Cubed, Ground, Frozen or Marinated? 4 Scientists Talk Brain Dissection Styles

Author: Lina Zeldovich

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Dead men don't talk, but their brains can be quite verbose. Depending on how and where one looks, dead brains hold clues to the mysteries of this complex organ in both health and disease. Here, four pathologists and anatomists tell the tales of brain preservation and dissection. Interestingly, it's not unlike cooking, they note—there's a lot of slicing and dicing involved. But whether cubed, ground, frozen or marinated in formalin, your brains can reveal a lot in their afterlife.

Fresh out of the skull

Judy Melinek, forensic pathologist and co-author of Working Stiff, says that with the exception of a scalpel and a saw, she bought all her equipment at a culinary supply store. "They gave me a trade discount after I told them what I do for a living!"

Getting brains out of their skulls takes some brawn. First, Melinek uses a scalpel to make a round cut in the head's flesh—from one ear to another and over the top of the head. From there, she cuts and peels off the galea, a tough thick layer of fibrous tissue attached to the skull, folding it inside-out—forward over the face and backward over the neck. Once the bone is exposed, she makes another circle cut—this time with the Stryker saw, which she describes as a "jacked-up kitchen hand blender fitted with a crescent-shaped buzz saw blade." The saw makes an "awful racket" and spews bone dust, smelling just like a dental drill (so it's a good idea to do it under a plastic cover), but when it's done, the top of the skull comes off like a lid on a pot.

Scooping the brain out of that "pot" requires more slicing. Melinek slips her left index and middle fingers under the bone of the brow, lifts up the brain's frontal lobes, and cuts everything that still ties the brain to its former home—nerves and arteries leading to the eyes and nose, a part of the dura matter called the brain tent, and the spinal cord. Then she slowly guides the organ out. "I cradle the back of the brain with my right hand while tipping it back with the fingers of my left," she says. "Sometimes, if the skull cap has come off just the right way, I can use it as a bowl to collect the brain."

Fresh brains are squishy like Jell-O and tricky to cut. "I gently squeeze its hemispheres together so they don't splay out while I slice off half-inch-thick slabs," Melinek says. "I take a look at each piece as it falls away from the brain to my right to see if there's anything unusual." For instance, trauma results in bleeding while lack of oxygen damages the nerves and causes swelling. A hypertension stroke, in which usually a small vessel in the center of the brain bursts, leaves "a bloody mess inside the normally white and grayish tissue." In an ischemic stroke caused by lack of blood flow from blocked arteries, the brain swells, turning mottled reddish-white where the blood could no longer reach. Thermal injuries from fire or superheated water can cook the brain, leaving it swollen, firm to the touch, and smelling like bad barbecue.

Damien Hirst, Autopsy with Sliced Human Brain, oil on canvas 2004

Fixed in formalin and placed under the microscope, brain slices reveal more clues. Oxygen-starved nerves appear bright pink or red. Although it's not possible to tell whether the person was strangled, had a drug overdose, or something else happened, Melinek notes. Certain poisons can also paint brains in vivid colors. Hydrogen sulfide can give it a green tinge, while carbon monoxide or cyanide can dye it a bright, cherry red shade. For a definitive diagnose, however, Melinek sends samples to forensic chemist at a toxicology lab.

Unlike fresh brains, formalin-fixed brains are firm. They are cut into shapely slices that are easy to move around on the cutting board for picture-taking. "For that I use another tool I bought at the culinary supply store," Melinek reveals. "The metal spatula a cook uses to flip burgers."

Marinated and sliced

At the Imperial College London, neuropathologist Steve Gentlemen gets his brains after they soak in formalin for four weeks and turn into hardened brownish globules. His job too has a detective element to it, although his culprits are various diseases that ruin the brain, from stroke to Alzheimer's. He begins to gather his clues the moment he holds the organ in his hands. An acute stroke leaves dead tissue on the brain surface which is soft to the touch. A past stroke makes a small cavity, because the brain will eventually clear dead cells but leave the empty spot. Meningitis, an infection of brain membranes, leaves a white pus-like substance under the arachnoid membrane. An asymmetry between the two brain hemispheres means that there may be a tumor inside shifting the midline.

Gentleman cutting a brain, image via Wellcome Collection

Sometimes Gentleman gets only half a brain to work with, because the other hemisphere is frozen somewhere for research. When he receives a full brain, Gentleman scoops out the brain stem and cerebellum from under the hemispheres, not unlike how a cook cleans out the chicken guts. He then examines the blood vessels at the base of the brain for yellow patches. "They're the arteriosclerotic debris," he explains. "When the little deposits built up, they can completely block the vessels or break off and cause the stroke higher up in the brain." He also examines the midbrain for substantia nigra, a normally darker brain tissue, which loses its color in Parkinson disease. "Even before I can get to a microscope, if this coloration is lost, I'm pretty sure what I am going to find."

With a blade akin to a large bread knife, Gentleman halves the brain in two and cuts it up—except he makes his cuts horizontally, not vertically: the hemisphere lies on its freshly cut side while the knife slices it from bottom up to assure uniformity. The resulting pieces look just like slices of a bread loaf with a roundish top. At that point you can spot some atrophied and shrunk tissue, Gentleman says. "The brain folds get narrower and it begins to look like a pickled walnut." The cuts are then embedded in paraffin, sliced seven microns thin, and stained with various chemicals for research.

