Several types of cells in the human body do not undergo mitosis. These are classified as permanent (postmitotic) cells, and the three main examples are neurons, cardiac muscle cells, and skeletal muscle fibers. A few other specialized cells, like mature red blood cells, also fall outside the mitotic cycle for different reasons. Understanding which cells can and cannot divide explains a lot about why certain injuries heal easily while others cause lasting damage.
Three Categories of Cell Division
Cells in the human body fall into three broad groups based on their ability to divide. Labile cells divide continuously throughout life. These include skin cells, the cells lining your gut, and bone marrow cells. At any given moment, more than 1.5% of cells in these tissues are actively dividing. That’s why a scrape on your skin heals in days.
Stable cells sit quietly in a resting state but can be triggered to divide when needed. Liver cells are the classic example: after injury or surgical removal of part of the liver, the remaining cells re-enter the division cycle and regenerate lost tissue.
Permanent cells are the ones that cannot re-enter the division cycle. Once they mature, they exit the cell cycle for good. When these cells are lost, the body typically fills the gap with scar tissue rather than functional replacements.
Neurons
Mature neurons are the textbook example of cells that do not divide. After a neuron finishes developing, it enters a resting phase called G0, essentially a permanent exit from the cell cycle. The molecular machinery that would allow it to divide is shut down, and reactivating it doesn’t lead to a new cell. Instead, if a mature neuron receives signals pushing it toward division, the process is rerouted into a self-destruct pathway. The neuron dies rather than proliferates. This is why spinal cord injuries and many forms of brain damage cause permanent loss of function.
There is one notable exception. A small region of the brain involved in memory, the hippocampus, retains a limited population of progenitor cells that can produce new neurons even in adulthood. Recent single-nucleus RNA sequencing studies have confirmed the existence of immature neurons and proliferating progenitors in the adult human hippocampus, and their numbers appear to decline in Alzheimer’s disease. Still, this is a narrow exception. The vast majority of your roughly 86 billion neurons will never be replaced.
Cardiac Muscle Cells
Heart muscle cells were long considered completely postmitotic, and functionally they almost are. In a healthy adult heart, these cells do not divide in any meaningful way. After a heart attack, some cardiac cells near the damaged zone show molecular signs of re-entering the cell cycle, but the actual rate of division is vanishingly small. Studies of infarcted human hearts found a mitotic index of just 0.08% in tissue adjacent to the infarct and 0.03% in regions farther away. For context, that means fewer than one in a thousand cells near the injury site was dividing.
This negligible capacity for self-renewal is why heart attacks cause permanent damage. Dead heart muscle is replaced by stiff scar tissue (fibrosis) that cannot contract. The surviving muscle cells can enlarge to compensate, roughly doubling in size, but once that limit is reached, no further adaptation occurs. The heart simply has less functional muscle to work with.
Skeletal Muscle Fibers
Mature skeletal muscle fibers are large, multinucleated cells that do not divide. However, skeletal muscle has a repair strategy that the heart and brain largely lack: satellite cells. These are stem cells that sit between the outer membrane of a muscle fiber and the surrounding tissue. In their normal state, satellite cells are dormant. When muscle is damaged or stressed (through exercise, for instance), they activate, multiply, and either fuse with existing fibers to donate fresh nuclei or fuse together to form entirely new fibers.
This is the mechanism behind muscle growth from resistance training. The muscle fibers themselves never split in two. Instead, satellite cells proliferate and merge into the fiber, increasing its size and the number of nuclei it contains. The distinction matters: the mature fiber is postmitotic, but the tissue as a whole can regenerate thanks to its resident stem cell population.
Mature Red Blood Cells
Red blood cells cannot undergo mitosis for a fundamentally different reason than the cells above. During their final stage of development in the bone marrow, red blood cell precursors physically eject their nucleus in a process called enucleation. The resulting cell is essentially a membrane-bound sac of hemoglobin, optimized for carrying oxygen. Without a nucleus, it has no DNA to replicate and no way to divide. Red blood cells also lose their mitochondria and most of their internal machinery during maturation.
This is not a problem for the body because the bone marrow continuously produces new red blood cells at a staggering rate, roughly 200 billion per day. Each red blood cell lives about 120 days before being broken down and recycled. So while individual red blood cells are permanently postmitotic, the supply chain that produces them is one of the most active dividing tissues in the body.
Lens Fiber Cells
The lens of the eye contains another population of non-dividing cells. Cells at the edge of the lens can still divide, but as they migrate inward, they differentiate into lens fibers and lose that ability. These fibers are never removed or replaced. They simply accumulate, which is why the lens grows heavier throughout your entire life. The number of dividing cells in the lens epithelium also decreases with age, contributing to the gradual stiffening and clouding that leads to presbyopia and cataracts.
Why This Matters for Injury and Disease
The practical consequence of having non-dividing cells is straightforward: when they die, the body cannot grow new ones to take their place. Instead, connective tissue fills the gap. In the heart, this means fibrosis after a heart attack. In the brain, it means permanent neurological deficits after a stroke. In skeletal muscle, satellite cells offer a workaround, but even that system has limits, particularly with aging, when satellite cell numbers and responsiveness decline.
This distinction between cell types also explains why cancers of permanent cells are relatively rare in adults. Cancer requires uncontrolled cell division, and cells that have exited the cycle entirely are far less likely to become malignant. Brain tumors in adults, for example, typically arise not from mature neurons but from glial cells (the support cells of the nervous system) that retain some capacity to divide.