Senile plaques are clumps of a sticky protein fragment called amyloid-beta that accumulate between nerve cells in the brain. They are one of two hallmark features of Alzheimer’s disease, the other being neurofibrillary tangles. First described by Alois Alzheimer in 1911, who called them “miliary foci,” these deposits have since become central to how scientists understand, diagnose, and attempt to treat Alzheimer’s.
What Senile Plaques Are Made Of
The core ingredient of a senile plaque is amyloid-beta, a small protein fragment ranging from 37 to 49 amino acids long. It comes from a much larger molecule called amyloid precursor protein (APP), which sits in the membrane of nerve cells throughout the brain. APP itself isn’t harmful. The problem begins when enzymes slice it apart in a specific sequence.
Two enzymes do the cutting. The first, called beta-secretase, snips the outer portion of APP. The second, gamma-secretase, cuts the remaining stub within the cell membrane. What’s left over is the amyloid-beta fragment. The two most common versions are 40 amino acids long (Aβ40) and 42 amino acids long (Aβ42). That small two-amino-acid difference matters enormously: the 42-amino-acid form is more toxic, clumps together much faster, and is the dominant form found in Alzheimer’s plaques.
Once released, these amyloid-beta fragments begin sticking to each other. Individual fragments form small clusters called oligomers, which grow into longer chains called protofibrils, and eventually mature into the dense, insoluble fibrils that make up a visible plaque. This process can unfold over years or even decades before symptoms appear.
Not All Plaques Are the Same
Under a microscope, senile plaques fall into distinct categories that reflect different stages of development and different levels of damage to surrounding tissue.
Diffuse plaques are the earliest and most common type. They appear as loose, wispy deposits of amyloid-beta without a well-defined structure. Crucially, they lack the damaged nerve fibers and dense cores seen in more advanced plaques. In people who are just at the threshold of cognitive decline, roughly 85% of their brain plaques are this diffuse type.
Neuritic plaques are more destructive. They contain not only amyloid-beta but also swollen, abnormally shaped nerve cell projections called dystrophic neurites, a sign that the surrounding brain tissue is actively being damaged. Some neuritic plaques develop a hard, compact core of densely packed amyloid, making them especially prominent on brain imaging and in autopsy studies. These cored neuritic plaques are the type most strongly associated with Alzheimer’s disease and make up about 15% of plaques in early dementia.
How Plaques Damage the Brain
For years, scientists assumed the large, visible plaques themselves were the primary source of brain damage. The picture now is more nuanced. The smaller, soluble clusters of amyloid-beta, particularly oligomers and protofibrils, appear to be the most directly toxic forms. These smaller clusters interfere with the signaling machinery at synapses, the junctions where nerve cells communicate.
Specifically, amyloid-beta oligomers disrupt receptors that control the flow of calcium into nerve cells. When too much calcium floods in, it triggers a cascade of problems: synapses weaken, the brain’s ability to strengthen connections (a process essential for learning and memory) gets suppressed, and nerve cells eventually die. Amyloid-beta also causes a buildup of the signaling chemical glutamate outside of cells, which overstimulates surrounding neurons and compounds the damage.
The brain’s immune cells, called microglia, add another layer. Amyloid-beta activates these cells, which initially try to clean up the deposits. But chronic activation turns them inflammatory, and they can begin killing healthy neurons in the process.
Plaques and Cognitive Decline
The relationship between plaque buildup and memory loss is real but not as straightforward as “more plaques, worse symptoms.” Amyloid accumulation is now considered an early initiating event in Alzheimer’s, one that can begin 15 to 20 years before any noticeable cognitive problems. The spread of a second abnormal protein, tau, through the brain’s memory and thinking regions is what tracks more closely with actual symptom severity.
Still, having significant amyloid buildup dramatically increases your risk. In studies of cognitively normal older adults, those with high amyloid levels on brain scans were nearly four to five times more likely to progress to mild cognitive impairment or dementia within three years compared to those without amyloid. In one large study, 26% of amyloid-positive but otherwise healthy older adults progressed to a clinical diagnosis within three years, compared to just 7% of those who were amyloid-negative.
An estimated 50% of people over 65 have some degree of amyloid deposition in their brains. Roughly half of those people show no cognitive symptoms at all. This means plaques are common in aging, but the combination of plaques plus other forms of brain degeneration is what most reliably predicts decline.
How Plaques Are Detected
Until relatively recently, the only way to confirm senile plaques was through a brain autopsy. That changed with the development of amyloid PET imaging, a type of brain scan that uses specialized radioactive tracers to bind to amyloid deposits and make them visible.
The first research tracer, known as Pittsburgh Compound B, paved the way for clinical versions. The FDA has approved tracers including florbetapir (marketed as Amyvid) and flutemetamol (Vizamyl) for use in patients. These scans can reveal whether a person’s brain contains significant amyloid deposits, helping doctors distinguish Alzheimer’s from other causes of memory loss. The scans detect plaque-form amyloid specifically, so they’re most useful for identifying moderate to advanced buildup.
Amyloid can also be measured indirectly through cerebrospinal fluid, obtained via a spinal tap, or through newer blood tests that detect amyloid-beta ratios. These alternatives are less expensive than PET scans and are increasingly used in clinical settings.
Treatments That Target Plaques
A new class of drugs, monoclonal antibodies designed to bind to amyloid-beta, represents the first treatments that directly address plaque buildup rather than just managing symptoms. Three have received FDA approval: aducanumab, lecanemab, and donanemab. Each targets a slightly different form of amyloid-beta.
Lecanemab, for example, preferentially binds to protofibrils (those intermediate-sized clusters) with 100 times greater affinity than it has for individual amyloid-beta fragments. Donanemab targets a chemically modified form of amyloid-beta found only in established plaques. Despite these differences in targeting, all three work through the same basic mechanism: they flag amyloid deposits so the brain’s immune cells can engulf and break them down.
The results are measurable. In lecanemab’s large phase III trial, treated patients showed a reduction of about 55 centiloids on amyloid PET scans (a standardized measure of plaque density), while those on placebo showed a slight increase. In an earlier trial, 81% of participants had their plaque levels reduced below the detection threshold. These drugs slow cognitive decline modestly, on the order of 25 to 35% compared to placebo over 18 months, but they do not stop or reverse it. They also carry a risk of brain swelling and microbleeds, which requires regular monitoring with MRI scans.
The fact that clearing plaques slows but doesn’t halt decline reinforces what the research on plaque density and symptoms suggests: amyloid-beta is a critical early driver, but by the time plaques are abundant, other disease processes, particularly tau accumulation and neuroinflammation, are already well underway.