Unit 6 Reflection: The Skeletal System

In this unit, we learned about the skeletal system. The essential questions from this unit were:

• What are the microscopic and macroscopic characteristics of the skeletal system?
• How do disorders of the skeletal system affect different types or parts of bones?
• What are the effects of vitamins, cells, and the environment on bone density?
• How does a bone repair itself after being fractured?
• What are the defining characteristics of different types of joints?

The macroscopic features of bones include the diaphysis (shaft), which surrounds a hollow medullary cavity, and the proximal and distal epiphyses (flared ends). Compact, or cortical bone, is the smooth, dense bone, while spongy bone is more porous, with cross-hatching supports called trabeculae that help the bone resist stress. There are also two types of bone marrow stored in the bones: red, which produces blood cells and is found in spongy bone, and yellow, which stores energy as fat and is found in the medullary cavity.

At the microscopic level, osteons are what make up bone. They are cylinder-shaped weight-bearing units that run parallel to a bone's axis. They are made of tubes within tubes called lamellae between which are tiny spaces called lacunae, which hold osteocytes (mature bone cells that maintain bone). They also have a central canal that holds nerves and blood vessels to support all of the cells in the bone.

anatomy of a bone (Wikipedia)

Bones are assembled into skeletons, which can be divided into the axial and appendicular skeleton. The axial skeleton is made of the bones along the axis of a body: skull, ribs, sternum, and vertebrae. The appendicular skeleton includes all the rest of the bones, which are attached to the axial skeleton. Bones can also be classified by shape: long, short, flat, and irregular. To help us learn about the skeleton, we did a lab where we dissected an owl pellet, classified the bones we found in it, and tried to assemble them into a skeleton.

Contrary to popular belief, bones do not require just calcium to grow. They also need vitamin K, which can be found in animal-based foods, and vitamin D, which the skin can make when exposed to sunlight. Vitamin D is especially important, since it is needed to absorb calcium so that the calcium can be used to make bones stronger. Severe vitamin D deficiency can lead to rickets, the softening of bones to the point where they may become crooked. Lack of calcium can exacerbate osteoporosis, where the bones become more porous and brittle.

Bone fractures can be classified into many different types. A greenstick, or incomplete, fracture occurs when the bone does not break all the way through. A fissured, or hairline, fracture is a incomplete fracture along the axis of the bone. A comminuted fracture is when the bone breaks into three or more pieces. A transverse fracture crosses a bone perpendicular to its axis, an oblique fracture crosses a bone at an angle, and a spiral fracture occurs when a large twisting force breaks the bone. Simple, or closed, fractures are those that do not puncture the skin, whereas compound, or open, fractures are when the end(s) of the fractured bone(s) break the skin.

After a bone breaks, bone cells called osteoclasts, which build up new bone, and osteoblasts, which break down old bone can repair the bone. Osteoclasts and osteoblasts are also used in bone remodeling, such as when a bone is growing.

 

bone remodeling

Bones attach together at joints, which can be classified by function (syn-, amphi-, and di-arthroses) or by structure (fibrous, cartilagenous, and synovial). Synarthroses are immoveable, amphiarthroses are slightly moveable, and diarthroses are freely moveable. When most people think about joints, like the elbow or the knee, they are thinking about synovial joints, which are diarthrotic. Sutures in the skull are an example of a fibrous synarthrotic joint.

I learned a lot of new things this unit, but there are still a few things I wonder about. For example, if the bone marrow in the medullary cavity is just fat, then is the bone marrow fancy restaurants serve really just fat? What happens to the bone marrow of starving people? Does the body utilize the energy stored in the fat there, and what does this mean for the bones? I would also be interested in learning more in detail about the other types of joints, not just synovial joints.

So far, I have been reaching my New Year's goals of getting eight hours of sleep everyday and not procrastinating on studying for my tests. I hope to continue to keep fulfilling these goals throughout the rest of the year.

2016.02.22 Owl Pellet Lab

Today we dissected an owl pellet to study the bones inside. Using a probe and forceps, we broke apart an owl pellet, separated the bones from the fur, categorized them, then tried to identify the organism that was in the pellet.

our owl pellet

After studying the bones in our owl pellet, we concluded that the organism in our pellet was a shrew. We determined by comparing our bones with an identification chart. In particular, we used the skull, mandible, and tibia and fibula to determine that our organism was a shrew. Our organism's skull and mandible resembled the skull and mandible of the shrew in the diagram; furthermore, the skull was 14 mm long and the mandible was 10 mm long, the average lengths of a shrew's skull and mandible. Also, the fused-together tibia and fibula (bottom-right-most bone in the picture on the right below) resembled the tibia and fibula of the shrew in the identification chart.

some bones from the pellet

bones sorted in a chart

Compared to a human skeleton, our shrew skeleton had the same basic parts: skull, vertebral column, ribs, and limbs. These bones all fit in pretty much the same way, and they all perform the same function a human's skeleton does. Some of the bones, like the skull, are made of several fused-together parts, just like a human's.

However, compared to a human skeleton, a shrew skeleton has many differences. For example, the skull and mandible of a shrew are shaped very differently from the skull and mandible of a human. Also, a shrew's pelvis is shaped very differently from a human's (it is longer), since a shrew crawls around on all four feet while a human walks upright. A shrew's tibia and fibula are fused together, where as a human's tibia and fibula are separate bones.