Bone Histology and Movement

Module #4

Characteristics of Bone Tissue

Called Osseous tissue

Is a type of connective tissue

Two types :1. Compact – strength2. Spongy – lighter weight

Characteristics of compact bone tissue:

Composed of:

1. Hydroxyapatite - ionic compound composed of calcium and phosphorus. Ca10(PO4)6(OH)2

2. Ground substance (water and other proteins)

3. Collagen

The characteristics of compact bone:

Hydroxyapatite and other minerals give bone its hardness and compressive strength

Collagen (protein) fibers give bone its flexibility.

Osteon

Compact bone - Functional unit called an osteon.

Osseous Tissue

Bone contains 4 types of cells:

1. Osteogenic 2. Osteoblasts3. Osteocytes4. Osteoclasts

1. Osteogenic cellsOsteogenic cells are stem cells that divide to produce osteoblasts.

Assist in repairing bone fractures.

Overproduction causes osteogenic sarcoma (cancer).

Osteogenic Sarcoma

2. Osteoblast cellsOsteoblasts are immature bone cells that secrete the matrix

When osteoblasts are surrounded by bone, they mature and become osteocytes.

3. Osteocyte cellsOsteocytes are mature bone cells that maintain the bone matrix.

Each osteocyte lives in a lacuna.

Canaliculi through the lamellae allow osteocytes to obtain necessary nutrients.

3. Osteocyte cells continued

Osteocytes do not divide.

The main functions of osteocytes are:

1. to maintain the protein and mineral content of the matrix.

2. to help repair damaged bone.

Osteocyte

Osteocyte

4. Osteoclast cells

Osteoclasts are very large cells that remodel bone.(break down old bone)

Note:Bone building and bone recycling must be kept in balance. When osteoclasts break down bone faster than osteocytes build bone, bones become weak.

Osteoclast

Osteoclast

Note:Wolff's law : Bones get stronger with exercise, stress causes osteocytes to build more bone.

Bone remodeling: Bone is broken down because it deteriorates over time and must be replaced. Also minerals are released that the body needs.

Osseous Tissue

Structure of compact bone

Osteon –basic unit.

Osteocytes are arranged inconcentric circles (concentric

lamella)around a central canal called theHaversian canalwhich contains theblood vessels.

Structure of compact bone

Connecting the central canal isthe perforating canals called thecanaliculi carry nutrients to theosteocytes Lacunae – spaces in the rings

where osteocytes are found

Structure of compact bone

In between the osteons is interstitial lamellae or “packing material”

Structure of compact bone:

Circumferential lamallae - A bony layer that underlies the periosteum or endosteum.

Structure of compact bone Perforating canal – canals in the

bone that blood vessels pass sideways between Haversian canals

Structure of compact bone

All osteons in long bones run the length of the bone, strengthening the bone in that direction.

Structure of compact bone

All osteons in long bones run the length of the bone, strengthening the bone in that direction.

Spongy bone

Found in the center of bones and on the ends

Light weight

Structure of spongy boneSpongy bone does not have osteons. Its matrix forms an open network of trabeculae.

Structure of spongy bone

no blood vessels

Spaces are filled with red bone marrow which has blood vessels and supplies nutrients to the osteocytes.

Structure of spongy bone

Red bone marrow is red because it forms red blood cells.

In other bones, spongy bone may hold yellow bone marrow, which is yellow because it stores fat.

Bone formationHuman bones grow until about age 25.

The process of replacing cartilage tissue with bone is called ossification.

Growth in length of long bone

Epiphyseal plate is the site of growth in the long bone. Located between epiphysis and diaphysis.

Epiphyseal plate is composed of hyaline cartilage with chondrocytes (reproductive).

Growth in long bone:1.At epiphyseal plate chondrocytes will reproduce.2.New chondrocytes are laid down on the epiphysis end of bone3.Old chondrocytes on the diaphysis end will elongate the lacunae.4.Calcium salts will be laid down around elongated lacunae.5.Chondrocyte is cut off from nutrients and will die.

Growth in long bone:

6.Blood vessels will grow in lacunae of dead chondrocytes and bring osteoblasts.7.Osteoblasts will produce and maintain bone matrix and mature to osteocytes.8.Therefore, spongy bone will of epiphysis is remodeled into compact bone and diaphysis increases in length.

Growth in long bone

NOTE: Epiphysis can grow in the same way except from the articular cartilage at the end of bone.

Growth in long bone

Appositional bone growth – growth in width of the bone.

Osteoblast lay down new matrix on top of old matrix expanding

diameter.

Bone Remodeling

Old bone broken down by osteoclast and rebuilt by osteoblast.

Reasons:1.Change spongy bone to compact2.Increase/decease mass of bones 3.Repair damaged4.Replace worn out collagen

Bone Remodeling

How:Osteoclasts secrete acids and proteolytic enzymes that break down and dissolve bone salts and digest proteins (collagen).

Osteoblast brought in by damaged blood vessels to lay down new matrix.

Diseases of the bone:1. Rickets – low calcium and phosphorous levels in the blood leading to a lack of hydroxyapatite in the bones resulting in the bones becoming rubbery.

Cause: lack of Vitamin D in the diet which regulates Ca and P

Diseases of the bone:

2. Brittle Bone Disease Osteogenesis

imperfecta –

Result of decreased amount of collagen making the bones brittle.

Cause – genetic

Fractures

Fractures are repaired in 4 steps:Step 1. Hematoma forms

Bleeding produces a clot called the fracture hematoma

Bone cells in the area die.

Fractures Fractures are repaired in 4 steps:

Step 2. Callus forms A callus (mass of tissue that connects the ends of broken bone) is formed.

Internal callus forms new bone tissue and is found between the breaks

External callus stabilizes the bone while it begins to heal and is found around the outside of the bone

FracturesFractures are repaired in 4 steps:

Step 3 Callus is ossified

Osteoblasts replace the central cartilage of the external callus with spongy bone.

FracturesFractures are repaired in 4 steps:

Step 4. External callus removed and remodeled

Osteoblasts and osteocytes continue to remodel the fracture for up to a year, reducing the bone calluses.