Chapter 6 The Skeletal System: Bone Tissue

Question 1

Name the six main functions of the skeletal system

Answer 1

1. Support
2. Protection
3. Movement
4. Mineral homeostasis (storage and release)
5. Blood cell production
6. Triglyceride storage

Question 2

1. Support

Answer 2

 Structural framework for body
 Supports soft tissues
 Attachment points for tendons

Question 3

2. Protection

Answer 3

 Protects important internal organs from injury e.g. cranial bones -> brain, rib cage -> heart and lungs

Question 4

4. Mineral homeostasis (storage and release)

Answer 4

 Bone tissue store minerals e.g. calcium, phosphorus that give bones strength
 99% of calcium stored in bone tissue
 Bone releases minerals into blood to maintain homeostasis and distribute to rest of the body

Question 5

3. Movement

Answer 5

 Skeletal muscles attach to bones, contract -> movement

Question 6

5. Blood cell production

Answer 6

 Hemopoiesis (blood making): within bones, red bone marrow -> red blood cells, white blood cells, platelets
 Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, macrophages within a reticular fiber network
 Red bone marrow present in fetus’ developing bones and some adult bones like pelvic bones, ribs, sternum, vertebrae, skull, ends of humerus and femur
 In newborn all bone marrow is red and involved in hemopoiesis, with age changes from red to yellow

Question 7

6. Triglyceride storage

Answer 7

 Yellow bone marrow consists of adipose cells storing triglycerides = potential chemical energy reserve

Question 8

Diaphysis

Answer 8

 bone shaft/body-long, cylindrical, main portion of bone

Question 9

Epiphyses

Answer 9

 proximal/distal ends of bone

Question 10

Metaphyses

Answer 10

 Regions between diaphysis and epiphyses
 Growing bone: epiphyseal (growth plate) = hyaline cartilage allows diaphysis to grow. When a bone stops growing, cartilage in epiphyseal plate replaced by epiphyseal line

Question 11

Articular cartilage

Answer 11

 Thin later of hyaline cartilage covering joints
 Reduces friction, absorbs shock
 Lacks perichondrium and blood vessels thus limited repair

Question 12

Periosteum

Answer 12

 Tough connective tissue sheath and blood supply surrounding bone not covered by articular cartilage
 Outer fibrous layer of dense irregular connective tissue
 Inner osteogenic layer of cells. Some cells allow bone to grow in thickness
 Protects bonem fracture repair, nourishes bone tissue, attachment point for ligaments + tendons
 Attached to underlying bone by perforating or Sharpey’s fibers = thick bundles of collagen extending from periosteum to extracellular matrix

Question 13

Medullary cavity or marrow cavity

Answer 13

 Hollow cylindrical space within diaphysis containing fatty yellow bone marrow and blood vessels in adults
 Minimizes bone weight by reducing dense bony material where least needed thus design provides maximum strength with minimum weight

Question 14

Endosteum

Answer 14

 Thin membrane lining medullary cavity containing single layer of bone forming cells and small amt of connective tissue

Question 15

Explain why bone tissue is classified as a connective tissue.

Answer 15

Contains abundant extracellular matrix that surrounds widely separated cells

Question 16

Describe the cellular composition of bone tissue

Answer 16

o Extracellular matrix: 15% water, 30% collagen fibers, 55% crystallized mineral salts
o Most abundant mineral salt = calcium phosphate [Ca3(PO4)2] combine with calcium hydroxide [Ca(OH2)2] -> crystals of hydroxyapatite [Ca10(PO4)6(OH)2] which combine with other mineral salts like calcium carbonate (CaCO3) and ions like magnesium, fluoride, potassium, sulfate
o As mineral salts are deposited in framework of collagen fibers of extracellular matrix, they crystallize and tissue hardens = calcification
o Crystallized salts + collagen fibers = bone characteristics

Question 17

Osteoprogenitor cells

Answer 17

 Unspecialized bone stem cells derived from mesenchyme
 Only bone cells to undergo cell division, resulting in osteoblasts
 Found along inner periosteum, in endosteum, and canals containing blood vessels

