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Skeletal System
Biology 105 - Lecture 11
Chapter 5
Copyright © 2009 Pearson Education, Inc.
Outline
I. Overview of the skeletal system
II. Function of bones
III. Bone structure
IV. Bone cells
V. Cartilage
VI. Tendons and ligaments
VII.Joints
VIII.Bone development
IX. Hormonal regulation of bone growth
X. Homeostasis
XI. Disorders of the skeletal system
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Skeletal System
The skeletal system is composed of different
types of connective tissues:
Bones – rigid structure
Cartilage – soft, cushions the joints
Ligaments – attach bone to bone
Tendons – attach muscle to bone
Tendons link the skeletal and the
muscular systems.
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1. Supports and gives shape to the body.
2. Protects soft body parts.
3. Produces blood cells.
4. Stores minerals (calcium and phosphate).
5. Stores fat.
6. Along with the muscles, permits flexible
body movement.
Functions of Bone
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Bone Structure
Bones contain different types of tissue:
Calcified tissue = osseous tissue
Blood vessels
Nerves
Contain collagen fibers that give flexibility
This combination of calcified tissue and collagen
fibers makes the bone strong but flexible.
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Bone Structure
All bones have two layers:
Compact bone – dense mineralized outer
layer
Spongy bone –
inner layer with
open network filled
with marrow
Figure 5.1e
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Typical Long Bone Architecture
Epiphysis – rounded end of the typical long
bone, composed mainly of red bone marrow
where blood cells are made
Diaphysis – shaft of the typical long bone, has a
cavity filled with yellow marrow where fat is
stored
Periosteum – fibrous outer covering of the bone
that contains nerves, blood vessels, and
lymphatic vessels
Functions in bone repair and growth
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Typical Long Bone Architecture
Figure 5.1a
Spongy bone
(spaces contain
red bone
marrow)
Compact
bone on
surface
Yellow
bone
marrow
Blood
vessel
Periosteum
Central cavity
(contains
yellow bone
marrow)
(a) A long bone, such as the femur of the
leg, consists of a shaft and two heads,
or enlarged ends. Compact bone is
located on the outer surface of the bone.
Spongy bone is found in the heads.
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Bone is Living Tissue!
Figure 5.1b
Blood vessels
and nerve in
central canal
Spongy bone Compact bone
Osteon
Bone-forming cells
(osteoblasts)
(b) The structural unit of compact bone is
an osteon. Mature, living bone cells
(osteocytes) are found in small spaces within the hard matrix.
Periosteum
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Compact Bone Structure
Osteon – structural unit of the compact bone
Osteon structure:
Central canal (Haversian) – contains blood
vessels and nerves
Osteocytes – mature bone cells that maintain
bone structure, density, and minerals
Found in cavities called lacunae
Canaliculi are canals that connect the lacunae
to the central canal
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Figure 5.1c The structure of bone
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Figure 5.1d The structure of bone
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Bone Cells: Osteoblasts
Osteoblasts = bone-forming cells
Immature cells that secrete material called
“bone ground substance”:
This is non-cellular matrix that surrounds the cells.
Osteoblasts mature into osteocytes after they
have secreted enough material.
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Bone Cells: Osteoclasts
Osteoclasts = bone-absorbing cells
These are bone cells on the outer edge of
bones.
They release enzymes that eat away at the
bone, releasing minerals.
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Q: The arrow is pointing to:
1. Diaphysis
2. Epiphysis
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Q: Immature cells that secrete calcified material are:
1. Osteoclasts
2. Osteocytes
3. Osteoblasts
Q: Osteocytes are found in cavities called:
1. Central canals
2. Collagen
3. Lacunae
4. Vellus
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The skeleton begins to form at about 6 weeks
after conception.
Most bones grow through adolescence, but
some bones continue to grow through about
age 25.
Bone continue to change throughout life:
Osteoclasts remove the bone matrix, while
osteoblasts build up the matrix.
