Bones and Skeletal Tissue

Chapter 6

Bones and Skeletal Tissue

Pages 172-191

Important Vocabulary

Hyaline cartilage / Sesamoid bones / Epiphyseal Plate
Elastic cartilage / Flat bones / Periosteum
Fibrocartilage / Irregular bones / Osteoblasts
Appositional growth / Spongy bone / Osteoclasts
Interstitial growth / Compact bone / Osteogenic Cells
Appendicular skeleton / Trabeculae / Endosteum
Long bones / Diaphysis / Dipole
Short bones / Epiphyses / Red Marrow
Osteon / Canaliculi / Membrane bone
Haversian system / Interstitial lamella / Endochondral bone
Lamella / Circumferential lamella / Periosteal bud
Haversian Canal / Osteoid / Osteomalacia
Volkmann’s Canal / Ossification / Osteoporosis

I. Basic Structure, Types and Locations of Skeletal Cartilage

Skeletal Cartilage contains no blood vessels. It has a high water content which is responsible for its resilience. Surrounding the cartilage is a perichondrium. This helps hold the cartilage together and contains blood vessels which nourish the avascular cartilage through diffusion. This limits the thickness of cartilage. All three types of cartilage contain the following components:

v  Cells called chondrocytes that reside in lacunae

v  Extracellular matrix

The three types of cartilage are:

v  Hyaline cartilage is the most common and contains collagen fibers. It is found:

o  Articular surfaces

o  Costal cartilages

o  Respiratory cartilage

o  Nasal cartilage

v  Elastic cartilage, contain elastic fibers instead of collagen. Found only on the ears and epiglottis. It can withstand repeated bending.

v  Firboelastic cartilage is highly compressible and contains both collagen and elastic fibers. It can withstand great pressure and is found on the menisci of the knee and vertebral discs.

II. Classification of Bones

The 206 bones of the skeleton is divided into the axial skeleton and the Appendicular skeleton.

1. The Appendicular skeleton consists of the upper and lower limbs and girdles. Its role is involved with locomotion.

2. The Axial skeleton forms the long axis of the body and contains the skull, vertebral column and rib cage. It is involved in protecting and supporting the internal organs.

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3. Bones can be further classified by shape. There are four types:

v  Long bones are longer than they are wide. Typical examples are the limb bones.

v  Short bones are cuboidal in shape. The carpals and tarsals are the common examples. The sesamoid bone is a special case.

v  Flat bones are curved and flattened in one dimension. Examples include the ribs, scapula and most of the skull.

v  Irregular bones have a complicated shape. The vertebrae and hip bones are common examples.

III. Function of the Bones

1. Support: Provides support for body and soft organs.

2. Protection: Fused bones of the skull protect the bone.

3. Movement: Provide levers for muscle motion.

4. Mineral and fat storage: Bones are the major site of calcium and phosphate storage. Fat is stored in the marrow of long bones.

5. Blood cell formation: Hematopoiesis occurs in the “red” marrow of bones.

IV. Bone Structure

1. Gross Anatomy

Bone markings are the site of muscle, ligament and tendon connections. Fossa serves as openings for blood vessels and nerves. Examples include trochanters, spines, foramina and grooves.

2. Compact and Spongy Bone

Spongy bone is a honeycomb structure made up of needle like projections called trabeculae. The spaces are filled with either red or yellow marrow.

Compact Bone / Spongy Bone

Compact bone is dense and appears solid and has a smooth outer surface.

Both types are found in the bones of the skeleton.

3. Structure of the Long Bone

All long bones have the same basic structure.

A) Diaphysis is the long shaft of the bone. It is made primarily of compact bone and has a central medullary cavity. In adults this contains yellow marrow (fat).

B) Epiphyses are the expanded ends of the long bone. They contain compact bone surrounding spongy bone. The outer surfaces usually have a layer of hyaline cartilage.

C) Membranes cover both the exterior and interior surfaces of the long bones.

The periosteum covers the exterior surface of the bone and contains two layers. The outer fibrous layer of dense irregular connective tissue and the inner osteogenic layer which contains the bone forming cell the osteoblasts. These are responsible for laying down the bone matrix. The bone destroying cells, osteoclasts and the osteogenic cells which are stem cells giving rise to osteoblasts.

The periosteum is held tightly to the bone by perforating fibers called Sharpey’s fibers.

The endosteum lines the inner bone surfaces and consists of a single thin connective tissue membrane. It contains bone forming and destroying cells.

4. Structure of Short, Irregular and Flat Bones.

These bones all have thin plates of compact bone with spongy bone sandwiched in between. In flat bones the spongy bone is called the diploe. These bones also have the periosteum and endosteum. The spongy bone marrow is filled with hemapoietic tissue.

V. Microscopic Anatomy of Bone

Four cell types populate the bone tissue: osteogenic cells (osteoblasts precursor), osteoblasts, osteocytes and osteoclasts.

A) Compact Bone Microscopic Structure

The structural unit of the compact bone is known as the osteon or the Haversian system. Each of these is an elongated cylinder oriented along the long axis. Running through the center of the osteon is a central canal or Haversian canal. This carries blood vessels and nerves. Branches from these vessels and nerves go through perforating canals called Volkman’s canals. These canals are lined with the endosteum. Surrounding the canal is the matrix tube called the lamella.

