Anatomy
Chapter 7, Part I – Skeletal System
Lecture Notes
I. Skeletal System
A. Consists of
1. Bones – 206 in average adult
2. Associated Connective Tissue
a. cartilage
b. tendons
c. ligaments
II. Bone Structure – differ greatly in size & shape, yet are similar in structure,
development, & function
A. Long bone structure
1. epiphysis – expanded portion at each end of bone which articulate with
another bone
a. articular cartilage - a layer of hyaline cartilage which coats the
outer portion of articulating surface
2. diaphysis – shaft of the bone located between the epiphyses
3. periosteum – tough, vascular covering of fibrous tissue surrounding bones
except for articular cartilage at ends of bone
a. firmly attached to bone
b. attachment site for muscles via tendons
c. periosteal fibers are continuous with ligaments & tendons that
connect to the membrane
d. helps form & repair bone tissue
4. compact bone (cortical bone) – tightly packed tissue
a. diaphyses are largely made of compact bone
b. has a continuous matrix with no gaps
5. spongy bone (cancellous bone) – consists of numerous branching bony
plates
a. spongy bone make up epiphyses
b. covered with thin layer of compact bone on the surface
c. irregular, connecting spaces between plates help reduce bone’s
weight
d. bony plates are most highly developed in epiphyses that are
subjected to compressive forces
6. medullary cavity – hollow chamber within in diaphysis and small spaces
of epiphyses formed by compact bone arranged in a semirigid tube &
small chambers in spongy bone
a. tube is continuous with spaces of the spongy bone
b. marrow – specialized type of soft connective tissue in medullary
cavities
7. endosteum – thin layer of cells that lines medullary cavity
B. Microscopic Structure
1. osteocytes – mature bone cells
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2. lacunae – small, bony chambers which form concentric circles around
osteonic canal
3. lamellae –mineralized matrix arranged in concentric circles around central
canal
4. osteonic canal (Haversian canal) – tube through bony tissue which hold
blood vessels and nerves
a. extend longitudinally through bone
b. perforating canals (Volkmann’s canals) – horizontal canals that
connect osteonic canals
1) contain larger blood vessels and nerves by which smaller
blood vessels and nerve fibers in osteonic canals
communicate with surface of bone & medullary cavity
5. canaliculi – tunnels between osteocytes though which cell processes
extend
a. allow osteocytes to communicate with each other
6. Compact bone – composed of osteons (Haversian systems) – cylindershaped units
a. consists of osteocytes and layers of intercellular material
concentrically clustered around an osteonic canal
b. many of these units cemented together form substance of
compact bone
7. Spongy bone - composed of trabeculae-osteocytes & intercellular
material
a. osteocytes are not arranged around osteonic canals
b. substances diffuse into canaliculi to nourish bone cells
III. Bone development and growth
A. ossification – the process of changing cartilage into bone
B. formation of bone begins during first few weeks of prenatal development
C. continue to develop and grow into adulthood
D. form by replacing existing connective tissues in either of two ways:
1. intramembranous ossification – broad, flat bones of skull
a. membranelike layers of connective tissues appear at the sites of
future bones
b. some of connective tissue cells enlarge & differentiate into boneforming cells called osteoblasts
c. ostoeblasts become active within membranes and deposit bone
matrix around themselves
d. results in spongy bone tissue forming in all directions within layers
of connective tissues
e. membranous tissue outside the developing bone become the
periosteum
f. osteoblasts on inside of periosteum form a layer of compact bone
over newly formed spongy bone
g. when matrix completely surrounds osteoblasts, they are called
osteocytes
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2. endochondral bones – most bones of the skeleton
a. develop from masses of hyaline cartilage shaped like future bony
structure
b. cartilage cells grow rapidly and then begin to change
1) in long bones, change begins in center of diaphysis
2) cartilage breaks down and disappears
c. at same time, periosteum forms from connective tissue that
encircles the developing diaphysis
d. blood vessels & osteoblasts from periosteum invade disintegrating
cartilage & spongy bones forms in its place
e. primary ossification center – region of original bone formation in
center of diaphysis
f. osteoblasts from periosteum deposit thin layer of compact bone
around primary ossification center
g. epiphyses of bone remain cartilaginous & continue to grow
h. secondary ossification centers – growth areas that appear later
within epiphyses
i. spongy bone forms in secondary ossification centers and branches
in all directions
j. epiphyseal disk - band of cartilage between two ossification
centers
1) cells (chondrocytes) undergo mitosis & produce new cells
2) new cells enlarge & matrix forms around them causing
the disk to thicken which lengthens the bone
3) calcium salts accumulate in matrix adjacent to oldest
cartilaginous cells
4) as matrix calcifies, cells begin to die
5) osteoclasts – large, multinucleated cells that break down
the calcified matrix
a) originate in bone marrow when certain white blood
cells fuse
b) secrete an acid that dissolves the inorganic
component of calcified matrix
c) their lysosomal enzymes digest the organic
components
d) cleans the matrix
6) osteoblasts invade region & deposit bone tissue in place
of calcified cartilage
7) osteocytes – when osteoblasts are surrounded by matrix
k. long bone continues to lengthen while cartilage in epiphyseal disks
are active
l. bone growth in length is complete when ossificaion centers meet
and epiphyseal disks ossify
