Skeletal system

Anatomy and Physiology | Tutorial Notes

Skeletal System

Learning objectives

After study of this chapter, the student will:

1. Classify bones according to their shapes and name an example from each group.

2. Describe the macroscopic and microscopic structure of a long bone, and list the functions of these parts.

3. Distinguish between intramembranous and endochondr al bones, and explain how such bones develop and grow.

4. Describe the effects of sunlight, nutrition, hormonal secretions, and exercise on bone development and growth.

5. Discuss the major functions of bones.

6. Distinguish between the axial and appendicular skeletons, and name the major parts of each.

7. Locate and identify the bones and the major features of the bones that comprise the skull, vertebral column, thoracic cage, pectoral girdle, upper limb, pelvic girdle, and lower limb.

8. Describe the differences between male and female skeletons.

9. Describe life-span changes in the skeletal system.

tutorial outline

I. Bone Shapes (figure 7.1)

A. Long bones consist of a shaft with two ends.

Examples include:

1. upper limb: clavicle, humerus, radius, ulna, metacarpals, phalanges

2. lower limb: femur, tibia, fibula, metatarsals, phalanges

B. Short bones are cube-like.

Examples include:

1. wrist bones = carpals

2. ankle bones = tarsals

C. Flat bones are thin and usually curved.

Examples include:

1. Some skull bones: frontal, parietal, occipital, temporal

2. Bones of thoracic cage: sternum, ribs

3. shoulder blades: scapulae

D. Irregular bones are not long short or flat.

Examples include:

1. vertebrae

2. auditory ossicles

3. sphenoid and ethmoid bones

C. Sesamoid bones develop within a tendon.

1. The patella is a human sesamoid bone.

II. Parts of a Long Bone (figure 7.2 and figure 7.3a)

A. Diaphysis = shaft.

1. Consists of a central medullary cavity (filled with yellow marrow)

2. Surrounded by a thick collar of compact bone.

B. Epiphyses (sing. = epiphysis) = expanded ends

1. Primarily composed of spongy bone surrounded by a thin layer of compact bone.

2. proximal epiphysis vs. distal epiphysis

3. Epiphyseal line = remnant of epiphyseal plate.

4. Epiphyseal plate

·  Present in developing skeleton during childhood

·  cartilage at the junction of the diaphysis and epiphyses (growth plate).

5. Periosteum = outer layer of dense Connective Tissue covering diaphysis.

·  Periosteum is richly supplied with blood & lymph vessels nerves (nutrition): Nutrient Foramen = perforating canal allowing blood vessels to enter and leave bone.

·  Inner Osteogenic layer contains osteoblasts (bone-forming cells) and osteoclasts (bone-destroying cells)

·  Periosteum serves as insertion for tendons and ligaments.

6. Endosteum = inner lining of medullary cavity.

·  Contains layer of osteoblasts & osteoclasts.

7. Articular cartilage = pad of hyaline cartilage on the epiphyses where long bones articulate or join.

·  "shock absorber”

III. Bone Histology (figure 7.4 and 7.5)

A. Compact Bone is solid dense and smooth. The structural unit of compact

bone is the Osteon or Haversian system.

1. Compact bone is composed of elongated cylinders cemented

together to form the long axis of the bone.

2. Components of Osteon (Haversian system)

·  osteocytes (spider-shaped bone cells that lie in "lacunae") that have laid down a matrix of collagen and calcium salts in

·  concentric lamellae (layers) around a

·  central canal (Haversian canal) containing

·  blood vessels and nerves

3. Communicating canals within compact bone:

·  Canaliculi connect the lacunae of osteocytes

·  Perforating (Volkmann's) canals connect the blood & nerve supply of adjacent osteons together.

o  Run at right angles to and connect adjacent central canals (Haversian canals).

B. Spongy (Cancellous) Bone

See Fig 7.3b and c page 202.

1. Consists of poorly organized trabeculae (small needle-like pieces of bone)

with a lot of open space between them

2. Nourished by diffusion from nearby Haversian canals

3. filled with red bone marrow, so hematopoiesis (bone cell formation) occurs in spongy bone

4. located within flat bones and in epiphyses of long bones

7.2 BONE DEVELOPMENT AND GROWTH (Osteogenesis/ossification)

A.  Introduction

1. The "skeleton" of an embryo is composed of fibrous CT membranes (formed from mesenchyme and hyaline cartilage) that are loosely shaped like bones.

2. This "skeleton" provides supporting structures for ossification to begin.

3. At about 6-7 weeks gestation ossification begins and continues throughout adulthood.

4. Ossification follows one of two patterns:

a. intramembranous ossification or endochondral ossification

b. Both mechanisms involve the replacement of preexisting

connective tissue with bone.