Question 18

Osteoblasts (osteoBlasts Build bone)

Answer 18

 Bone building cells
 Synthesize/secrete collagen fibers + organic components to build extracellular matrix, initiate calcification
 Osteoblasts surround themselves w/ extracellular matrix, becoming trapped in secretions and become osteocytes

Question 19

Osteocytes

Answer 19

 Mature bone cells, main cells in bone tissue, maintain daily metabolism (nutrient/waste exchange in blood)
 No cell division

Question 20

Osteoclasts (osteoClasts Carve out bone)

Answer 20

 Huge cells from fusion of 50 monocytes (white blood cell), concentrated in endosteum
 On side of cell facing bone surface, plasma membrane deeply folded into ruffled border where cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components of extracellular bone matrix. Process = bone resorption part of normal development, maintenance, bone repair

Question 21

Compact bone tissue

Answer 21

 Contains few spaces, strongest form of bone tissue
 Found beneath periosteum of all bones, makes up bulk of diaphysis
 Provides protection/support, resists stresses produced by weight/movement
 Composed of osteons

Question 22

Osteons

Answer 22

 Concentric lamellae, circular plates of mineralized extracellular matrix around osteonic/central canal with blood vessels and nerves
 Small spaces between concentric lamellae called lacunae containing osteocytes
 branching off of lacunae are tiny canaliculi that connect lacunae with each other and central canals -> canal system for nutrients, oxygen, and waste
 osteons aligned in same direction and parallel to length of diaphysis -> resists bending/fracturing (lines of stress in bone not static)
 areas between neighbouring osteons contain interstitial lamellae
 blood vessels/nerves from periosteum penetrate compact bone through interosteonic canals
 circumferential lamellae around entire circumference of long bone shaft (external circumferential lamellae and internal circumferential lamellae)

Question 23

Spongy bone tissue

Answer 23

 No osteons, in interior of bone protected by compact bone
 Consists of lamellae in irregular pattern of thin columns called trabeculae
 Between trabeculae are spaces filled w/ red bone marrow (produces blood cells) and yellow bone marrow (adipose tissue), both have blood vessels for osyeocyte nourishment
 Each trabecula has concentric lamellae, osteocytes in lacunae, and canaliculi radiating out from lacunae
 Precisely oriented along lines of stress to resist stresses (located where bones not heavily stressed or stressed from many directions)

Question 24

Spongy bone is different from compact bone tissue in 2 ways

Answer 24

1. Spongy bone tissue is light, reducing bone weight -> allows bone to move easily
2. Trabeculae support/protect red bone marrow

Question 25

Blood vessels

Answer 25

abundant in bone w/ red bone marrow, pass into bone from periosteum

Question 26

Periosteal arteries

Answer 26

small arteries w/ nerves, enter diaphysis through interosteonic canals supplying periosteum and outer compact bone

Question 27

Nutrient artery/foramen

Answer 27

Near diaphysis center, large nutrient artery passes through hole in compact bone called nutrient foramen. On entering medullary cavity, nutrient artery divides into proximal and distal branches coursing towards each end of bone, supplying inner compact bone tissue of diaphysis and spongy bone tissue of red bone marrow

Question 28

Metaphyseal arteries

Answer 28

enter metaphyses of long bone and with nutrient artery supply red bone marrow and bone tissue of metaphyses

Question 29

Epiphyseal arteries

Answer 29

enter epiphyses of long bone and supply red bone marrow and bone tissue of epiphyses

Question 30

Veins carrying blood away from long bones evident in 3 places

Answer 30

1. One or two nutrient veins accompany nutrient artery and exit through diaphysis
2. Numerous epiphyseal veins and metaphyseal veins accompany and exit through respective arteries
3. Many small periosteal veins accompany respective arteries and exit through periosteum

Question 31

Nerve supply

Answer 31

 Nerves accompany blood vessels that supply bones
 Periosteum rich in sensory nerves sensitive to tearing/tension

