Skeletal System

Functions of Skeletal System

Support
Protection
Movement
Storage
Blood Cell Production

Four Components of Skeletal System

Bones
Cartilage
Tendons (Muscle to Bone Attachment)
Ligaments (Bone to Bone Attachment)

Cartilage

A strong, flexible connective tissue that protects your joints and bones
Three types of Cartilage:

Hyaline Cartilage
Fibrocartilage
Elastic Cartilage

Hyaline Cartilage

Hyaline cartilage is most intimately associated with bone.
The growth in length of bones and bone repair often involves the production of hyaline cartilage, followed by its replacement with bone.
Chondroblasts: developmental cartilage cells
Chondrocyte: mature cartilage cells that maintain the cartilage structure
Lacuna: space within the matrix
Perichondrium: a double-layered connective tissue sheath covering most cartilage

Cartilage Growth

Appositional Growth:

Appositional growth is the addition of new cartilage matrix on the surface of cartilage.
Chondroblasts in the inner layer of the perichondrium lay down new matrix on the surface of the cartilage under the perichondrium.
When the chondroblasts are surrounded by matrix, they become chondrocytes in the new layer of cartilage.

Interstitial Growth:

Interstitial growth is the addition of a new cartilage matrix within cartilage.
Chondrocytes within the tissue divide and add more matrix between the cells.
The addition of new cells and matrix increases the thickness of the cartilage.

Articular Cartilage

A cartilage covering the ends of bones where they come together to form joints
Has no perichondrium, blood vessels or nerves

Bone Matrix

Responsible for the characteristics of bone
Mature bone matrix normally is approximately 35% organic and 65% inorganic material

Organic material: Collagen and Proteoglycans
Inorganic material: Hydroxyapatite

The collagen and mineral components are responsible for the major functional characteristics of bone

Collagen: Flexible Strength
Mineral: Matrix Compression Strength

Bone Cells

Osteoblasts: produce new bone matrix
Ossification or osteogenesis: is the formation of bone by osteoblasts
Osteocytes: are mature bone cells that maintain the bone matrix
Lacunae: spaces occupied by the osteocyte cell bodies
Canaliculi: spaces occupied by the osteocyte cell processes
Osteoclasts: are responsible for the resorption, or breakdown, of bone

Bone Tissue Classifications

According to Organization of Collagen Fibers:

Woven Bone:

Randomly oriented in many fibers
First formed during fetal development or during the repair of a fracture
Bone Remodeling (from woven to lamellar)

Lamellar Bone:

Mature bone that is organized into thin sheets or layers

According to the Amount of Matrix:

Spongy Bone:

Spongy bone, or cancellous (lattice) bone, appears porous. It consists of thin rods or plates of interconnecting bone called trabeculae with spaces between the trabeculae.

Compact Bone:

Compact bone is denser and has fewer spaces than spongy bone. Blood vessels enter the substance of the bone itself, and the lamellae of compact bone are primarily oriented around those blood vessels.

Bone Shapes

Long: Femur, Ulna, Radius
Short: Capitate (carpal) bone, Talus
Flat: Scapula, Sternum
Irregular: Sphenoid bone, Vertebra

Structure of a Long Bone

The diaphysis is the shaft of a long bone, and the epiphysis is separated from the diaphysis by the epiphyseal plate or line.
The epiphyseal plate is the site of bone growth in length. The epiphyseal plate becomes the epiphyseal line when all of its cartilage becomes bone.
The medullary cavity is a space within the diaphysis (contains yellow marrow).
Red bone marrow is the site of blood cell production, and yellow bone marrow consists of fat.
The periosteum covers the outer surface of the bone. The outer layer contains blood vessels and nerves. The inner layer contains osteoblasts, osteoclasts, and osteochondral progenitor cells.
The endosteum lines cavities inside bone and contains osteoblasts, osteoclasts, and osteochondral progenitor cells.

Structure of Flat, Short, and Irregular Bones

Flat, short, and irregular bones have an outer covering of compact bone surrounding spongy bone.
Spaces contain bone marrow and blood vessels.

Bone Development

Bones develop in the fetus by intramembranous or endochondral ossification. Both processes involve the formation of bone matrix on preexisting connective tissue. Both methods initially produce woven bone, which is then remodeled. After remodeling, bone formed by intramembranous ossification cannot be distinguished from bone formed by endochondral ossification.

Intramembranous Ossification:

Some skull bones, part of the mandible, and the diaphysis of the clavicles develop from membranes.
Within the membrane at centers of ossification, osteoblasts produce bone along the membrane fibers to form spongy bone.