B. Intramembranous Ossification is when a bone forms on or within a fibrous CT membrane.

1.  Flat bones are formed in this manner (i.e. skull bones, clavicles)

2.  See Fig 7.8, page 205 and left side of Table 7.1, page 205.

C.  Endochondral Ossification occurs when a bone is formed from a hyaline

cartilage model.

1. Most bones of the skeleton are formed in this manner.

2. Primary Ossification center hardens as fetus and infant.

3. Secondary Ossification centers develop in child and harden during

adolescence and early adulthood.

4. See Fig 7.9, page 206, and right side of Table 7.1, page 205.

* See Fig 7.6, page 204 which show both patterns of ossification in a fetus.

* During infancy and childhood long bones lengthen entirely by growth at the epiphyseal plates (called longitudinal growth) and all bones grow in thickness by a process called appositional growth.


C. Growth at the Epiphyseal Plate See Fig 7.10 page 206.

1. Structure of the Epiphyseal Plate or Disc (4 zones):

a. Zone of resting cartilage

·  near epiphysis

·  small scattered chondrocytes

·  anchor plate to epiphysis.

·  Does not add new cartilage.

b. Zone of proliferating cartilage

·  larger chondrocytes that resemble a stack of coins

·  Chondrocytes divide to replace those that die at the diaphyseal surface of the epiphysis.

c. Zone of Hypertrophic cartilage

·  extremely large chondrocytes that are arranged in columns

·  maturing cells and thickening cartilage

d. Zone of calcified cartilage

·  only a few cells thick

·  consists of dead cells within calcified matrix

·  This calcified matrix is destroyed by osteoclasts.

§  Osteoclasts are large multinucleated cells that originate from white blood cells called monocytes, and are responsible for bone resorption. See Fig 7.11, page 207.

·  secrete lysosomal enzymes that digest the organic matrix

·  secrete acids that decompose calcium salts into Ca2+ and PO4- ions which can then enter blood.

o  This matrix is then invaded by bone-building cells, osteoblasts, which lay down bone on the calcified cartilage that persists.

o  As a result the diaphyseal border of the plate is firmly cemented to the bone of the diaphysis.

2. The epiphyseal plate allows for bone lengthening. As a child grows

a. Cartilage cells are produced by mitosis on epiphyseal side of plate.

b. They are then destroyed and replaced by bone on diaphyseal side of plate. Therefore thickness of the plate remains almost constant while the bone on the diaphyseal side increases in length.

c. The cartilage of epiphyseal plate is replaced by bone forming the epiphyseal line.

d. Ossification of most bones is completed by age 25.

* See box and Figure 7.12 on page 207 re: growth plate damage in childhood and Ossification Timetable 7.2, page 207.

D. Bone Thickening / Appositional Growth

Along with increasing in length bones increase in thickness or diameter.

1. occurs in osteogenic layer of periosteum

2. Osteoblasts lay down matrix (compact bone) on outer surface.

3. This is accompanied by osteoclasts destroying the bone matrix at the endosteal surface.

E. Homeostasis of Bone Tissue

1. Once bones are formed, the actions of osteoclasts and osteoblasts continually remodel them.

2. Bone remodeling occurs throughout life.

a. Osteoclasts resorb bone

b. Osteoblasts replace the bone

c. These opposing processes are highly regulated so that total mass of bone tissue in adult skeleton normally remains constant, even though 3%-5% of bone calcium is exchanged each year.

F. Factors Affecting Bone Development Growth and Repair

A number of factors influence bone development, growth, and repair. These include nutrition, exposure to sunlight, hormonal secretions, and physical exercise.

1. Nutrition

a.  Vitamin D greatly increases intestinal absorption of dietary calcium & retards its urine loss.

o  Deficiency causes rickets in children and osteomalacia in adults.

o  Vitamin D is found in dairy products

o  Vitamin D is synthesized in the skin with exposure to sunlight.

b.  Vitamin A is required for bone resorption controls the activity distribution and coordination of osteoblasts & osteoclasts during development.

c.  Vitamin C helps maintain bone matrix (collagen synthesis)

o  Deficiency causes scurvy.

2. Exposure to sunlight – See F.1.a (above)

3. Hormonal Secretions:

a. human Growth Hormone (hGH) from the pituitary gland is responsible for the general growth of all tissues. In bone it stimulates reproduction of cartilage cells at epiphyseal plate.

o Hyposecretion in childhood results in pituitary dwarfism.

o Hypersecretion in childhood results in pituitary gigantism.

o Hypersecretion in adulthood results in acromegaly.

These disorders are discussed in greater detail in Chapter 13.


F. Factors Affecting Bone Development Growth and Repair

3. Hormonal Secretions:

b. Thyroid hormones

 T4 = Thyroxine stimulates replacement of cartilage by bone in epiphyseal plate.

