Frequently Asked Questions

Bone Biology

The Human Skeleton

Skeletal Morphology

Science

Bone Biology

• How do bones form?
  
  The process by which bones form, beginning in the eighth week of an embryo's development, is known as osteogenesis or ossification. The majority of the skull bones and the clavicle develop through intramembranous ossification, in which the bones form directly from an embryonic tissue known as mesenchyme. All other bones of the body develop through endochrondral ossification, in which the bones are first formed in hyaline cartilage which then is gradually replaced by bone tissue. Endochrondral ossification of long bones begins in the diaphysis, or shaft, of the bone; the last stage is the ossification of the epiphyses, or the ends. This last stage occurs around the time of birth. Ossification continues throughout life, with the epiphyses finally fusing to the diaphysis in adulthood. Until then, a section of hyaline cartilage known as the epiphyseal plate remains between the diaphysis and the epiphysis.
• How do bones grow?
  
  After birth, increase in the length of bone is caused by growth of hyaline cartilage which is then replaced by bone tissue. Increase in the width of bone is caused by appositional growth, in which bone tissue is added to the surface of the existing bone. This new bone tissue is secreted from the deep layer of the periosteum, a connective tissue membrane that covers the bone.
• What types of cells form bone?
  
  There are three types of bone cells: osteoblasts, osteocytes, and osteoclasts. Osteoblasts form new bone by hardening the protein collagen with minerals. Osteocytes maintain bone by exchanging nutrients and wastes between blood and bone tissues. Osteoclasts destroy bone and release minerals back into the blood.
• What is the structure of bone?
  
  Bones are actually made up of two different types of bone. Cortical bone, or compact bone, forms the outer thick layer. Cancellous bone, or spongy bone, is formed out of trabeculae, bony struts that provide support. In living bone, cancellous bone contains the red bone marrow.
• What materials make up bone?
  
  Bone is made up of both organic and inorganic components. Protein collagen provides the flexibility of bones; the mineral component of bones is mostly hydroxyapatites, formed primarily of calcium and phosphorus. Small amounts of sodium, magnesium, fluorides, and carbonates are also present. One-third of living bone is water.
• How do muscles attach to bones?
  
  Muscle fibers attach to bone via strong fibrous connective tissues, either directly through muscle fascicles or indirectly through tendons. The connective tissue joins the outer layer of the periosteum, a connective tissue membrane that covers the bone. At the points where tendons attach to bones, raised areas of bone (such as tubercles, trochanters, and crests) may develop to provide extra attachment area.
• What is the function of bone?
  
  Bone has three main functions. It supports the body and stores minerals that are needed for metabolic functions. In organisms that have exoskeletons, shells, or bony scales, bone provides protection from predators.

The Human Skeleton

• How many bones are in the human body?
  
  On average, there are 206 bones in the human skeleton. A baby has over 300 bones, some of which fuse together as the individual grows. Variation means that an adult human may have between 200 and 210 bones in their body.
• Do men and women have different numbers of ribs?
  
  No. Most adult men and women have 12 pairs of ribs. However, there is variation in this number and some people may have an extra pair or ribs or an extra vertebra.
• What is the longest bone in the human body?
  
  The longest bone in the human body is the femur, sometimes called the thighbone.
• What is the smallest bone in the human body?
  
  The smallest bones are the ear ossicles, three bones that are found in the middle ear. They are called the stapes ("stirrup"), incus ("anvil"), and malleus ("hammer").

Skeletal Morphology

• What are the differences between a human skeleton and a chimpanzee skeleton?
  
  There are many differences between the human and the chimpanzee skeleton. The following are some examples:
  • The volume of the human brain case averages 1400 cm³, while the chimpanzee brain case averages 400 cm³.
  • Human canines are small in both sexes; chimpanzee canines are larger in males.
  • Chimpanzee metacarpals are longer and more curved for climbing in trees and knuckle-walking on the ground.
  • The human thumb is relatively longer and has well-developed musculature for fine manipulation.
  • The chimpanzee clavicle angles upward; the human shoulder joint is lower.
  • The thorax of the chimpanzee is broad compared to the human thorax.
  • The human spine is curved for balance during bipedal locomotion.
  • The human pelvis has a short, broad ilium and a short ischium. This is also an adaptation for bipedality.
  • The human femur is distally angled inwards towards the midline. This is called a valgus knee and helps with balance during walking.
  • Human toes are shorter than chimpanzee toes.
  • The big toe of the human is adducted, or drawn in line with the rest of the toes, compared to the divergent big toe of the chimpanzee.
    You can compare human and chimpanzee bones on the Comparative Anatomy page!
• Why do teeth come in different shapes?
  
  Teeth vary in size and shape according to how to the animal is using them. Teeth perform different functions such as biting, slicing, chewing, crushing, cracking, gnawing, grooming, digging, defending, and communicating. Teeth that are long with broad, flat surfaces are useful for grinding plant food. Teeth with sharp, pointed cusps can tear apart leaves and insects. The narrow, sharp molars and carnassials of canines are used for cutting meat and crushing bone. Large canines, like those in baboons, can be shown to competitors to display dominance and aggression.

Science

• What is evolution?
  
  Evolution is the process by which organisms acquire and pass on novel traits from generation to generation. The four mechanisms that act upon evolution are natural selection, random mutation, genetic drift, and gene flow.
• What is a fossil?
  
  A fossil is formed when hard elements of a skeleton, such as shells, teeth, and bones, are quickly buried by sand or mud. These sediments are turned into rock over millions and during this time the organic components of the skeleton are replaced with minerals. This transition from bone to stone forms a fossil.