Download Skeletal System

Anatomy & Physiology
101-805
Unit 6
The Skeletal
System
Paul Anderson
2017
Skeletal System: Major Functions:
1.  Support of soft tissues.
2.  Body shape.
3.  Protection of vital organs
(brain by cranium, heart & lungs by rib cage, spinal cord by spine.
4. Allows for adaptive (i.e. homeostatic) movements
- forms attachment sites for muscles (affects bone shape, bone markings)
- has moveable joints
- forms levers with muscles: movement occurs when muscles pull on bones
5. Production of blood cells (Hemopoiesis)
- in adult red bone marrow forms all red blood cells, platelets & most white
blood cells
6.  Storage of minerals (Ca/P) in bone matrix & fat (in yellow marrow)
- minerals harden bones for their normal functions
- Bone deposition & resorption are controlled by hormones (e.g. PTH)
- vit D required for absorption of Ca/P
Diet
Vit D
Blood
Ca+2
PO4 -3
Bone deposition
Calcitonin
Bone resorption
PTH
Ca/P stored as
Hydroxyapatite,
Ca5(PO4)3OH
in bone matrix
2
Skeletal System: Components
AXIAL SKELETON
APPENDICULAR SKELETON
• Bones major organs of system, have all
costal (rib)
functions of system.
cartilages
• Cartilages connect & protect bones at
joints, form smooth articular surfaces of
ribs
moveable bones.
sternum
• Ligaments connect bones at joints &
ulna
stabilise joints.
• Joints (Articulations): regions where
bones meet: form fulcrum of levers.
Moveable
fulcrum of
lever
ligament
Elbow joint
patella
femur
tibia
Knee joint
Meniscus
cartilage
Patellar
ligament
cranium
Clavicle
scapula
Vertebral
column
humerus
pelvis
radius
femur
patella
tibia
Martini &
Bartholomew
fig 1-2b
3
Muscles, Bones & Joints form Levers
The skeleto - muscular system is a system of levers
(devices for performing work).
A Lever is a rigid bar (in the body, a bone) that turns
about an axis of rotation or a fulcrum (in the body, a
joint).
•  The Power point (P) of the lever is the insertion point of a
muscle.
•  The Fulcrum (F) of the lever is the moveable joint at
which movement occur.
•  The Resistance (R) is the weight of the part being moved.
Resistance (R),
R
Weight of body
part moved
F
Fulcrum (F),
Joint that allows
movement
bone
P
Power point (P),
4
where tendon of
muscle pulls on bone
Tissues in the Skeletal System
Bones are organs consisting of several tissues.
• Bone (Osseous Tissue)
• Cartilage
• Fibrous (Dense) Connective Tissue: in the surface
layer of bones (periosteum), cartilages (perichondrium) & in
ligaments.
• Hemopoietic (“blood forming”) or Myeloid) Tissue
in red bone marrow of vertebrae, sternum, ribs, skull,
scapula, pelvis & proximal epiphyses of femur & humerus.
• Adipose Tissue (in yellow marrow)
- source of energy
- can form red marrow in emergencies.
Cartilage vs Bone Tissue in the Skeletal System
Cartilage and bone are distributed according to their properties.
Cartilage
Bone
• Properties of resiliency & strength.
• Non – calcified matrix -firm but not
rigid.
• Mature cartilage cells chondrocytes.
• Non vascular so thin & slow to heal.
• Grows by appositional (surface) &
interstitial (internal) growth.
• Forms most of embryonic skeleton.
• In adults found mainly at joints and
in the upper respiratory tract.
• Properties of rigidity, hardness &
strength.
• Calcified matrix- hard and rigid.
• Mature bone cells osteocytes.
• Vascular (vessels run in canals).
• Grows by appositional growth only,
due to its calcified matrix.
• In development bone gradually
replaces cartilage to form the
major tissue of all adult bones.
• Resiliency is due to the organic matrix ground substance.
• Strength is due to the organic matrix collagen fibers.
• Hardness & rigidity are due to calcified inorganic matrix.
Two Types of Growth Occur in Skeletal Tissues:
• Appositional Growth means growth at a skeletal surface.
• Appositional Growth occurs by cell division of osteogenic (or
chondrogenic) cells in the periosteum or endosteum of bone or in the
perichondrium of cartilage.
