SKELETAL TISSUE

CHAPTER OBJECTIVES
When you have completed this chapter you should be able to:
Describe the function of the skeleton.
Describe the process of bone growth and the nutrients needed.
Describe the different types of bone tissue, bone shape and surface irregularities, giving examples.

The skeleton is a strong but bendable framework composed of 206 bones and their associated cartilages, connected by ligaments and muscles. It has 4 functions;

1. To support and protect the soft organs.
2. To give muscles somewhere to attach and something to pull against.
3. To manufacture blood cells.
4. To provide storage for phosphorus and calcium.

Bone development

Osteoblasts are cells that convert soluble calcium chloride into insoluble calcium phosphate forming the basis of bone. This process is called ossification of which there are two types, intramembranous and intracartilaginous ossification. Intramembranous ossification occurs in the skull bones and is when the osteoblasts replace connective tissue with calcium phosphate.  Intracartilaginous ossification occurs in the majority of the skeleton and is when the osteoblasts replace hyaline cartilage with calcium phosphate.   

Name	Ossification method	Location
Intramembranous ossification	Connective tissue is replaced by calcium.	Skull bones.
Intracartilaginous ossification	Hyaline cartilage is replaced by calcium.	Most other bones.

There are also cells called osteoclasts that function to absorb calcium phosphate.  They work with the osetoblasts to remodel bone durng growth and throughout life. For example, in a long bone the osteoblasts originate in the tough outer covering of the cartilage called the periosteum and secrete bone onto the outer surface; simultaneously the osteoclasts remove bone from the inner surface, to create the medullary cavity and to prevent the bone becoming too thick and heavy.

Clinical Considerations

Osteoporosis

When too much bone tissue is absorbed by the osteoclasts the bone becomes weakened and more likely to fracture. It is often common in women after the menopause and is known as osteoporosis.

Bone growth and repair

Calcium, phosphorus and vitamins C and D are essential to bone growth. Major phases of bone growth occur before birth, as a child grows and when recovering from injury or bone disease. The effects of these nutrients are summarised in the table below;

Nutrient	Role in bone development	Foods it is found in;	Deficiency causes
Calcium	Needed to form calcium phosphate.	Milk, eggs, green vegetables.	Rickets in children and osteomalacia in adults (soft bones).
Phosphorous	Needed to form calcium phosphate.	Meat, fish, egg yolks.	Rickets in children and osteomalacia in adults (soft bones).
Vitamin C	Plays a part in laying down connective tissue.	Fresh fruit (particularly citrus fruit), green vegetables, tomatoes, potatoes.	Bone and cartilage are deficient in collagen, scurvy (ulceration and haemorrhage throughout the body).
Vitamin D	Allows calcium and phosphate to be absorbed into the intestine.	Animal fat, fish oils, can be converted from ergosterol by UV-rays from the sun on the skin.	Rickets in children and osteomalacia in adults (soft bones).

Exercise also affects bone growth and repair as it increases blood supply to muscles and bones, stimulating growth. Muscles pulling on a bone will also affect the shape of a bone.

Types of bone tissue

There are two types of bone tissue, compact and spongy;

Compact bone forms the outer surface of bones and consists of Harversian systems.  Harversian systems are formed by tiny concentric plates of bone called lamellae which surround Harversian canals like the rings of a tree trunk.  Each canal contains blood vessels, nerves and lymphatic vessels.  In compact bone the lamellae and Haversian systems are packed closely together with only small spaces between the lamellae to house osteocytes and between each Harversian system for lymph.

Spongy bone fills the substance of bone and also consists of Haversian systems.  Spongy bone differs from compact bone in that the Harversian canals are larger and there are larger gaps between the lamellae. The spaces this creates are filled with red and yellow bone marrow, which is a mixture of fat and red blood cells. The spaces also help to reduce the weight of the bone.

Types of bone

Bones can be classified in terms of their shape and have been divided into four categories; long, flat, irregular and short bones.

Long bones

Long bones such as the femur phalanges have a long shaft with two extremities; their main differences are associated with size. The shaft has an outer layer of compact bone with a hollow cavity called the medullary canal that contains yellow bone marrow for fat storage. Long bones are covered in periosteum and receive a rich blood supply.

Ossification

Long bones develop from three areas; the first to ossify is the shaft and is called the diaphysis. There are then two secondary ossification centres at either end of the shaft called epiphyses, which develop after birth. From each centre of ossification, bone tissue gradually spreads through the cartilage until they meet. There remains a layer of epiphyseal cartilage between the diaphysis and the epiphyses that allows the shaft to continue growing until the age of 18 - 25. At this time the epiphyseal cartilage will ossify and the bone will stop growing.

Flat bones

Flat bones are plates formed by two layers of compact bone held together by spongy bone. Examples include the squamous bones of the skull, the scapula, and the sternum. They are smooth and flat to protect delicate organs and provide attachment for muscles.

Irregular bones

Irregular bones are a mass of spongy bone surrounded by compact bone. Examples include vertebrae, middle ear bones and the sphenoid bone of the skull.

Short bones

Short bones are roughly cube-like and are approximately equal in all dimensions. Examples include the carpal and tarsal bones.

Surface irregularities

Bones display certain types of irregularities that are identified in the table below;

Name	Articular (joint surfaces)	Non-articular (for muscle attachments)	Projection	Depression	Description	Example
Head	Articular	-	Projection	-	Spherical or disc-like end to a bone	Femur: head
Condyle	Articular	-	Projection	-	Rounded but oval	Femur: medial condyle
Fossa	Articular	-	-	Depression	Shallow depression	Humerus: coronoid fossa
Process	-	Non-articular	Projection	-	Rough projection for muscle/ligament attachment	Radius: styloid process
Spine	-	Non-articular	Projection	-	Pointed rough projection	Scapula: spine
Tuberosity	-	Non-articular	Projection	-	Broad rough projection	Tibia: tibial tuberosity
Trochanter	-	Non-articular	Projection	-	Large rough projection	Femur: greater trochanter
Tubercle	-	Non-articular	Projection	-	Small rough projection	Humerus: lesser tubercle
Crest	-	Non-articular	Projection	-	Long narrow projection	Hip bone: iliac crest
Fossa	-	Non-articular	-	Depression	Notch	Hip bone: iliac crest
Groove	-	Non-articular	-	Depression	Long narrow depression	Tibia: groove for tibialis posterior tendon
Foramen	-	Non-articular	-	Depression	Hole	Occipital bone: foramen magnum
Sinus	-	Non-articular	-	Depression	Cavity within the bone	Frontal bone: frontal sinus

SELF - TEST

Complete the following questions before you go onto the next section:

Name the nutrients needed for healthy bone growth

Describe and give examples of different types of bone

Name and give examples of 9 different surface irregularities of bone

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