Bone Lecture

Question 1

Bone (5 details, And What it’s composed of)

Answer 1

A. One of the hardest tissues of the body.
B. Main constituent of the skeleton.
C. Provides support for all soft tissues and protects the vital organs (i.e. cranial and thoracic cavities).
D. Contains bone marrow - where blood cells are formed.
E. Forms a system of levers which multiply the forces generated during skeletal muscle contraction.
F. Composed of:
1. Bone Matrix
2. Osteocytes
3. Osteoblasts
4. Osteoclasts

Question 2

Bone Matrix (4 big details)

Answer 2

A. Like other connective tissues, bone is composed of cells which are widely separated by a great deal of intercellular material.
B. About 50-70% of matrix dry weight = inorganic matter = hydroxyapetites (calcium phosphate and calcium carbonate)
- calcium and phosphorus are very abundant and form crystals called hydroxyapetite
C. Organic matter is about 30-50% of matrix dry weight
- organic matter is about 95% collagenous fiber; and also amorphous ground substance containing acid mucopolysaccharides.
D. It is the association of hydroxyapatite with the collagenous fibers that is responsible for the “hardness” characteristic of bone.
- remove inorganic calcified component (e.g. decalcification by treatment with acid) and bone will become very pliable
- remove organic (collagen) component (e.g. by burning) bone becomes very brittle.

Question 3

Osteocytes (3 details)

Answer 3

A. Found in lacunae within the bone matrix.
B. Canaliculi - tiny passageways which radiate from lacunae. Since intercellular matrix is not readily permeable to diffusion, canaliculi connect osteocytes with neighboring cells, internal and external bone surfaces, and blood vessels.
C. Believed that these cells assist in maintaining matrix.

Question 4

Osteoblasts (3 details)

Answer 4

A. Responsible for synthesis of organic components of bone matrix (collagen and glycoproteins)
B. Exclusively located at the surfaces of bone tissue.
- arranged side by side almost like simple epithelium.
- have a more cuboidal shape when intensely engaged in matrix synthesis.
C. When osteoblast becomes surrounded by newly formed matrix = osteocyte

Question 5

Osteoclasts (3 details)

Answer 5

A. Very large motile multinucleated cells.
B. Appear on bone surfaces wherever bone resorption occurs.
C. Responsible for breaking down existing bone.

Question 6

Periosteum and Endosteum

Answer 6

Internal and external surfaces of bones are covered by layers of connective tissue.

• surface not covered by these layers or by osteoblasts is subject to resorption due to osteoclasts which arrive in area immediately.
• thus care must be taken in surgery.

Question 7

Periosteum (vs. Endosteum)

Answer 7

1. 2 layers: outer layer is “fibrous layer”
   inner layer is “osteogenic layer”
2. layer of dense C.T. - very fibrous externally but more cellular and vascular near bone.
3. cells have morphology of fibroblasts but are transformed into osteoblasts - thus play important role in bone growth and repair.

Question 8

Endosteum (vs. Periosteum)

Answer 8

• same components as periosteum, but much thinner and do not have 2 layers.
• contain osteoblasts.

Question 9

(Types of Bone) Compact Bone

Answer 9

dense without cavities

Question 10

(Types of Bone) Spongy Bone

Answer 10

• areas with numerous interconnecting cavities. Connected by trabeculae.
• however the bone tissue itself is very similar in these two types

Question 11

(Types of Bone) Long Bones

Answer 11

epiphysis - spongy bone covered by thin layer compact bone.

diaphysis - hollow shaft of almost totally compact bone - only some spongy bone around marrow cavity.

Question 12

(Types of Bone) Short Bones and Irregular Bones

Answer 12

core of spongy bone surrounded by compact bone.