• Interstitial Growth means internal growth by cell division &
production of new matrix internally, expanding the tissue from
within.
• Interstitial Growth occurs in cartilage but not in bone since bone
matrix is calcified (i.e. hardened).
Growth in
Skeletal
Tissues
Appositional
(surface)
Growth
Interstitial
(internal)
Growth
At surface of
Bone & Cartilage
Tissues
In Cartilage
Tissue only
Appositional Growth in Cartilage
Perichondrium
(surface membrane
of cartilage)
Fibrous layer:
outer protective
collagenous layer
Cellular layer: inner
chondrogenic layer
with fibroblasts
New
matrix
Collagen
fibers
fibroblasts
form
New
chondrocytes
Dividing
fibroblast
new matrix
new matrix
old
matrix
Martini,
Fundamerntals of
A & P, 8th ed.
Fig. 4-13
Appositional Growth occurs in Bone & Cartilage
Interstitial Growth in Cartilage
• Interstitial Growth means internal growth by cell
division and production of new matrix internally, thus
expanding the tissue from within.
• Interstitial Growth occurs in cartilage but not in
bone since bone matrix is calcified (i.e. hardened).
Non - calcified
matrix
old
matrix
Dividing Chondrocyte
Martini, Fundamerntals of A & P,
8th ed., Fig. 4-13
• Cartilage has a non -calcified matrix so can grow by Interstitial Growth.
• Bone has a calcified matrix so can only grow by Appositional Growth.
Cartilage Tissues
in the Skeletal
System
Cartilages
in nose
Articular cartilage
of a joint
Costal
Cartilages
Cartilages in
Intervertebral
disk
Two types of
cartilage occur in
the skeletal system.
• Hyaline Cartilage
Meniscus
(cartilage pad)
in knee joint
• Fibrous Cartilage
Articular
Cartilage of a
Joint
Pubic
symphysis
Hyaline
Cartilages
Fibrocartilages
Marieb, 5th ed., Figure 7-1
Types of Cartilage : Hyaline Cartilage
• There are
three types of
cartilage.
• Only Fibrous
and Hyaline
Cartilage occur
in the skeletal
system.
Martini & Bartholomew, Figure 4-11(a)
articular
cartilage
Hyaline Cartilage
Hyaline Cartilage
• Matrix appears smooth but
has many thin collagen fibers.
Perichondrium
(surface
membrane)
• Provides firmness, resiliency,
strength and a smooth
surface for joints.
• Found in articular surfaces of
bones, embryonic skeleton,
costal cartilages, upper
respiratory tract.
Hyaline
Cartilage
Martini, 6th ed. Figure 4-15
11
Intervertebral
Disk
Meniscus
Fibrous Cartilage
• Pubic
Symphysis
Intervertebral
Disk
- Matrix has many thick parallel collagen fibers
- Provides extreme strength
- Fibrous Cartilage is found in Skeletal System
• Pubic Symphysis
• Intervertebral Disks
• Menisci cartilages in knee joint
Martini & Bartholomew, Figure 4-11(c)
12
12
Bone Tissue
In Bone tissue the extracellular matrix secreted
by bone cells consists of an organic component
(osteoid) and a hardened inorganic component.
• The organic component (osteoid) consists of collagen
fibers in a firm glycoprotein ground substance.
• Collagen provides strength to withstand tensile forces
(due to stretching, bending and twisting) without
breaking.
• The inorganic component consists mainly of
hydroxyapatite, Ca5(PO4)3OH, a hard mineral.
• The inorganic component provides hardness and rigidity
to withstand compression forces without bending.
13
Bone Tissue: Components & Properties
Bone Tissue
Extracellular
Matrix
Organic Matrix
(Osteoid)
Ground
Substance
(glycoprotein)
Bone Cells
(Osteocytes)
Inorganic Matrix
(hydroxyapatite)
Collagen
Fibers
Strength
Hardness & rigidity
Properties of bone tissue
14
Bone Cells
• Mature bone cells (Osteocytes) are connected to each other and to the
nearest blood supply via many cytoplasmic processes which run through
tiny canals (Canaliculi) in the extracellular matrix.
• Unlike cartilage, bone matrix is impermeable so does not allow diffusion,
except via canaliculi which are therefore “lifelines” for bone cells.
