Science Matters

The biggest brain ever mapped—a fruit fly’s

Boyce Rensberger

(12/2024) What you see in the picture above is the wiring diagram of the brain of a fruit fly. Creating it was a landmark event in neuroscience. This is, in fact, the largest and most complete map of any animal brain to date.

Each dot is a brain cell, or neuron, and each line represents a communication link from cell to cell. These links are extensions from each neuron called axons and dendrites. Actually, the lines are links between each neuron and a great many others. As in the human brain, each neuron "talks to" many others. And, in turn, each one receives messages from many others.

As you may be able to make out, we are looking at a three-dimensional structure with the neurons in front obscuring our view of many, many more neurons behind. The bug’s brain is made up of about 139,000 neurons which connect to one another through 34 million synapses (points where one brain cell talks to another).

This brain map is the product of hundreds of scientists at nearly 50 laboratories around the world. Each lab worked on a tiny portion of the tiny insect’s tiny brain.

Neuroscientists call this kind of diagram a connectome, following the wording pattern of "genome," the complete list of genes in an organism.

Keep in mind how small a fruit fly is—small enough to fit inside this O, wings and all. You may have seen them around bananas that have been on the counter too long. Fruit flies have been a favorite research subject for more than a century. If you’d like to look deeper into this, I recommend the scientists’ website at flywire.ai. Anyone can use their online tools to examine the little brain.

What neuroscientists would love to have is the connectome of the human brain, but creating that will be the work of many decades, if not a century.

Earlier this year, for example, an international team of 31 researchers published the connectome of one cubic millimeter of a human brain. It contained 57,000 neurons that made 150 million connections among themselves. The little cube also contained an astounding nine inches of capillaries.

The scientists found all this by using a special type of electron microscope to examine a bit of human brain cortex that was removed during surgery needed in a deeper part of the brain to treat epilepsy. The cortex includes the outer layers of neurons, where much of intelligent thinking takes place. (The patient is fine.)

The technique is something like that of a CT scan, producing a series of stacked slices, each with millions of voxels (the three-dimensional counterparts to pixels). A computer examines each slice and fills in connections to the slices above and below. In total, the analysis produced 1.4 petabytes of data. That’s 1.4 quadrillion bytes or 1400 terabytes. Still not impressed? That’s enough data to play high-definition movies continuously for more than 11 years.

Needless to say, it took some major computing power to analyze the data and create a three-dimensional reconstruction of that one bit of brain. Figure that the average human brain has a volume 1.4 million times as much as that tiny bit, and you get a sense of the task ahead.

A Monster Centipede

Imagine a centipede as big as an adult alligator. Had you been around during the Carboniferous period, 300 million years ago, you would not have had to imagine. Just such animals roamed Earth, the largest land-dwelling arthropod discovered to date. (Arthropods are the group with no internal skeleton that includes insects, spiders, crabs and the many-legged wormy animals we call centipedes and millipedes.)

Paleontologists (the two-legged creatures who study prehistoric life) discovered these giants three years ago, but they had only fragmentary fossils. Just recently a French-British group has analyzed nearly complete specimens and discovered that this species, called Arthropleura, looks like a hybrid of centipedes and millipedes. Their report is in the latest issue of Science Advances.

The difference between the two groups, by the way, is not that one has a hundred legs and the other a thousand, though the names falsely imply that. Centipedes have one leg on each side of a body segment, and millipedes have two on each side.

The analysis revealed that these giant critters are something like hybrids, having two legs on each side of each segment (like millipedes) but a head and mouth parts typical of centipedes. The mouth part differences are too detailed for me to follow; suffice it to say that the experts know the differences. The researchers also note that recent DNA studies show the two groups to be much more closely related than was once supposed. So, if you get the two confused, you’re really not so far off base.

The point is that this prehistoric beast would be pretty scary if it had survived to our time.

Breast Feeding Mystery

When a new mother breast feeds, she needs to pack her milk with enough calcium to build the child’s growing skeleton. Where does she get the mineral? Some will be in her diet, but also her body pulls calcium out of her bones.

That’s been known for a long time, but it has posed a mystery. Why don’t such young mothers develop osteoporosis. Osteoporosis typically affects older women whose bodies normally make less estrogen, a hormone that helps maintain strong bones. Furthermore, a new mother’s estrogen level normally drops after giving birth. The combination of factors suggested that something else must be happening to keep those women’s bones strong.

A predominantly female team of scientists has now found the answer, and they say it may lead to a new treatment for people with osteoporosis. It may also help broken bones heal faster in anyone.

Researchers at the University of California at both the San Francisco and Davis campuses have discovered that once a woman gives birth, her brain makes a previously unknown hormone that takes the place of the reduced estrogen. It keeps the mother’s bones from losing too much calcium. Also, it probably extracts more of the element from the mother’s food to supply her and the baby.

The U.C. researchers tested those hypotheses by giving doses of the hormone, which they call Maternal Brain Hormone (CCN3), to mice with broken bones. (Yes, the scientists deliberately snapped the animals’ bones, but they gave a local anesthetic first.) It turned out that the bones healed much faster than without the hormone.

Scientists also experimented on lactating mouse mothers whose bodies had been blocked from making the hormone. Those mice rapidly lost bone mass and their babies began to lose weight. But when the researchers injected the mothers with CCN3, mothers and babies returned to normal.

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