Ossification – formation of bones
Bone Development and Growth
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Figure 5.3 Steps of bone formation in long and short bones,
from an embryo into childhood
2 months
Fetal Development
Cartilage
model
3 months 9 months
Bone
collar Deteriorating
cartilage
matrix
Calcified
cartilage
Blood
vessel
Birth
Spongy
bone
formation
Childhood
Spongy
bone
Compact
bone
Step 2:
Osteoblasts form a collar of bone around the shaft of the model.
Step 4: Secondary centers of bone formation develop in the ends of the bone.
Step 5: Cartilage remains only on the surfaces that rub against other bones and in the cartilage growth plates.
Cartilaginous
surface
Step 1:
A cartilaginous model of the future bone forms.
Cartilaginous
(epiphyseal)
growth plate
Step 3: The shaft of the cartilage model begins to hollow out, and spongy bone fills the space. Blood vessels continue to penetrate the area, and the region of bone formation expands.
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Cartilage in Bones
Two regions of cartilage remain at each end of
the long bone:
The cap that covers the surfaces that rub against
other bones.
A plate of cartilage called the epiphyseal plate, or
growth plate.
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Bone Growth
Bone growth is stimulated by growth hormone during childhood.
Thyroid hormones ensure that the skeleton grows with the proper proportions.
At puberty, increasing levels of male or female sex hormones initially stimulate cartilage cells to divide, but eventually allow for the growth plates to fuse, and bone can no longer increase in length.
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Growth hormone (GH) – directly stimulates
growth of the epiphyseal plate and stimulates
bone growth in general
Vitamin D – converted to a hormone that
causes the intestines to absorb calcium
Sex hormones – adolescents experience a
growth spurt due to an increased level of
hormones
Hormonal Regulation of Bone Growth
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Parathyroid hormone (PTH) – produced by the parathyroid gland
Accelerates bone recycling and increases blood calcium
Calcitonin – produced by the thyroid gland
Decreases blood calcium levels, and deposits calcium into the bone
Homeostasis – maintaining a balance of calcium levels in blood and in bone
Hormonal Regulation of Bone Calcium Levels Hormonal Regulation of Bone Calcium Levels
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Healing Broken Bones
Broken bones and fractures are the same thing.
Bone fractures are healed by fibroblasts and
osteoblasts.
When a break occurs, there is bleeding
followed by a clot.
Fibroblasts secrete collagen fibers that form a
callus linking the two parts of the bone.
This is later replaced by bone.
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Bone Fractures are Healed by Fibroblasts and
Osteoblasts
Figure 5.4 (1 of 2)
Blood
clot
(hematoma)
Step 1: Within hours
after the fracture, a
blood clot forms.
Step 2: A cartilaginous
callus is formed by
invading fibroblasts.
Formation of
blood clot
Spongy
bone
struts
Formation of
cartilaginous callus
Cartilaginous
callus
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Bone Fractures are Healed by Fibroblasts and
Osteoblasts
Figure 5.4 (2 of 2)
Step 3: Osteoblasts form
new bone, converting the
cartilaginous callus to
a bony callus.
Step 4: The fracture is
healed and bone is
remodeled, restoring
bone to original shape.
New
blood
vessels
Formation of
bony callus Bone remodeling
Bony
callus Healed
fracture
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Q: This hormones accelerates bone recycling and
increases blood calcium:
1. Growth hormone
2. Calcitonin
3. Parathyroid hormone
Q: Calcitonin is produced by the ______ gland.
1. Parathyroid
2. Thyroid
3. Anterior pituitary
4. Hypothalamus
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Cartilage
Cartilage – flexible connective tissue that is not
as strong as bone tissue
Function: cushions joints, provides flexibility
Where found:
Ends of long bones, nose, ends of ribs, larynx and
trachea, disks between vertebrae, knee joint, ear
flaps, and epiglottis
Cartilage cells are called chondrocytes.
Cartilage lacks blood vessels, so it is very slow
to heal!
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Tendons and Ligaments
Ligaments – connective tissue that connects
bone to bone
Tendons – connective tissue that connects
muscle to bone
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Tendons and Ligaments
Figure 5.13b
Quadriceps
tendon
Femur
Fibula
Tibia
Posterior cruciate
ligament
Anterior cruciate
ligament
Medial collateral
ligament
Patellar ligament
Patella
(kneecap)
(b) Ligaments hold bones together, support the joint,
and direct the movement of the bones.
