WEEK 1: HISTOLOGY OF BONE

Question 1

Outline the functions of bone.

Answer 1

• Provides support
• Protects vital organs
• Reservoir of calcium and other ions

Question 2

State the 4 types of cells that make up the bone tissue.

What are their functions?

Answer 2

*Osteocytes
*Osteoblasts
*Osteoclasts
*Osteoprogenitor cells

1.Osteoprogenitor cells that differentiate into osteoblasts

2. Osteoblasts are growing cells which synthesize and secrete the organic components of the matrix.
   They form either bone lining or osteocytes. They are found in surfaces of new bones.
3. Osteocytes are found in cavities (lacunae) between bone matrix layers (lamellae), with cytoplasmic processes in small canaliculi (L. canalis, canal) that extend into the matrix. They are formed from osteoblasts. Are responsible for maintaining the bone matrix. They are mature cells.
4. Osteoclasts are giant, multinucleated cells involved in removing calcified bone matrix and remodeling bone
   tissue. They are found in bone surfaces and sites of old, injured or unneeded bone.

Question 3

Describe the formation of bone matrix by osteoblast.

Answer 3

1.Osteoblasts secrete type I collagen, several
glycoproteins, and proteoglycans.

2.Osteocalcin and certain glycoproteins, bind Ca2+ with high affinity, thus raising the local concentration of these ions.

3.Osteoblasts also release very small membrane-enclosed matrix vesicles with which alkaline phosphatase
and other enzymes are associated.

4.These enzymes hydrolyze PO4 ions from various
macromolecules, creating a high concentration of these ions locally.

5.The high ion concentrations cause crystals of CaPO4
to form on the matrix vesicles.

6.The crystals grow and mineralize further with formation of small growing masses of hydroxyapatite [Ca10(PO4)6(OH)2] which surround the collagen fibers and all other macromolecules.

7.Eventually the masses of hydroxyapatite merge as a confluent solid bony matrix as calcification of the matrix is
completed.

Question 4

Differentiate between osteoclast and osteoblast.

Answer 4

OSTEOBLASTS
*Are developed from osteogenic/ osteoprogenitor cells.
*Small and mononucleated.
*Involved in bone formation.

OSTEOCLAST
*Are developed from macrophages and monocytes
*Large and multinucleated
*Involved in bone resorption

Question 5

What is osteoid?

Answer 5

An organic matrix of bone not yet mineralized that is laid down by osteoblasts.

Question 6

How do osteoclasts digest bone?

Answer 6

1.At the osteoclast sealing zone, integrins tightly bind to the cell bone matrix.

2.The sealing zone surrounds the ruffled border of microvilli and other cytoplasmic projections close to the matrix.

3.The sealed space between the cell and the matrix is acidified by proton pumps in a ruffled border of the cell membrane and receives secreted matrix METALLOPROTEASES and other HYDROLYTIC ENZYMES.

4.The acid promotes dissolution of hydroxyapatite from the bone and stimulates activity of the PROTEIN HYDROLASES producing matrix resorption.

5.The breakdown products of collagen and other polypeptides are endocytosed by the osteoclasts and further degraded in lysosomes.

6.Calcium is released directly and taken up by the blood

Question 7

What is periosteum?

Answer 7

It is the outer sheath outside the bone that supplies them with blood, nerves and cells that help them to grow and heal.

Question 8

State the two layers of periosteum.

Answer 8

1.Outer fibrous layer: Contains irregular, dense connective tissue with more collagenous matrix and less number of cells.

2.Inner cellular layer
-Made of osteoprogenitor cells.

Question 9

What is endosteum?

Answer 9

A membrane that lines the center of the bone that contain bone marrow.

Question 10

State the 3 classes of endosteum.

Answer 10

1.Cortical endosteum: lining bone marrow cavity
2.Osteon endosteum: lining the osteon
3.Trabeculae endosteum: lines the trabeculae near the developing part of the bone.

Question 11

Differentiate between endosteum and periosteum.

Answer 11

ENDOSTEUM
*Thin
*Has one cellular layer of connective tissue
*Main cells present are fibroblasts and hematopoietic cells
*Found in medullary canal, Volkman’s canal’ harversian canal and spongy bone.
*Responsible in bone growth, repair and remodeling.

