section 1, chapter 7
The Skeletal System
The science of bones is called osteology
Functions of bone
1. Support & protect organs
• The brain is protected by the skull and the heart and
lungs are protected the ribs & sternum
2. Movement
• Muscles attach to skeleton
3. Inorganic salt storage
• Stores calcium and phosphate
4. Blood cell production
• Red bone marrow forms new blood cells
Components of bone
The extracellular matrix of bones is composed of
1. hydroxyapatite – a calcium phosphate salt that provides
the hardness of bones
2. collagen fibers – provides bone with some pliability
The cells associated with bones include:
1. Osteocytes = cells that maintain bone
2. Osteoblasts = cells that deposit new bone. Once
mature, osteoblasts become osteocytes.
3. Osteoclasts = cells that dissolve bone. Osteoclasts
originate from white blood cells and they secrete an
acid that dissolves the inorganic salts of bone.
Bones may be classified by their shape.
1. Long bones = elongated diaphysis
• humerus
radius
• femur
tibia
• metatarsals
metacarpals
2. Short Bones = cube-shaped
• carpals
• tarsals
3. Flat Bones = plate-like
• sternum
ribs scapula
• parietal and frontal bones
ulna
fibula
phalanges
Bone Classification continued
4. Irregular bones = variety of shapes
• vertebrae
• mandible
maxilla
• ethmoid bone
sphenoid bone
5. sesamoid (or round) bone = develops
within tendons
• patella
Parts of a long bone
1. Diaphysis = shaft of long bone
• Lined with compact bone
2. Epiphysis = expanded ends of bone
• Filled with spongy bone
• Proximal epiphysis & distal epiphysis
• Sites of articulation (joint)
3. Epiphyseal plates
• Remnants of bone growth
4. Articular cartilage
• Hyaline cartilage
• Covers epiphyses
Parts of a long bone
5. Medullary Cavity
• Cavity within diaphysis
• Filled with bone marrow, blood
vessels and nerves
6. Endosteum
• Membrane that lines medullary cavity
• Contains osteoblasts
7. Periosteum
• Tough membrane covering bone
• Continuous with tendons and ligaments
• Osteoblasts, blood vessels, and nerves
Parts of a long bone
7. Compact bone
• Lines the Diaphysis
• Composed of osteons
8. Spongy bone
• Fills the epiphyses
• Trabiculae = thin bony plates
• Osteocytes lie within trabiculae
Figure 7.3
Compact Bone
Osteon = Structural & functional unit of compact bone
1. Lamella = concentric rings of bone
2. Central Canal = blood vessels and nerves
3. Lacunae = bony chamber that contains an osteocyte
4. Canaliculi = canals with cellular processes
• Pathway for nutrient and waste diffusion
Figure 7.5 Scanning electron micrograph
of a single osteon in compact bone.
Osteon continued
Perforating Canal = conveys blood
from periosteum towards individual
osteons
Figure 7.4 Compact bone is
composed of osteons
cemented together by bone
matrix.
Figure 7.4c Canaliculi allow nutrients
and waste to diffuse between the
central canal and individual osteocytes.
Bone Development and Growth
Parts of the skeletal system begin to develop during the first
few weeks of prenatal development
Bone formation = ossification
Bones replace existing connective tissue in one of two ways:
As intramembranous bones
As endchondral bones
Intramembranous Bones
Intramembranous Bones
Broad, flat bones of the skull
Formed by replacing layers of
connective tissue (mesenchyme)
with bone
Osteoblasts within mesenchyme
deposit bony matrix in all directions
Osteoblasts become osteocytes
once surrounded by bone
Endochondral Bones
Endochondral Bones
Most of the bones in the skeleton are endochondral
Bone formation begins with a hyaline cartilage model
Cartilage decomposes and is replaced by bone.
Figure 7.6a stained bones of a 14week fetus showing intramembranous
and endochorndal bones.
Endochondral Ossification
1. Hyaline cartilage forms model of
future bone
2. Cartilage degenerates and
periosteum surrounds bone
3. Osteoblasts from periosteum invade
the degenerating tissue
4. Osteoblasts beneath periosteum
form compact bone at diaphysis =
primary ossification center
5. Later, Osteoblasts form spongy bone
at epiphyses = secondary ossification
center
Endochondral Ossification continued
Figure 7.8 Major stages of endochondral ossification. (ad fetal, e child, f adult)
Endochondral Ossification
Two areas of endochondral bone retain
cartilage after ossification.
1. Articular cartilage
• surrounds the epiphyses for joints
2. Epiphyseal plates
• retain cartilage for bone growth
Articular
cartilage
Growth at the Epiphyseal Plate
Epiphyseal Plate
• Band of hyaline cartilage that remains
between the two ossification centers
• Bone growth continues at epiphyseal
plates until adulthood.
