It's all about access when it comes to the brain. A strong defense system allows what the brain needs in and keeps out toxins and pathogens. It's an vital function that also stops promising drugs from reaching their target (think: a tumor) and prevents neuroscientists from probing how the brain works. Scientists at Caltech have now engineered viruses that can bypass this barrier in mice and deliver genes to many different types of cells.

Why it matters: In the past few decades, a wealth of tools have been created to study the brain — neurons can be individually labeled and imaged, genes can conceivably be edited, and the brain's cells can be manipulated with light in order to see what effect it has on functions like memory. But that all hinges on getting into the brain.

This new work opens up the brain's intricacies for unprecedented study and possibly treatment of neural disorders (like Parkinson's and Huntington's diseases), brain cancers, and Friedreich's ataxia and other peripheral nervous system diseases.

Read more here.

• Cancer advance: In 2 small studies of melanoma patients, personalized cancer vaccines show promise for defeating tumors.
• Traffic report: When Jakarta cancelled the city's HOV lanes, commuting chaos ensued. Like it or not, carpooling seems to work.
• Infant mortality: While the overall rate in the U.S. has fallen, there is an increasing disparity between the death rates of African-American babies and non-Hispanic white ones.
• Last laugh: We now know whose laugh we sent into space on the Voyager probes.

Humans have stared at the sky for millennia trying to figure out what is out there and what it tells us about our place in the universe. And there — in the gaps between planets and people and particles — hums something that scientists and philosophers have spent a long time trying to understand: space. For as fundamental to our existence and experience as space is, it's hard to describe and define it.

So, we asked four experts a deceptively simple question: What exactly is space?

• Sean Carroll, theoretical physicist, California Institute of Technology: Space is overrated.
• Izabella Laba, mathematician, University of British Columbia: We can work with space without understanding its essence.
• Bridget Falck, astrophysicist, University of Oslo: Space is a relation between things, not a thing itself.
• David Albert, philosopher, Columbia University: Maybe there is no space.

• In danger: The Atlantic horseshoe crab. A population of one of Earth's oldest, most resilient, and medically important (their blood is key to testing for bacterial contamination in vaccines) creatures has declined by 30% over the past 40 years.
• Scaly skin: An Icelandic company has developed a patch from fish skin for healing chronic wounds, which is a $50 billion industry in the U.S. due to an aging population and a rise in diabetes and obesity.
• Inspiring: Read Quanta's profile of a Princeton mathematics professor who came to the game late — and is killing it. Bonus: author Kevin Hartnett offers a beautiful explanation of some complex math.

The majority of life on Earth can't be seen with the naked eye. Bacteria, amoeba, archaea found on hydrothermal vents, single-celled algae, and other unicellular organisms outnumber us multicellular types and have existed on the planet at least 2 billion years longer. Some, like the green algae pediastrum shown above, form colonies from identical cells, each self-sufficient despite their communing.

History lesson: Some of these early occupiers of Earth eventually joined together to share the work of life and cooperate to survive — a key step in life's evolution on Earth (and one that could also inform the search for alien life beyond the planet).

Go deeper: Biologist Sally Warring collects samples of unicellular organisms from New York City's ponds and documents the extraordinary lives of single cells at Pondlife.