It is a good day for sperm.

Today Nature published work by Sato et al. on growing functional mammalian sperm in a culture dish. This was never thought to be possible due to limitations and complexity of spermatogenesis (taking more than a month to form sperm from spermatogonial stem cells!). By tweaking the in vitro (petri dish, or “out of the organism”) growth methods they could grow healthy sperm cells that can be maintained for up to 2 months in serum-free culture media (generally sugar water, picture below). This is a major step for elucidating the mechanisms behind spermatogenesis and male infertility.

Thin sections of sperm cells cultured for 38 days (right). I’m not generally knowledgeable in looking at sperm formation - but they say this shows the well-developed seminiferous tubules which indicates spermatogenesis.
Sato T, Katagiri K, Gohbara A, Inoue K, Ogonuki N, Ogura A, Kubota Y, & Ogawa T (2011). In vitro production of functional sperm in cultured neonatal mouse testes. Nature, 471 (7339), 504-7 PMID: 21430778

I’m currently reading Rebecca Skloot’s novel “The Immortal Life of Henrietta Lacks” – this is topical because Henrietta Lacks (right) unwillingly & unknowingly had donated her cervical cancer cells to labs all around the world for decades after her death. This immortalized cell line is called HeLa cells and is found in every biological lab I’ve seen to date… GREAT BOOK, bt-dubs.


Science Brief Today:

Blue-light influences what time you wake up:

Im & Taghert published in Science this week that known blue light-sensing photoreceptor cells activate the firing of neurons in circadian rhythms in two distinct pathways. CRYPTOCHROME (CRY) is the light-sensitive protein that each day helps to reset the fly’s circadian clock, without this protein flies to not respond to light:dark cycles (these mutants are called cry-baby and affect downstream proteins called JETLAG - are you seeing that Drosophilia researchers have a strange sense of humor?). They found that this CRY protein has two distint means of signaling light information (see picture below). Most interestingly they showed there was a very VERY low amount of light required for “re-setting” the circadian rhythm - in contrast CRY induces the same neuronal activity with higher level of lights (through a different pathway). Therefore, the multifunctional CRY protein mediates both aspects of this neuronal photosensitivity.

If I haven’t convinced you that the CRY protein is badass – I’ll just let you know that in coral CRY is part of the mechanism that triggers coordinated spawning for a few nights after a full moon in the spring (CRY wikipedia). IN ADDITION, this protein modulates the circadian clock in monarch butterflies!

In the picture above they show very low light levels at dawn & dusk (left) reset the circadian clock in neuronal pacemakers by changing levels of TIM protein. Higher light levels at mid-morning (right) modulate electrical activity in the same neurons without TIM involvement. The multifunctional CRY protein mediates both aspects of this neuronal photosensitivity.
Im SH, & Taghert PH (2011). Neuroscience. A CRY to rise. Science (New York, N.Y.), 331 (6023), 1394-5 PMID: 21415342


Putting 'radioactive particles' into perspective, with comix

XKCD, unsurprisingly, does a good job of showing how different sources of everyday radiation compare to those found in the current nuclear 'situation' and past ones.



Two weeks ago, I would have told you that not many people cared about my undergraduate nuclear engineering degree. I currently study radiation therapy and radiation-based imaging, and I’ve pushed all my long hours studying reactor thermal hydraulics to the back of my brain. But the crisis at the Fukushima Daiichi nuclear power plant in Japan has suddenly made this all very relevant.

With these types of events being so rare, it is understandable that they are not very well understood. I certainly don’t claim to be an expert in power plant operations or radiological cleanup. But I have been seeing a lot of “frequently asked questions” and misconceptions about what is happening over there.

So to go with the theme of this fine blog, I am going to address the “smaller questions” about nuclear power. Hopefully this can give some perspective as you read news reports and try to stay up-to-date on the events in Japan.

Why did the reactors continue producing power after the earthquake? Why didn’t they shut them down?

This question gets at the heart of one of the fundamental differences between nuclear and other power sources. The answer is that they did turn the reactors off. Nuclear power plants (NPP) don’t use their own power to operate; rather, they take offsite power from the grid to run the reactor systems. When the earthquake/tsunami hit, they lost offsite power, and immediately dropped the control rods to terminate the fission chain reaction.

