The skeletal system is made up of bones and cartilages that provide structure and support to the body. Bones are living tissues composed of cells, collagen fibers, and mineral salts. The skeletal system provides structure, protects organs, allows body movement, stores minerals, and produces blood cells. Bones form through two processes - intramembranous and endochondral ossification - and are constantly remodeled through the actions of osteoblasts and osteoclasts. Fractures occur when bones break but then heal through several phases.

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The Skeletal System: Bone Tissue
Principles of Anatomy and Physiology

2. Bones are complex organs that contain many elements
including blood vessels, nerves, cartiladge and specialized
bone cells.
The entire framework of the body is made up of bones and
their cartilages, which constitute the skeletal system.
Bone
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3. Provides support
Protects the internal organs (brain, heart, etc.)
Assists body movements
(in conjunction with muscles)
Provides mineral homeostasis stores and releases
calcium and phosphorus
Participates in blood cell production (hemopoiesis)
Functions of the Skeletal System
Stores triglycerides in adipose cells
of red marrow
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4. Structure of Bone
Metaphysis
Diaphysis
Metaphysis
Distal
epiphysis
Proximal
epiphysis
Articular cartilage
Articular cartilage
Periosteum
Medullary cavity
Endosteum
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5. Histology of Bone Vocabulary
Prefix Suffix
• Blast = precursor
(secretes matrix)
• Cyte = mature cell
• Osteo = bone
• Chondro = cartilage
• Peri = peripheral
• Endo = inner lining
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6. Bone contains an abundant extracellular
matrix that surrounds widely separated cells.
Histology of Bone
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7. Histology of Bone
15%
55%
Crystallized
mineral salts
Water
• Calcium phosphate
• Calcium hydroxide
• Hydroxyapatite
30%
Collagen
Extracellular matrix
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8. 4 Types of Bone Cells
Osteocytes
(mature bone cells)
Osteoblasts
(bone-building cells
that secrete matrix)
Osteoclasts
(remodel bones
and cause them
to release calcium)
Osteoprogenitor cells
(bone stem cells able
to differentiate into
other types of cells)
Derived from Derived from
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9. Histology of Bone
Trabecular (spongy)
bone is lightweight
and provides tissue
support.
Cortical (compact) bone is good at
providing protection and support.
Medullary cavity
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10. Blood Supply of Bone
Epiphyseal
artery/vein
Metaphyseal
artery/vein
Nutrient veins exit
via the same canal.
A nutrient artery enters the
center of the diaphysis
through a
nutrient foramen
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11. Ossification (osteogenesis) is the process of
bone formation.
Bone Formation
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12. Bone Formation
Embryological and fetal
development
Bone growth
Fracture healing
Bone remodeling
Bones form in 4 situations:
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13. Ossification
Intramembranous
Takes place in 2 forms:
Endochondral
Occurs in flat bones
(e.g., skull)
Takes place in most other bones
(e.g., long bones)
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14. Bone Formation
1 2
4
3
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15. Bone Formation
Osteoid
Mesenchymal
cells
Ossification
center
Osteoid
1 2
Osteoblast
Osteoblast
New bone
matrix
Osteocyte
Collagen fibers
Development of
ossification center
Calcification
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16. Bone Formation
3 4
Fibrous
periosteum
Osteoblast
Compact
bone
Spongy
bone
Mesenchymal
cells
Trabeculae
Blood vessel
Development of the
periosteum
Formation of trabeculae
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17. Endochondral ossification replaces
cartilage with bone in the developing
embryo and fetus.
Bone Formation
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18. Bone Formation
Hyaline
cartilage
Calcified
matrix
Uncalcified
matrix
Primary
ossification
center
Nutrient
artery
Spongy
bone
Perichondrium
1. Development of cartilage
model
2. Growth of cartilage model 3. Development of primary
ossification center
Mesenchymal cells develop
into chondroblasts
Chondroblasts secrete
extracellular matrix and
mature into chrondrocytes
Chondrocytes are replaced
with osteoblasts
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19. Bone Formation
5. Development of secondary
ossification centers
6. Formation of articular cartilage
and epiphyseal plate
Osteoclasts break down the
center of the bone to form the
cavity
Introduction of an artery
into the epiphysis
Spongy bone forms in the
epiphysis
Uncalcified
matrix
Medullary
cavity
Periosteum
Calcified
matrix Uncalcified
matrix
Secondary
ossification
center
Articular
cartilage
Spongy
bone
Epiphyseal
plate
Artery/
vein
Artery/vein
4. Development of medullary
(marrow) cavity
Artery/vein
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20. Endochondral ossification also occurs in epiphyseal plates of long bones as they grow in length.
