Bones grow through two main processes: they get longer by converting cartilage into bone at specialized regions called growth plates, and they get wider by adding new bone tissue to their outer surface. These processes begin in the womb, accelerate during childhood and puberty, and don’t fully stop until the mid-20s, when bone density reaches its lifetime peak.
Two Ways Bones Form
Not all bones grow the same way. Your body uses two distinct methods depending on which bone it’s building.
Long bones like the femur, tibia, and the bones in your arms grow through a process that starts with a cartilage template. Early in development, your skeleton is mostly flexible cartilage. Over time, specialized bone-building cells called osteoblasts move in and begin replacing that cartilage with hard bone tissue. They do this by secreting a soft protein matrix, mostly collagen, that then binds calcium and hardens. As osteoblasts become surrounded by the matrix they’ve produced, they transform into osteocytes, the mature cells that live permanently within bone.
Flat bones like the skull and collarbones skip the cartilage step entirely. Instead, early stem cells transform directly into osteoblasts, cluster together, and start laying down bone tissue right within sheets of connective tissue. First they create spongy, porous bone laced with blood vessels. Then a membrane called the periosteum forms on the surface, and osteoblasts beneath it add dense, compact layers on top. This is why a baby’s skull has soft spots at birth: the flat bones haven’t finished filling in the gaps between ossification centers yet.
How Growth Plates Make Bones Longer
The growth plate is a thin disc of cartilage near each end of a long bone. It’s the engine behind your height increases during childhood and adolescence. The plate works like a conveyor belt, continuously producing new cartilage on one side while converting it to bone on the other.
Inside the growth plate, cartilage cells (chondrocytes) are organized into distinct zones, each with a specific job:
- Resting zone: A reservoir of stem-like cells that are relatively inactive, producing collagen and waiting for signals to divide.
- Proliferative zone: Cells begin dividing rapidly, stacking into columns and pumping out collagen and other structural proteins. This is where most of the lengthening action happens.
- Hypertrophic zone: Cells stop dividing and swell dramatically in size, sometimes to five or ten times their original volume. They begin producing a different type of collagen and start mineralizing the surrounding tissue.
- Calcification zone: The enlarged cartilage cells die off. Blood vessels invade, bringing bone-building osteoblasts that replace the calcified cartilage with true bone tissue.
This cycle repeats continuously throughout childhood. Each round pushes the end of the bone slightly further from the shaft, adding length bit by bit.
How Bones Get Wider
While growth plates handle length, bones increase in diameter through a process called appositional growth. Osteoblasts on the periosteum, the tough membrane wrapping the bone’s outer surface, lay down new layers of compact bone on the outside. At the same time, cells on the inner surface can resorb bone to widen the marrow cavity, keeping the bone from becoming too heavy.
This process differs between sexes during puberty. In males, androgens drive the periosteum to expand outward while the inner cavity stays roughly the same size, producing thicker cortical bone. In females, periosteal expansion slows while bone formation shifts to the inner surface. This is one reason male bones tend to be wider and thicker by adulthood.
The Three Cell Types That Build and Reshape Bone
Bone is living tissue, and three types of cells keep it functioning:
Osteoblasts are the builders. They secrete the collagen-rich protein matrix that forms the structural scaffold of bone, then help mineralize it with calcium and phosphate. Once an osteoblast becomes fully enclosed in the matrix it created, it transforms into an osteocyte.
Osteocytes are the most abundant cells in mature bone. Trapped inside tiny spaces called lacunae, they act as sensors, detecting mechanical stress and fluid flow within the bone. When you run, jump, or lift something heavy, osteocytes register those forces and send chemical signals to surrounding cells, telling them where to add or remove bone.
Osteoclasts are the demolition crew. These large, multinucleated cells dissolve bone tissue by releasing acid and enzymes into small pits on the bone surface. This sounds destructive, but it’s essential. Osteoclasts clear out damaged or unnecessary bone so osteoblasts can replace it with fresh tissue. The cycle of removal and rebuilding is called remodeling, and it continues your entire life.
Hormones That Control Bone Growth
Growth hormone, released by the pituitary gland, is the primary driver of bone elongation during childhood. It doesn’t act on growth plates directly for the most part. Instead, it stimulates the liver and local tissues to produce a secondary signal called IGF-1. In the growth plate, growth hormone nudges resting cartilage cells to begin differentiating, while IGF-1 triggers the rapid cell division in the proliferative zone that actually adds length. The two work as a relay system.
Sex hormones play a dual role during puberty. Estrogen and testosterone both accelerate bone growth initially, which is why adolescents shoot up in height. But estrogen is also responsible for eventually closing the growth plates. It triggers the final mineralization of the cartilage, replacing it entirely with bone and ending any further lengthening. This is why girls, who produce more estrogen earlier, typically stop growing before boys do.
Thyroid hormones, vitamin D, and parathyroid hormone also contribute. Vitamin D helps the gut absorb calcium, making it available for mineralization. Parathyroid hormone regulates calcium levels in the blood and, when released in certain patterns, can actually stimulate new bone formation on both the inner and outer bone surfaces.
When Growth Plates Close
Growth plates don’t all close at once. Different bones fuse on different schedules, and girls consistently reach each milestone earlier than boys. At the knee, for example, complete fusion of the lower femur begins around age 16 to 17 in females and 17 to 18 in males. By age 20 to 21 in females and 21 to 22 in males, all subjects in one large study showed full fusion at the knee.
Once a growth plate fuses completely, that bone can no longer grow longer. However, reaching your final height doesn’t mean your skeleton is done developing. Bone density continues to increase into your 20s. Females reach peak bone mass around age 22 on average, while males reach it later, around age 23 to 27 depending on the measurement. Everything you do before that point, nutrition, exercise, hormonal health, contributes to your lifetime reserve of bone strength.
How Mechanical Stress Shapes Bone
Bones adapt to the forces placed on them. This principle, known as Wolff’s Law, means that bones subjected to regular loading become denser and stronger, while bones that aren’t stressed weaken over time. The duration, magnitude, and rate of force all matter.
The mechanism relies on osteocytes. When you bear weight or when a muscle pulls on bone through a tendon, it changes the flow of fluid through microscopic channels in the bone tissue. Osteocytes detect this fluid shift and convert the mechanical signal into chemical messages that activate osteoblasts to deposit new bone in the areas under stress. This is why weight-bearing exercise builds bone density, and why astronauts lose bone mass in microgravity where those mechanical signals disappear.
Nutrients That Support Bone Growth
Calcium is the most important mineral for bone, and 99% of the body’s calcium is stored in the skeleton. The recommended daily intake ranges from 1,000 to 1,300 mg depending on age and sex, with adolescents needing the higher end during their rapid growth years.
Phosphorus partners with calcium to form hydroxyapatite, the crystal that gives bone its hardness. Most people get enough phosphorus through a normal diet since it’s abundant in dairy, meat, and grains. Magnesium, though it makes up only about 1% of total bone mineral content, plays a structural role in the crystal lattice and acts as a cofactor for enzymes involved in bone metabolism.
Vitamin D is critical because without it, your intestines absorb only a fraction of the calcium you eat. During periods of rapid growth, insufficient vitamin D can lead to soft, poorly mineralized bones. Other micronutrients, including zinc, copper, and manganese, serve as cofactors for the enzymes that build and maintain the collagen scaffold of bone.