Mitosis occurs in nearly every tissue in an animal’s body, with the important exception of a few cell types that permanently stop dividing after they mature. Any cell that isn’t a sex cell (sperm or egg) can potentially undergo mitosis, but the rate varies enormously depending on the tissue. Some cells divide every day, others sit quietly for years and only divide when damage triggers them into action, and a small number never divide again once they’re formed.
Cells That Divide Constantly
The most mitotically active tissues in an animal are the ones that experience constant wear and tear. These are classified as “labile” cell types, meaning they proliferate continuously throughout life to replace cells that die or slough off. The major examples are skin, the lining of the digestive tract, salivary gland tissue, and blood-forming cells in the bone marrow.
In the intestine, mitosis is concentrated in tiny pockets called crypts at the base of the intestinal lining. Cells born in these crypts migrate upward along finger-like projections called villi, doing their absorptive work before being shed into the gut a few days later. In the duodenum (the first section of the small intestine), the full cell cycle from one division to the next takes roughly 17 to 21 hours, making it one of the fastest-renewing tissues in the body.
In the skin, stem cells in the deepest layer of the epidermis and in the bulge region of hair follicles divide to produce new cells that gradually push upward, flatten, and eventually flake off the surface. This turnover cycle for human skin takes about a month from new cell to shed cell, though the mitotic divisions that fuel it happen continuously at the base.
Bone Marrow and Blood Cell Production
Bone marrow is one of the most prolific sites of mitosis in any animal. Hematopoietic stem cells, nestled along the inner bone surface and near blood vessels in spongy bone, activate in response to signals from their surrounding environment. Once activated, they divide to produce progenitor cells that progressively specialize into red blood cells, white blood cells, and platelets. Each step in this hierarchy involves further rounds of mitosis, amplifying a small pool of stem cells into the billions of blood cells an animal needs daily. This tightly regulated process of activation, proliferation, and differentiation keeps the blood supply continuously replenished.
Growing Bones in Young Animals
In young, still-growing animals, the growth plates near the ends of long bones are critical sites of mitosis. These plates are made of cartilage and contain distinct zones. In the proliferative zone, cartilage cells divide rapidly in organized columns, producing new rows of cells that secrete a structural matrix around themselves. As these cells mature, they stop dividing, enlarge dramatically, and eventually die. The matrix they leave behind is then replaced by actual bone tissue, which is how bones lengthen over time.
This process continues until an animal reaches skeletal maturity, at which point the growth plates close and are fully replaced by bone. It’s why a broken growth plate in a young animal can be more serious than a simple fracture: disrupting the mitotic activity there can permanently alter bone length.
Stem Cell Niches in Adult Animals
Adult animals maintain small reservoirs of stem cells in specific locations throughout the body. These niches act as local repair stations, housing cells that can divide when replacement cells are needed. The major stem cell niches identified in mammals include the bone marrow, skin (hair follicle bulge and the base of the outer skin layer), intestinal crypts, brain, and muscle.
In the brain, neural stem cells persist in two small regions: the walls of fluid-filled cavities deep in the brain and a zone within the hippocampus, a structure involved in memory. These cells can divide to produce new neurons, though at a far slower rate than skin or gut cells. In muscle, stem cells called satellite cells sit along the surface of muscle fibers in a dormant state. When muscle is damaged, satellite cells activate, divide, and fuse together to repair or replace injured fibers.
Wound Healing and Tissue Repair
When an animal is injured, mitosis ramps up in multiple cell types to close the wound. The process unfolds in three overlapping stages: inflammation, proliferation, and remodeling. Fibroblasts, the cells responsible for building connective tissue, play a central role throughout all three stages. They divide and migrate into the wound site, producing the structural proteins that form scar tissue and restore skin integrity.
The liver offers one of the most dramatic examples of repair-driven mitosis. Hepatocytes (liver cells) are normally quiescent, sitting in a non-dividing state for long stretches. But if a large portion of the liver is lost to injury or surgical removal, the remaining mature hepatocytes re-enter the cell cycle and begin dividing. Unlike most organ repair, this process doesn’t rely on a dedicated stem cell population. Instead, the ordinary working cells of the liver handle the job themselves, proliferating until the organ reaches its original mass. Once sufficient liver tissue has been restored, signaling pathways shut proliferation back down and the cells return to their resting state.
Embryonic Development
The most intense period of mitosis in any animal’s life is embryonic development. After a fertilized egg forms, it immediately begins a rapid series of divisions called cleavage. In mammals, the first division splits the single-celled embryo along a line running from top to bottom, and subsequent divisions follow in roughly perpendicular planes. These early cell cycles can be remarkably fast, though the timing varies. Some embryos show division cycles as short as dozens of minutes, while others take a day or more between divisions.
During this stage, mitosis is happening everywhere in the embryo because every cell is dividing. There’s no specialization yet, just a growing ball of increasingly smaller cells. As development progresses and tissues begin to differentiate, mitosis becomes more localized to specific zones where organs and structures are actively forming.
Where Mitosis Does Not Occur
A few cell types in animals are considered permanently post-mitotic, meaning they exit the cell cycle and never divide again. The two most notable examples are heart muscle cells and most neurons in the central nervous system.
Heart muscle cells (cardiomyocytes) actively divide during embryonic development, but shortly after birth they progressively lose the ability to complete cell division. Many of these cells end up with two nuclei, a sign that they began dividing but couldn’t finish the final step of splitting into two separate cells. This is a major reason heart attacks cause permanent damage: the lost muscle cells simply aren’t replaced through mitosis. Skeletal muscle cells face a similar limitation, though satellite stem cells in the surrounding tissue can generate new muscle fibers to some extent.
Neurons in the adult brain are largely non-dividing as well, with the exception of the two small stem cell niches mentioned above. This is why neurodegenerative diseases and traumatic brain injuries have such lasting consequences. The brain has extremely limited capacity to replace lost neurons through mitosis.