noise and the extrapolation of the known behaviour of slowly
rotating pulsars.Comment: 9 pages, 6 figure
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