3
Bone Tissue
Chapter 6
CHAPTER SUMMARY
After a brief review of the functions of bone tissue and the skeletal system, this chapter provides a thorough description of the general structure of bones. The histology of bone tissue is described and the key features of compact bone tissue and spongy bone tissue are examined. The blood and nerve supply of bones is concisely summarized. The processes by which bone formation (ossification) occurs are described. The mechanisms of bone growth and bone remodeling (replacement) are portrayed. Fractures and the repair of bone fractures are thoroughly explained. The effects of exercise on bone tissue and the effects of aging on bone tissue are also described. A glossary of key medical terms associated with bone tissue is provided. This chapter ends with a thorough study outline, an excellent self-quiz, critical thinking questions, and answers to questions that accompany chapter figures.
STUDENT OBJECTIVES
1. Discuss the functions of the skeletal system.
2. Describe the parts of a long bone.
3. Describe the histological features of compact and spongy bone tissue.
4. Describe the blood and nerve supply of bone tissue.
5. Describe the steps involved in intramembranous and endochondral ossification.
6. Describe how bones grow in length and width.
7. Describe the processes involved in bone remodeling.
8. Describe the sequence of events in repair of a fracture.
9. Describe how exercise and mechanical stress affect bone tissue.
10. Describe the effects of aging on bone tissue.
LECTURE OUTLINE
A. Functions of the Skeletal System (p. 145)
1. Bones are organs which, along with their associated cartilages, form the skeleton system.
2. Bone tissue and the skeletal system perform a variety of major functions:
i. support
ii. protection
iii. assistance in movement
iv. mineral storage and release
v. site of blood cell production: hemopoiesis, which occurs in red bone marrow
vi. triglyceride storage in yellow bone marrow
B. Anatomy of a Bone (p. 145)
1. A typical long bone consists of the following regions:
i. diaphysis
ii. epiphyses
iii. metaphyses, which are regions that include the epiphyseal plate
- latter is replaced by epiphyseal line when bone lengthening stops
iv. articular cartilage
v. periosteum
vi. medullary (marrow) cavity
vii. endosteum
C. Histology of Bone Tissue (p. 145)
1. Bone or osseous tissue contains abundant matrix surrounding widely separated cells.
2. The matrix of this connective tissue contains abundant mineral salts, primarily hydroxyapatite and some calcium carbonate, deposited in a framework of collagen fibers; these features give bone two important characteristics:
i. calcification (mineralization) of bone tissue gives bone hardness
ii. collagen fibers (and other organic molecules) give bone flexibility and tensile strength
3. There are four major types of cells present:
i. osteogenic cells give rise to osteoblasts
ii. osteoblasts produce the bone tissue matrix
iii. osteocytes maintain the bone tissue
iv. osteoclasts function in bone resorption
4. There are two major types of bone tissue:
i. compact (dense) bone tissue (p. 147)
a. forms the external layer of all bones
b. provides protection and support and helps bones resist stress
c. consists of numerous neighboring osteons (Haversian systems)
ii. spongy (cancellous) bone tissue (p. 148)
a. consists of trabeculae arranged in an irregular latticework
b. contains spaces that are sometimes filled with red bone marrow
c. forms most of the interior of short, flat, and irregular bones as well as most of the interior of epiphyses of long bones
D. Blood and Nerve Supply (p. 150)
1. Bones have a rich blood supply; portions of bone containing red bone marrow have an especially rich supply of blood vessels.
2. Blood supply to a bone is provided by the following arteries (and accompanying veins):
i. periosteal arteries
ii. nutrient artery that enters a bone via a nutrient foramen
iii. metaphyseal arteries
iv. epiphyseal arteries
3. Nerve supply to a bone is not extensive, consisting primarily of nerves to blood vessels, and
sensory nerves (concerned with pain) supplying the periosteum.
E. Bone Formation (p. 150)
1. Ossification or osteogenesis begins around the sixth week of embryonic life and continues throughout adulthood.
2. Bone formation occurs via one of two processes in which preexisting connective tissue is replaced by bone tissue:
i. intramembranous ossification occurs within fibrous connective tissue membranes
ii. most bones are formed by endochondral ossification which occurs within a hyaline cartilage model
F. Bone Growth (p. 152)
1. Lengthening of a bone occurs at an epiphyseal plate, which consists of four regions:
i. zone of resting cartilage
ii. zone of proliferating cartilage
iii. zone of hypertrophic (maturing) cartilage
iv. zone of calcified cartilage
2. Growth in bone diameter occurs via appositional growth, a process in which osteoblasts from the periosteum add new bone tissue to the outer surface.
3. Ossification of most bones is usually completed by age 25.
G. Bone Remodeling (p. 155)
1. Remodeling is a continuous process in which worn and injured bone tissue is replaced by new bone tissue.
2. Osteoclasts resorb old bone tissue and osteoblasts form the new bone tissue.
3. Normal bone growth and bone replacement depend on the presence of:
i. several minerals (e.g., calcium, phosphorus, etc.)
ii. several vitamins (e.g., A, D, etc.)
iii. several hormones (e.g., human growth hormone, sex hormones, etc.
H. Fracture and Repair of Bone (p. 156)
1. A fracture is any break in a bone; repair of a fracture may take months.
2. Fractures can be classified into several categories including open (compound), closed (simple), comminuted, greenstick, impacted, Pott’s, Colles’ and stress fractures.
3. Repair involves the following steps:
i. formation of a fracture hematoma
ii. development of a procallus and subsequently a fibrocartilaginous callus
iii. conversion into a bony callus
iv. remodeling of the callus in which spongy bone is replaced by compact bone
4. Setting a fracture without surgery is called closed reduction; setting a fracture with surgery is called open reduction.
I. Exercise and Bone Tissue (p. 158)
1. Bone has the (limited) ability to alter its strength in response to mechanical stress.
2. When placed under mechanical stress, bone tissue becomes stronger due to increased deposition of mineral salts and production of collagen fibers.
3. Removal of mechanical stress causes a weakening of bone tissue due to demineralization and collagen reduction.
4. Bone mass is built and maintained by weight-bearing activities such as walking and weight-lifting.
K. Aging and Bone Tissue (p. 159)
1. Until young adulthood, more bone is produced than is lost; until middle age, bone deposition and resorption are about the same; during and after middle age, bone resorption outpaces bone deposition.
2. Aging has two principal effects on bone tissue:
i. demineralization of bone matrix (the loss of calcium is a major factor in development of osteoporosis)
ii. decrease in rate of protein (mainly collagen) synthesis results in loss of tensile strength, making bones more brittle and susceptible to fracture.
L. Key Medical Terms Associated with Bone Tissue (p. 160)
1. Students should familiarize themselves with the glossary of key medical terms.