A microsome is a tiny membrane-enclosed vesicle, roughly 100 to 200 nanometers in diameter, that forms when cells are broken apart in a laboratory. Microsomes are not structures you’d find inside a living cell. They’re fragments of the cell’s internal membrane system, primarily the endoplasmic reticulum, that spontaneously reseal into small bubbles during the process of grinding up tissue. Despite being artificial, microsomes retain most of the biochemical machinery of the original membranes, which makes them extraordinarily useful for studying how cells process drugs, build proteins, and manufacture hormones.
How Microsomes Form
When researchers homogenize tissue (essentially blending it into a slurry), the force breaks apart the cell’s internal membranes, including the plasma membrane and the extensive network of the endoplasmic reticulum. These membrane fragments don’t stay open. They immediately reseal into small, closed spheres, much like how a soap bubble reforms when you pop part of it. The resulting vesicles derived from the endoplasmic reticulum are what scientists call microsomes.
To collect microsomes, researchers use a technique called differential centrifugation, spinning the cell slurry at progressively higher speeds. Heavier components like intact nuclei and mitochondria settle out at lower speeds. Microsomes, being much smaller and lighter, require very high centrifugal force to pellet out of the liquid. The key detail is that the interior of each microsome is biochemically equivalent to the inside of the endoplasmic reticulum in a living cell, preserving the orientation of enzymes and other proteins embedded in the membrane.
Rough vs. Smooth Microsomes
Not all microsomes look or behave the same. When the rough endoplasmic reticulum (the part studded with protein-building machinery called ribosomes) breaks apart, the resulting vesicles keep those ribosomes attached to their outer surface. These are called rough microsomes. Smooth microsomes, by contrast, lack ribosomes and come from the smooth endoplasmic reticulum. But smooth microsomes can also originate from fragments of the plasma membrane, the Golgi apparatus, endosomes, or even mitochondria. The exact mix depends on the tissue being studied.
This distinction matters because rough and smooth microsomes carry different enzymatic toolkits. Rough microsomes are useful for studying how cells manufacture and fold new proteins. Smooth microsomes are particularly rich in the enzymes responsible for metabolizing drugs and synthesizing lipids and hormones.
Why Liver Microsomes Matter in Drug Development
The liver is the body’s primary chemical processing plant, and liver microsomes are one of the most widely used tools in pharmaceutical research. The liver microsomal drug-metabolizing system consists of two core protein components and a structural lipid. One protein acts as the binding site for both oxygen and the drug molecule being processed, while the other shuttles electrons to power the chemical reaction. Together, they form the engine behind what pharmacologists call Phase I metabolism: the initial chemical modification of a drug that typically makes it easier for the body to eliminate.
The enzymes most associated with liver microsomes belong to the cytochrome P450 family. These enzymes are remarkably versatile. They help synthesize steroid hormones, cholesterol, bile acids, and signaling molecules like prostacyclin and thromboxane. They also break down fatty acids, retinoic acid, and steroids the body no longer needs. On the defensive side, they degrade foreign chemicals, including pharmaceutical drugs and cancer-causing compounds. Multiple forms of cytochrome P450 exist, each with different properties, which helps explain why drug metabolism varies so much between species, sexes, age groups, and even individuals.
How Researchers Use Microsomes to Test Drugs
In early drug discovery, pharmaceutical companies need to know how quickly the body will break down a new compound. A drug that’s metabolized too fast won’t stay in the bloodstream long enough to work. One that’s metabolized too slowly could accumulate to dangerous levels. Microsomal stability assays answer this question efficiently.
The basic approach is straightforward: researchers incubate a candidate drug with liver microsomes and measure how quickly the parent compound disappears over time, yielding a half-life estimate. At many research centers, rat liver microsomes serve as a first-pass screen, with human liver microsomes used as a more refined follow-up. The results help teams rank and prioritize compounds, weeding out molecules with poor metabolic profiles before investing in expensive animal studies or clinical trials. These assays are high-throughput, meaning hundreds of compounds can be screened in a short period.
Machine learning models now complement these lab assays. Pharmaceutical companies train predictive algorithms on large datasets of microsomal stability results, then use those models to estimate metabolic stability before a compound is even synthesized. Researchers have found that combining rat and human microsomal data improves the accuracy of these predictions, particularly for identifying compounds that would be rapidly degraded in the liver.
Limitations Compared to Other Test Systems
Microsomes are cost-effective and easy to automate, but they capture only a slice of what happens to a drug in the body. Because they come from the endoplasmic reticulum, they contain only the enzymes housed there, primarily the cytochrome P450 family and certain enzymes involved in one type of Phase II reaction (a later step in drug processing that attaches a chemical tag to help the body excrete the compound). They miss the enzymes that live in other parts of the cell, including several that have gained recognition as major contributors to metabolism for certain drug types.
A broader alternative is the S9 fraction, which includes both the microsomal components and the soluble enzymes from the cell’s cytoplasm. S9 fractions capture both Phase I and Phase II metabolism more completely. In comparative studies, S9 fractions produce a metabolic profile closer to what intact liver cells (hepatocytes) generate. For example, certain metabolic byproducts that appear in both S9 fractions and hepatocytes are completely absent when the same compound is tested with microsomes alone. Relying only on microsomes could cause researchers to miss important metabolic pathways, potentially overlooking a compound’s vulnerability to breakdown.
Hepatocytes, intact liver cells grown in culture, remain the gold standard because they contain the full complement of cellular machinery. But they’re more expensive, harder to work with, and less suited to screening thousands of compounds at once. In practice, most drug discovery programs use microsomes as an early, fast filter, then follow up with S9 fractions or hepatocytes for compounds that advance further in development.
Handling and Storage
Commercially prepared microsomes, typically sourced from pooled human or animal liver tissue, are stored at negative 80 degrees Celsius. They can be refrozen up to twice without losing significant enzymatic activity, which gives researchers some flexibility during experiments. Once thawed for use, microsomes are mixed with the test compound and necessary cofactors, then incubated at body temperature while samples are collected at timed intervals to track how fast the drug is broken down.