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4 Bone Classes
1. Bone
2. Cartilage (3 Types)
a) Hyaline
b) Fibrocartilage
c) Elastic
3. Tendons
4. Ligaments
Hyaline Cartilage
  • Consists of specialized cells that produce matrix
Chondroblasts: form matrix Chondrocytes: surrounded by matrix; are lacunae (a rounded cell that occupies a space within the matrix)

  • Matrix. Collagen fibers for strength, proteoglycans for resiliency
  • Perichondrium. Double-layered C.T. sheath. Covers cartilage except at articulations
  1. Inner: More delicate, has fewer fibers, contains chondroblasts
  2. Outer: Blood vessels and nerves penetrate.  No blood vessels in cartilage itself

  • Articular cartilage. Covers bones at joints; has no perichondrium
  • Growth
  1. Appositional. New chondrocytes and new matrix at the periphery
  2. Interstitial. Chondrocytes within the tissue divide and add more matrix between the cells.
Why does damaged cartilage take a long time to heal? What are the advantages of articular cartilage having no pericardium, blood vessels, or nerves?
In the absence of blood supply, nutrient, chemicals, and cells involved in tissue repair enter cartilage tissue very slowly.  As a result, the ability of cartilage to undergo repair is poor.  Within a joint, the articular cartilage of one bone presses against and moves against the articular cartilage of another bone.  If they were covered by pericardium, or contained blood vessels or nerves, the resulting presure and friction could damage these structures.
5 Functions of Skeletal System
  1. Support
  2. Protection
  3. Movement
  4. Storage
  5. Blood Cell Production
In general, the bones of the elderly people break more easily than the bones of younger people. Give possible explanations.
In the elderly, the bone matrix contains proportionately less collagen than hydroxyapatite, compared with the bones of younger people.  Collagen provides bones with flexible strength, and a reduction in collagen results in brittle bones.  In addition, the elderly have less dense bones with less matrix.  The combination of reduced matrix that is more brittle results in a greater likelihood of bones breaking.
Bone Structure
  • Organic (35%)
collagen and proteoglycans
  • Inorganic (65%)
Hydroxyapatite (calcium phosphate crystal CaPO4

If Mineral Removed: Bendable
If Collagen Removed: Brittle
Bone Cells
  • Osteoblasts - perform bone ossification by forming collagen in its E.R. and golgi and releasing it via exocytosis. Also, hydroxyapatite precursors stored in vesicles and released via exocytosis. They communicate through gap junctions (small channel between cells that allow the passage of ions and small molecules between cells).
  • Osteocytes - Mature bone cells that maintain the matrix. Lacunae are the spaces occupied by the osteocyte cell body and canaliculi are the canals occupied by the osteocyte cell processes.  Nutrients diffuse through lacunae and canaliculi from one cell to the next via gap junctions.
  • Osteoclasts - Breakdown bone by a ruffled border at matrix and plasma membrane (H+ pumped across border and produce acid that decalcifies).
They release enzymes that digest bone.

Derived from monocytes (formed from stem cells in red marrow).

Multinucleated and arise from fusion of a number of cells.
Origin of Bone Cells
Stem Cells from Mesenchymal cells replicate and become Osteochondral progenitor cells.  They then become osteoblasts or chondroblasts.
Woven vs. Lamellar Bone
Woven Bone -
Collagen fibers randomly oriented. Formed during fetal development and fracture repair. During remodeling, woven bone is remodeled into lamellar.

Lamellar Bone -
Mature bone in sheets called lamellae.  Fibers are oriented in one direction in each layer, but in different directions in different layers for strength.
Cancellous vs. Compact Bone
Cancellous -
consists of interconnecting rods or plates of bones called trabeculae. ("scaffolding" filled with marrow, covered with endosteum, oriented along stress lines).

Compact Bone -
Dense, fewer spaces

Central (or Haversian) canals: contain blood vessels, nerves, and loose connective tissue.

Concentric - Surround Central Canal
Circumferential - Thin plates around outer surfaces of compact bone.
Interstitial - Remnants from remodeling in between osteons.

Osteon or Haversian System:
Central canal, contents, assoc. concentric lamellae, and osteocytes.

Perforating (or Volkmann's) Canals:
Perpendicular to long axis of bone that contain blood vessels.  ALSO, direct flow of nutrients from one cell to the other.
Circulation in Bone
Perforating Canals (Volkmanns) from periosteum.

Central Canal vessels

Nutrients and Wastes travel to and from osteocytes via:
1. interstitial fluid of lacunae and canaliculi
2. osteocyte to osteocyte via gap junctions.
Long Bones and their Features

Diaphysis- Shaft/ Compact Bone

Epiphysis- End of Bone/ Cancellous. Ends of bone covered by articular cartilage.

Epiphyseal Plate- Growth Plate made of Hyaline Cartilage/ Present until growth stops. Then becomes Epiphyseal Line (plate ossified).

