Longitudinal growth, the process of increasing human height, is a precisely regulated biological phenomenon occurring primarily during childhood and adolescence. This increase in stature is achieved by the lengthening of the body’s long bones, such as the femur, tibia, and humerus. Understanding how these bones extend provides insight into the mechanics of human development and why height changes cease upon reaching adulthood.
The Site of Skeletal Elongation
The specific location where long bones grow in length is a specialized structure called the epiphyseal plate, which is more commonly known as the growth plate. This thin, dedicated layer is composed of hyaline cartilage and exists near the ends of the long bones. Anatomically, the growth plate is situated between two major sections of the bone: the epiphysis, which is the widened, rounded end, and the diaphysis, which is the long, central shaft.
This cartilaginous plate serves as a temporary construction zone, allowing the bone to be pushed outward from its center. New cartilage is produced on one side of the plate while old cartilage is simultaneously converted to bone on the other side. This mechanism allows the bone shaft to lengthen without altering the size of the joint at the bone’s end.
How Cartilage Becomes Bone
The lengthening process is driven by endochondral ossification, a sequence of events where a cartilage model is systematically replaced by bone tissue. This activity is organized into distinct zones within the growth plate, each performing a specific function. The process begins in the zone of proliferation, where specialized cartilage cells, called chondrocytes, rapidly divide. These daughter cells stack into columns that face the diaphysis, effectively creating new cartilage material and pushing the epiphysis away from the bone shaft.
As the chondrocytes are pushed away from the epiphysis, they enter the next phase, the zone of hypertrophy, where they enlarge significantly. This cellular swelling causes the lacunae, the spaces they occupy, to expand, further increasing the length of the plate. Following this enlargement, the chondrocytes in the zone of calcification begin to die, and the surrounding cartilage matrix mineralizes with calcium deposits. This calcified tissue forms a rigid framework that acts as a scaffold for the final stage of bone formation.
In the final zone of ossification, blood vessels and bone-forming cells called osteoblasts invade the spaces left by the dead chondrocytes. The osteoblasts then deposit new bone tissue onto the calcified cartilage remnants, fully replacing the temporary cartilage structure with permanent osseous tissue.
When Longitudinal Growth Stops
The period of length growth concludes with epiphyseal closure, also known as growth plate fusion. This event is triggered primarily by hormonal changes during puberty. Rising levels of sex hormones, specifically estrogen in both males and females, signal the end of the growth period.
Estrogen accelerates the programmed aging, or senescence, of the growth plate’s chondrocytes. This acceleration causes the cartilage cells to lose their ability to proliferate and divide, which halts the production of new cartilage. Once the rate of cartilage production slows, the process of bone replacement overtakes the remaining cartilage layer.
The remaining cartilage of the growth plate is completely replaced by bone tissue, resulting in the fusion of the epiphysis and the diaphysis. This fused structure is visible on an adult bone as a thin, bony remnant called the epiphyseal line. Once this fusion is complete, typically around 14 to 15 years for females and 15 to 17 years for males, the bone can no longer lengthen, and growth in height ceases.