 Calcitonin acts on several sites to decrease blood calcium levels (see 7.3, C. 3. on next page)

c. Parathyroid Hormone (PTH)

 from parathyroid glands stimulates osteoclasts to resorb bone.

d. Sex hormones from the male and female gonads (testosterones and estrogens, respectively)

·  Sex Hormones are abundant at puberty and cause considerable long bone growth (aid osteoblast activity and promote new bone growth)

·  Sex Hormones also degenerate cartilage cells in epiphyseal plate (i.e. close epiphyseal plate).

 Estrogen’s effect is greater than androgen effect

4. Physical Exercise

a. Physical stress stimulates bone growth = hypertrophy.

b. Lack of exercise = atrophy or bone tissue waste.

c. See Fig 7.13, page 210, re: muscle attachments on bones with exercise.

* See Clinical Application 7.1, Fractures on pages 208-209 and Clinical Application 7.2, Preventing Fragility Fractures, page 212.


A. Support, Protection, and Movement

1. Support:

a. The bones in legs and pelvis support the trunk

b. The atlas (1st vertebra) supports the skull etc.

2. Protection of underlying organs

a. The skull protects the brain

b. The rib cage protects the heart and lungs etc.

3. Body Movement – discussed in Chapters 8 and 9

a. Skeletal muscles are attached to bones by tendons.

b. serve as levers to move bones

B. Blood Cell Formation - Hematopoiesis

a. Embryo – Blood Cells are formed in the liver and yolk sac

b. After Birth - blood cells are formed in red bone marrow

·  In Child – all bones contain red bone marrow

·  In Adult – red bone marrow is limited to flat bones (skull, sternum, ribs, hips) and irregular bones (vertebrae), and proximal end of long bones.


C. Inorganic Salt Storage

1.  Hydroxyapatite, which is primarily calcium phosphate [Ca3(PO4)2.(OH)2]

gives bone its hardness or rigidity.

2. Calcium is required for many metabolic processes, including

a. muscle contraction

b. nerve impulse conduction

3. Blood calcium homeostasis must be maintained

a. Bone remodeling directly affects blood calcium homeostasis because the primary matrix of bone is calcium.

Once a bone is formed it is continuously being remodeled throughout life. This process involves the action of osteoblasts and osteoclasts (see above) two hormones (calcitonin & parathyroid hormone) and in turn affects blood calcium homeostasis. See Figure 7.14, page 211.

Parathyroid hormone (PTH) which is secreted by the parathyroid glands when blood calcium levels are low:

 stimulates osteoclast activity (resorption of bone occurs) which releases Ca2+ into the blood

o  causes kidney tubules to reabsorb Ca2+ back into the blood

o  causes intestinal mucosa to increase dietary absorption of Ca2+ and therefore

o  causes an increase in blood calcium levels (back to normal).

Calcitonin which is secreted by the thyroid gland when blood calcium levels are high:

 inhibits bone resorption increases osteoblast activity (i.e. causes a deposition of bone matrix)

 causes the kidney tubules to secrete excess Ca2+ into the urine and therefore

 results in a decrease in blood calcium levels (back to normal).


C. Inorganic Salt Storage

3. Blood Calcium Homeostasis: See Fig 7.14 page 211.

Add arrows to complete negative feedback loop below.

Thyroid Gland Hormone: Calcitonin

1.  Osteoclasts are inhibited.

2.  Osteoblasts use excess Ca++ to lay down bone matrix.

Stimulus: ↑ blood Ca++ 3. Kidney tubules secrete excess Ca++ into urine.

↓ blood Ca++

Normal Blood Ca++

Set Point = 8.4-10.3dL

↑ blood Ca++

Stimulus: ↓ blood Ca++

1. Osteoclasts resorb bone matrix.

2. Kidney tubules reabsorb Ca++

back into bloodstream.

3. Intestinal mucosa absorbs Ca++.

Parathyroid Glands

Parathyroid Hormone


Skeletal System

A. Avascular

B. Composed of stratified squamous epithelium

C. Thickness of skin varies from region to region

1. Epidermis is thickest on soles of feet and palms of the hand

2. Epidermis is thinnest on eyelids

D. Epidermis contains 4 – 5 layers, depending on the thickness

Arranged from Deep skin to superficial surface:

1. Stratum Basale

i. Stratum basale is the deepest layer of epidermis, 1 – 2 Cell Layers thick

iii. Composed of basal keratinocytes (stem cells of the epidermis)

iv. Keratinocytes of stratum basale divide to form the keratinocytes of the stratum spinosum.

v. Melanocytes within the stratum basale produce the pigment melanin.

§  Melanin provides skin color

§  Melanin absorbs UV radiation in sunlight, protecting cells from UV damage, which causes mutations in the DNA of skin cells

§  Differences in skin color result from the differences in the amount of melanin produced by the melanocytes.

2. Stratum Spinosum

i. Stratum spinosum has a prickly appearance

ii. Keratinization begins in the stratum spinosum