1. The Skeletal System: Bone Tissue Chapter 6

3. The Skeletal System: Bone Tissue
Functions of Bone and Skeletal System
Structure of Bone
Histology of Bone Tissue
Blood and Nerve Supply of Bone
Bone Formation
Bone’s Role in Calcium Homeostasis
Exercise and Bone Tissue
Aging and Bone Tissue

4. Functions of Bone and Skeletal System
Support
Protection
Assistance in Movement
Mineral Homeostasis
Blood Cell Production
Triglyceride Storage

5. Functions of Bone and Skeletal System
Support
Structural framework of the body
Supports soft tissues
Provides attachment points for tendons of skeletal muscle
Protection
Protects important internal organs
Cranium protects brain
Vertebrae protects spinal cord
Ribs protect lungs and heart

6. Functions of Bone and Skeletal System
Assistance in Movement
Skeletal muscle attaches to bone
Skeletal muscle contraction pulls on bone producing movement
Mineral Homeostasis
Bone tissue stores several minerals
Acts to serve as a reservoir of critical minerals
Calcium (99% of body’s content)
Phosphorus

7. Functions of Bone and Skeletal System
Blood Cell Production
Red bone marrow produces (Hemopoiesis)
Red blood cells
White blood cells
Platelets
Triglyceride Storage
Yellow bone marrow
Triglycerides stored in adipose cells
Serves as a potential chemical energy reserve

8. Structure of Bone Long Bone Anatomy (Humerus) Diaphysis Epiphysis
Metaphysis
Epiphyseal growth plate
Articular cartilage
Perforating fibers
Periosteum
Medullary cavity
Endosteum

11. Histology of Bone Tissue
Extracellular matrix surrounding widely separated cells
Matrix
15% water
30% collagen fibers
55% crystallized mineral salts
The most abundant mineral salt is calcium phosphate

12. Histology of Bone Tissue
A process called calcification is initiated by bone-building cells called osteoblasts
Mineral salts are deposited and crystallize in the framework formed by the collagen fibers of the extracellular matrix
Bone’s flexibility depends on collagen fibers

13. Histology of Bone Tissue
Four types of cells are present in bone tissue
Osteogenic cells
Undergo cell division (stem cells); the resulting cells develop into osteoblasts
Osteoblasts
Bone-building cells
Synthesize extracellular matrix of bone tissue
Osteocytes
Mature bone cells
Exchange nutrients and wastes with the blood

14. Histology of Bone Tissue
Osteoclasts
Release enzymes that digest the mineral components of bone matrix (resorption)
Regulate blood calcium level

15. Histology of Bone Tissue
Bone may be categorized as:
Compact
Spongy

16. Histology of Bone Tissue
Compact Bone
Resists the stresses produced by weight and movement
Components of compact bone are arranged into repeating structural units called osteons or Haversian systems
Osteons consist of a central (Haversian) canal with concentrically arranged lamellae, - lacunae, osteocytes, and canaliculi

20. Histology of Bone Tissue
Osteon
Central canals run longitudinally through bone
Around the central canals are concentric lamellae
Rings of calcified matrix (like the rings of a tree trunk)
Between the lamellae are small spaces called lacunae which contain osteocytes
Radiating in all directions from the lacunae are tiny canaliculi filled with extracellular fluid and the plasma membrane extensions of the osteocyte

21. Histology of Bone Tissue
Osteon
Canaliculi connect lacunae, forming a system of interconnected canals
Providing routes for nutrients and oxygen to reach the osteocytes
The organization of osteons changes in response to the physical demands placed on the skeleton

22. Histology of Bone Tissue
Spongy Bone
Lacks osteons
Lamellae are arranged in a lattice of thin columns called trabeculae
Spaces between the trabeculae make bones lighter
Trabeculae of spongy bone support and protect the red bone marrow
Hemopoiesis (blood cell production) occurs in red bone marrow of spongy bone

23. Histology of Bone Tissue
Spongy Bone
Within each trabecula are lacunae that contain osteocytes
Osteocytes are nourished from the blood circulating through the trabeculae within the red bone marrow.
Interior bone tissue is made up primarily of spongy bone
The trabeculae of spongy bone are oriented along lines of stress
helps bones resist stresses without breaking while keeping them light.