Endochondral Ossification:

Chondroblasts produce a cartilage model that is surrounded by perichondrium, except where joints will form.
The perichondrium of the diaphysis becomes the periosteum, and a bone collar is produced. Internally, the chondrocytes hypertrophy, and calcified cartilage forms.
A primary ossification center forms as blood vessels and osteoblasts invade the calcified cartilage. The osteoblasts lay down bone matrix, forming spongy bone.
The process of bone collar formation, cartilage calcification, and spongy bone production continues. Calcified cartilage begins to form in the epiphyses. A medullary cavity begins to form in the center of the diaphysis.
Secondary ossification centers form in the epiphyses of long bones.
The original cartilage model is almost completely ossified. Unossified cartilage becomes the epiphyseal plate and the articular cartilage.
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.

Stages of Bone Repair

Hematoma formation: Blood released from damaged blood vessels forms a hematoma.
Callus formation: The internal callus forms between the ends of the bones, and the external callus forms a collar around the break.
Callus ossification: Woven, spongy bone replaces the internal and external calluses.
Bone remodeling: Compact bone replaces woven bone and part of the internal callus is removed, restoring the medullary cavity.

Bone Growth Mechanisms

General Principle:

Unlike cartilage, bones cannot grow by interstitial growth.
Bones increase in size only by appositional growth, the formation of new bone on the surface of older bone, or cartilage.
For example, trabeculae grow in size by the deposition of new bone matrix by osteoblasts onto the surface of the trabeculae.

Bone Growth in Length:

Chondrocyte replication and hypertrophy result in interstitial cartilage growth.
Interstitial cartilage growth increases the length of the bone.
Ossification of calcified cartilage produces additional bone on the diaphyseal side of the epiphyseal plate.
The thickness of the epiphyseal plate remains unchanged because the addition of cartilage through interstitial cartilage growth occurs at the same rate as the addition of bone through ossification of calcified cartilage.

Bone Growth in Width:

Osteoblasts beneath the periosteum lay down bone to form ridges separated by grooves. Blood vessels of the periosteum lie in the grooves.
The groove is transformed into a tunnel when the bone built on adjacent ridges meets. The periosteum of the groove becomes the endosteum of the tunnel.
Appositional growth by osteoblasts from the endosteum results in the formation of a new concentric lamella.
The production of additional concentric lamellae fills in the tunnel and completes the formation of the osteon.

Factors Affecting Bone Growth

Genetic factors determine bone shape and size. The expression of genetic factors can be modified.
Factors that alter the mineralization process or the production of organic matrix, such as deficiencies in vitamin D, can affect bone growth.
Growth hormone, estrogen, and testosterone stimulate bone growth.
Estrogen and testosterone cause closure of the epiphyseal plate.

Common Diseases and Disorders of Bones

Bursitis

Description: Inflammation of a bursa, which is a fluid-filled sac that cushions tendons. It occurs most commonly in elbow, knee, shoulder and hip.
Causes: Overuse of trauma and joints, Bacterial infections
Signs and Symptoms: Joint pain and swelling, Tenderness in the structures surrounding the joint
Treatment: Bed rest, Pain medications, Steroid injections, Aspiration of excess fluid from bursa, Antibiotics

Carpal Tunnel Syndrome

Description: Median nerve in the wrist is excessively compressed. Typists, assembly-line workers, painters, and people who play sports such as racquetball are most likely to develop carpal tunnel syndrome.
Causes: Overuse of the wrist
Signs and Symptoms: Weakness and numbness in the hand, and pain in the wrist, hand or elbow
Treatment: Wrist splints, Pain medication, Steroid injection

Achondroplasia

A person with a nearly normal-sized trunk and head but shorter than normal limbs.
Achondroplasia is an autosomal-dominant trait, caused by a mutation of a gene regulating bone growth.

Gigantism

Giantism is a condition of abnormally increased height that usually results from excessive cartilage and bone formation at the epiphyseal plates of long bones.

Acromegaly

Acromegaly involves the growth of connective tissue, including bones, after the epiphyseal plates have ossified.

Scoliosis

Scoliosis is a sideways curve of the spine. Everyone has normal curves in the spine, and when looked at from behind, the spine appears straight. However, children and teens with scoliosis have an abnormal S-shaped or C-shaped curve of the spine.

Osteoporosis

Porous bone, results from a reduction in the overall quantity of bone tissue.
Occurs when the rate of bone resorption exceeds the rate of bone formation.

Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) (imperfect bone formation) is a rare disorder caused by any one of a number of faulty genes that results in either too little collagen formation or a poor quality of collagen.
As a result, bone matrix has decreased flexibility and is more easily broken than normal bone.
Osteogenesis imperfecta is also known as the "brittle bone" disorder. In mild forms of the disorder, children may appear normal except for a history of broken bones.