Anatomy & Physiology
Chapter 7: Skeletal System
1
Introduction to Skeletal System
• Human skeleton is initially cartilage and fibrous membranes
• By age 25 the skeleton is completely hardened
• 206 bones make up the adult skeleton (20% of body mass)
• 80 bones of the axial skeleton
• 126 bones of the appendicular skeleton
•The organs of the skeletal system include the bones and
structures that connect bones to other structures
including ligaments, tendons, and cartilages.
2
Bone Classification
 Bone Classification:
• Long bones
ex. femur
• Short bones
ex. tarsals
•Flat bones
ex. skull
• Irregular bones
ex. vertebrae
• Sesamoid bones
ex. patella
(b)
(c)
(d)
(a)
(e)
3
Parts of a Long Bone
• Epiphysis
• Distal
• Proximal
• Diaphysis
• Metaphysis
• Compact bone
• Spongy bone
• Articular cartilage
• Periosteum
• Endosteum
• Medullary cavity
• Trabeculae
• Bone marrow
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Epiphyseal plates
Articular cartilage
Proximal
epiphysis
Spongy bone
Space containing
red marrow
Endosteum
Compact bone
Medullary cavity
Yellow marrow
Diaphysis
Periosteum
Distal
epiphysis
• Red marrow and yellow marrow
Femur
4
Parts of a Long Bone
Diaphysis = shaft
a. consists of central medullary cavity
b. surrounded by a thick collar of compact bone
Epiphyses = expanded ends
a. consist mainly of spongy bone
b. surrounded by a thin layer of compact bone
c. proximal epiphysis vs. distal epiphysis
Epiphyseal line = remnant of epiphyseal disc/plate
a. cartilage at the junction of the diaphysis and
epiphyses (growth plate)
5
Parts of a Long Bone
Periosteum = outer protective covering of diaphysis
a. supplied w/ blood, lymph vessels & nerves (nutrition)
b. osteogenic layer contains osteoblasts (bone-forming cells) and
osteoclasts (bone-destroying cells)
c. serves as insertion for tendons and ligaments
Endosteum = inner lining of medullary cavity
a. contains layer of osteoblasts/osteoclasts
Articular cartilage = pad of hyaline cartilage on the epiphyses
where long bones articulate or join
a. “shock absorber”
6
Parts of a Flat Bone
Flat bones
1. covered by periosteum – covered compact bone
2. surrounding endosteum – covered spongy bone
3. In a flat bone the arrangement looks like a sandwich:
a. spongy bone (meat) sandwiched between
b. two layers of compact bone (bread)
 Hematopoetic tissue (red marrow) is located in the spongy bone
within the epiphyses of long bones and flat bones
7
Microscopic Structure:
Chemical Composition of Bone
 Organic components (approx. 35%)
Cells:
osteoprogenitor cells
1. can undergo mitosis and become osteoblasts
osteoblasts
1. form bone matrix by secreting collagen
2. cannot undergo mitosis
osteocytes
1. mature bone cells derived from osteoblasts
2. principle bone cell
3. cannot undergo mitosis
4. maintain daily cellular activities
(ie. exchange of nutrients & wastes with blood)
8
Microscopic Structure:
Chemical Composition of Bone
 Organic components…cont.
Cells:
Osteoid
1. primarily collagen (90% of bone protein) which
gives bone its high tensile strength
2. other bone proteins include osteocalcin,
osteonectin, and osteopontin
3. also contains glycolipids and glycoproteins
 Inorganic components
Hydroxyapatite (mineral salts) which is primarily
a. calcium phosphate [Ca3(PO4)2(OH)2]
b. gives bone its hardness or rigidity
9
Microscopic Structure:
Compact Bone
 Compact bone is solid, dense, and smooth
Structural unit = Haversian system or osteon
a. elongated cylinders cemented together to form
the long axis of a bone
b. components of Haversian system
osteocytes (spider shaped bone cells in
“lacunaea”
that have laid down a…
matrix of collagen and calcium salts in…
concentric lamellae (layers) around a…
central Haversian canal containing…
blood vessels and nerves.
CONTINUED NEXT SLIDE
10
Microscopic Structure: Compact Bone
c. Communicating canals with compact bone
-canaliculi connect the lacunae of osteocytes
-Perforating (Volkmann’s) canal connect the
blood & nerve supply of adjacent Haversian
systems together.
11
Compact Bone
• Osteon
• Haversian System
• Central canal
• Perforating canal
• Volkmann’s canal
• Osteocytes
• Lamellae
• Lacunae
• Bone matrix
• Canaliculi
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Osteon
Central canal
containing blood
vessels and nerves
Endosteum
Periosteum
Nerve
Blood
vessels
Pores
Central
canal
Perforating
canal
Compact
bone
Nerve
Blood
vessels
Nerve
Trabeculae
Bone matrix
Canaliculus
Osteocyte
Lacuna
(space)
12
Microscopic Structure: Spongy Bone
 Consists of poorly organized trabeculae
( small needle-like pieces of bone)
with a lot of open space between them