25. Blood and Nerve Supply of Bone
Bone is richly supplied with blood
Periosteal arteries accompanied by nerves supply the periosteum and compact bone
Epiphyseal veins carry blood away from long bones
Nerves accompany the blood vessels that supply bones
The periosteum is rich in sensory nerves sensitive to tearing or tension

26. Bone Formation The process by which bone forms is called ossification
Bone formation occurs in four situations:
1) Formation of bone in an embryo
2) Growth of bones until adulthood
3) Remodeling of bone
4) Repair of fractures

27. Bone Formation Formation of Bone in an Embryo
Bone formation follows one of two patterns
Intramembranous ossification
Flat bones of the skull and mandible are formed in this way
“Soft spots” that help the fetal skull pass through the birth canal later become ossified forming the skull
Endochondral ossification
The replacement of cartilage by bone
Most bones of the body are formed in this way including long bones

28. Bone Formation Formation of Bone in an Embryo
Cartilage formation and ossification occurs during the sixth week of embryonic development

29. Bone Formation - Intramembranous ossification

30. Bone Formation - Intramembranous ossification

31. Bone Formation - Intramembranous ossification

32. Bone Formation - Intramembranous ossification

33. Bone Formation - Endochondral ossification

34. Bone Formation - Endochondral ossification

35. Bone Formation - Endochondral ossification

36. Bone Formation - Endochondral ossification

37. Bone Formation - Endochondral ossification

38. Bone Formation - Endochondral ossification

40. Bone Growth During Infancy, Childhood and Adolescence
Growth in Length
The growth in length of long bones involves two major events:
1) Growth of cartilage on the epiphyseal plate
2) Replacement of cartilage by bone tissue in the epiphyseal plate

41. Bone Growth During Infancy, Childhood and Adolescence
Osteoclasts dissolve the calcified cartilage, and osteoblasts invade the area laying down bone matrix
The activity of the epiphyseal plate is the way bone can increase in length
At adulthood, the epiphyseal plates close and bone replaces all the cartilage leaving a bony structure called the epiphyseal line

43. Bone Growth During Infancy, Childhood and Adolescence
Growth in Thickness
Bones grow in thickness at the outer surface
Remodeling of Bone
Bone forms before birth and continually renews itself
The ongoing replacement of old bone tissue by new bone tissue
Old bone is continually destroyed and new bone is formed in its place throughout an individual’s life

44. Bone Growth During Infancy, Childhood and Adolescence
A balance must exist between the actions of osteoclasts and osteoblasts
If too much new tissue is formed, the bones become abnormally thick and heavy
Excessive loss of calcium weakens the bones, as occurs in osteoporosis
Or they may become too flexible, as in rickets and osteomalacia

45. Factors Affecting Bone Growth and Bone Remodeling
Normal bone metabolism depends on several factors
Minerals
Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling
Vitamins
Vitamin A stimulates activity of osteoblasts
Vitamin C is needed for synthesis of collagen
Vitamin D helps build bone by increasing the absorption of calcium from foods in the gastrointestinal tract into the blood
Vitamins K and B12 are also needed for synthesis of bone proteins

46. Factors Affecting Bone Growth and Bone Remodeling
Hormones
During childhood, the hormones most important to bone growth are growth factors (IGFs), produced by the liver and bone tissue (under influence of hGH)
IGFs stimulate osteoblasts, promote cell division at the epiphyseal plate, and enhance protein synthesis
Thyroid hormones (T3 and T4) also promote bone growth by stimulating osteoblasts
Insulin promotes bone growth by increasing the synthesis of bone proteins

47. Factors Affecting Bone Growth and Bone Remodeling
Hormones
Estrogen and testosterone cause a dramatic effect on bone growth
Cause of the sudden “growth spurt” that occurs during the teenage year
Promote changes in females, such as widening of the pelvis
Ultimately-Shut down growth at epiphyseal plates
Parathyroid hormone, calcitriol, and calcitonin are other hormones that can affect bone remodeling

48. Bone’s Role in Calcium Homeostasis
Bone is the body’s major calcium reservoir
Levels of calcium in the blood are maintained by controlling the rates of calcium resorption from bone into blood and of calcium deposition from blood into bone
Both nerve and muscle cells depend on calcium ions (Ca2+) to function properly
Blood clotting also requires Ca2+
Many enzymes require Ca2+ as a cofactor

