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Smithsonian National Museum of Natural History
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skull

Bone Basics

 

An Inside Look at Bone
 
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Skeletons are good age markers because teeth and bones mature at fairly predictable rates. For toddlers to teenagers up to age 21, teeth are the most accurate age indicators. Some of the best indicators of adult age are in the pelvis.

Tibia and fibula of a 15 year-old, with partially fused growth plates and a healed fracture with surgical plate on the fibula.
Tibia and fibula of a 15 year-old, with partially fused growth plates and a healed fracture with surgical plate on the fibula.  Image courtesy of: Smithsonian Institution

In Children

A baby's bones begin to grow in the womb. At birth, the skeleton is partially formed. Many bones are still in "parts." The ends (epiphyses) and bony shafts (diaphyses) of long bones form separately in the womb. At birth, the ends of the long bones are mainly cartilage, with centers of bone beginning to form inside. As a child grows, the shafts get longer, and bone gradually replaces the cartilage epiphyses. Through the growing years, a layer of cartilage (the growth plate) separates each epiphyses from the bone shaft.

clavicle
 
Clavicle.  Image courtesy of: Smithsonian Institution

Between 17 and 25 years, normal growth stops. The development and union of separate bone parts is complete. At this point, you and your skeleton are as tall as you are going to get - with many fewer bone parts than you started with!

Facts:

  • The clavicle (collar bone), pictured above, is the last bone to complete growth, at about age 25.
  • Measuring the length of long bones can give an estimate of age for children, but this technique is useful only until bones have stopped growing.
  • The tibia completes growth at about age 16 or 17 in girls, and 18 or 19 in boys.
  • For toddlers to teenagers up to age 21, teeth are the most accurate age indicators. Learn more here!

In Adults

Skeletons record an adult's age in several ways. The surfaces of the cranium, pubic bones, and rib ends hold clues. At the microscopic level, investigators can see the bone "remodeling" that takes place throughout life, as well as age-related bone breakdown.

Bone "Remodeling"

femur cross sections of adults ages 24 (left) and 77 (right)
Femur cross sections of adults ages 24 (left) and 77 (right). Images courtesy of: Smithsonian Institution

Even after childhood growth stops, bone "remodeling" continues. Throughout a lifetime, bone makes new osteons — minute tubes containing blood vessels. Microscopic exams show these changes, which can indicate adult age to within 5 to 10 years. Younger adults have fewer and larger osteons. Older adults have smaller osteons and more osteon fragments, as new ones form and disrupt older ones.

Clues in the Cranium

craniums of a 20 year-old and a 70 year-old
Craniums of a 20 year-old (left) and a 70 year-old (right).  Images courtesy of: Smithsonian Institution

The bones that enclose the brain grow together during childhood along lines called cranial sutures. During adulthood, bone "remodeling" may gradually erase these lines, at variable rates. Closure of cranial sutures gives general information about a person's age. It is best used with additional indicators to estimate age, or when other age indicators are unavailable.

Other Age-Related Changes

Arthritis on the spine as evidenced by "lipping" of the vertebrae.
Arthritis on the spine as evidenced by "lipping" of the vertebrae.  Image courtesy of: Smithsonian Institution

Wear and tear on a body throughout a lifetime affects the skeleton. Arthritis of the spine and joints can reflect increasing age. Scientists also recognize many other clues to aging, such as the appearance of the rib ends and the cartilage that joins them to the sternum. In a young adult, the rib end walls are thick and smooth, with a scalloped or rounded edge. In an older adult, the walls are thin, with sharp edges, and the rim often has bony, irregular projections.

 

 

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