Lateral
collateral
ligament
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Freely movable joints held together by ligaments.
Synovial membrane – produces synovial fluid, an excellent lubricant for the joints
Bursae – fluid-filled sacs, reduce friction
Menisci – cartilage between the bones, acts as a cushion
Cartilage is also found at the ends of the bones.
Synovial Joints
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Synovial Joints
Figure 5.13a
A layer of cartilage on the
articulating surfaces of the bones
reduces friction as the bones move.
Femur
(a) Synovial joints, such as the knee shown here, permit a
great range of movement.
A bursa is a fluid-filled sac
that cushions certain joints
and reduces friction between
tendons and ligaments.
The joint cavity is filled with
synovial fluid, which serves as a
shock absorber and lubricant.
The synovial membrane forms
the inner surface of the joint cavity
and secretes synovial fluid.
Tibia
Fat pad
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Q: Cartilage cells are called:
1. Fibrocytes
2. Osteocytes
3. Chondrocytes
Q: This type of connective tissue connects bone to
bone:
1. Ligaments
2. Tendons
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Q: Fluid-filled sacs are called:
1. Synovia
2. Menisci
3. Bursae
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Osteoarthritis – deterioration of cartilage at joints
Rheumatoid Arthritis – chronic inflammatory
disorder of the joints
Sprain – when ligaments are torn or stretched
Bursitis – inflammation of bursa
Tendonitis – inflammation of tendon sheath
Fracture – broken bone:
Simple = bone breaks but does not penetrate
through the skin
Compound = bone breaks and protrudes through
the skin
Disorders of the Skeletal System
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Osteoporosis
Occurs when more bone is reabsorbed than is formed:
Homeostasis is not maintained, and more calcium is taken out of the bones than is replaced.
Estrogen and testosterone help maintain bone density.
After menopause, women produce less estrogen and this can lead to reduced bone density.
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Risk Factors for Osteoporosis
Post-menopausal
Not enough exercise
Poor diet, not enough calcium in the diet
Smoking
Low vitamin D levels
Certain hormonal disorders
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Treatment
Exercise
Increased calcium in the diet:
Calcium supplements may be needed.
The type of calcium supplement is important.
Estrogen replacement therapy
Prescription drugs
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Recommended Calcium Intakes*
Age Amount mg/day
Birth - 6 months 210
6 months - 1 year 270
1-3 500
4-8 800
9-13 1300
14-18 1300
19-30 1000
31-50 1000
51-70 1200
70 or older 1200
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Decreased Calcium Absorption
Oxalic acid – potent inhibitor of calcium
absorption
Found in high concentrations in spinach and
rhubarb, and in somewhat lower concentrations in
sweet potato and dried beans
Sodium – increased sodium intake results in
increased loss of calcium in the urine
Protein – as dietary protein intake increases, the
urinary excretion of calcium also increases
Caffeine – caffeine in large amounts increases
urinary calcium content for a short time
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Important Concepts
Read chapter 5
What are the four components of the skeletal
system, and what are their functions?
What are the functions of bone?
What are the two layers of bone?
What is the periosteum, and what is its
function?
What is the structure of the osteon, and what
are the features found in the osteon?
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Important Concepts
What are the three types of bone cells and
their functions?
What are the functions of cartilage, tendons,
and ligaments?
What are the parts of synovial joints and their
functions?
What are the hormones involved in bone
growth and homeostasis? Which glands
produce calcitonin and parathyroid hormones?
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Important Concepts
What are the disorders of the skeletal system
we discussed in this lecture?
How do bones heal?
What is osteoporosis, what are the risk factors
for osteoporosis, and what can you do to
prevent it?
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Definitions
Compact bone, osteon, central canal
(Haversian canal), spongy bone, epiphysis,
diaphysis, periosteum, lacunae, callus,
fibroblasts, osteoblasts, osteocytes,
osteoclasts, chondrocytes, growth hormone,
parathyroid hormone (PTH), calcitonin, vitamin
D, epiphyseal plate, growth plate