Osseous Tissue & Bone Structure

Chapter 6

The Skeletal SystemCartilages

Ligaments (connects bone to bone)

Joints

Bone

Five functions of the skeletal system

Support for the whole body including anchoring of internal organs and soft tissues

Storage of Ca+, Phosphate and Lipids

Red and White Blood Cell production

Protection of organs and tissues

Leverage for skeletal muscle which allows for movement

CartilageInitially the human skeleton is made up of

cartilages and other fibrous membranes.

Most of these early supports are replaced by bone.

Some cartilage remains in adults and is found mainly in regions where flexible skeletal tissue is needed.

Cartilage consists mainly of water which is why it is so resilient.

Cartilage contains no nerves or blood vessels and relies on surrounding tissue for nutrient exchange.

Hyaline Cartilage

The most abundant of the skeletal cartilages.

Made solely of fine collagen fibers.

Hyaline cartilages include:

-Articular cartilages (most joints)

-Costal cartilages (connect ribs to sternum)

-Respiratory cartilages (larynx and trachea)

-Nasal cartilages (supports the external nose)

Elastic Cartilages

Contain more elastic fibers so better able to withstand repeated bending.

Found in only two locations:

-External ear

-Epiglottis

Fibrocartilage

Highly compressible with great tensile strength.

Intermediate composition between Hyaline and Elastic.

Occur in sites that are subjected to both heavy pressure and stretch.

-Menisci

-Intervertebral disks

BoneBecause bones contain various types of

tissue (specialized cells, protein fibers, ground substance, nervous tissue and more), bones are referred to as organs.

The hard structure of bone results from Calcium Phosphate which makes up >66 % of the weight of a bone.

Bone is classified by shapelong, short, flat, irregular, sutural, seasamoid

Short bones = short and boxy with roughly equal dimensions (carpals)

Flat bones = thin roughly parallel surfaces (parietal)

Irregular bones = complex shapes (vertebra)

Sutural bones = between flat bones of the skull (can vary)

Sesamoid bones = small, flat, and shaped somewhat like a sesame seed usually at joints and number can vary (patella)

Long BoneLonger than wide.

Have a central shaft (Diaphysis) Heavy wall of compact bone

Also have an expanded portion on each end (Epiphysis).

The diaphysis surrounds a central cavity (medullary cavity) which contains bone marrow.

The “growth plate” is the area between the diaphysis and the epiphysis (Metaphysis).

Figure 6-2a Bone Structure

Epiphysis

Diaphysis(shaft)

Metaphysis

Medullarycavity

Compactbone

Spongybone

Epiphysis

Metaphysis

The structure of a representativelong bone (the femur) inlongitudinal section

Compact Bone is a relatively solid outside layer.

Spongy Bone is the internal “honeycomb” or porous layer.

Trabeculae are the “little beams” that make up the honeycomb.

Bone Marrow is a loose connective tissue which fills the epiphysis (red and yellow).

-Red bone marrow is where red and white blood cells are formed.

Periosteum surrounds bone and isolates it from surrounding tissues.

-The periosteum provides a route for nutrient supply and aids in bone growth and repair.

Endosteum lines the inner surface of the bone and is active during growth and repair.

Bone Cells Osteoprogenitor cells are stem cells that produce

daughter cells that differentiate into osteoblasts.

Osteocytes are mature bone cells (completely surrounded osteoblast).

They maintain bone structure by recycling Ca+ salts in the matrix and assist in repair.

Osteoclasts are large multinucleated cells which dissolve the bony matrix (osteocytes) and release stored minerals.

Osteoblasts are the cells that produce new bone.

Osteoblasts and Osteoclasts are constantly building up and breaking down bone which helps maintain whole body homeostasis.

Bone and Skeletal GrowthAll skeletal elements begin as cartilage.

When embryo is 6 weeks old bone growth (osteogenesis and ossification) begins and continues until your mid-twenties.

Osteogenesis is the formation of new bone and ossification is when other tissues are replaced with bone.

There are two types of ossification intramembranous and endochondral.

Intramembranous OssificationWhen bone develops from a fibrous

membrane.

Results in the formation of cranial bones of the skull and the clavicles.

All bones formed this way are flat bones.

Usually begins at about the 8th week of development.

Ossification begins at an ossification center which is an area where cells differentiate into osteoblasts.

Osteoblasts then form bone and mature into osteocytes.

Endochondral OssificationForms most of the bones of the skeleton.

Occurs when bone is developed by replacing hyaline cartilage.

