section 1, chapter 7

The Skeletal System

The science of bones is called osteology

Functions of bone1. Support & protect organs

• The brain is protected by the skull and the heart and lungs are protected the ribs & sternum

2. Movement• Muscles attach to skeleton

3. Inorganic salt storage• Stores calcium and phosphate

4. Blood cell production• Red bone marrow forms new blood cells

Components of bone

The extracellular matrix of bones is composed of 1. hydroxyapatite – a calcium phosphate salt that

provides the hardness of

bones

2. collagen fibers – provides bone with some pliabilityThe cells associated with bones include:1. Osteocytes = cells that maintain bone

2. Osteoblasts = cells that deposit new bone. Once mature, osteoblasts become osteocytes.

3. Osteoclasts = cells that dissolve bone. Osteoclasts originate from white blood cells and they secrete an acid that dissolves the inorganic salts of bone.

Bones may be classified by their shape.

1. Long bones = elongated diaphysis• humerus radius ulna• femur tibia fibula• metatarsals metacarpals phalanges

2. Short Bones = cube-shaped• carpals • tarsals

3. Flat Bones = plate-like• sternum ribs scapula• parietal and frontal bones

Bone Classification continued

4. Irregular bones = variety of shapes• vertebrae• mandible maxilla• ethmoid bone sphenoid bone

5. sesamoid (or round) bone = develops within tendons• patella

Parts of a long bone

2. Epiphysis = expanded ends of bone • Filled with spongy bone• Proximal epiphysis & distal epiphysis• Sites of articulation (joint)

1. Diaphysis = shaft of long bone• Lined with compact bone

4. Articular cartilage• Hyaline cartilage • Covers epiphyses

3. Epiphyseal plates• Remnants of bone growth

Parts of a long bone5. Medullary Cavity

• Cavity within diaphysis• Filled with bone marrow, blood

vessels and nerves

6. Endosteum• Membrane that lines medullary cavity• Contains osteoblasts

7. Periosteum• Tough membrane covering bone• Continuous with tendons and ligaments• Osteoblasts, blood vessels, and nerves

Parts of a long bone

7. Compact bone• Lines the Diaphysis• Composed of osteons

8. Spongy bone• Fills the epiphyses• Trabiculae = thin bony plates• Osteocytes lie within trabiculae

Figure 7.3

Compact BoneOsteon = Structural & functional unit of compact bone 1. Lamella = concentric rings of bone

2. Central Canal = blood vessels and nerves

3. Lacunae = bony chamber that contains an osteocyte

4. Canaliculi = canals with cellular processes• Pathway for nutrient and waste diffusion

Figure 7.5 Scanning electron micrograph of a single osteon in compact bone.

Osteon continued

Perforating Canal = conveys blood from periosteum towards individual osteons

Figure 7.4 Compact bone is composed of osteons cemented together by bone matrix.

Figure 7.4c Canaliculi allow nutrients and waste to diffuse between the central canal and individual osteocytes.

Bone Development and Growth

Parts of the skeletal system begin to develop during the first few weeks of prenatal development

Bone formation = ossification

Bones replace existing connective tissue in one of two ways: As intramembranous bones As endchondral bones

Intramembranous Bones Intramembranous Bones

Broad, flat bones of the skull

Formed by replacing layers of connective tissue (mesenchyme) with bone

Osteoblasts within mesenchyme deposit bony matrix in all directions

Osteoblasts become osteocytes once surrounded by bone

Endochondral Bones Endochondral Bones

Most of the bones in the skeleton are endochondral Bone formation begins with a hyaline cartilage model Cartilage decomposes and is replaced by bone.

Figure 7.6a stained bones of a 14-week fetus showing intramembranous and endochorndal bones.

Endochondral Ossification

1. Hyaline cartilage forms model of future bone

2. Cartilage degenerates and periosteum surrounds bone

3. Osteoblasts from periosteum invade the degenerating tissue

4. Osteoblasts beneath periosteum form compact bone at diaphysis = primary ossification center

5. Later, Osteoblasts form spongy bone at epiphyses = secondary ossification center

Endochondral Ossification continued

Figure 7.8 Major stages of endochondral ossification. (a-d fetal, e child, f adult)

Endochondral Ossification

Two areas of endochondral bone retain cartilage after ossification.

1. Articular cartilage • surrounds the epiphyses for joints

2. Epiphyseal plates • retain cartilage for bone growth

Articular cartilage

Growth at the Epiphyseal Plate

Epiphyseal Plate• Band of hyaline cartilage that remains between the two ossification centers

• Bone growth continues at epiphyseal plates until adulthood.