From Martini, 8th ed. Figure 6-3
Osteocyte
Extracellular
calcified matrix
non - dividing mature bone cell
Nucleus of
osteocyte
CO2
wastes
Lacuna: space
surrounding
osteocyte
Cytoplasmic
processes of
osteocytes in
canaliculi
O2
nutrients
Canaliculi:
allow diffusion
between osteocytes
& blood vessels
15
Compact Bone Tissue
Two types of bone tissue occur, Compact Bone and Spongy Bone.
Compact Bone occurs externally on bones and has:
•  No marrow spaces
•  Parallel Osteons that resist forces primarily in one direction
Blood vessels
in Central
Canal
External
membrane
(periosteum)
of bone organ
Osteocyte
in Lacuna
Concentric
layers of
bone tissue
(Lamellae)
Haversian
System
(Osteon)
Martini & Bartholomew Fig. 6-3,
16
Compact Bone Tissue: Haversian Systems
• Haversian Systems (Osteons) consist of concentric
layers (Lamellae) of bone tissue surrounding a
central (Haversian) canal containing blood vessels.
• Radiating Canaliculi connect the Osteocytes with
the central canal.
calcified
Fig. 4-12, Martini & Bartholomew;
17
Spongy Bone Tissue
• Contains many spaces with marrow
• Spongy bone reduces the weight of bones and (where it contains red
marrow) is a source of blood cells.
• Has Lamellae but no osteons
• Consists of irregular bony bars or struts (Trabeculae) aligned to
withstand forces from many directions.
• Occurs internally in bones
Opening of
canaliculi
Trabeculae
Spaces
containing
marrow
Lamellae but
no osteons
Internal
surface
(Endosteum)
Osteocyte in
lacuna
Martini, 8th ed. Figure 6-6
18
Bone Tissues - 1
Compact Bone tissue in long bones contains Circumferential Lamellae
surrounding the bone, Concentric Lamellae surrounding osteons and
Interstitial Lamellae between osteons.
Perforating or Volkmann’s canals connect
Haversian canals to the main blood supply.
Fig. 6-3, Martini & Bartholomew;
Fig. 6-5, Martini, 8th ed.
19
Bone Tissues - 2
Central
Canal
Concentric
Lamellae
Endosteum
lines
internal
surfaces of
bones
Haversian
System (osteon)
Alternate Collagen
fiber orientation
strengthens osteon
Concentric
Lamellae
Fig. 6-3, Martini & Bartholomew;
Fig. 6-5, Martini, 8th ed.
20
Structure of a
Long Bone
• Long bones consist of a long
shaft (the DIAPHYSIS) the
walls of which are mostly of
Compact Bone.
Proximal
Epiphysis
• The DIAPHYSIS is hollow
with a Marrow Cavity
containing fatty Yellow
Marrow.
• Each end of a long bone
expands to form EPIPHYSES.
Diaphysis
• EPIPHYSES form joints with
other bones and contain
Spongy Bone.
• The EPIPHYSIS is covered by
a thin cortex of compact bone
and, at the joint surfaces, by
an Articular Cartilage (Hyaline
Cartilage).
Articular
cartilage
Distal
Epiphysis
Fig. 6-2, Martini & Bartholomew;
Fig. 6-13, Martini, 8th ed.
21
Structure of a Long Bone -2
Spongy Bone
•  The DIAPHYSIS of a long bone with its
Compact Bone withstands forces mainly
parallel to the bone.
•  The Spongy Bone in the EPIPHYSES is
able to withstand forces from many
directions and contains Yellow or Red
Marrow in the spaces between
Trabeculae.
•  In adults Red Marrow occurs in the
proximal epiphyses of the femur and
humerus.
Endosteum
Compact Bone
Periosteum
•  External surfaces of bones (except at
joint surfaces) are covered by the
Periosteum and internally by the
Endosteum.
Fig. 6-2, Martini & Bartholomew;
22
Fig. 6-13, Martini, 8th ed.
Structure of a
Long Bone -3
epiphysis
Fig. 6-2, Martini &
Bartholomew;
Martini, Fundamerntals of
A & P, 8th ed., Fig. 6-13
metaphysis
Epiphyseal Plate
(immature bone) or
Epiphyseal Line
(mature ossified bone)
diaphysis
• The METAPHYSIS is the junction between diaphysis and epiphysis.