m. thickening of bones
1) compact bone is deposited on outside beneath periosteum
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2) as compact bone forms on surface, osteoclasts erode bone
tissue on the inside
3) space created by osteoclasts becomes medullary cavity
n. bone in center of epiphyses & diaphysis remain spongy & hyaline
cartilage on ends of epiphyses remain as articular cartilage
C. Homeostasis of bone tissue
1. resorption – continual destruction of bone matrix by osteoclasts
throughout life
2. deposition – continual replacement of bone matrix by osteoblasts
3. regulated by hormones
4. total mass of bone tissue within adult skeleton remains nearly constant
5. 3-5% of bone calcium is exchanged each year
IV. Bone Function
A. Support and protection
B. Body movement – created with interaction of muscles
1. form levers
C. Blood cell formation
1. hematopoiesis – blood cell formation
2. occurs in bone marrow
a. soft, netlike mass of connective tissue
b. found in medullary cavities of long bones, spaces in spongy bones,
& larger osteonic canals of compact bone
c. two types of bone marrow
1) red marrow – functions in formation of red blood cells,
white blood cells, & platelets
a) hemoglobin – oxygen carrying pigment that gives it
red color
b) occupies cavities of most bones in infants
c) in adults, most red marrow is replaced by yellow
marrow;
d) in adults, found primarily in spongy bone of skull,
ribs, sternum, clavicle, vertebrae, & pelvis
2) yellow marrow – stores fat
a) active in blood cell production
D. Storage of inorganic salts
1. intercellular matrix is rich in calcium salt (calcium phosphate)
2. when calcium in blood is low, osteoclasts break down bone tissue
releasing calcium into blood
3. magnesium, sodium, potassium, & carbonate ions also stored
V. Joints (articulations) – functional junctions between bones
A. vary in structure & function
B. Can be classified by degree of movement
1. immovable (skull bones)
2. slightly movable (joints between vertebrae)
3. freely movable (elbows & knees)
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C. Can be classified by type of tissue that binds bones together; most common
classification
1. Fibrous joints – lie between bones that closely contact one another
a. thin layer of dense connective tissue joins bones
b. sutures – fibrous joints between bones of skull
c. no movement or very limited movement
2. Cartilaginous joints – disks of fibrocartilage or hyaline cartilage connect
bones
a. slightly movable (intervetebral joints)
3. Synovial joints - most joints
a. Characteristics of synovial joints
1) freely movable
2) joint capsule – tubular capsule of dense connective tissue that
holds articular ends of bones together
a) outer layer is composed of ligaments
b) inner layer is composed of synovial membrane
i. secretes synovial fluid to lubricate joint
3) menisci – flattened, shock –absorbing pads of fibrocartilage
between the articulating surfaces in some joints
4) bursae – fluid-filled sac
a) lined with synovial membrane which may be continous with
synovial membrane of joint cavity
b) located between the skin & underlying bony prominences
c) aid movement of tendons
b. Types of synovial joints – based on shapes of parts and movement
they allow
1) ball-and-socket – made of bone with ball-shaped head that
articulates with a cup-shaped cavity of another bone
a) allows wide range of motion including rotation around a
central axis
b) examples – shoulder & hip
2) chondyloid joint – oval-shaped condyle of one bone fits into an
elliptical cavity of another bone
a) permits movement on a variety of planes, but does not
include rotation
b) example – joints between metacarpals & phalanges
(knuckles)
3) gliding joints – almost flat or slightly curved
a) allow sliding & twisting movement
b) examples – wrists and ankles
4) hinge joint – the convex surface of one bone fits into to
concave surface of another
a) permits movement in one plane only
b) examples – elbows and joints of phalanges (fingers)
5) pivot joint – cylindrical surface of one bone rotates within a
ring formed of bone & ligament
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a) movement is limited to rotation around a central axis
b) example – joint between proximal ends of radius & ulna
6) saddle joint – forms between bones whose articulating surfaces
have both concave & convex regions
a) surface of one bone fits the complementary surface of the
other
b) permits variety of movements
c) example – base of thumb
c. Movements of synovial joints
1) produced by skeletal muscles
a) insertion – movable end pulled by muscle contraction
b) origin – fixed end of muscle
2) flexion – bending parts at a joint so that the angle between them
decreases & parts become closer together
3) extension – straightening parts at a joint so that the angle
between them increases & the parts move farther apart
4) dorsiflexion – bending the foot at the ankle toward the shin
(standing on heels)
5) plantar flexion– bending the foot at the ankle toward the sole
(pointing toes)
6) hyperextension – excess extension of the parts at a joint beyond
the anatomical position
7) abduction – moving a part away from the midline
8) adduction – moving a part toward the midline
9) rotation – moving a part around an axis
10) circumduction – moving a part so that its end follows a circular
path
11) pronation – turning the hand so that the palm is
downward
or turning the foot so that the medial margin is lowered
12) supination – turning the hand so that the palm is upward or
turning the foot so that the medial margin is raised
13) eversion – turning the foot so that the sole is outward
14) inversion – turning the foot so that the sole is inward
15) retraction – moving a part backward
16) protraction – moving a part forward
17) elevation – raising a part
18) depression – lowering a part
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