Bone Intro 2024

The Skeletal System

The skeletal system is primarily responsible for supporting the body and protecting vital organs.  We are born with more than 270 bones that eventually fuse together as we grow, leaving adult humans with 206 bones.  Bones are made up of a complex arrangement of inorganic minerals and a variety of tissues including bone, bone marrow, nerves, blood vessels, endothelial, and cartilage.  They come in a variety of shapes and sizes depending on their location and function, but all bones are lightweight, strong, and hard.

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The Skeletal System - Background

Multiple Choice

How many bones are in the human body?

• 206
• 1000
• 500
• 100

Multiple Choice

 Bones are made up of a complex arrangement of inorganic minerals and a variety of tissues including bone, bone marrow, nerves, blood vessels, endothelial, and what?

• ligaments
• muscles
• tendons
• cartilage

 Bone has a variety of functions that include:

• Protection of organs (skull protects the brain, ribs protect the heart, etc.)

• Support and framework for the human body

• Movement by providing attachment points for muscles

• pH balance of the blood by absorbing or releasing bone minerals

• Hematopoiesis (blood production) in blood marrow

• Fat storage in yellow bone marrow

• Sound transduction through small bones located in the ear canal

• Storage of growth factor in bone matrix

• Removal of heavy metals or foreign chemicals to detoxify the blood and release slowly for excretion

• Mineral storage of calcium and phosphorous

• Production of hormones such as osteocalcin

Match

Match the following bone function component

• Protection

• Support

• Movement

• Hematopoiesis

• Storage

Skull protects the brain, ribs protect t

• Make up framework for the human body

• Providing attachment points for 

muscles

Make blood cells

Holds marrows, calcium, and phosphorus

• Skull protects the brain, ribs protect t

• • Make up framework for the human body

• • Providing attachment points for 

  muscles

  
  

• Make blood cells

• Holds marrows, calcium, and phosphorus

 Bone Structure

Bone tissue is created from several minerals, most notably calcium and phosphorus, that form carbonated hydroxyapatite with the chemical formula Ca10(PO4)6(OH)2. 
Bone mineral is created by osteoblasts and allows bones to withstand large amounts of compressional force. The other major component of the bone matrix is organic collagen, which is a protein that gives bone the ability to withstand stretching forces.
The major cells that contribute to building and breaking down bone structure are osteoblasts, osteocytes, and osteoclasts.  Osteoblasts are responsible for creating a bone matrix and, therefore, building bone.  Once osteoblasts have become trapped in the bone matrix that they have created, they become osteocytes. Osteocytes function to maintain the bone matrix and calcium homeostasis. They are unable to move from their assigned location or space which is called the lacunae.  Osteoclasts are large cells that are capable of reabsorbing bone minerals and, therefore, remodeling bone structure. Osteoclasts also remove minerals from the bloodstream for a variety of bodily functions, such as muscle contraction.

 Bone Structure

The bone matrix can be arranged into two classifications of bone; compact & trabecular bone. 
Compact bone, also known as dense or cortical bone, is extremely hard and compact with very little space.  Bone mineral in compact bone is arranged into tight circles called osteons, with nerves and blood vessels passing through the center.  Compact bone accounts for 80% of the total bone mass.

Trabecular bone, also known as spongy or cancellous bone, is porous and more like a network that allows nerves, blood vessels, and bone marrow to easily fill trabecular bone.  Stress on the trabecular bone causes it to create new and stronger networks, making it extremely adaptable.  Although trabecular bone only accounts for 20% of the total bone mass, it has a much greater surface area than compact bone.  

 Bone Types

 There are five main types of bone based on their shape.  These include long bones, short bones, irregular bones, sesamoid bones, and flat bones.  The following table provides examples of these bone types.

 Bone Types

Long Bones
Bones are longer than they are wide and are made up primarily of compact bone.  Examples include arm bones, leg bones, and phalanges.

 Bone Types

Short Bones
Cube-shaped with a thin layer of compact bone.  Examples include wrist and ankle bones.

 Bone Types

Flat Bones
Thin and curved with parallel layers of compact bone.  Examples include the sternum and bones of the skull.

 Bone Types

Sesamoid Bones
Bones embedded in tendons.  Examples include the patella and pisiform.

 Bone Types

Irregular Bones
Bones that do not fit in any of the other categories.  Examples include the vertebra and bones of the sinus

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Bone Tissue Structure

​Bone is made up of 65% inorganic minerals, primarily calcium and phosphorus, and 35% organic tissues.  While bone is considered a connective tissue, it is actually composed of several different types of tissues.  They include bone, nerves, blood, cartilage, and epithelial tissues.  Because bone is made up of these different types of tissues, it is technically considered an organ. There are two types of bone structures in humans - the inner spongy bone and the outer compact bone.  

Spongy bone is made up of a dense spider web-like netting of bone called trabeculae.  The spaces between the trabeculae webbing are filled with bone marrow.  Bone marrow functions in hematopoiesis, and, in a healthy adult, produces more than 500 billion blood cells a day.  Platelets, red blood cells, and white blood cells are all produced and stored within the red bone marrow.  Yellow bone marrow is primarily made up of fat and found in the diaphysis (middle) of long bones. Bone marrow makes up 4% of the total body mass.

Externally, bones have a protective tissue layer called the periosteum.  The periosteum contains cells that resist force and that also respond to external stress to initiate bone remodeling.  Fibers called Sharpey’s fibers to connect the periosteum to the outer compact bone. The trabeculae of spongy bone also have a thin protective membrane called the endosteum.  

The Cells of Bone

Bone Remodeling

Bone remodeling occurs throughout the human lifetime and is the process by which minerals and tissues are constantly added / removed from the bone. The formation of new bone is called ossification, and the removal of bone is called resorption. Bone remodeling is important in the regular growth and development of skeletal structure, but it is also necessary following injuries such as fractures. Mechanical demand on bones can also increase or decrease the rate of bone remodeling on specific bones.  When the remodeling procedure becomes imbalanced, bone diseases such as Paget’s disease or osteoporosis can result.