• During growth years a layer of hyaline cartilage occurs on the
epiphyseal side of the metaphysis called the EPIPHYSEAL PLATE.
• This is responsible for longitudinal growth (lengthening of long bones)
until maturity by INTERSTITIAL GROWTH of cartilage and
APPOSITIONAL GROWTH of bone.
• At maturity the epiphyseal plate is completely ossified and is now
called the EPIPHYSEAL LINE.
23
Bone Tissues: Periosteum
• The Periosteum has two layers, an outer collagenous Fibrous Layer
and an inner Cellular Layer.
- The outer Fibrous Layer of the periosteum protects bones and
binds tendons and ligaments to bones (via perforating or Sharpey’s
fibers).
- The inner Cellular Layer of the Periosteum is osteogenic, i.e.
functions in appositional growth, remodeling and repair of bones.
Fibrous
layer
Periosteum
Cellular
layer
Anchors bones to
ligaments & tendons
Protects
bones
Forms new bone
by Appositional
Growth
• Growth
• Remodeling
• Fracture repair
24
The Periosteum
contains
• Stem Cells
• Osteoblasts
• Osteoclasts
anchor
periosteum to
bone tissue
Figs. 6-2, 6-3, Martini & Bartholomew;
Fig. 6-6, Martini 8th ed.
25
The Endosteum
• Internally the cavities
of bones are lined with
a thin cellular
Endosteum which
functions like the
cellular layer of the
periosteum.
• It therefore contains
- stem cells
(osteoprogenitor cells)
- osteoblasts, the
source of new bone
tissue
- osteoclasts which
destroy bone tissue.
Osteoid
organic bone matrix
is first to form.
Figs. 6-3, Martini & Bartholomew
Martini, Fundamerntals of A & P, 8th ed., Fig. 6-8
26
Figs. 6-3, Martini &
Bartholomew
Fig. 6-3, Martini, 8th
ed.
Endosteum
Compact bone
Osteocyte
Types of
Bone Cells
Periosteum
Osteoblast
forms
Osteoprogenitor cell
forms
Osteoclasts
lower pH to
dissolve
inorganic matrix
Osteocytes
cannot
Osteoblasts
normally divide raise pH to form
inorganic matrix
Osteoclast
Bone
Deposition
Bone
Resorption
27
Functions of Bone Cells
• Osteoclasts are multinucleated cells derived
from hemopoietic stem cells in bone marrow.
•  Osteoclasts are mobile cells that move to sites
of injury or resorption.
•  Osteoclast activity is increased by PTH which
raises blood [Ca+2].
28
Appositional Growth of Bone Tissue
• Osteogenic (bone – forming) cells in the periosteum or endosteum
are called osteoblasts.
• Osteoblasts exchange chemicals via cytoplasmic processes
connecting the cells.
• Osteoblasts secrete the organic matrix first and then create the
alkaline environment causing precipitation of the inorganic matrix
of hydroxyapatite around themselves.
• The osteoblasts are now mature non- dividing cells called
osteocytes and are imprisoned within their lacunae in the calcified
matrix and connected to their blood supply via canaliculi.
Osteoprogenitor
cell
secretes
Figs. 6-3, Martini & Bartholomew
Martini, Fundamerntals of A & P,
8th ed., Fig. 6-8
Osteoid
Organic Matrix
Osteoblast
Osteocyte
Calcified
Bone Matrix
Raises pH
to form
29
Appositional Growth of Long Bone Diaphysis
Increased diameter of marrow cavity due
to resorption by osteoclasts in endosteum.
Increased bone circumference due to
appositional growth by osteoblasts in periosteum.
Fig. 6-6, Martini & Bartholomew;
30
Ossification
Bone Formation occurs by Ossification and by
Appositional Growth, processes that occur
• during the growth of the skeleton to maturity
• during remodelling of bone throughout life
• in the repair of bones.
• Ossification is the replacement of cartilage or
fibrous connective tissues with bone tissue.
• Ossification occurs in development of the
skeleton to maturity & in fracture repair.
31
Development of Skeleton
In the embryo most
bones are first formed as
hyaline cartilage bone
“models” which are
gradually replaced by
bone (osseous) tissue at
maturity in a process
called Endochondral
Ossification.
Intramembranous
bones
Skeleton of
16 week old
fetus
A few bones however, are
formed instead as fibrous
membrane models (most
skull bones and the
clavicle) in a different
process called
Intramembranous
Ossification.