Bone Formation
Resting cartilage
Proliferating cartilage
Hypertrophic cartilage
Calcified cartilage
Epiphyseal
growth plate:
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21. Bone Formation
<<<<<
Growth plate
zones
Changes in
chondrocytes
Matrix
production
Resting zone
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22. Bone Formation
<<<<<
Mitosis
Proliferative
zone
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23. Bone Formation
<<<<<
Lipids, glycogen,
and alkaline
phosphatase
accumulate; matrix
calcifies
Maturation
and
hypertrophy
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24. Bone Formation
<<<<<
Cell
death
Calcified
matrix
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25. Bones thicken thanks to the cooperative action of osteoblasts and osteoclasts.
Bone Formation
1 2 3 4
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26. As osteoblasts deposit bone on the outer surface,
osteoclasts widen the medullary cavity from within.
Bone Formation
Infant Child Young
adult
Adult
Bone formed
by osteoblasts
Bone destroyed
by osteoclasts
Medullary
cavity
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27. Response to Stress in Adult Bones
Bone remodeling redistributes bone tissue
along lines of stress:
• Under slight stress, bone tissues will be
resorbed.
• Under heavy stress, bone mass increases.
Femur of an
inactive person
Femur of an
athlete
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28. Bone Growth and Remodeling Factors
Androgens (e.g., testosterone)
2 main hormones that control bone growth:
Estrogens
Lengthen the bone Stop bone modeling
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29. A fracture is a break in a bone. Many different
types of fractures may occur.
Fracture
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30. The healing process involves 3 different phases in 4 steps:
Fracture and Repair of Bone
Healed fracture
1. Reactive phase 2. Reparative phase 3. Bone remodeling phase
Early inflammatory phase 1. Formation of a
fibrocartilaginous callus
2. Bony callus
Last step as the bony callus
is remodeled
Bony callus of spongy bone
Hematoma
New blood vessels
External
callus
Spongy
bone
trabecula
Internal
callus
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31. Common Fractures Open vs. Closed
Open
Broken ends of bone protrude through skin
Closed
Broken ends do not break skin
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32. Common Fractures Comminuted
• Bone splintered, crushed, or
broken into pieces at the site of
impact
• Smaller bone fragments lie
between 2 main fragments
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33. Common Fractures Greenstick
• Partial fracture
• Only 1 side of the bone is broken;
the other is bent
• Occurs only in children, whose
bones are not fully ossified
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34. Common Fractures Impacted
One end of a fractured bone is
forcefully driven into the interior
of the other.
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35. Common Fractures Pott
• Fracture of the distal end of
the lateral leg bone (fibula)
• Serious injury of the distal
tibial articulation
fracture
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36. Common Fractures Colles
• Fracture of the distal end of the
lateral forearm bone (radius)
• Distal fragment displaced
posteriorly
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37. Role in Calcium Homeostasis
Parathyroid glands (located on the back of the thyroid gland)
Intestine
Bone
Kidney
Blood vessel
Ca2+
Ca2+
Ca2+
PTH*
Vitamin D
PTH*
+
Low Ca2+
levels
stimulate the
parathyroid
glands
-
High Ca2+
levels inhibit
the para-
thyroid
glands
*PTH: parathyroid hormone
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38. In a Nutshell
✓ The skeletal system includes all of the
bones of the body.
✓ The functions of the skeletal system are to:
provide support, protect internal organs,
assist in body movements, help establish
mineral homeostasis, produce red blood
cells, and store triglycerides.
✓ Bone tissue contains 4 cell types:
osteocytes, osteoblasts, osteoclast, and
osteoprogenitor cells.
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39. In a Nutshell
✓ There are 2 types of bone formation:
intramembranous and endochondral
ossification.
✓ Homeostatic imbalance in the skeletal
system can include bone fracture and
certain diseases, such as osteoporosis.
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