Chapter 7: Bone Tissue

Question 1

Bone Tissue

Answer 1

• often associated with death
  
  • bones and teeth are durable and remain once other tissues are gone
• a living skeleton is made of dynamic tissue, full of cells, and permeated with nerves & blood vessels

Question 2

Tissues and Organs of the Skeletal System

Answer 2

• Osteology: the study of bones.
• the skeletal system consists of:
  
  • bones
  • cartilage:
    forerunner of
    most bones;
    covers many joint
    surfaces of mature
    bones
    - ligaments:
    holds bones
    together at joints
  • (tendons attach to muscles at bones)

Question 3

Functions of the Skeleton

Answer 3

1. support: holds up the body, supports muscle and teeth (mandible and maxilla)
2. protection: brain, spinal cord, heart, lungs
3. movement: action of muscle on bone enables limb movement and breathing
4. electrolyte balance: calcium and phosphate ions
5. acid-base balance: buffers blood against excessive pH changes
6. blood formation: red bone marrow is the chief producer of blood cells

Question 4

Bones as Organs

Answer 4

• individual bones are organs that consist of bone tissue plus blood, bone marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue.
  
  • metabolically active
  • permeated with nerves and blood vessels, which attest to its sensitivity and metabolic activity.
  • constantly changing in response to the environment
  • interacts physiologically with all the other organ systems

Question 5

Categories of Bones (the organs)

Answer 5

• flat bones: curved but wide and thin; protects soft organs (brain)
• long bones: longer than wide; rigid levers acted upon by muscles
• short bones: equal in length and width; glide across one another in multiple directions
• irregular bones: elaborate shapes don’t fit into any category; unity of shape and function

Question 6

features of long bones

Answer 6

• epiphysis: enlarged end of long bone
  
  • strengthens joint and allows room for ligaments and tendons to attach
• diaphysis (shaft): cylinder of compact bone to provide leverage
• epiphyseal plate (growth plate): area of hyaline cartilage between marrow spaces of the epiphysis and diaphysis
  
  • epiphyseal plate: in children/young people
  • epiphyseal line: in adults (a bony scar left where the growth plates used to be)

Question 7

features of long bone cont.

Answer 7

• articular cartilage: layer of hyaline cartilage that covers the surfaces of joints (where bones meet)
  
  • allows joints to move more freely and relatively friction-free
• nutrient foramina: minute holes in the bone surface that allow blood vessels to penetrate
• medullary (marrow) cavity: marrow-containing chamber in diaphysis

Question 8

feature of long bone cont.

Answer 8

• periosteum: external sheath, covers bone except where there is articular cartilage
  
  • outer fibrous layer
    
    • strong link between bone –> tendon–> muscle
  • inner osteogenic layer
    
    • bone-forming cells important to growth of bone and healing of fractures
• endosteum: thin layer of reticular connective tissue lining the medullary cavity and covering spongy bone tissues

Question 9

General Features of Flat Bone

Answer 9

• sandwich-like construction
• 2 layers of compact bone enclosing a middle layer of spongy bone
  
  • both surfaces of flat bone covered with periosteum
• diploe: spongy layer in cranium
  
  • absorbs shock
  • marrow spaces lined with endosteum

Question 10

Composition of Bone (Osseous) Tissue

Answer 10

• connective tissue with few cells, lots of matrix
• matrix: 1/3 organic, 2/3 inorganic
• organic materials: collagen and protein-carbohydrate complexes
  
  • collagen fibers synthesized by bone building cells (osteoblasts)
  • gives bones some flexibility
  • osteogenesis imperfecta (brittle bone disease): defect in collagen deposition
• inorganic materials: minerals
  
  • 85% hydroxyapatite (crystallized calcium phosphate salt)
  • 10% calcium carbonate (plus some F, Na, Mg, etc. ions)
  • gives bone strength
  • rickets = diminished mineral deposition (due to vitamin D deficiency)
    –> soft bones

Question 11

two types of bone tissue

Answer 11

• compact bone: dense outer shell of bone
• spongy (cancellous) bone: under the compact bone in the ends of long bone and in the middle of other bones. space is filled with marrow
• together these make bone strong with minimal weight
  
  • 3/4 of skeletal dry weight is from compact bone

Question 12

compact vs spongy bones

Answer 12

• compact bone is composed of osteons
• spongy bone has few osteons, bone tissue is in thin spicules and trabeculae with spaces between

Question 13

histology of compact bone

Answer 13

• each osteon is actually cylinder of matrix tissue organized around the central canal
  
  • collagen fibers wrap around in opposite directions, making it stronger
• perforating (Volkmann) canal: diagonal canals that connect central canals to each other

Question 14

histology of compact bone cont.