Endochondral
bones
Fig. 6-4, Martini & Bartholomew
32
Intramembranous Ossification
• In both types of ossification, centers of bone forming tissue
(called ossification centers) are formed within the model in a
vascularised environment.
• Osteoblasts first secrete the organic matrix followed by
calcification.
• Ossification then proceeds by appositional growth away from the
ossification center.
• In Intramembranous Ossification embryonic
connective tissue (mesenchyme) cells
differentiate to become osteoblasts (bone
forming cells) within a fibrous membrane
under the skin.
Fig. 6-2, Martini, 8th ed.
Periosteum
• Osteoblasts then form spongy bone to which
is added compact bone, externally, beneath
the newly formed periosteum.
• Bones formed in this way are called “Dermal
bones” & include the flat bones of the skull,
the mandible & clavicle.
33
Endochondral Ossification: Summary
• For most bones embryonic mesenchyme cells first
differentiate to become chondroblasts surrounded by
a perichondrium.
• Hyaline cartilage is formed which grows by
interstitial and appositional growth forming a cartilage
bone “model”.
• In Endochondral Ossification calcification of the
hyaline cartilage bone model occurs which causes death
of most chondrocytes.
• The bone model is then invaded by vascularised tissue
which form Ossification Centers within the model.
• Cartilage is replaced by spongy bone tissue as
ossification spreads by Appositional Growth away from
the ossification centers.
34
Endochondral Ossification: Initial Steps
Fig. 6-5, Martini &
Bartholomew;
1.
Death of
chondrocytes
in cartilage
model
Blood
vessels
2.
Bone Collar
Formation
(Bone collar)
• In the center of the diaphysis the cartilage model calcifies and
chondrocytes die.
• Increased vascularisation of the perichondrium forms a periosteum
which forms an external collar of compact bone around the diaphysis
of the cartilage model.
• This collar spreads along the diaphysis.
35
Endochondral Ossification: Steps
Periosteal
bud
Spongy
bone
3.
Formation of
Primary
Ossification
Center in
center of
diaphysis
4.
Ossification
of diaphysis
Spongy
bone
Fig. 6-5, Martini &
Bartholomew;
• Vascularised bone - forming tissue from the periosteum (a periosteal
bud) invades the center of the diaphysis forming a Primary Ossification
Center.
• Osteoclasts digest away the calcified cartilage while osteoblasts
form new spongy bone around the remnants of the cartilage within the
diaphysis.
• Calcification of cartilage continues within the diaphysis and
36
ossification follows this, spreading along the diaphysis.
Endochondral Ossification: Steps
5.
Formation of Secondary
Ossification Centers in
Epiphyses
Secondary
ossification
center
Marrow
cavity
• Osteoclasts erode the newly formed
spongy bone to create a marrow cavity
in the center of the diaphysis which
spreads along the diaphysis.
• Calcification of cartilage then occurs
in each epiphysis at or after birth,
followed by invasion by a periosteal
bud, forming a Secondary Ossification
Center in each epiphysis.
Fig. 6-5, Martini &
Bartholomew;
37
Endochondral Ossification:
Final Steps
• Ossification forms spongy bone
in each epiphysis between a
layer of epiphyseal cartilage
(the Epiphyseal Plate) and
articular cartilage.
• Epiphyseal plate cartilage
continue to grow by interstitial
growth as fast as it replaced by
bone tissue on the diaphyseal
side of the plate.
Fig. 6-8, Martini
6.
Ossification
(closure) of
Epiphyseal Plates
by end of puberty
• Therefore the long bone
continues to grow in length until
maturity when the epiphyseal
plate is completely ossified
(“closure of epiphyses”)
forming an epiphyseal line.
• After this stage no further
lengthening of bones can occur
but bones continue to increase
in thickness.
38
Development of the Skeleton - 1
Human Embryo at 7 weeks.
The skeleton consists entirely
of cartilage and membrane
Intramembranous
ossification
forming dermal
bone
Hyaline
cartilage
bone models
are sites for
endochondral
ossification
Secondary ossification
Centers in epiphyses
39
Development of the Skeleton - 2
Articular
cartilage
Epiphyseal plates shrinking
40
Development of the Skeleton - 3
• Epiphyseal
Plate almost
completely
ossified.