nourished by diffusion from nearby Haversian canals
13
Spongy Bone
• Spongy bone is aka cancellous bone
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Spongy
bone
Compact
bone
(a)
(b)
(c)
Spongy
bone
Compact
bone
a: © Ed Reschke; b,c: Courtesy of John W. Hole, Jr.
Spongy bone Compact bone
Remnant of
epiphyseal plate
14
Bone Development
and Growth
 Introduction
The “skeleton” of an embryo is composed of fibrous CT
membranes ( formed from mesenchyme and hyaline cart) that is
loosely shaped like bone.
This skeleton provides supporting structures for
ossification to begin.
At about 6-7 wks gestation ossification begins and
continues throughout adulthood.
15
Bone Development
and Growth
 Ossification follows one of two patterns
1. Intermembranous Ossification
When bone forms on or within a fibrous CT membrane
ex. Flat bones are formed in this manner
2. Endochondral ossification
Occurs when a bone is formed from a hyaline cartilage model.
a. most bones of the skeleton are formed this way
b. Primary ossification center hardens as fetus or infant
c. Secondary ossification centers develop in child and harden during
adolescence and early adulthood
During infancy and childhood long bones lengthen entirely by growth at the
epiphyseal plates (longitudinal growth)
Bones grow thicker by “appositional growth”
16
Endochondral Ossification
• Hyaline cartilage model
• Primary ossification center
• Secondary ossification centers
Cartilaginous
model
Developing
periosteum
• Epiphyseal plate
• Osteoblasts vs. osteoclasts
Remnants of
epiphyseal
plates
Secondary
ossification
center
Compact bone
developing
Spongy
bone
Epiphyseal
plates
Blood
vessel
Calcified
cartilage
(a)
(b)
Medullary
cavity
(c)
Medullary
cavity
Compact
bone
Medullary
cavity
Remnant of
epiphyseal
plate
Epiphyseal
plate
Primary
ossification
center
Secondary
ossification
center
(d)
Articular
cartilage
Spongy
bone
Articular
cartilage
(e)
(f)
17
Growth at the Epiphyseal Plate
 The epiphyseal plate allows for bone lengthening until adulthood.
As a child grows…
a. cartilage cells are produced by mitosis on the epiphyseal side of the plate
b. they are then destroyed and replaced by bone on the diaphyseal side of the
plate
*therefore the thickness of the plate remains almost constant while
the bone on the diaphyseal side increases in length.
c. the cartilage of the epiphyseal plate is replaced by bone forming the
epiphyseal line.
d. ossification of most bones is completed by age 25.
18
Bone Thickening:
Appositional Growth
 Along with increasing in length bones increase in thickness or dia.
1. occurs in osteogenic layer of periosteum
2. Osteoblasts lay down matrix (compact bone) on outer surface
3. This is accompanied by osteoclasts destroying the bone matrix
at the endosteal surface.
19
Homeostasis of Bone Tissue
 Once bones are formed, the actions of osteoclasts and osteoblasts
continually remodel them
Bone remodeling occurs throughout life
a. osteoclasts resorb bone
b. osteoblasts replace the bone
c. these opposing processes are highly regulated so that total
mass of bone tissue in adult skeleton normally remains constant
even though 3-5% of bone calcium is exchanged each year.
20
Homeostasis of Bone Tissue
• Bone Resorption – action of osteoclasts via stimulation from
parathyroid hormone (PTH)
• Bone Deposition – action of osteoblasts and via stimulation
from calcitonin
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Developing
medullary
cavity
Osteoclast
21
© Biophoto Associates/Photo Researchers, Inc.
Bone Function
Support
a. Bones in legs and pelvis support trunk
b. The atlas (1st vertebra) supports the skull etc

Protection of underlying organs
a. The skull protects the brain
b. The rib cage protects the heart and lungs etc.

Body Movement
a. Skeletal muscles attached to bones by tendons
b. Serve as levers to move bones.

Hematopoiesis
Blood cell formation
a. All blood cells formed in the red marrow of certain bones
22
Divisions of the Skeleton
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Axial Skeleton
• Skull
• Spine
• Rib cage
Cranium
Skull
Face
Hyoid
Clavicle
Scapula
Sternum
Humerus
Ribs
Vertebral
column
• Appendicular Skeleton
• Upper limbs
• Lower limbs
• Shoulder girdle
• Pelvic girdle
Vertebral
column
Hip
bone
Carpals
Sacrum
Radius
Coccyx
Ulna
Femur
Metacarpals
Phalanges
Patella
Tibia
Fibula
Tarsals
Metatarsals
Phalanges
(a)
(b)
23
7.6: Skull
• Is composed of the cranium (brain case) and the
facial bones
24
7.7: Vertebral Column
• The vertebral column, or spinal column, consists of
many vertebrae separated by cartilaginous intervertebral
discs.
25