• New cartilage is added towards the epiphysis and cartilage is ossified towards diaphysis

• Once the epiphyseal plates ossify the bones can no longer be lengthened

4 Layers (zones) of growth at epiphyseal Plate

1. Zone of resting cartilage• Cartilage cells near epiphysis• Do not participate in bone growth• Anchor epiphyseal plate to epiphysis

2. Zone of proliferating cartilage• Young chondrocytes undergoing

mitosis• Adds new cartilage to plate

(b)

4 Layers (zones) of growth at epiphyseal Plate

3. Zone of hypertrophic cartilage• Older cells enlarge and thicken the

epiphyseal plate• Osteoblasts invade and calcify the

cartilaginous matrix.

4. Zone of calcified cartilage• Dead cells & calcium matrix

Ossified bone• Osteoclasts dissolve and phagocytize the matrix

• Osteoblasts invade the region and deposit new bone.

Figure 7.9aEnd of Section 1, Chapter 7

Section 2, Chapter 7

Bone Homeostasis

Homeostasis of Bone Tissue

Calcium is constantly exchanged between the blood and bone.

Bone resorption = Osteoclasts breakdown bone releasing calcium into the blood. Bone resorption occurs when blood [Ca2+] is low and it’s stimulated by parathyroid hormone (PTH).

Bone deposition = Osteoblasts deposit new bone from calcium in the blood stream. Bone deposition occurs when blood [Ca2+] is high and it’s stimulated by the hormone calcitonin.

Vitamin D – promotes Ca2+ absorption in small intestine• Vitamin D deficiency = softened and deformed bones

• Osteomalacia in adults• Rickets in children

Vitamin A – balances bone resorption and deposition• Vitamin A deficiency = retards bone development

Vitamin C – is required for collagen synthesis.• Vitamin C deficiency = results in fragile bones

Nutrients that effect bone homeostasis

Calcitonin• Secreted from thyroid gland• Promotes bone deposition

Parathyroid Hormone (PTH)• Secreted from parathyroid glands• Promotes bone resorption

Figure 7.13 Hormonal regulation of blood calcium and resorption

Hormones that affect bone homeostasis

Growth Hormone (GH)• Secreted from pituitary gland• Promotes bone growth at epiphyseal

plates

Pituitary Gigantism over secretion of GH during childhood

Pituitary Dwarfism insufficient GH during childhood

Acromegaly • Over secretion of GH as an adult • Occurs after epiphyseal plates have sealed• Enlargement of hands, feet, nose

Hormones that affect bone homeostasis

Sex Hormones (testosterone & estrogen)• Promotes long bone growth at puberty• Sex hormones also stimulate ossification at

epiphyseal plates.

Effects of Exercise on bone homeostasis

Contracting muscles pull on bones and promotes bone thickening

Figure 7.12 The thickened bone on the left is better able to withstand forces from muscle contractions.

Hormones that affect bone homeostasis

Bone Fractures

Incomplete Fractures

Greenstickfracture

Fissuredfracture

Spiralfracture

Comminutedfracture

Obliquefracture

Transverse fracture

Bone Fractures

Complete Fractures

Repair of a fractureWhen a bone breaks blood vessels rupture and the periosteum tears. The repair of a broken bone occurs in 5 general steps.

Step 1. hematoma formation

Blood soon forms a hematoma (blood clot).

Hematoma in foot

Step 3. cartilaginous callus

Fibroblasts deposit a mass of fibrocartilage “cartilaginous callus”

& Phagocytes remove hematomaOsteoclasts remove bony debris

Step 2. temporary spongy bone

Osteoblasts invade from periosteum and deposit temporary spongy bone.

Repair of a fracture

Step 5. bone remodeling Osteoclasts remove excess bone, remodeling the bone the bone close to its original shape.

Step 4. bony callus

Osteoblasts replace the cartilaginous callus with bone, forming a bony callus

Repair of a fracture

Disorders of Bone

Osteopenia “low bone mass”• Progresses towards osteoporosis

Osteoporosis “porous bone”• Bones develop spaces and canals• Bones are fragile and easily broken• Common in menopausal women (from the low estrogen levels)

Over time, osteoclasts outnumber osteoblasts, and more bone is resorbed than can be deposited. Bone mass decreases as a result.

Bone loss is rapid in menopausal women due to reduced estrogen

End of Chapter 7, Section 2

Ways to delay or prevent osteoporosis:

1. Exercise daily.

2. Consume enough calcium and vitamin D every day.

3. Do not smoke.