Optimism in the Face of Reality

A comet not far from Earth has shed some light on how our planet could have come by much of its water, a new study has revealed.

Writing in Nature, Paul Hartogh, who is based at Germany's Max Planck Institute for Solar System Research, used ESA's orbiting Hershel Space Observatory to study a small 1.5 kilometre-wide comet called 103P/Hartley 2, which formed originally out in the Kuiper Belt beyond the orbit of Pluto and now follows a 6.5 year elliptical orbit in the region of space between the Earth and Jupiter.

Hartogh and his colleagues probed the comet's "coma" - the pall of dust and debris thrown off by the body as it is warmed by the Sun - for the telltale-traces of water.  Specifically, they were looking at the relative proportions of "heavy" hydrogen - or deuterium - that were present.

Water on Earth contains about one atom of deuterium for every 6500 atoms of "light" hydrogen. This a close match for the levels seen in asteroids and many meteorites but about half as large as the levels seen in other comets that have been measured far out in deep space in a region called the Oort cloud.

Consequently, space scientists had concluded that when the Earth first formed it was a hot, dry and barren rock that, as it cooled sufficiently for liquids to condense, was progressively wetted by the arrival of water-laden asteroids. Comets, they thought, must have been a relatively rare contributor to the planet's early water supply.

However, the observations from 103P/Hartley turn this idea on its head. The comet shows a deuterium-hydrogen fingerprint that is almost exactly the same as Earth's own water. It also reveals some interesting details about the birthplaces of the comets themselves and therefore the structure of the early solar system.

Comets like 103P/Hartley are thought to have formed in the Kuiper Belt, about 50 times further from the Sun than Earth is, and then migrated inwards later. Conversely, more distant comets like those now in the Oort Cloud and over 5000 times the Earth-Sun distance away, are thought originally to have been born near Jupiter before being gravitationally booted into the outer reaches of the solar system later.

But because these Oort Cloud comets formed closer to the Sun originally, and therefore would have been exposed to higher temperatures, they should have a lower deuterium-hydrogen ratio than their Kuiper Belt counterparts that formed under cooler conditions.

Instead the scientists have found the opposite. This argues that something is out of kilter in our understanding of the formation of the early Solar system, but it will take further cometary chemical forensics, like that announced by Hartogh, to ultimately reveal the answer...

Researchers have discovered a chemical signal secreted by blood cells that prevents arteries from furring up again following injury or angioplasty. 

Called cathelicidin, the substance, which also has antimicrobial properties, is secreted by white blood cells called neutrophils that flock to sites of tissue injury and inflammation.

In tests on mice, University of Munich researcher Christian Weber and his colleagues found that removal of the neutrophils from circulation following an arterial injury caused vessels to become blocked by an overgrowth of tissue called neointimal material, the growth of which was triggered by the damage.

But when neutrophils were present, the researchers found, the cells clung to the injured parts of the artery and deposited cathelicidin locally. This then recruited a separate population of blood vessel-rebuilding cells called early outgrowth cells. These proliferated and secreted repair factors, reassembling the endothelial lining that paves and seals the inside surfaces of arteries. Re-establishing the endothelium in this way appears to prevent the potentially lethal neointimal overgrowth.

This means the discovery could also have implications for the process of angioplasty in which doctors use a tiny balloon to open up blocked blood vessels. Usually during this procedure a small metal cage called a stent is inserted to hold open the artery afterwards. But this can trigger inflammation, leading to neointimal overgrowth and re-stenosis, or blockage, of the artery.

To get around this problem, the latest generations of stents are impregnated with anti-proliferative drugs designed to block the growth of the neointimal tissue. But whilst they are very effective in this role, ironically 30% of patients who receive these so-called "drug-eluting stents" develop complications including the formation of blood clots inside the vessel. Doctors think this occurs because the drugs don't only block the growth of the neointimal tissue but also prevent the smooth endothelial lining being re-established. So a cathelicidin-secreting stent might be able to overcome these difficulties.

With this in mind, the team, who have presented the work in Science Translational Medicine, engineered a cathelicidin-impregnated stent that was inserted into the carotid arteries of a group of experimental mice. A second group of control animals received stents that lacked cathelicidin but were otherwise identical.

When the arteries from the two groups of animals were compared 4 weeks later, the arteries of the cathelidicin-stented mice showed significantly less narrowing.

According to the team, "although it remains to be seen whether this effect will be reproduced in humans, cathelicidin may prevent stents from causing the very problem they are supposed to treat and thus improve therapy for severe atherosclerosis."