Question 32

Intramembranous ossification – bone forms directly within mesenchyme

Answer 32

 1. Development of ossification center
 At bone development site called ossification center, specific chemical message cause mesenchyme cells to cluster and differentiate into osteoprogenitor cells then osteoblasts which secrete organic extracellular matrix until surrounded
 2. Calcification
 Extracellular matrix secretion stops and osteocytes line in lacunae and extend into canaliculi
 Calcium and other mineral salts deposited and extracellular matrix hardens/calcifies
 3. Formation of trabeculae
 As bone extracellular matrix forms, develops into trabeculae that fuse to form spongy bone (connective tissues in trabeculae differentiate into bone marrow)
 4. Development of periosteum
 Along with trabeculae formation, mesenchyme condenses and develops into periosteum
 Thin layer of compact bone replaces surface of spongy bone, spongy bone stays in center
 Much of new bone is remodeled as bone is transferred to adult shape and size

Question 33

Endochondral ossification – bone forms within hyaline cartilage that develops from mesenchyme (replacement of cartilage into bone)

Answer 33

 1. Development of cartilage model
 At bone development site, specific chemical messages cause cells in mesenchyme to cluster into general shape of future bone, then develop into chondroblasts
 Chondroblasts secrete cartilage extracellular matrix, producing a cartilage model (future diaphysis) of hyaline cartilage
 Perichondrium develops around cartilage model
 2. Growth of cartilage model
 Once chondroblasts become deeply buried in cartilage extracellular matrix they’re called chondrocytes
 Cartilage model grows in length by chondrocyte cell division with further cartilage extracellular matrix secretion
 This interstitial growth results in an increase in length (growth within)
 Growth of cartilage in thickness mainly due to appositional growth (outer surface)
 As cartilage model continues growing, chondrocytes increase in size and cartilage extracellular matrix begins to calcify
 3. Development of the primary ossification center
 Primary ossification proceeds inward
 Nutrient artery penetrates perichondrium and calcifying cartilage model through a nutrient foramen in midregion of cartilage model -> osteoprogenitor cells in perichondrium differentiate into osteoblasts
 Once perichondrium starts to form bone, it’s called periosteum
 Near middle of model, periosteal capillaries grow into disintegrating calcified cartilage, causing growth of primary ossification center (region where bone tissue replaces most cartilage)
 Osteoblasts begin to deposit extracellular matrix over remains of calcified cartilage, forming spongy bon trabeculae
 Primary ossification spreads from central location toward both ends of cartilage model
 4. Development of medullary (marrow cavity)
 As primary ossification center grows towards ends of bone, osteoclasts break down new spongy bone trabeculae, leaving the medullary (marrow) cavity in diaphysis
 Eventually most diaphysis wall replaced by compact bone
 5. Development of the secondary ossification centers
 Secondary ossification centers develop when epiphyseal artery branches enter epiphyses (around time of birth)
 Spongy bone remains in epiphyses interior
 Proceeds outward from epiphyses center towards outer bone surface
 6. Formation of articular cartilage and the epiphyseal (growth) plate
 Hyaline cartilage covering epiphyses becomes articular cartilage

Question 34

Growth in length

Answer 34

 Epiphyseal plate: a layer of hyaline cartilage in metaphysis of growing bone consists of four zones

 1. Zone of resting cartilage: layer nearest epiphysis consists of small, scattered chondrocytes. Resting b/c cells don’t function in bone growth instead anchor epiphyseal plate to epiphysis of bone
 2. Zone of proliferating cartilage: slightly large chondrocytes stacked. Undergo interstitial growth as they divide (to replace dead cells at diaphyseal side of epiphyseal plate) and secrete extracellular matrix
 3. Zone of hypertrophic cartilage: consists of large maturing chondrocytes in columns
 4. Zone of calcified cartilage: final zone only few cells thick, consists mostly of dead chondrocytes b/c surrounding extracellular matrix has calcified. Osteoclasts dissolve, calcified cartilage, osteoblasts, and capillaries from diaphysis invade area. Osteoblasts lay down bone extracellular matrix, replacing calcified cartilage in process of endochondral ossification (replacing cartilage w/ bone) thus zone of calcified cartilage becomes new diaphysis firmly cemented to rest of diaphysis