PERIOSTEUM
*Thick
*Has 2 layers: Outer fibrous layer and inner cellular layer
*Mature osteoblasts are the main cells.
*Found on the outer surface of all bones except sesamoid bones and at the surfaces of joints.
*Forms the attachment for tendons and ligaments and nourishes compact bone.

Question 12

State the 2 main types of bones.

Answer 12

1.Woven bone
2.Lamellar bone

Question 13

State the 2 types of lamellar bone and their % of lamellar bone.

Answer 13

1.Compact bone 80%
2.Cancellous bone 20%

Question 14

What are the synonyms for woven bone?

Answer 14

Immature bone; primary bone; bundle bone

Question 15

What are the synonyms for lamellar bone?

Answer 15

Mature bone; secondary bone

Question 16

Differentiate between Woven and lamellar bone.

Answer 16

WOVEN BONE
*Newly calcified
*Irregular and random arrangement of cells and
collagen
*Lightly calcified
*LOCATION: Developing and growing bones; hard callus of bone fractures.

LAMELLAR BONE
*Remodeled from woven bone
*Parallel bundles of collagen in thin layers (lamellae), with
regularly spaced cells between.
*Heavily calcified
*LOCATION: All normal regions of adult bone.

Question 17

Describe compact bone.

Answer 17

1.Also called Cortical bone.
2. Makes ~80% of all lamellar bone
3.Parallel lamellae or densely packed osteons, with
interstitial lamellae
4.LOCATION: Thick, outer region (beneath periosteum) of bones.
5.It is compact with less pores.
6.Contain yellow bone marrow: site for fat storage.
7.Can withstand heavy weights.
8.Aids in movement and structural support.

Question 18

What is an osteon?

Answer 18

A structural unit of compact bone that is parallel to the long axis of bone.

Question 19

Describe structures of an osteon.

Answer 19

1.central canal/ harversian canal: Contain blood supply in between.

2.Circumferential lamellar: Prevent twisting of long bone

3.Interstitial lamellar: Fills gaps between osteons

4.Perforating/ Volkmann’s canal:

-Perpendicular to the long axis of bone and interconnect the central canal and also create a continuous blood flow path between the periosteum and medullary cavity.

Question 20

Describe cancellous bone.

Answer 20

1.Also called Spongy bone; trabecular bone; medullary bone
2.Makes ~20% of all lamellar bone
3.Contains Interconnected thin spicules or trabeculae covered by endosteum.
4. LOCATION: Inner region of bones, adjacent
to marrow cavities
5.It is porous
6.Contain red bone marrow: site for hematopoiesis

Question 21

What are the functions of spongy bone?

Answer 21

1.Reduces weight of the skeleton
2.Site for erythropoiesis
3.Mineral storage
4.Storage for bonre marrow.

Question 22

State the two ways of bone formation.

Answer 22

1.Endochondrial ossification
2.Intramembranous ossification

Question 23

Describe the process of endochondrial ossification.

Answer 23

1.Mesenchymal cells condense and differentiate to form chondrocytes which form a hyaline cartilage.

2.The chondrocytes hypertrophy and the extracellular matrix surrounding the cell become calcified.

3.Blood vessels invade the center of the cartilage and cause the perichondrium to differentiate into periosteum.

4.Chondrocytes are converted into osteoprogenitor cells.

5.Osteoprogenitor cells convert to osteoblast.

6.Bone matrix secreted by the osteoblast forms bone collar.

7.Nutrients are prevented from reaching the hypertrophied chondrocytes causing them to degenerate.

8.Osteoclasts break the bone and form holes on the collar bone allowing passage of periosteal buds.

9.The periosteal buds consist of blood vessels and osteoprogenitor cells and hematopoietic stem cells.

10.Osteoprogenitor cells form more osteoblasts which continue to form the bone matrix on the surface of the calcified cartilage.

11.The bone matrix calcifies forming a calcified cartilage bone complex

12.The bone collar grow in either direction towards the epiphyses.

13.Osteoclasts resorb the cartilage calcified bene complex to widen the marrow cavity.

14.Secondary ossification center develops at the epiphyses of long bones. The osteoblasts continue to secrete the matrix on the cartilage framework.