So if they shut the reactor off, why do they have to keep cooling the core? The mechanism here is actually the same physical process that powers deep space satellites, which use a radioactive battery like the one pictured below. When an isotope decays, it releases an energetic particle. In the end, all the energy of this particle is converted to heat. Inside the fuel rods of the nuclear reactor, an immense amount of radioactive material is being created from fission, and as they decay they release a significant amount of heat.

The common term for this is “decay heat.” At the instant the reactor is turned off, the heat due to radioactive decay will produce about 7% of the power at full operation. In the case of Fukushima Daiichi, this amounts to about 50 megawatts of power. One household in the US uses about 1 kilowatt of power, so the heat that needs to be removed from the core is roughly equivalent to the power consumed by a small city. As the very short-lived fission products decay away rapidly, the decay heat drops to around 1% of full power, and stays at this level for some time.

A pellet of 238PuO2 glows red hot due to radioactive decay (above).

The decay heat phenomenon means that the spent fuel must be actively cooled with water for a few years after it has been used. Currently, there are several places at the Fukushima Daiichi NPP that contain spent fuel. There are 6 reactors at the plant: reactors 1-3 were in operation at the time of the earthquake, and reactors 4-6 were in cold shutdown. Each reactor has its own spent fuel pool that contains fuel rods removed from the core within the last 2 years. Finally, there is a common fuel storage pool that contains older spent fuel from all 6 reactors. All 7 of these pools must have water actively pumped through them.

is a nuclear meltdown?

If you ask most people what a “meltdown” is, they start having flashbacks to when their SimCity was destroyed in a fiery atomic explosion. Okay, maybe not most people… maybe most 20-something video game players. Most people think a meltdown results in a Hiroshima-like event, destroying cities, creating Godzilla, etc. The truth is that “meltdown” is a bit of technical jargon that is widely misused when people discuss accidents.

To understand what a meltdown is, you need to know a little bit about nuclear fuel. In a standard NPP, the most basic unit of fuel is the fuel pin. At the center is uranium oxide, which is the actual fuel of the reactor. The oxide is a good form for the fuel because it has a very high melting point (~3,000 K). To hold the uranium oxide together, you surround it in cladding. It is typical for these to be made of some type of zirconium alloy.

In between the fuel and the clad is the gap. When the fuel is made, it is generally filled with helium. The gap is where the gaseous fission products are contained. The clad is what keeps the (dangerous, radioactive) fission products out of the coolant.

A meltdown occurs when you cannot adequately cool the fuel rods, for whatever reason. In the case of Fukushima Daiichi, there was no power to pump the coolant through the core. The decay heat raises the temperature of the fuel rods until the clad or fuel melts. This is a meltdown.

This is bad because it means that the radioactive fission products are no longer contained in the fuel pin. They can leak into the reactor coolant, contaminating it. This exacerbates other accident scenarios because now the release of the coolant (such as steam from the reactor) could potential lead to exposure in the environment.

A meltdown is a very serious incident, but it is by no means catastrophic. Very likely, whoever owns the reactor will be out a couple billion dollars, but reactors are designed to withstand these kinds of events. When the accident occurred at Three Mile Island, the fuel melted down, but there was no environmental release because the reactor containment structures kept everything inside.

What caused the explosions at Fukushima Daiichi? Can a nuclear reactor explode like an atomic bomb?

I think anyone’s heart stops for a second when they hear the words “explosion” and “nuclear power plant” in the same sentence. As of this writing, the reactors at Fukushima Daiichi have suffered three explosions at reactors #1, #2, and #3. I cringe when I remember the plot of that season of 24 where the terrorists had a device that would make every nuclear power plant explode, because of all the horrible misconceptions it caused. The good news is that it is impossible for a nuclear power plant to explode like a nuclear bomb.