• Eventually
closes to
become
Epiphyseal
Line.
• No more
lengthening
of bones can
then occur.
Articular
cartilage
remains
throughout
life
41
Bone Remodeling
Bone remodeling refers to the changes in shape and
thickness of bones throughout life due to two
antagonistic processes, bone deposition and bone
resorption at the periosteum and endosteum.
Bone deposition and resorption occur
•  in response to the need for a constant blood calcium
level
•  in response to mechanical stresses
•  for repair of fractures and
•  during bone growth to maturity.
42
Bone Deposition & Bone Resorption
• Bone Deposition means appositional growth of new
bone tissue by the activity of osteoblasts.
• Osteoblasts first secrete the organic matrix then
help form the inorganic matrix.
• Formation of inorganic matrix is catalysed by locally
increased concentrations of Ca+2 and PO4-3 and by the
creation of an alkaline environment for enzymes
secreted by the osteoblasts.
• Bone Resorption refers to the digestion of bone
matrix by enzymes and acids secreted by osteoclasts
which also phagocytose the cellular products.
43
Bone Tissue Responds to Mechanical Stress
Bone is a living tissue and is constantly responding to the
needs of the body to resist mechanical stress (Wolff’s law).
Wolff’s law states that bones respond to the level
of mechanical stress by structural changes (e.g.
trabeculae of spongy bones form along lines which will
resist the most stress).
• The fetal skeleton is relatively featureless.
• Athletes show increased bone mass.
• Astronauts & bedridden patients show decreased bone
mass (Atrophy of Disuse).
• Bone remodeling occurs in response to the mechanical
stresses placed upon bones.
• Bone: “Use it or Lose it”!
44
Bone Tissue Responds to Body’s Needs for Ca/P
Bone is a living tissue and is constantly responding
to the needs of the body for adequate blood levels
of Ca+2 and PO4 -3.
Two antagonistic hormones (Parathyryroid Hormone,
PTH and Calcitonin) respond to the needs for adequate
blood calcium and therefore affect bone remodeling.
• PTH raises blood calcium levels so
is hypercalcemic
• Calcitonin lowers blood [calcium] so
is hypocalcemic
45
How PTH & Calcitonin Control Blood Ca levels
stimulus
↓ Blood
[Ca+2]
-ve
feedback
Parathyroid
Gland
Thyroid
Gland
↑Blood
[Ca+2]
-ve
feedback
Parathyroid
Hormone
(PTH)
+
osteoclasts
↑Blood Ca
stimulus
↑ Bone
Resorption
Calcitonin
osteoclasts
↑ Bone
deposition
+
↓ Blood Ca
46
PTH Causes Bone Resorption & Increases Blood Ca
Ca+2 returned
to blood
PTH with
PTH
calcitriol
Ca+2
Intestine
Ca+2 absorbed
into to blood
Stimulus for PTH
↓ Blood [Ca]
After Martini, 8th ed. Figure 6-16
Ca+2
Ca+2
Ca+2
PTH
Ca+2 removed
from bone
Bone
Resorption
Kidney
↑Blood Ca
Bone
• PTH is released when blood calcium levels fall.
• PTH stimulates osteoclasts so increasing Bone Resorption.47
Calcitonin Causes Bone Deposition & Lowers Blood Ca
Ca+2 excreted
calcitonin
Ca+2
Ca+2
Ca+2
calcitonin
Stimulus for
Calcitonin
↑Blood [Ca]
After Martini, 8th ed. Figure 6-16
Ca+2 added
to bone
Bone
Deposition
Bone
Kidney
Ca+2 lost
in urine
↓ Blood [Ca]
• Calcitonin is released when blood calcium levels rise.
• Calcitonin inhibits osteoclasts allowing osteoblasts to
increase Bone Deposition.