Question 13

(Types of Bone) Flat Bones

Answer 13

• 2 layers of compact bone - plates with middle layer of spongy bone - diploe
  *cavities within bone contain bone marrow
  2 kinds: Red marrow - site of blood cell formation
  Yellow marrow - mainly fat cells

Question 14

(Histologically - 2 kinds of bone) Primary

Answer 14

(immature or woven)

• collagenous bundles are randomly organized
• in bone formation and repair - first bone tissue to appear.
• replaced in adults by secondary bone except in just a few places (cranial sutures, tooth sockets, some tendon insertions)
• smaller content of minerals (more easily penetrated by x-rays)
• higher percentage of osteocytes

Question 15

(Histologically - 2 kinds of bone) Secondary

Answer 15

(mature or lamellar)

• collagenous fibers arranged in lamellae
• either parallel or concentrically organized around vascular canal - Haversian Canal
• complex called Haversian System (Osteon)
• osteocytes are found between and within the lamellae.

1. Haversian System - long, often bifurcated cylinder running parallel to the diaphysis
2. Haversian Canals - communicate with the marrow cavity and with the periosteum and with each other via transverse or oblique canals called:
3. Volkmann’s Canals - Volkmann’s canals do not have concentric lamellae
   - collagenous fibers of different lamella run at right angles to each other.

Question 16

Histogenesis: 2 types and about them

Answer 16

Intramembranous and Endochondral

• in both processes, primary bone tissue appears first. It is temporary bone and in soon replaced by the definitive lamallar variety.
• during bone growth, areas of primary bone, areas of resorption, and areas of lamallar bone appear side by side. Process of synthesis and removal occurs throughout life. (although slower in adults)

Question 17

Intramembranous Ossification (6 details)

Answer 17

• takes place within membranes of C.T. (e.g. flat bones some bones of skull) (also is a factor that contributes to short bones growth and long bone’s thickening.)
• within C.T. a primary ossification center develops as osteoblasts differentiate from firbroblasts
• as osteoid synthesis and calcification continue, osteoblasts become osteocytes
• matrix forms in “spicules” that fuse to give a spongy appearance.
• remaining C.T. is penetrated by B.V.’s and mesenchymal cells which give rise to bone marrow cells.
• portion of C.T. that does not differentiate or ossify forms the periosteum and endosteum.

Question 18

Endochondral Ossification (4 details)

Answer 18

1. takes place over a piece of hyaline cartilage whose shape resembles a small version of the bone to be formed.
2. principally responsible for formation of short and long bones.
3. Formation of a Long Bone (example of endochondral ossification)
   - Epiphyseal plate consists of 4 zones
   * the epiphyseal plate is only mechanism for lengthening the diaphysis.
   - this will allow lengthening of diaphysis until early adulthood.
   * growth in diameter accompanies growth in length as bone is resorbed to enlarge marrow cavity - periosteum continues to lay down bone on outside.

Question 19

Fracture Repair (4 steps, and 6 types)

Answer 19

A. Blood flows into the fracture site producing a fracture hematoma.
B. Fibroblasts migrate from the adjacent periosteum into the hematoma and transform into chondroblasts that form a fibrocartilage callus at the fracture site.
C. Chondrocytes transform into osteoblasts and begin to form immature or woven bone forming a boney callus that initially resembles spongy bone
D. immature bone is gradually transformed into mature bone. The callus is reduced in size, but remains after the bone has healed. Compact bone forms at the periphery of the bone.

E. Frequent types of fracture:

1. Comminuted fracture – bone fragmented into multiple pieces
2. Compression – bone is crushed – often occurs in vertebrae
3. Spiral – created by excessive twisting motion on a long bone
4. Epiphyseal – Separation at the epiphyseal plate separating the diaphysis from the epiphysis
5. Depression fracture – normally a skull fracture where a cranial bone is pushed inward.
6. Greenstick – Bone is broken, but not separated into separate pieces. Normally in a long bone.

Question 20

Osteoporosis (What is this?)

Answer 20

chronic condition where osteoclast activity outperforms osteoblast activity causing a weakening of the bone.