The Skeletal System: Introduction to the Skeletal System and Skeletal Tissues

1. Introduction the Skeletal System
a. Organs
i. Bones
1. Each bone is an individual organ.
ii. Joints
1. Each joint is an individual organ.
iii. Cartilages
1. Each cartilage is an individual organ.
b. Basic Functions
i. Structural support
1. The skeleton is a frame that other soft tissues are attached to and use for
structural support.
ii. Movement
1. Bones and muscles work together to produce skeletal movement.
iii. Protection
1. Skull- protects brain
2. Vertebrae- protects spinal cord
3. Ribcage- protects thoracic organs
4. Pelvis- protects urinary and reproductive organs
iv. Storage
1. Energy storage
a. Yellow marrow is stored fat
2. Mineral storage
a. Calcium (Ca2+ ), Magnesium (Mg 2+ )
v. Blood cell formation
1. All blood cells (red blood cells and white blood cells) are produced in red bone
marrow.
2. Cartilage
a. Cartilage tissue
i. All types of cartilage have a solid matrix containing:
1. A ground substance containing chondroitin sulfate which produces a solid gel
despite being 60% to 80% water.
a. The high percentages of water allow all cartilages to do well at resisting
compression.
2. Protein fibers like collagen fibers
ii. Contains cartilage cells called chondrocytes in tiny spaces called lacunae.
b. The Three Types of Cartilage
i. Hyaline Cartilage
1. Most abundant cartilage type.
2. Contains collagen fibers only.
3. Rigid supporting cartilage that is good at resisting compression

4. Locations
a. Costal cartilages, larynx, airways, articular cartilages, nose
ii. Elastic Cartilage
1. Contains collage fibers and lots of visible elastic fibers
2. Flexible cartilage that allows for elastic recoil
a. Elastic recoil- the tissue can be bent or stretched and return to its original
shape.
3. Locations
a. Cartilage in the external ear
b. Epiglottis in the larynx
iii. Fibrocartilage
1. Contains lots of collagen fibers.
2. The best at resisting compression, also good at resisting pulling forces.
3. Locations
a. Intervertebral discs
b. Pubic symphysis
c. Menisci in the knee joint
3. Bone
a. Compact Bone vs. Spongy Bone
i. Compact bone- solid dense bone tissue.
1. Complex microscopic structure, contains structures called osteons.
2. Very strong but also very heavy.
3. Makes up outer layer of all bones.
ii. Spongy bone- porous bone tissue.
1. Made of lots of interconnected columns of bone tissue called trabeculae with lots
of spaces between them.
2. Simple microscopic structure, does not contain osteons.
3. Strong and light.
4. Makes up inner layer of all bones.
5. The spaces contain red bone marrow
b. Bone Connective Tissues
i. Just like all other organs bones are surrounded and protected by connective tissue
layers. There are two bone connective tissues:
1. The Periosteum- continuous, sheet-like connective tissue layer surrounding the
outer surface of the bone.
a. Covers entire surface except the articular cartilages at the tips of the bone.
b. Contains two layers:
i. Outer fibrous layer- Dense irregular CT surrounding the entire bone
ii. Inner cellular layer- Contains fibroblasts and bone remodeling cells
and bone stem cells.
c. Contains blood vessels and nerves
2. The Endosteum- non-continuous connective tissue layer on the inner surface of
the bone.

a. Non-continuous = not sheet-like, this layer is in patches throughout the
inside of the bone.
i. Must be like this because it covers spongy bone tissue.
b. Contains bone remodeling cells (osteoblasts and osteoclasts) and bone
stem cells.
c. Bone Tissue Structure
i. Bone tissue (simplified) = Bone cells + Matrix (hydroxyapatite + collagen fibers)
ii. Bone tissue Matrix
1. Inorganic components
a. 65% of bone matrix by mass
b. Mineral salts in the ground substance
i. Primarily calcium and phosphate containing crystals called
hydroxyapatite crystals that make the matrix solid.
1. Bone starts off as a liquid called osteoid, this liquid solidifies
when the calcium is added to form the hydroxyapatite
crystals in the matrix.
2. Makes bone rigid and resists compression.
3. The hydroxyapatite crystals can be dissolved by acids.
2. Organic components
a. 35% of bone matrix by mass
b. Bone cells, fibers, and organic components of ground substance
c. Bone matrix contains lots of collagen fibers suspended in the solid matrix
in arrangements that increase the tensile strength of the bone.
i. The collagen fibers allow bone tissue to resist pulling/stretching
ii. The collagen fibers can be destroyed by exposing them to very high
temperatures.
iii. Bone Cell Types
1. Osteogenic/Osteoprogenitor Cells
a. Bone stem cells that divide by mitosis to produce a new osteoprogenitor
cell and a new osteoblasts.
i. Responsible for producing new osteoblasts and osteocytes =
responsible for bone growth.
b. In periosteum and endosteum. Most active in the periosteum.
2. Osteoblasts
a. Bone cells that produce and secrete the liquid pre-cursor to bone matrix
called osteoid.
i. Osteoblasts secrete osteoid all around themselves, when calcium is
added it forms the layers of solid bone matrix around them. These
osteoblasts are now trapped in a lacuna in the newly formed layers of
bone matrix. The trapped osteoblasts differentiate into osteocytes.
1. The layers of bone matrix are called lamellae.
b. In periosteum and endosteum, but most active in the periosteum.
3. Osteocytes
a. Bone cells in lacunae.

b. Maintain the solid bone matrix and keep it healthy.
4. Osteoclasts
a. Bone cells that dissolve old or damaged bone matrix by secreting
hydrochloric acid (HCl).
b. Found in periosteum and endosteum but most active in the endosteum.
iv. Microscopic Structure of Bone Tissue
1. Microscopic Structure of Compact Bone- See slide 9
a. The outermost layers of compact bone are formed by cylinders of bone
matrix that wrap around the entire bone called circumferential lamellae.
i. The osteocytes in lacunae are between the layers of circumferential
lamellae.
b. Deeper layers of compact bone consist of many cylindrical structures
called osteons.
i. Structure of an osteon
1. Each osteon has a hollow cylinder in the center called the
central canal.
a. Each central canal is lined by endosteum and contains
blood vessels and nerves that support the living bone
tissue.
2. The central canal is surrounded by many concentric layers of
bone matrix called concentric lamellae.
a. The osteocytes in lacunae are between the layers of
concentric lamellae.
3. The gaps between the cylindrical osteons are filled with
layers of bone matrix called interstitial lamellae.
a. The osteocytes in lacunae are between the layers of
interstitial lamellae.
4. Canaliculi
a. Tiny canals (tubes) through all three types of lamellae.
b. Contain extensions of osteocytes where they connect
to each other via small membrane channels called gap
junctions. This allows all osteocytes to share
cytoplasm so the osteocytes near the blood vessels can
share nutrients with osteocytes far away from the
blood vessels.
5. Perforating/Volksmann Canals
a. Canals perpendicular to the osteons.
b. Connect central canals to each other, to the cavity in
the center of the bone and to the periosteum.
c. Allow blood vessels and nerves to branch into central
canals.
2. Spongy Bone
a. The microscopic structure of spongy bone is very simple:

i. Several layers of lamellae and osteocytes in lacunae form each
trabecula.
ii. The trabeculae are too small to contain osteons or vessels.