Answer 14

• osteon: the structural and functional unit of the bone
  
  • concentric lamellae: layers of matrix, laid in concentric circles around a central canal
  • central canal (haversian or osteonic canal): contains blood vessels and nerves
  • osteocytes in lacunae
• slide of compact bone contains matrix but no cells
  
  • osteocytes are destroyed in the slide-making process
  • empty lacunae are left behind

Question 15

histology of spongy bone

Answer 15

• spongy bone consists of: slivers of bone called spicules, thin plates of bone called trabeculae, spaces filled with red bone marrow
• provides strength with minimal weight
• few osteons and no central canal
  
  • all osteocytes are close to bone marrow

Question 16

bone marrow

Answer 16

• soft tissue that occupies the marrow cavities in bone
• red bone marrow: occupies the empty spaces within the spongy bone, produces blood cells (hemopoietic), in almost every bone of children, in adult: persists of skull, vertebrae, ribs, sternum, hips, proximal heads of humorous and femur
• yellow marrow: in adults, replaces red marrow in rest of skeleton; becomes fatty, no more hemopoiesis

Question 17

bone cell types

Answer 17

• osteogenic cells, osteoblasts, osteocytes
  
  • same cells at different stages of differentiation
  • builds bone
• osteoclasts: develop from different stem cells (in marrow); dissolve bone
• osteogenic cell: type of cell found in endosteum and inner periosteum; multiply continuously to produce new osteoblasts
• osteoblasts: differentiated, non-mitotic bone-forming cells; single layer of cells under endosteum and periosteum; synthesizes soft organic matter matrix (collagen) to be hardened by mineral deposition

Question 18

bone-forming by osteoblasts

Answer 18

• osteoblasts produce collagen fibers that spiral the length of the osteon
• mineral deposition: calcium, phosphate and other ions from blood plasma (via. tissue fluid) are spontaneously deposited along the collagen fibers as these minerals precipitate (come out of solution)
• form hydroxyapatite crystals
• the matrix hardens as the collagen fibers become encrusted with the crystals

Question 19

osteocytes

Answer 19

• former osteoblasts that have become trapped in the hardened matrix they have deposited
• lacunae: tiny cavities where osteocytes reside
• canaliculi: cytoplasmic channels that allow communication between lacunae

Question 20

osteoclasts

Answer 20

• bone-dissolving cells on bone surface
• develop from the bone marrow stem cells that give rise to blood cells (not osteogenic cells)
• formed from the fusion of several stem cells
• typically, very large; have 3-50 nuclei
• sit in resorption bays which are pits on the surfaces of bone

Question 21

mineral deposition and resorption

Answer 21

• mineral deposition/resorption exerts a profound influence over the rest of the body by exchanging minerals with tissue fluid (tissue fluid occupies space that is not in blood vessels nor within cells)
• mineral deposition: Ca^2+ (calcium ion) and PO4^3- (phosphate ion) concentration in tissue fluid + various other signals > crystal formation
  
  • first few crystals (seed crystals) attract more and more calcium and phosphate from solution (positive feedback!!)
  • ectopic ossification: abnormal calcification occurring in lungs, brain, eyes, muscles, tendons, or arteries (arteriosclerosis)
    
    • calculus: calcifies mass in a soft organ such as the lung

Question 22

mineral deposition and resorption cont.

Answer 22

• mineral resorption: the process of dissolving bone and releasing minerals into tissue fluid –> blood
• performed by osteoclasts at their ruffled borders
  
  • hydrogen pumps in membranes secrete H+ into space between the osteoclasts and bone surface
  • chloride ions follow by electrical attraction
  • hydrochloric acid (HCI, pH 4) dissolves bone minerals
  • protease enzyme digests the collagen
• orthodontic appliances (braces) reposition teeth: tooth moves because osteoclasts dissolve bone ahead of the tooth & osteoblasts deposit bone behind the tooth

Question 23

ossification (osteogenesis)

Answer 23

• formation of bone; development and growth of bone that starts in the human fetus
  bone develops by two methods:
• endochondral ossification: develops from hyaline cartilage model mesenchyme (embryonic CT) –> hyaline cartilage –> bone; produces most bones (limbs, pelvic girdle, ribs, etc.); occurs from 6 weeks (fetal development) until full height is achieved
• intramembranous ossification: develops from mesenchyme surrounding blood vessels; produces flat bone of the skull and clavicle.

Question 24

endochondral ossification

Answer 24

1. early cartilage model: mesenchyme develops into area of hyaline cartilage covered by perichondrium which chondrocytes
2. chondrocytes inflate, calcify & die leaving cavity; perichondrium is now periosteum, creates osteoblasts which deposits a ring of bone
3. osteoclasts arrive & dissolve calcifies cartilage; osteoclasts thicken the outer bone
4. At birth: chondrocytes death has also occurred at an epiphysis, forming a secondary marrow cavity
5. in infancy and childhood: epiphyses fill with spongy bone; remaining cartilage = epiphyseal plate and articular cartilage

Question 25

intramembranous ossification

Answer 25

1. mesenchyme cells line up around blood vessels, become osteoblasts; secrete osteoid tissue (includes collagen fibers)
2. minerals crystalize on collagen fibers; osteoblasts become osteocytes
3. continue mineralization –> spongy bone; mesenchyme at surface forms periosteum
4. osteoblasts beneath periosteum form zone of compact zone