Medullary Cavity- Large internal space in diaphysis.  Filled with marrow.  Red marrow is site of blood cell formation (in children). Yellow marrow is mostly adipose tissue in limb bones and skull (in adults). Some red marrow remains to exist in proximal part of arm and thigh bones.

Periosteum- Connective tissue membrane that covers the outer surface of a bone.  Outer = fibrous, Inner = cellular (osteoblasts, osteoclasts, and osteochondral proogenitor cells).

Endosteum- Cellular, lines internal spaces.
Flat Bones
No diaphyses or epiphyses

Sandwich of cancellous bone between compact bone.
Compact bone has perforating and central canals.  Why isn't such a canal system necessary in cancellous bone?
Cancellous bone consists of trabeculae with spaces between them. Blood vessels can pass through these spaces.  In compact bone, the blood vessels pass through the perforating and central canals.  The trabeculae in cancellous bone are thin enough that nutrients and gases can diffuse from blood vessels around the trabeculae to the osteocytes in the canaliculi.
Short and Irregular Bones
Compact bone that surrounds cancellous bone center; similar to structure of epiphyses of long bones

No diaphyses and not elongated. Some small epiphyses.

Some flat and irregular bones of skull have air-filled spaces, sinuses, lined by mucous nembranes.
Bone Development

Intramembranous Ossification & Endochondral Ossification

Both Methods initially produce woven bone, which is then remodeled. After remodeling, formation cannot be distinguishes as one or the other.
Intramembranous Ossification:

8th week of development until 2 yrs.

Takes place in connective tissue membrane formed by embryonic mesenchyme.

Forms many skulls bones, part of mandible, diaphyses of clavicles.

Centers of ossification:  The locations in the membrane where ossification begins. (contain oldest and youngest bone).

Fontanels:  larger, membrane covered spaces between developing skull bones that have not yet been ossified. Close by 2 yrs. of age.

Endochondral Ossification:

Bones at base of skull, part of the mandible, epiphyses of the clavicles, and most remaining bones of the skeletal system.

Cartilage formation begins at end of 4th week of development.

Some ossification beginning at about 8th week; some does not begin until 18-20 yrs of age.

  1. A cartilage model is produced by chondroblasts. The cartilage model is surrounded by perichondrium except where the joints will form.
  2. The perichondrium of the diaphyses becomes the periosteum and a bone collar is produced. Internally, the chondrocytes grow excessively, and calcified cartilage is formed.
  3. A primary ossification center is formed as blood vessels and osteoblasts invade the calcified cartilage.  The osteoblasts lay down bone matrix, forming cancellous bone.
  4. The process of bone collar formation, cartilage calcification, and cancellous bone production continues.  Calcified cartilage begins to form in the epiphyses.  A medullary cavity begins to form in the diaphyses.
  5. Secondary ossification centers form in the epiphyses of long bones.
  6. The original cartilage model is almost completely ossified.  Unossified cartilage becomes the epiphyseal plate and the articular cartilage.
  7. In a mature bone, the epiphyseal plate has become the epiphyseal line and all the cartilage in the epiphysis, except the articular cartilage, has become bone.

During endochondral ossification, calcification of cartilage results in the death of chondrocytes.  However, ossification of the bone matrix does not result in the death of osteocytes. Explain.
Chondroblasts are surrounded by cartilage matrix and receive oxygen and nutrients by diffusion through the matrix.  When the matrix becomes calcified, diffusion is reduced the the point that the cells die.  When osteoblasts form bone matrix, they connect to one another by their cell processes.  Thus, when the matrix is laid down, canaliculi are formed.  Even though the ossified bone matrix is dense and prevents significant diffusion, the osteocytes can receive gases and nutrients through the canaliculi or by movement form one osteocyte to another.
Growth in Bone Length
Occurs at epiphyseal plate

Involves the formation of new cartilage by:

1. interstitial cartilage growth (situated between the cells)
2. appositional growth on the surface of the cartilage

Closure of epiphyseal plate - becomes ossified, termed epiphyseal line. Between 12-25 years of age.

Articular Cartilage - does not ossify and persists through life.


(Epiphyseal Side)

1. Zone of resting cartilage - cartilage attaches to the epiphysis

2. Zone of proliferation -
new cartilage is produced on the epiphyseal side of the plate as the chondrocytes divide and form stacks of cells.

3. Zone of hypertrophy -
chondrocytes mature and enlarge

4. Zone of calcification -
matrix is calcified and chondrocytes die.

5. Ossified bone -
the calcified cartilage on the diaphyseal side of the plate is replaced by bone. (the epiphyseal plate remains unchanged).