Question 24

Long bones increase in length at the secondary ossification centers.
State the 5 zones of ossification in the epiphyses.

Answer 24

1.Zone of calcium reserve: Contains randomly arranged mitotically active chondrocytes.

2.Zone of proliferation; Chondrocytes are proliferating and form isogenous groups in rows oriental parallel to the long axis of the bone.

3.Zone of hypertrophy: Cells move towards the diaphysis, hypertrophy, mature and collect glycogen with their cytoplasm. The chondrocytes undergo apoptosis as they migrate.

4.Zone of calcification: Calcium ions brought to the epiphyses through blood vessels calcify the cartilage matrix surrounding the dying chondrocytes.

5.Zone of ossification: Osteoprogenitor cells arrive and become osteoblasts which secrete bone matrix on the calcified cartilage. Bone tissue is formed as the matrix becomes calcified.

Question 25

Describe the radial growth of bone.

Answer 25

During radial bone growth formation of new bone at the periosteal surface occurs concurrently with bone removal at the endosteal surface around the large medullary, enlarging this marrow-filled region and not greatly increasing the bone’s weight.

Question 26

What s bone remodeling?

Answer 26

The process of resorption followed by bone replacement of bone with limited change in shape.

Question 27

Describe the process of bone remodeling.

Answer 27

1.Osteoblast release RANKL.

2.RANKL binds to RANK receptors on monocytes and activate them to become osteoclasts.

3.Osteoclasts release lysosomal enzymes (collagenase) which dissolve collagen on the bone matrix forming Howship’s lacunae and HCL which dissolve hydroxyapatite into calcium and phosphate ions.

4.Osteoblasts will the secrete OSTEOPROTEGERIN which binds to RANKL and preventing it from activating RANK receptors, hence slows down activation of osteoblasts.

5.Osteoclasts die by apoptosis after their function.

6.Osteoblasts will the secrete osteoid seam on the lacunae.

7.Calcium and phosphate ions deposit on the seam forming hydroxyapatite.

8.The osteoblasts are trapped on the matrix and become osteocytes.

Question 28

Describe the process of bone repair.

Answer 28

(1) Blood vessels torn within the fracture release blood that clots to produce a large fracture hematoma.

2.The blood vessels at the broken ends are sealed by the clotting process. Bone cells are derived of nutrients and begin to die.

3.Capillaries grow into the hematoma, while phagocytic cells clear the dead cells away.

4.Osteoblasts and fibroblasts enter the area and begin to form bone.

5.Fibroblasts produce collagen fibers that connect the broken ends while osteoblasts start to reform spongy bone.

6.A fibrocartilaginous callus forms which is then converted to spongy bone.

7.The spongy bone then undergoes bone remodeling.

8.A compact bone is added to make it similar to the original.

Question 29

Describe the Hormonal impact on bone remodeling.

Answer 29

PROMOTE RESORPTIONA

1.PTH
-Increase the level of calcium ions in the body.
-Has an indirect action on osteoclasts by increasing the activity of RANKL resulting in high bone resorption.
2.GH stimulate both.

PROMOTE FORMATION
1.ESTROGEN
2.Calcitonin
3.Glucocorticoids
4.Thyroid hormone
5.Vitamin D

Question 30

Outline structures of a long bone.

Answer 30

1.Periosteum
2.Diaphysis, metaphysis, epiphyses
3.Compact bone, spongy bone
4.Endosteum
5.Medullary cavity
6.Yellow bone marrow, red bone marrow
7.Articular cartilage.

Question 31

State the types of bone fractures.

Answer 31

1.Open or compound fracture; Broken bones pierce the skin

2.Spiral fracture:

3.Transverse fracture: This type of fracture has a horizontal fracture line.

4.Oblique fracture: This type of fracture has an angled pattern

5.Comminuted fracture: In this type of fracture, the bone shatters into three or more pieces.

6.Linear fracture: Vertical fracture parallel to the bone long axis.

7.Greenstick fracture: A Greenstick fracture is an incomplete fracture in which the bone is bent. This type occurs most often in children.

8.Segmental fracture

9.Avulsion: An avulsion fracture is a bone fracture which occurs when a fragment of bone tears away from the main mass of bone as a result of physical trauma.