This may seem counterintuitive. A nuclear power plant is based off the same physics as a bomb, so why is it wrong to think that you might one day see a mushroom cloud over a NPP? There are several issues at play here. Remember back in 2006 when North Korea detonated its nuclear weapon? This test was generally regarded as a “fissile,” although the DPRK press certainly claimed otherwise. The challenge in making an atom bomb exploded is that you have to keep the nuclear material together long enough to allow the fission chain reactor to occur. The longer you hold it together, the more powerful the blast. Ask a North Korean nuclear physicist, and they will tell you that it’s hard to keep the nuclear material together and avoid a fissile. It requires extremely precise engineering and components designed specifically for this purpose.

The problem is that, as heat and energy builds, the components want to separate from one another. Materials naturally expand as the temperature goes up. NPPs are especially sensitive to this because they contain so much water. If for some reason there is a spike in reactor power, the volume of water skyrockets as it changes to steam. This is very damaging to a nuclear reactor, but it separates the components of the fuel and stops the chain reaction.

Another safety factor at work here has to do with the materials in the reactor. You may have heard terms like “uranium enrichment” or “weapons-grade uranium.” There are two isotopes of uranium that naturally occur, 235U and 238U (or as I will call them, good uranium and bad uranium). Good uranium is what actually undergoes fission inside a NPP to produce power. Good uranium is also what is enriched to high concentrations to make bomb material. Bad uranium is bad, from the perspective of a nuclear reactor, because it absorbs the neutrons that cause fission. Bad uranium makes it harder for a chain reaction to occur. But here is where “bad” uranium can be “good” during an accident. As bad uranium heats up, it absorbs more neutrons. This is negative feedback! If a reactor loses cooling and starts to heat up, the bad uranium makes it harder for the reactor to get any hotter. If you tried to make a reactor explode like a bomb, Jack Bauer style, the bad uranium would stop you in your tracks.

So then what caused the explosions at reactors 1-3? This turns out to be a fairly simple process. The outer cladding of the fuel rods is made of zirconium. If zirconium gets very hot, above 2,000ºC or so, it starts to oxidize in the presence of water. This process releases hydrogen gas. When the engineers eased the pressure of the reactors, they vented steam (containing hydrogen) into the reactor building. Something caused the hydrogen to ignite, which caused the explosions. These explosions did not occur inside the reactor, or inside the super beefy containment structure designed to prevent the release of radiation to the environment. They happened in the reactor building, which isn’t designed to protect the reactor, apart from protecting the workers from the rain.

Where can I find good information about Fukushima Daiichi?

As you’ve probably noticed, most media outlets aren’t too concerned with accurately representing what is happening at Fukushima. It’s our nature to respond more enthusiastically to headlines that evoke a more fearful response, and the cable news networks know this.

Surprisingly, the Wikipedia article for the accident is very well maintained and had a lot of information about the current status of the reactors.

Another good source of information is the International Atomic Energy Agency. They provide updates a few times a day on the latest confirmed reports of what is happening.

- Tripp Jones


Happy Saint Patrick's Day: Here is some Green-Fluorescence!

Dubey & Ben-Yehuda published last month in Cell a new form of communication between bacterial cells. Using Bacillus subtilis, they found bacteria could transfer fluorescent molecules between adjacent cells. In addition, they found that plasmids, circular extra-chromosomal DNA, that do not possess transfer proteins could be transferred from one cell to another. Using electron microscopy they found these bacteria were forming tubular extensions, or nanotubes, for exchange of cytoplasmic material!!

The paper equates these nanotubes to eukaryotic neural synapses or gap junctions in epithelial cells.

I had to update about this because the pictures were phenomenol.

Figure 1:
They visualized calcein (a nongenetically encoded cytoplasmic fluorophore, or a protein that the bacteria would not make hangin' out in it's cytoplasm – fluorescing green in picture below) traveling from one bacterium to the next in a gradient form (check out C - there's green everywhere!):

Figure 2:
These electron microscopy photographs depict the bacterial nanotubes that are forming between cells. The green arrows point to the nanotubes. In addition, E-G depict a protein tagged with a gold particle (black dots) traversing the bacterial nanotube:

Dubey GP, & Ben-Yehuda S (2011). Intercellular nanotubes mediate bacterial communication. Cell, 144 (4), 590-600 PMID: 21335240