48
Homeostatic Control of Blood Ca by Calcitonin & PTH
Thyroid Gland
Secretes
Calcitonin
Increased excretion
of calcium
in kidneys
Calcium deposition in
bone (inhibition
of osteoclasts)
Blood calcium
levels decline
HOMEOSTASIS
DISTURBED
Rising calcium
levels in blood
HOMEOSTASIS
DISTURBED
Falling calcium
levels in blood
Parathyroid
Glands secrete
Parathyroid
Hormone (PTH)
HOMEOSTASIS
Normal calcium
levels
(8.5-11 mg/dl)
Release of stored
calcium from bone
(stimulation of
osteoclasts, inhibition
of osteoblasts)
Enhanced
reabsorption
of calcium in kidneys
Stimulation of
calcitriol production
at kidneys;
enhanced Ca2+, PO43absorption by
digestive tract
HOMEOSTASIS
RESTORED
HOMEOSTASIS
RESTORED
Blood calcium
levels
increase
Martini & Bartholomew
Figure 10-10
49
Factors Affecting Growth of the Skeleton
Growth of the skeleton requires Extrinsic Factors
and Intrinsic Factors.
Extrinsic Growth Factors include
• vitamin D (for Ca/P absorption & inorganic matrix formation)
• sunlight (for synthesis of vitamin D)
• vitamin C (for collagen synthesis)
• vitamin A (for osteoblast activity & normal bone growth in children)
• vitamins K & B12 (for protein synthesis)
• dietary protein (for synthesis of organic matrix)
• dietary calcium and phosphorus (for inorganic matrix)
• carbohydrate (supplies energy for growth).
Intrinsic Growth Factors include Genes, Growth
Hormone (GH), Thyroid Hormones & Sex Hormones.
50
Factors Affecting Growth of the Skeleton - 2
I
vit C
Amino
acids
collagen
energy
• Vit C promotes formation of collagen & organic matrix.
• Vit D promotes Ca/P absorption & formation of inorganic matrix.
51
Formation & Functions of Vitamin D
•  Vitamin D is synthesed from cholesterol in
the skin in the presence of sunlight.
•  Vitamin D promotes Ca and P absorption
from the diet.
•  Vit D is converted by the kidneys into the
hormone calcitriol which targets the small
intestine, increasing Ca/P absorption.
Bones &
teeth
via blood
Increased
absorption
of Ca/P
Integumentary System
Martini & Bartholomew
fig 1-2a
diet
Cholesterol
in skin
sunlight
Vit D precursor
(vit D3)
via blood
Calcitriol
Kidney
via blood
Liver
stores vit D3
Small
Intestine Kidney activates vit D3 to hormone
Calcitriol (activated vit D)
Calcitriol released into blood by PTH
Rickets: a Childhood Bone Disorder due to
Calcium or Vitamin D Deficiency
Softening of bones
causes bowing of legs &
other bone deformities
2 years later,
with calcium & vit D
supplements
53
Scurvy: a Connective Tissue Disorder From
Deficiency of Vitamin C
Inadequate collagen
causes bruising &
bleeding under the skin
Inadequate collagen
causes loose teeth &
bleeding gums,
54
Effect of Growth Hormone on the Skeleton
• Growth hormone (GH) causes
protein synthesis and cell
division of osteogenic cells
throughout the growing years.
• Growth Hormone causes the
Juvenile Growth Spurt and
general growth of the skeleton
to maturity .
• Pituitary Dwarf has short
stature with normal skeletal
proportions.
From Ganong, Review of
Medical Physiology
Hypopituitary Dwarf
• ↓Height
• Normal proportions
55
Effect of Thyroid Hormone on the Skeleton
• Thyroid hormone (Thyroxine)
causes energy release for
bone growth and controls
changes in skeletal
proportions as bones grow.
• Thyroid hormone (thyroxine)
works synergistically with GH
to cause skeletal growth and
controls changes in skeletal
proportions.
From Ganong, Review of
Medical Physiology
Hypothyroid Dwarf
• ↓Height
• Infantile proportions
• Hypothyroid Dwarf has short
stature and infantile skeletal
proportions.
56
Effect of Sex Hormones
on the Skeleton
Deterioration
of vertebral
support
• Sex Hormones
(Androgens & Estrogens)
cause
• Growth spurt of puberty.
• Ossification ("closure")
of the epiphyseal plates
at the end of puberty
• Secondary Sexual
skeletal changes
Bones
porous &
brittle
Normal spongy
bone
Bone with
Osteoporosis
• Bone Deposition
throughout life
(preventing osteoporosis).