• New cartilage is added towards the epiphysis
and cartilage is ossified towards diaphysis
• Once the epiphyseal plates ossify the
bones can no longer be lengthened
4 Layers (zones) of growth at
epiphyseal Plate
1. Zone of resting cartilage
• Cartilage cells near epiphysis
• Do not participate in bone growth
• Anchor epiphyseal plate to epiphysis
2. Zone of proliferating cartilage
• Young chondrocytes undergoing
mitosis
• Adds new cartilage to plate
4 Layers (zones) of growth at
epiphyseal Plate
3. Zone of hypertrophic cartilage
• Older cells enlarge and thicken the
epiphyseal plate
• Osteoblasts invade and calcify the
cartilaginous matrix.
4. Zone of calcified cartilage
• Dead cells & calcium matrix
Ossified bone
• Osteoclasts dissolve and phagocytize
the matrix
• Osteoblasts invade the region and
deposit new bone.
(b)
End of Section 1, Chapter 7
Figure 7.9a
Section 2, Chapter 7
Bone Homeostasis
Homeostasis of Bone Tissue
Calcium is constantly exchanged between the blood and bone.
Bone resorption = Osteoclasts breakdown bone releasing
calcium into the blood. Bone resorption occurs when
blood [Ca2+] is low and it’s stimulated by parathyroid
hormone (PTH).
Bone deposition = Osteoblasts deposit new bone from
calcium in the blood stream. Bone deposition occurs
when blood [Ca2+] is high and it’s stimulated by the
hormone calcitonin.
Nutrients that effect bone homeostasis
Vitamin D – promotes Ca2+ absorption in small intestine
• Vitamin D deficiency = softened and deformed bones
• Osteomalacia in adults
• Rickets in children
Vitamin A – balances bone resorption and deposition
• Vitamin A deficiency = retards bone development
Vitamin C – is required for collagen synthesis.
• Vitamin C deficiency = results in fragile bones
Hormones that affect bone homeostasis
Calcitonin
• Secreted from thyroid gland
• Promotes bone deposition
Parathyroid Hormone (PTH)
• Secreted from parathyroid glands
• Promotes bone resorption
Figure 7.13 Hormonal regulation of
blood calcium and resorption
Hormones that affect bone homeostasis
Growth Hormone (GH)
• Secreted from pituitary gland
• Promotes bone growth at epiphyseal
plates
Pituitary Gigantism
over secretion of GH during childhood
Pituitary Dwarfism
insufficient GH during childhood
Acromegaly
• Over secretion of GH as an adult
• Occurs after epiphyseal plates have sealed
• Enlargement of hands, feet, nose
Hormones that affect bone homeostasis
Sex Hormones (testosterone & estrogen)
• Promotes long bone growth at puberty
• Sex hormones also stimulate ossification at
epiphyseal plates.
Effects of Exercise on bone
homeostasis
Contracting muscles pull on bones
and promotes bone thickening
Figure 7.12 The thickened bone on
the left is better able to withstand
forces from muscle contractions.
Incomplete Fractures
Greenstick
fracture
Fissured
fracture
Complete Fractures
Transverse
fracture
Oblique
fracture
Comminuted
fracture
Spiral
fracture
When a bone breaks blood vessels rupture and the periosteum tears.
The repair of a broken bone occurs in 5 general steps.
Step 1. hematoma formation
Blood soon forms a
hematoma (blood clot).
Hematoma in foot
Step 2. temporary spongy bone
Step 3. cartilaginous callus
Osteoblasts invade from
periosteum and deposit
temporary spongy bone.
Fibroblasts deposit a mass of
fibrocartilage “cartilaginous
callus”
&
Phagocytes remove hematoma
Osteoclasts remove bony debris
Step 4. bony callus
Step 5. bone remodeling
Osteoblasts replace the
cartilaginous callus with bone,
forming a bony callus
Osteoclasts remove excess
bone, remodeling the bone
the bone close to its original
shape.
Over time, osteoclasts outnumber osteoblasts, and
more bone is resorbed than can be deposited. Bone
mass decreases as a result.
Osteopenia “low bone mass”
• Progresses towards osteoporosis
Osteoporosis “porous bone”
• Bones develop spaces and canals
• Bones are fragile and easily broken
• Common in menopausal women
(from the low estrogen levels)
Bone loss is rapid in menopausal
women due to reduced estrogen
Ways to delay or prevent osteoporosis:
1. Exercise daily.
2. Consume enough calcium and
vitamin D every day.
3. Do not smoke.
End of Chapter 7,
Section 2