Question 26

Flat bones

Answer 26

• flat bones of the skull are formed by intramembranous ossification

Question 27

Bone growth: elongation

Answer 27

• in long bones, interstitial growth (interior growth) occurs at the epiphyseal plate
  
  • in the x-rays of long bone
  • x-ray shows epiphyseal in a child’s hand
• cartilage is precursor to bone
  
  • hyaline cartilage calcified and is replaced by bone
  • lengthening of long bones continues as long as the epiphyseal plates have cartilage
  • until about 16-20 yrs old (different ages for different bone)

Question 28

metaphysis

Answer 28

• transition area found on either side of the epiphyseal plate

Question 29

zones of the metaphysis

Answer 29

1. zone of reserve cartilage: typical histology of resting hyaline cartilage
2. zone of cell proliferation: chondrocytes multiplying and lining up in rows of small flattened lacunae
3. zone of cell hypertrophy: cessation of mitosis, enlargement of chondrocytes & removing of lacuna walls
4. zone of calcification: temporary calcification of cartilage matrix between columns of lacunae
5. zone of bone deposition: breakdown of lacuna walls, leaving open channels; death of chondrocytes; bone deposition by osteoblasts, forming trabeculae of spongy bones

Question 30

transformation

Answer 30

• growing bone to adult bone
• by late teens to early 20s, all remaining bone in the epiphyseal plate
  
  • gap between epiphyses and diaphysis closes leaving behind an epiphyseal line
  • single marrow cavity
• no more increase in height

Question 31

dwarfism

Answer 31

achondroplastic dwarfism:
- long bones stop growing in childhood
- normal torso; short limbs
- failure of cartilage growth in metaphysis
- due to spontaneous DNA mutation

pituitary dwarfism:
- lack of growth hormone
- normal proportions with short stature

Question 32

bone growth: widening and thickening

Answer 32

appositional growth = new bone tissue at surface
- this type of growth occurs throughout life
- occurs by intramembranous ossification
deposition of new bone occurs when osteoblasts on the inner side of periosteum deposit osteoid tissue.
- osteoblast become trapped as the tissue calcifies
- matrix is laid down in layers parallel to surface form circumferential lamellae over surface

as bone widens, osteoclasts dissolve bone tissue to increase the marrow cavity

Question 33

bone remodeling

Answer 33

a mature bone remains metabolically active
- involved in its own growth and constant remodeling
collaborative effort between osteoblasts (bone builders) & osteoclasts (bone dissolves)

wolfs law of bone: architecture of bone is determined by mechanical stresses placed on it
- trabeculae develop along the bones lines of stress
- bony processes grow larger in response to mechanical stress

Question 34

bone remodeling cont.

Answer 34

10% of skeleton is remodeled each year.
- renew old bones, repair microfractures, release minerals into the blood

osteoblasts strengthen bone
- increase in number with stress and fractures because osteogenic cells are stimulated to produce more osteoblasts to reinforce or rebuild bones

osteoclasts release minerals
- secrete proteases that dissolve the organic components
- secrete acids that dissolve the inorganic components

Question 35

importance of bone minerals

Answer 35

calcium and phosphate are used for much more than bone structure
- phosphate is a component of DNA, RNA, ATP, phospholipids, and pH buffers
- calcium needed in neuron communication, muscle contraction, blood clotting, and exocytosis

the skeleton acts as a storage unit for minerals
- minerals are deposited in the skeleton and withdrawn when they are needed for other purposes

Question 36

phosphate homeostasis

Answer 36

occurs as HPO4^2- (monohydrogen and dihydrogen phosphate ions)

phosphate levels are not regulated as tightly as Ca^2+
- no immediate functional disorders

regulated by calcitriol & parathyroid hormone (PTH)
- calcitriol (forms vitamin D) promotes phosphate absorption by the small intestine –> increase blood phosphate levels (making phosphate available for bone deposition)
- PTH promotes urinary excretion of phosphate –> lowers blood phosphate levels

Question 37

calcium homeostasis

Answer 37

99% of body calcium is in the skeleton
- most bone calcium is in hydroxyapatite – very stable
- small Amont is exchangeable calcium (Ca^2+) - easily released into tissue fluid <–> blood

normal blood calcium
- 45% is calcium that can diffuse across capillary walls and affect other tissues
- the rest is in reverse, bound to plasma proteins

balance between:
- dietary intake of calcium
- urinary and fecal losses of calcium
- exchange between osseous tissue & blood

regulated by 3 hormones:
- calcitriol
- parathyroid hormone (PTH)
- calcitonin

Question 38

calcium imbalances

Answer 38

hypocalcemia: calcium deficiency in blood
- even moderate hypocalcemia can cause problems
- can cause overexcitability of the nervous system and muscle systems resulting in muscle spasms, trousseau sign, tetany (inability of muscles to relax)

hypercalcemia: excessive calcium in blood
- rarely occurs
- can cause under-excitability of the nervous and muscle systems resulting in muscle weakness, reduced neural reflexes and non-responsiveness