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Class Notes: Human Osteology

Saturday: September 24, 2011

Week 2: Bone Growth and Joints & Skull


Bone development (osteogenesis) in the embryonic stage basically forms three layers: the ectoderm, which transforms into skin and teeth enamel; the mesoderm, which forms connective tissues and bone; and the endoderm, which turn into the organs. Mesenchyme in the mesoderm layer will begin to form a cartilaginous skeleton, known as an anlage. This will be followed by a layer of protein matrix which will build the bone shapes. This pre-bone matrix is an osteoid. Mineralization then takes place, creating immature bone. This turns into mature bone once the minerals and proteins become organized through the work of osteoblasts (bone forming cells) and osteoclasts (bone removal cells), to form osteons (organized bone matrix).

Bone repair works in much the same way, beginning with osteoids. If the periosteum becomes strained or torn (the layer on the outside of living bone), a fracture callus will form along with a hematoma (blood mass), allowing osteoids to patch the area until osteoblasts and osteoclasts begin the true remodeling.

There are three classifications of joints:

  • Synarthrosis (fibrous): allows almost no movement (such as that found in cranial sutures)
  • Amphiarthrosis (cartilaginous): allows limited movement (such as that found in gomphosis, which keeps teeth in place)
  • Diarthrosis (synovial): allows much movement (found in synovial joints, which are the typical joints)

Synovial joints come in several forms: ball and socket (ex. hip), hinge (ex. elbow), pivot (ex. shaking head to say “no”), gliding (ex. wrist movement), condyloid (ex. finger joints), and saddle (ex. thumb joint). A synovial joint includes the bones involved, which have a layer of articular cartilage on each of their articular surfaces. This cartilage, along with a synovial membrane, forms the joint, which is filled with synovial fluid. The joint is covered by a fibrous capsule and ligaments keep the capsule in place.


 The first skull appears about 500mya in fish, although it was more osteodentin than true bone. There were no moveable parts, and it acted as armor to protect the notochord. Placoderms derived from this and were the first jawed fish. Chondrichthyes began having a cartilaginous skeleton in addition to the skull plate. Osteichthyes developed a skin covering over the plate, and the plate covered the head, leaving the orbits, nasal, and mouth free. Early tetrapods began the mineralization of the cartilaginous skeleton, and moved out of the water. Their skulls now housed a full brain and teeth.

Today, we see three common types of skulls: anapsids have the boney plate still, but underneath is another boney portion protecting their brain. This set up is heavy, so it works best in aquatic or slow moving animals such as a turtle. Diapsids are similar, except that their boney plate opened up windows to lighten the load. They can move faster on land, and this is seen in alligators. Synapsids opened the window so much that there truly is no longer a window. Instead, synapsids skull is almost entirely only the inner boney portion found in anapsids and diapsids. In fact, the only skull plate left in a synapsid skull is the zygomatic arch (cheek bone). Humans are synapsids.

There are 29 bones in an average human adult skull, along with 32 teeth. In class, we covered the frontal bone, both parietal bones, both temporal bones, the occipital bone, the sphenoid, and the ethmoid.


In lab, we examined skull bones to be able to identify features and side fragments.

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