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How Human Bones Grow
October 15, 2009

Most people I speak to are surprised to find out that the bones in the arms and legs are separated into three pieces in children and only fuse together late into the teen years. However, this well documented fact helps pediatricians determine the biological maturity (age) of a child by analyzing the X-ray of the child's hand and wrist (see, for example, the Hand Bone Age book). Further, a company by the name of BoneXpert has created an automated system to accomplish this task! What follows is a brief description of how human bones grow.

7 years old boy both hands and wrists X-ray photo
X-ray photo of 7 year old boy. Note how the knuckles are separate from the rest of the hand bones (metacarpals) and how the wrist bones near the thumb are just beginning to form.

I am sure by this point many of you are wondering, especially given the image above, what is holding the body together in a child if there is no bone? The answer is cartilage, a tough elastic tissue that serves as a model for the shape of the bone.

Before we discuss how bones are built, let us take this opportunity to describe the types of bones and their structure. Human bones are generally classified as long bones, flat bones, and irregular bones. Long bones include the bones of the arms, legs, hands, and feet. Flat bones are found in the cranium surrounding the brain. Irregular bones are those which cannot be described by either of these terms. They include the vertebrae and the bones of the wrists and ankles.

Regardless of the bone type, all bones are arranged in two types of structures visible to the unaided eye (macroscopically). Cortical or compact bone is found mainly in the shafts of long bones. It is very dense bone, lacking visible spaces to the unaided eye. It's function is mostly mechanical, giving the shaft of the bone its strength. The other kind of bone is spongy, trabecular, or cancellous bone, which is organized as a network of branching fibers called trabeculae. The images below show examples of both types of structures.

Cortical bone of the radius Spongy bone of the radius
Cross section of a human radius bone showing the structure of cortical bone. Proximal end of a human radius with damage to the outer layer, revealing the spongy bone beneath.
CT slice of femur revealing cortical bone structure. CT slice of femur revealing spongy bone structure.
CT slice of femur along the shaft. Cortical bone generates a strong response in the image. CT slice of femur near the knee (note the patella above). Spongy bone does not appear as bright in the CT image due to its reduced mineral density.

Microscopically, there are also two types of bones. Woven bone is a mechanically weak arrangement characterized by the random organization of fibres. It is very atypical, appearing in the beginning of bone development and in fracture repair. The more common kind is called lamellar bone, which is very strong due to its highly-organized, parallel layers (called lamella) of minerals.

Cortical bone has a special arrangement of lamella called osteons. In an osteon, the lamella are organized in a cylindrical fashion around a canal which carries blood vessels and nerves (the Haversian canal). Some of the osteoblasts become trapped in small crevices (lacunae) between the lamella and are then known as osteocytes. The osteocytes between each lamellar layer are in contact with each other through tiny canals (canicullae) which allows them to exchange nutrients within the otherwise stiff mineral structure of the bone. This structure is not present in the bony trabeculae since osteocytes receive their nutrients from the bone marrow.

The process of bone generation is called ossification. There are actually two types of ossification:

endochondral ossification:
where the bone replaces the cartilage model. Occurs mostly in the long bones.
intramembraneous ossification:
where there is no cartilage model. Occurs mostly in flat bones.

Let us describe intramembraneous ossification first, as it is a simpler process. Intramembraneous ossification occurs when a number of cells in the developing body differentiate into osteoblasts, or bone depositing cells. These cells secrete a compound called osteoid which later becomes calcified and results in the development of bony trabecula.

Endochondral ossification begins in the middle of the cartilagineous model of the bone shaft, also known as the diaphysis. Here, the intercellular matrix becomes calcified and prevents the diffusion of nutrients to the cells within the bone. As these cells die away, they leave a network of calcified cartilage behind. At this point, blood vessels arising from the tissue surrounding the cartilaginous bone model (the periousteum) penetrate the calcified cartilage. Osteoblasts use this calcified cartilage as a scaffold to begin laying down osteoid, forming trabecula. Osteoblasts also appear in the periousteum and begin to deposit bone against the shaft of the bony cartilage, producing a bone collar. This deposition results in a lamellar arrangement called circumferential system, which surrounds the outer surface of the bone (the missing outer layer in the image of the spongy bone of the radius above). Another kind of cell, the osteoclasts, appears inside the shaft and begins to break down the spongy bone from the inside, forming the medullary cavity in which the bone marrow resides.

Here, the bone begins to grow in two directions. It increases in thickness by laying down bone from the periousteum (appositional growth) and it increases in length as the osteoclasts break down the spongy bone toward the ends of the bone (the metaphysis). This entire region of growth is known as the primary ossification center.

At birth or even later, secondary ossification centers appear at the ends of the long bones, laying down spongy bone. These bony end pieces, the epiphyses, are separated from the metaphysis by a layer of cartilage called the epiphyseal plate. As the cartilage in the metaphysis is turned into bone, more cartilage is generated, thus lengthening the bone. At some point, near 20 years of age, this cartilage is no longer regenerated, and the cartilagenous epiphysial plate disappears. The diaphysis and epiphysis fuse together achieving the final shape of the bone.

However, changes in the bone structure continue throughout our lifetime. Osteoblasts and osteoclasts are continuously remodelling the bony structures, replacing old minerals and cells with new ones. New osteons are built whenever there is enough space for them to appear, even if the old osteon is not fully removed. This produces the third kind of lamellar arrangement within the bone called the interstitial system. Osteoporosis results when more mineral is removed by the osteoclasts than what is deposited by the osteoblasts.

The images below show the epiphyseal line, the line at which the diaphysis and epiphysis of the bones fuse.

illustration of femoral epiphysial line humerus with separated epiphysis and diaphysis
Distal epiphysial line of the femur. Matching humerus bones, with one of the bones separated at the epiphysial line.

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