Osteocytes occupy lacunae at the junctions of adjacent lamella. Hairs like canals called canaliculi connect the lacunae to other lacunae and to the haversarian canals. The osteocytes have long tentacle like projections which run through the canaliculi and connect to other osteocytes by gap junctions. This provides an avenue for the movement of nutrients from one osteocyte to another. The formation and relationship of the osteocyte to the other bone cells is shown below. The osteoblasts form the bone matrix and become trapped. They turn into osteocytes. The cytoplasmic extensions of the osteocytes are shown.

B) Spongy Bone Structure

The trabeculae of the spongy bone align along areas of stress. There are no osteons in the spongy bone. The bone forming elements are found in the endosteum surrounding the trabeculae.

VI. Chemical Composition of the Bone.

Bone has an inorganic and organic component. The osteoid makes up one third of the matrix and includes the ground substance (proteoglycans and glycoproteins) and collagen fibers. Both of these are formed by the osteoblasts. The inorganic portions of the bone is made of calcium phosphate (hydroxyapatites)

VII. Bone Development

Ossification is bone formation. In embryos ossifications leads to bone formation and in adults it leads to remodeling and repair. Ossification can take place in two different ways depending on the bone type, intramembranous and endochondral.

A) Intramembranous Ossification

This is found in the cranial bones and the clavicles. The process involves four steps.

1. Formation of the ossification center in a fibrous connective tissue membrane.

2. The formation of osteoid within the ossification center.

3. Trabeculae form within the osteoid tissue.

4. Lamellar (compact) bone forms on the surface of the bone just below the periosteum.

B) Endochondral Ossification

All the bones except those of the skull and clavicle are formed by this method. Essentially the bone is formed from a hyaline cartilage mold.

1. A bone collar is laid down over the diaphysis of the hyaline cartilage model.

2. Cartilage center calcifies and then breaks down forming a cavity.

3. A periosteal bud invades the internal cavity bringing with it, blood vessels, nerves and tissues which give rise to the endosteum.

4. The diaphysis elongates and the medullary cavity forms.

5. The epiphyses form in a manner similar to the diaphysis.

VIII. Postnatal Bone Growth

A) Longitudinal growth mimics endochondral ossification. Between the epiphysis and diaphysis lies a layer of hyaline cartilage called the epiphyseal plate. This is also known as the growth plate. On the epiphyseal side is the quiet zone. Progressing towards the diaphysis the cells proliferate pushing the epiphysis upward. On the side towards the diaphysis the hyaline cartilage is broken sown and turned into spongy bone.

B) Appositional Growth

This occurs between the periosteum and the endosteum. Osteoblasts of the periosteum secrete bone while the osteoclasts in the endosteum break it down causing the bone to widen.

C) Hormonal Control of Bone Growth

Growth hormone is important in bone growth in the infant and childhood. Sex hormones promote specific modeling of the bone.

IX. Bone Remodeling and Repair

Bone deposit and resorption occur at the surface of the periosteum and endosteum. Together they work to reform the bone as new stresses are added to it. Bone deposit occurs through the actions of the osteocytes while bone resorption occurs though the action of the osteoclasts. Remodeling of bone is controlled through two processes, hormonal and mechanical stresses.

A) Hormonal control

Parathyroid hormone stimulates the releases of calcium from the bone by stimulating the osteoclasts. The free calcium ions raise blood calcium levels, slowing parathyroid secretion. Excessive parathyroid secretion can lead to wasting of the bone.

B) Mechanical Stress

Wolff’s law governs how a bone remodels. Compression and tension work together to shape the bone. For example, a person’s dominate limb has thicker bone structure than the non dominate limb. Boney projections on the limbs become more prominent when muscle action is increased.

X. Bone Repair

Although bones are strong, excessive forces can lead to fractures. Fractures can be due to trauma or an underlying disease. Fractures can be classified as follows:

1.  Non displaced, where the bones retain their normal position or displaced where the bone ends are out of position.

2.  A complete fracture is when the bone is broken completely through or incomplete where it is only partially cracked.

3.  A linear fracture breaks along the long axis or transverse where it is broken along perpendicular to the axis.

4.  Fractures can be open where the skin is broken or closed where the skin is intact.

Types of bone fractures commonly seen are shown below.

Typically the bone is treated by reduction (realignment) of the bones. Sometimes this requires surgery.

A simple fracture heals as follows:

1.  A hematoma forms. This is the result of broken blood vessels. This causes the site to become swollen and inflamed.

2.  A fibro cartilaginous callus forms. This is the result of a variety of factors released by the injured tissue and blood clots. This soft tissue is invaded by capillaries and phagocytic cells begin to clean up debris.

3.  Bony callus now begins to form and gradually replaces the fibro cartilaginous callus.

4.  Bone remodeling occurs and the repaired bone then takes its appropriate shape. This last part could take a month or more to occur.

XI. Homeostatic Imbalances of the bone.

1.  Osteomalacia and Rickets.

This is also known as soft bones and is the result of a calcium imbalance. The osteoid or connective tissue matrix is laid down but the calcium is not laid down. The result is that the bones bend. In children this condition is called Rickets’ and is due t o a Vitamin D deficiency. People will suffer from pain in the bones and can lead to deformities of the pelvis, bowed leg and soft ribs.

2.  Osteoporosis

This is typically a disease of old age and is a major cause of “hip” fractures. In this condition the bones become fragile, the amount of matrix remains normal but the mass is reduced. Compression fractures of the vertebrae are common leading to a hunched over posture. A bone density test is usually done to diagnose this condition.

3.  Paget’s Disease

This disease is characterized by haphazard bone deposit and resorption. Typically the bone form is spongy. As the disease progresses, the osteoclasts activity wanes but the osteoblast continue to work filling cavities and foramen. The cause is unknown.

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