 Epiphyseal plate activity is the only way for diaphysis to increase length. As bone grows, chondrocytes multiply on epiphyseal side, new chondrocytes replace older ones destroyed by calcification, thus cartilage replaced by bone on diaphyseal side so thickness remains constant while length increases
 When adolescence ends, epiphyseal plate closes (epiphyseal cartilage cells stop dividing and replaced by bone), fades, leaving epiphyseal line (bone stops growing in length)

Question 35

Growth in thickness

Answer 35

 1. At bone surface, periosteal cells differentiate into osteoblasts which secrete collagen fibers/organic molecules that form bone extracellular matrix. Osteoblasts get surrounded by extracellular matrix and develop into osteocytes. Process forms bone ridges on either side of a periosteal blood vessel, ridges slowly enlarge and create groove for periosteal blood vessel
 2. Ridges fold together and fuse, groove becomes tunnel enclosing blood vessel. Former periosteum now becomes endosteum lining tunnel
 3. Osteoblasts in endosteum deposit bone extracellular matrix forming new concentric lamellae. Formation proceeds inward toward periosteal blood vessel so tunnel fills in and new osteon created
 4. As new osteon forms, osteoblasts under periosteum deposit new circumferential lamellae, further increasing bone thickness

Question 36

Bone remodeling

Answer 36

o Bone remodeling: the ongoing replacement of old bone tissue by new bone tissue
o Involves bone resorption: removal of minerals and collagen fibers from bone by osteoclasts
o Involves bone deposition: addition of minerals and collagen fibers to bone by osteoblasts

Question 37

Process of bone resorption

Answer 37

 osteoclast attaches tightly to bone surface at endosteum or periosteum and forms leakproof seal at edges of border
 Then releases protein-digesting lysosomal enzymes and several acids into sealed pockets (enzymes digest collagen fibers/organic substances while acids dissolve bone minerals)
 Several osteoclasts carve out a small tunnel in old bone
 Degraded bone proteins and extracellular matrix minerals (calcium and phosphorus) enter osteoclast by endocytosis, cross cell in vesicles, undergo exocytosis on opposite side
 In interstitial fluid, bone resorption products diffuse into blood capillaries
 Once small bone area resorbed, osteoclasts leave and osteoblasts move in

Question 38

Factors affecting bone growth and bone remodelling

Answer 38

 1. Minerals
 Large amts of calcium and phosphorus and smaller amts of magnesium, fluoride, and manganese needed while bones grow
 Minerals also needed during bone remodelling
 2. Vitamins
 Vitamin A stimulates osteoblast activity
 Vitamin c needed for collage synthesis
 Vitamin D helps build bone by increasing calcium absorption
 Vitamins K and B12 needed for bone protein synthesis
 3. Hormones
 Insulin like growth factors (IGFs) most important in childhood, produced by liver and bone tissue, stimulate osteoblasts, promote cell division, enhance protein synthesis to build new bone. IGFs produced in response to growth hormone (GH) secretion from anterior lobe of pituitary gland. Thyroid hormones (T3 and T4) from thyroid gland also promote bone growth by stimulating osteoblasts. Hormone insulin from pancreas promotes bone growth by increasing bone protein synthesis
 Sex hormones secreted at puberty cause dramatic effect on bone growth. E.g. estrogens (ovaries) and androgens (testes)

Question 39

Importance of calcium

Answer 39

 Nerve and muscle cells depend on stable calcium levels in extracellular fluid to function properly
 Blood clotting requires calcium
 Many enzymes require calcium as a cofactor
 Small changes in calcium concentration may cause cardiac/respiratory arrest