Martini , 8th ed. fig 6-19
57
Summary of Effects of Hormones on Bones
GH has indirect action on bones by acting as a trophic hormone for the liver
Growth
Hormone (GH)
Anterior
Pituitary
TSH
Thyroid
Gland
Gn
Gonads
gonadotropin
Thyroxine
(T4)
Liver
Bones
Insulin –like
Growth
Factor (IGF
hormone)
stimulates
osteoblasts
Androgens
Estrogens
• GH
- Juvenile Growth Spurt
• T4
- Normal Skeletal Proportions
- Energy for Growth
• Sex Hormones - Adolescent Growth Spurt
- Closure of Epiphyses
58
Changes in the Human Skeleton with Age
•  Bone mass is not constant but increases and decreases
during life due to two processes, bone deposition and
bone resorption.
•  Hormones help control these skeletal changes (along
with diet and normal bone use).
• During growth years (0 - 20} the rate of deposition
exceeds the rate of resorption and there is a net
increase in bone mass.
• During adulthood (ages 20 - 35 in females; 20 - 60 in
males) the rate of deposition equals the rate of
resorption and bone mass is fairly constant.
• During later years (35+ in females; 60 +in males) the
rate of deposition is less than the rate of absorption
and there is a net decrease in bone mass.
59
Changes in the Human Skeleton with Age - 2
growth years
(0 - 20yrs}
deposition >
resorption
Middle years: deposition = resorption
Closure of
epiphyses
Adolescent
growth spurt
Sex
menopause
Hormones
• GH
• Thyroxine
Thyroxine
• Relative importance of
hormones at various ages
Growth
hormone
• All continue to be
secreted throughout life
androgens
estrogens
-25%
-30
%
Old age:
deposition < resorption
↓Sex hormones
Juvenile
growth spurt
Bone mass
↓Bone mass
Osteopenia
Osteoporosis
Osteopenia:
normal decline
in bone mass
with age
//
70
Osteoporosis:
pathological
loss of bone
mass:
weakens
bones
60
Specific Skeletal Changes with Age
Changes in Body Proportions
• skull becomes proportionately smaller (rest of body grows faster)
• face becomes proportionately larger (compared to cranium)
• cranium becomes proportionately smaller (compared to face)
• legs become proportionately longer
• trunk becomes proportionately smaller
• female pelvis widens (during puberty)
• thorax changes shape (from round to elliptical)
Marieb fig 7-34
Face
1
1
Skull
4
2
61
Specific Skeletal Changes with Age - 2
Ossification and Closure of Epiphyseal Plates
(by 18 in females; 21 in males) due to sex hormones.
Epiphyseal plates of
growing long bones
Martini fig 6-11
Epiphyseal lines of fully
ossified long bones
62
Specific Skeletal Changes with Age - 3
Appearance of Secondary Spinal Curvatures.
• Cervical curvature by 3 months as infant lifts head up.
• Lumbar curvature by 1 year as child stands upright.
cervical
Primary
Spinal
Curvatures
Secondary
Spinal
Curvatures
lumbar
Primary Spinal
Curvature
1 - 3 years
Adult
spine
63
Specific Skeletal Changes with Age - 4
• Closure of Fontanelles of Skull and formation of Sutures.
- Fontanelles are fibrous connections between cranial bones allowing
cranial distortion during birth: fontanelles disappear by age 2.
• Sutures start to form when brain stops growing at age 5.
• Sutures fuse in old age.
Cranium of
newborn Infant
Martini fig 7-15
Martini & Bartholomew fig 6-15
Cranium of
adult
Martini fig 7-3b
64
Repair of Bone
Fractures - Early
Stages
1. Breakage of
blood vessels
cause death of
Bone Tissue &
Formation of
Fracture
Hematoma (< 48h)
2. Formation of
Internal &
External Callus
from Cells of
Periosteum &
Endosteum
(48h to 3-4
weeks)
Fibro
Callus: a bridge
of spongy bone &
fibrocartilage
between severed
bone segments
Martini & Bartholomew
pp. 150-151 (7th ed.):
fig 6-7 (6th ed.)
65
Repair of Bone
Fractures Later Stages
3. Fusion of Calluses
& Ossification of
Cartilage
(3/4 weeks - 2/3
months): cast can be
removed
4. Remodeling of Calluses
by osteoclasts responding
to stresses on bones (by
3 months upper
extremity- 6 months
lower extremity).
New spongy bone in center of diaphysis resorbed by osteoclasts
leaving marrow cavity - similar to ossification process during growth.
66