Gentleman says he feels terribly privileged being able to do his job. "These people have bequeathed their brains to us, and I am fulfilling their last will and testament," he says. And his wonder in the brain's inner workings never ceases. "I've been examining brains for 25 years and I am still fascinated by this kilo and a half of fat," Gentleman says.

Carved, ground and frozen

For Jean-Paul Vonsattel, who runs the Brain Bank at Columbia University in New York, timing and precision is everything. He dissects brains fresh, as soon as they're harvested from donors. Because formalin changes tissue, certain aspects of morphological and biomolecular research require brain flesh that is still "living." Donated brains arrive at Vonsattel's laboratory inside bags and boxes filled with ice, and are immediately processed. First, they're sliced in two precisely equal halves, one to be prepared for research and the other fixed in formalin. "You can use the fixed tissue for diagnosis, but you can no longer extract proteins from it," explains Vonsattel. "So more and more researchers are asking for fresh samples to study what was in the brain while it was still alive." Because left and right hemispheres specialize at different tasks, and may be responsible for different symptoms, Vonsattel's team alternates them: On an even day, the right half gets carved, on an odd day, the left one.

The fresh half is sectioned into blocks carved from various brain areas such as the amygdala, hippocampus, motor cortex and others, yielding about 150 samples in total. Precision is important, Vonsattel notes, otherwise you can carve out the wrong part and skew months of someone's research. Sandwiched between two chilled metal plates, the slices are dipped into liquid nitrogen at around -180 degrees Celsius, which freezes the water inside the flesh so quickly that it doesn't form ice crystals that would rapture tissue and turn it to Swiss cheese. When such a block is thawed, it would retain the quality and the chemical composition of the fresh brain it had once been. From that, investigators would slice seven to 10 micrometer-thick sections for research. For the biomolecular studies of brain chemistry, Vonsattle's team manually grinds the samples into mush with a mortar and pestle, and then mixes in liquid nitrogen. When defrosted, the amalgams let researchers study the cells' genetic and biochemical make-up. "From that pulverized tissue you can isolate as many proteins as possible to see which one is toxic," says Vonsattlel, "For example the abnormal tau protein," which is implicated in Alzheimer's disease.

The blocks go into plastic bags and the liquefied mixes into vials, all of which are barcoded and catalogued akin to library materials, except they're stored in massive locked freezers at about -80 degrees Celsius. When a researcher requests tissue with a specific disorder, it takes minutes to locate the sample in a catalogue and then trace it to a freezer, shelf, rack, box, and finally the plastic bag. To date, Vonsattle's freezer library holds about three hundred thousand samples from 4,000 brains. And as the neurodegenerative research will grow, so will the hunger for his fresh brain slices.

Reassembled and digitized

Jacopo Annese, a computational neuroanatomist with The Brain Observatory at The Institute for Brain and Society, dissects brains to rebuild them once again. "There's a universe inside our heads," he says, but we know little about it. So he wants to create a digital neuroanatomical library that would store high-resolution images of hundreds of human brains.

Building digital brains requires dissecting its human predecessors into extraordinary thin slices which then can be "stacked up" to reassemble the brain as it has once been, so researchers can see its connections and structures. To get that level of finesse, the brains are frozen, embedded in gelatin, and cut using a microtome machine (from the Greek "tom," which means "to slice.") Akin to a deli meat cutter that slices prosciutto so fine you can see through it, the microtome produces nearly transparent micron-thin brain peels, each taking only a few seconds. Under the microtome's super-sharp horizontal blade, the brain is kept precisely at -35 degrees Celsius. At a higher temperature the blade may mar the slices, at a lower one, the brain turns brittle. As the knife peels off each slice into a pale mass of bunched-up tissue, Annese swipes it off with a brush and gently unfolds it inside a solution. "I like using the brush," he says. "It makes me feel like an artist."

Jacopo Annese, The Brain Observatory.

Turning a brain into 2500 to 3000 peels is time-consuming and physically exhaustive, but it's the only way to gather a lot of detail about a brain that researchers really care about. A few years ago, Annese and his colleagues used their method on the brain of Henry Molaison, or Patient H.M., the man who lost his ability to form memories after a surgery and became a focus of brain research for the rest of his life. Immediately after his death, his brain was scanned and then extracted to be sliced by in Annese's microtome.

"When we sliced the famous brain we were going all night, for 50 hours without sleep," Annese says. "If I'd start to miss my timing, my assistant had to alert me with a code word 'prosciutto.'"

Once a brain is sliced, the layers are stained with chemicals to reveal various brain parts and their connections. For example, some dyes would stain neurons a certain color while silver would reveal Alzheimer's plagues. "Brain is like a book written with magic ink so you have to use magic chemicals to reveal what's actually in it," Annese says. Then the slices are digitized and assembled into colorful three-dimensional brains, allowing researchers to traverse the entire organ, front to back. Annese's current goal is to digitize 1000 brains to help scientists with understanding neurological diseases. He also hopes to uncover how our brains define us, but for that he would need many more brains, he says. "Every brain has a different story to tell."

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