Caumette
Caumette G,
Ouypornkochagorn S,
Scrimgeour CM,
Raab A, Feldmann J
Environ. Sci. Technol., 41
(8), 2673
-2679, 2007.
"Trace elements often accumulate in
keratin-rich tissues. Hair, nails, and horns grow steadily but
once formed are metabolically inactive and provide an archive of
trace element exposure when analyzed in segments. Here we
demonstrate the use of laser ablation ICP-MS for the
high-resolution monitoring of trace elements in the horns of
seaweed-eating sheep from North Ronaldsay, which live on grass
only during lambing time. Due to this peculiar husbandry/dietary
pattern and the fact that seaweed is rich in arsenic and iodine,
we hoped to use iodine and arsenic as markers for seaweed
ingestion. Cross sections and scans along the growing axis
(representing the first 8-10 months of the sheep's life) revealed
that these elements were not homogeneously distributed in the
horn, with arsenic representing the amount of seaweed intake. The
scans show the periods in which the lambs were fed on milk and
grass and the change to seaweed ingestion with the successive
replacement of milk with seaweed; this was supported by the
carbon and nitrogen isotope signatures ( 13C
and
15N)
of the horn and the arsenic speciation in the horn. The period of
low arsenic accumulation in the horn had terrestrial isotope
signatures and accumulated arsenic of mainly inorganic origin.
The period of high arsenic accumulation was characterized by
isotope signatures of marine origin, and the majority of
accumulated arsenic in the horn was the main arsenosugar
metabolite dimethylarsinic acid. Although we have investigated
only four different horns of individual sheep, this study shows
that arsenic is not significantly transported with milk. However,
the high concentration of arsenic in the oldest part of the horn,
which was formed in utero, points to a relatively high placental
transport of arsenic while the ewe was eating seaweed. In
contrast to arsenic, iodine is transported not only through milk
ingestion but also through the placenta in large quantities."
April 15, 2007
"The exposure of animals and humans to toxic elements and
compounds is routinely monitored by the analysis of blood, urine
or tissues. While this process is highly successful and
relatively easy to carry out, it is limited in that it only
provides a snapshot of exposure at one particular time.
Measurements can be repeated over the following months to follow
future trends but cannot be done retrospectively to establish
historical exposure.
"In the world of toxicology,
human hair has been used with some success to show the drug usage
history of addicts. Drugs are transported through the body and
accumulate in the hair, where they are relatively stable. When a
hair is cut into segments for separate analysis, the
concentrations of the drugs in each segment can be correlated to
the growth rate to estimate how long an addict has been using
drugs as well as the relative amounts of drugs taken.
"Now, scientists in Scotland
have applied the same principles to the analysis of arsenic and
iodine in the horns of sheep. They may seem strange elements to
be considering in the ovine context but one particular Scottish
breed feeds exclusively on seaweed for more than half the year.
The seaweed is mainly brown kelp which contains high levels of
arsenic (74 mg/kg) and iodine (5700 mg/kg).
"This primitive breed, the
North Ronaldsay, lives on the northernmost island of the Orkneys.
It used to roam the island freely but was exiled to the beaches
in 1832, when the island interior was reserved for cattle
grazing. Now, it has adapted completely to a seaweed diet, but is
allowed some variation when the herds are brought inland just
before lambing, where they remain for up to 5 months on a grass
diet. It is now widely accepted that the foreshore exile
preserved this ancient species by preventing cross breeding.
"There is always plenty of
seaweed around to feed on, especially during winter when the
stormy seas wash it ashore. However, it can be scarcer during the
other seasons and sheep have been known to swim out into the
shallows to graze on kelp attached to the rocks.
"Jorg Feldmann and colleagues
from the University of Aberdeen and the Scottish Crop Research
Institute reasoned that arsenic and iodine could be transported
through the bloodstream and be taken up in the fibrous keratin
tissues of the horns as they are formed. Since horn growth begins
before birth, accumulation of the two elements can be monitored
from the womb, through feeding on milk and grass, to the seaweed
diet.
"Horns from three sheep and
one control animal (from the Pyrenees) were cut into sections
representing the first 8-10 months of life and analysed by laser
ablation inductively coupled plasma mass spectrometry (LA-ICPMS).
The data measured across the cross section confirmed that iodine
was more concentrated at the outer cortex of the horn than in the
centre, with the arsenic distribution the other way round. This
was consistent with the growth profile of the horns, in stacked
cup-like layers, which gave a slanted elemental profile.
"Along the length of the
horns, the arsenic content showed a small peak right at the tip,
corresponding to first growth stage, which fell quickly to a very
low level. This was maintained for a few cm then the level grew
gradually with time. Following a speciation analysis by LC-ICPMS,
in which the arsenic species present were identified, the
researchers were able to correlate the arsenic compounds with the
sheep diet.
"At the beginning of horn
growth, inorganic arsenic dominated and its origin was presumed
to be ewes' milk or grass. After several months, the levels of
dimethylarsenic (DMA) species began to grow and this coincided
with exposure to the seaweed as the animals were removed to the
shoreline. Only a gradual increase in DMA was observed,
consistent with the gradual replacement of milk with seaweed.
Isotope ratio measurements on carbon and nitrogen, which could be
assigned to terrestrial or marine sources, supported these
conclusions.
"In contrast to arsenic, the
iodine levels remained relatively constant along the horn length,
indicating that the seaweed and milk/grass diets contained
similar amounts of iodine. The ewes accumulate iodine during
their time on the shoreline and this is stored in tissues and
released into the milk when they are grazing inland. Placental
transport of iodine during gestation also occurs, at levels
approximately 1.5-fold higher than iodine uptake from eating
seaweed.
"This proof of concept study
shows that sheep horns can be used to map exposure to toxic
elements from gestation, through suckling into early adulthood.
The results also have implications for arsenic exposure to
humans.
"It is believed that the
metabolism of arsenosugars, like those found in seaweed, is
similar in sheep and humans. So, extrapolating the sheep data
suggests that arsenic is not transported significantly in human
milk whereas placental transport is more serious, especially when
pregnant women have been exposed to high levels of arsenic as
found in seaweed and contaminated drinking water. Equally, the
massive placental transport of iodine is a cause for concern,
since it could trigger hypothyroidism and thyroid enlargement
(goiter) during pregnancy."
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