(Diaphyseal Side)
Growth in Bone Width
Osteoblasts beneath the periosteum lay down bone to form ridges that vessels lie in. The ridges close around the vessel and form a tunnel.  Appositional growth of osteoblasts form new concentric lamella that fills the tunnel and forms an osteon.
Explain why bones cannot undergo interstitial growth, as does cartilage.
Interstitial growth of cartilage results from the division of chondrocytes within the cartilage, followed by the addition of new cartilage matrix between the chondrocytes.  The resulting expansion of the cartilage matrix is possible because cartiage matrix is not too rigid.  Bones cannot undergo interstitial growth because bone matrix is rigid and cannot expand from within.  New bone must therefore be added to the surface by apposition.
A 15-year-old football player is tackled during a game, and the epiphyseal plate of the left femur is damaged.  What are the results of such an injury, and why is recovery difficult?
Damage to the epiphyseal plate interferes with bone elongation; as a result, the bone, and therefore the thigh, will be shorter than normal.  Recovery is difficult because cartilage repairs very slowly.
Growth at the epiphyseal plate stops when the epiphyseal cartilage becomes ossified.  The articular cartilage, however, does not become ossified when the growth of the epiphysis ceases.  Explain why it is advantageous for the articular cartilage not to be ossified.
Growth of articular cartilage results in an increase in the size of the epiphyses.  This is only one of the functions of articular cartilage; it also forms a smooth, resilient covering over the ends of the epiphyses within joints.  Ossified articular cartilage could not perform that function.
Factors affecting bone growth
Size and shape is genetic, however can be affected by nutrition and hormones.

  • Lack of calcium, protein and other nutrients during growth and development can cause bones to be small.
  • Vitamin D
  1. Necessary for absorption of calcium through the intestines.
  2. Can by eaten OR manufactured in the body
  3. Rickets: Lack of Vit D in childhood.
  4. Osteomalacia: Lack of Vit D in adulthood leading to softening of bones.
  • Vitamin C
  1. Necessary for collagen synthesis by osteoblasts
  2. Scurvy: Deficiency of Vit C
  3. Lack of Vit C also causes wounds not to heal and teeth to fall out.


  • Growth Hormone (GH) from anterior pituitary increases general tissue growth (interstitial cartilage growth and appositional bone growth)
  • Thyroid Hormone required for growth of all tissues.
  • Sex Hormones : Estrogen and Testosterone - Cause growth at puberty, but also causes closure of epiphyseal plates and the cessation of growth.
A 12-year-old female has an adrenal tumor that produces large amounts of estrogen.  If untreated, what effect will this have on her growth for the next 6 months? On her height when she is 18?
Her growth for the next few months increases, and she may be taller than a typical 12-year-old female.  Because the epiphyseal plates ossify earlier than normal, however, her height at age 18 will be less than otherwise expected.
Bone Remodeling
Converts woven into lamellar bone.

Caused by migration of Basic Multicellular Units (BMU's) (groups of osteoclasts and osteoblasts that remodel bones).

Involved in bone growth, changes in bone shape, adjustments in bone due to stress, bone repair, and Ca2+ ion regulation.

  • Epiphyseal Growth:

  1. Growth in cartilage surrounding epiphysis.
  2. Cartilage replaced by bone
  3. Bone remodeled.
  • Growth in Length
  1. Cartilage growth in the epiphyseal plate.
  2. Cartilage replaced by bone
  3. Bone remodeled
  4. Bone resorption
  • Growth in Diameter:
  1. Bone addition
  2. Bone resorption
(the diameter increases as a result of the bone growth on the outside of the bone, and the size of the medullary cavity increases because of bone resorption).
Why is mechanical stress important for bone strength?
It increases osteoblast activity in bone tissue.  In a situation where a person is paralyzed, osteoclast activity remains while osteoblast activity is reduced, resulting in a decrease in bone density.

Wolff's Law:
  • a bone grows or remodels in response to the forces or demands placed upon it.
  • Long bones are thickest midway along the shaft (where bending stress is greatest)
  • Curved bones are thickest where they are most likely to buckle.
Bone Repair Steps
  1. Hematoma Formation - localized mass of blood released from blood vessels and confined in a space.
  2. Callus Formation -      Internal: Blood vessels invade macrophages clean up debris, osteoclasts break down dead tissue, fibroblasts produce collagen and granulation tissue, chondroblasts produce cartilage, and osteoblasts form bone.                                   External: Bone/ cartilage collar stabilizes two pieces.
  3. Callus Ossification - Callus replaced by woven, cancellous bone through endochondral ossification.
  4. Bone remodeling - Replacing of cancellous bone and damaged material by compact bone.  Sculpting of site by osteoclasts.
Hormonal Regulation of Ca2+
When blood Ca2+ levels decrease:
  • The parathyroid glands release parathyroid hormone (PTH)
  • This causes an increase in PTH (which then takes calcium from the bone to the blood, increases osteoclast activity, and decreases osteoblast activity in order to get calcium from the bone).
  • Osteoclasts degrade bone matrix and release Ca2+ into the blood
  • Calcium levels rise to normal value.
When blood Ca2+ levels increase:
  • Thyroid gland releases calcitonin.
  • There is an increase of calcitonin (stimulates calcium salt deposit in bone) into the blood.  It decreases osteoclasts activity by binding to their receptors.
  • Blood calcium levels decrease to normal values.
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