Question 40

Calcium regulation

Answer 40

 Parathyroid hormone (PTH)
 secreted by parathyroid glands
 hormone increases blood calcium level
 negative feedback
 if stimulus decreases blood calcium level, parathyroid gland cells (receptors) detect and increase production of cyclic AMP, the gene for PTH within nucleus of parathyroid gland cell (control center) detects intracellular increases in cyclic AMP (input), thus PTH (output) released into blood. Higher PTH levels increases number/activity of osteoclasts (effectors), increasing bone resorption. The release of calcium from bone to blood returns levels to normal
 PTH stimulates formation of calcitriol (active form of vitamin D), a hormone promoting absorption of calcium from foods in gastrointestinal tract into blood
 Calcitonin (CT) secreted by parafollicular cells in thyroid gland, decreasing blood calcium level as it inhibits osteoclasts activity, speeds blood calcium uptake by bone, and accelerates calcium deposition into bones

Question 41

Calcium and phosphorus

Answer 41

Make bone extracellular matrix hard

Question 42

Magnesium

Answer 42

Helps form bone extracellular matrix

Question 43

Fluoride

Answer 43

Helps strengthen bone extracellular matrix

Question 44

Manganese

Answer 44

Activates enzymes involved in synthesis of bone extracellular matrix

Question 45

Vitamin A

Answer 45

Needed for the activity of ostebolasts during remodeling of bone, deficiency stunts bone growth; toxic in high does

Question 46

Vitamin C

Answer 46

Needed for synthesis of collagen, the main bone protein; deficiency leads to decreased collagen production, which slows down bone growth and delays repair of broken bones

Question 47

Vitamin D

Answer 47

Active form (calcitriol) is produced by the kidneys; helps build bone by increasing absorption of calcium from
gastrointestinal tract into blood; deficiency causes faulty calcification and slows down bone growth; may reduce the risk
of osteoporosis but is toxic if taken in high doses. People who have minimal exposure to ultraviolet rays or do not take
vitamin D supplements may not have sufficient vitamin D to absorb calcium. This interferes with calcium metabolism.

Question 48

Vitamins K and B12

Answer 48

Needed for synthesis of bone proteins; deficiency leads to abnormal protein production in bone extracellular matrix
and decreased bone density.

Question 49

Growth hormone (GH)

Answer 49

Secreted by the anterior lobe of the pituitary gland; promotes general growth of all body tissues, including bone,
mainly by stimulating production of insulin-like growth factors.

Question 50

Insulin-like growth factors (IGFs)

Answer 50

Secreted by the liver, bones, and other tissues on stimulation by growth hormone; promotes normal bone growth by
stimulating osteoblasts and by increasing the synthesis of proteins needed to build new bone

Question 51

Thyroid hormones (T3 and T4)

Answer 51

Secreted by thyroid gland; promote normal bone growth by stimulating osteoblasts

Question 52

Insulin

Answer 52

Secreted by the pancreas; promotes normal bone growth by increasing the synthesis of bone proteins

Question 53

Sex hormones
(estrogens and testosterone)

Answer 53

Secreted by the ovaries in women (estrogens) and by the testes in men (testosterone); stimulate osteoblasts and
promote the sudden “growth spurt” that occurs during the teenage years; shut down growth at the epiphyseal plates
around age 18–21, causing lengthwise growth of bone to end; contribute to bone remodeling during adulthood by
slowing bone resorption by osteoclasts and promoting bone deposition by osteoblasts

Question 54

Parathyroid hormone (PTH)

Answer 54

Secreted by the parathyroid glands; promotes bone resorption by osteoclasts; enhances recovery of calcium
ions from urine; promotes formation of the active form of vitamin D (calcitriol).

Question 55

Calcitonin (CT)

Answer 55

Secreted by the parathyroid glands; promotes bone resorption by osteoclasts; enhances recovery of calcium
ions from urine; promotes formation of the active form of vitamin D (calcitriol).

Question 56

Calcitonin (CT)

Answer 56

Secreted by the thyroid gland; inhibits bone resorption by osteoclasts.

Question 57

EXERCISE

Answer 57

Weight-bearing activities stimulate osteoblasts and, consequently, help build thicker, stronger bones and retard
loss of bone mass that occurs as people age

Question 58

AGING

Answer 58

As the level of sex hormones diminishes during middle age to older adulthood, especially in women after
menopause, bone resorption by osteoclasts outpaces bone deposition by osteoblasts, which leads to a decrease in
bone mass and an increased risk of osteoporosis.