Around the 6th to 8th week of development the cartilage “pattern” that has previously developed in the fetus begins to be replaced by endochondral bone.

Endochondral Ossification 1. Cells at the center of the cartilage enlarge and die

as the bone matrix forms.

2. New osteoblasts from the inner layer of the cartilage cover the diaphysis in a thin layer of bone.

3. Blood vessels penetrate the cartilage and new osteoblasts form a primary ossification center forming spongy bone.

4. The bone of the diaphysis thickens and cartilage near each epiphysis is replaced by bone as ossification continues from the middle of the diaphysis to either end.

5. Blood vessels invade the epiphyses and osteoblasts form secondary ossification centers which produce spongy bone inside each epiphysis.

Fig. 6-9a, Steps 1 and 2, p. 190

Fig. 6-9a, Steps 3 and 4, p. 190

Fig. 6-9a, Steps 5 and 6, p. 190

What is required for bone growth?Minerals including calcium salts

Vitamin DConverted to hormone calcitriol which

stimulates absorption of calcium and phosphate from digestive tract

Vitamins A & CNecessary for regulation of osteoblast activity

Various hormonesHGH, Thyroid, Estrogen, Testosterone and

more

Bone as a mineral reserveCa++ ions play a role in a variety of

physiological processes

Small deviations from normal affect cellular operations; larger changes can cause clinical crisis 30 % increase in body fluids = neurons and muscle

cells become unresponsive 35 % decrease = neurons hyper-excitable and

convulsions 50 % decrease = death

Hormones and Ca++ Balance (impacts bone, intestines, kidneys)

Ca ion concentrations in blood fall below normal PTH stimulates osteoclast activity Intestinal absorption of Ca ions increases (via

enhancement of calcitriol action) Decreases rate of excretion of Ca ions at the kidneys

Calcium ion concentrations in blood rise above normal Calcitonin released from thyriod Calcitonin inhibits osteoclast activity Calcitonin increases the rate of excretion of Ca ions at

the kidneys PTH (and/or calcitriol) levels decrease, decreasing rate

of intestinal absorbtion

Injury and Repair

1. Fracture results in broken blood vessels and bleeding, clot (fracture hematoma) forms to close off injured vessels. Results in dead bone at the area of the break.

2. Cells from the periosteum and endosteum proliferate and migrate to fracture zone forming an external and internal callus. Hyaline cartilage is formed from chondrocytes differentiated from the cells at the center of the external callus.

3. Osteoblasts replace cartilage with spongy bone forming a continuous brace of spongy bone at the fracture site.

4. Remodeling of spongy bone continues leaving living compact bone in it’s place.

Fig. 6-15, Steps 1 and 2, p. 199

Fig. 6-15, Steps 3 and 4, p. 199

Types of Fractures (named according to several criteria)

External AppearanceOpen or Closed

LocationExamples; Pott’s, Colles’

Nature of BreakTransverse, spiral, comminuted

Completeness of BreakComplete or incomplete

Usually named using several of the above criteria

Fig. 6-16a, p. 200

*Affects both medial and lateral malleolus

Fig. 6-16b, p. 200

Fig. 6-16c, p. 200

Fig. 6-16d, p. 200

Fig. 6-16e, p. 200

Fig. 6-16f, p. 200

Fig. 6-16g, p. 200

Fig. 6-16h, p. 200

Fig. 6-16i, p. 200

Osteopenia and Osteoporosis

Osteopenia Due to inadequate ossification Normal result of aging (begins around 30 years old) Osteoblast activity declines and osteoclast activity

remains the same Women 8%/decade; Men 3%/decade Epiphyses, vertebrae, and jaws are most affected

Results in fragile limbs Reduction in height Tooth loss

Osteopenia and Osteoporosis

Osteoporosis 29% of women over 45 18% of men over 45 Bone loss compromises normal function Sex hormones play a vital role Results in fragile weak bones “dowagers hump”, hip fractures etc… Bone loss and cancer

Cancerous tissues release osteoclast-activating factor This stimulates osteoclasts Produces severe osteoporosis

Fig. 6-17, p. 201

Exercise plays an important role in prevention and regular bone health

Exercise stresses bones and stimulates osteoblast activity Hypothesized that this is due to small electrical fields

It is important to do some form of weight bearing exercise throughout your life to maximize bone density when young and slow bone loss as you age

Additionally, exercise (especially resistance training) improves strength and balance thus reducing the incidence of falls and fractures

Go to ACSM.ORG for their position stand on osteoporosis and exercise recommendations