49. Bone’s Role in Calcium Homeostasis
Actions that help elevate blood Ca2+ level
Parathyroid hormone (PTH) regulates Ca2+ exchange between blood and bone tissue
PTH increases the number and activity of osteoclasts (i.e., resorption)
PTH acts on the kidneys to decrease loss of Ca2+ in the urine
PTH stimulates formation of calcitriol (active form of Vitamin D) a hormone that promotes absorption of calcium from foods in the gastrointestinal tract

50. Bone’s Role in Calcium Homeostasis

51. Bone’s Role in Calcium Homeostasis
Actions that work to decrease blood Ca2+ level
The thyroid gland (i.e., parafollicular cells) secretes calcitonin (CT) which inhibits activity of osteoclasts
The result is that CT promotes bone formation and decreases blood Ca2+ level
See Table 6.2

52. Fracture and Repair of Bone
Fracture Types
Open (compound) fracture
The broken ends of the bone protrude through the skin
Closed (simple) fracture
Does not break the skin
Comminuted fracture
The bone is splintered, crushed, or broken into pieces
Greenstick fracture
A partial fracture in which one side of the bone is broken and the other side bends
Impacted fracture
One end of the fractured bone is forcefully driven into another
Pott’s fracture
Fracture of the fibula, with injury of the tibial articulation
Colles’ fracture
A fracture of the radius in which the distal fragment is displaced
Stress fracture
A series of microscopic fissures in bone

53. Anatomical appearance – like breaking a green twig
Fracture and Repair
Anatomical appearance – like breaking a green twig
Greenstick

54. Fracture and Repair
Anatomical appearance – the distal part is shoved up into the proximal part.
Impacted

55. Fracture and Repair
Anatomical appearance – though not seen here, one or both bones are “open” to the outside.
Open (compound)

56. Fracture and Repair
Eponyms – Colles’ is a fracture of the distal radius ± ulna.
Colles’

57. Fracture and Repair of Bone
Calcium and phosphorus needed to strengthen and harden new bone after a fracture are deposited only gradually and may take several months
The repair of a bone fracture involves the following steps
1) Formation of fracture hematoma
Blood leaks from the torn ends of blood vessels, a clotted mass of blood forms around the site of the fracture
2) Fibrocartilaginous callus formation
Fibroblasts invade the fracture site and produce collagen fibers bridging the broken ends of the bone
3) Bony callus formation
Osteoblasts begin to produce spongy bone trabeculae joining portions of the original bone fragments
4) Bone remodeling
Compact bone replaces spongy bone

58. Spongy bone
Osteoblast
Osteoclast
New compact
bone
Bony callus formation
Bone remodeling
Osteocyte 3 4
Compact bone
Periosteum
Fracture hematoma
Fracture
hematoma
Bone
fragment
Red blood
cell
Blood vessel
Formation of fracture hematoma
Phagocyte
Osteon 1
Fibroblast
Fibrocartilaginous
callus
Collagen fiber
Chondroblast
Cartilage
Fibrocartilaginous callus formation 2
Bony callus
Bony callus
Spongy bone
Osteoblast
Bony callus formation
Osteocyte 3
Compact bone
Periosteum
Fracture hematoma
Fracture
hematoma
Bone
fragment
Red blood
cell
Blood vessel
Formation of fracture hematoma
Phagocyte
Osteon 1
Fibroblast
Fibrocartilaginous
callus
Collagen fiber
Chondroblast
Cartilage
Fibrocartilaginous callus formation 2
Phagocyte
Osteoblast
Fibroblast
Fibrocartilaginous
callus
Collagen fiber
Chondroblast
Cartilage
Fibrocartilaginous callus formation 2
Compact bone
Spongy bone
Periosteum
Fracture hematoma
Fracture
hematoma
Bone
fragment
Osteocyte
Red blood
cell
Blood vessel
Formation of fracture hematoma
Osteon 1
Compact bone
Spongy bone
Periosteum
Fracture hematoma
Fracture
hematoma
Bone
fragment
Osteocyte
Red blood
cell
Blood vessel
Formation of fracture hematoma
Phagocyte
Osteon 1

59. Fracture and Repair Once a bone is fractured, repair proceeds in
a predictable pattern:
The first step, which occurs 6-8 hours after injury, is the formation of a fracture
hematoma, as a result of
blood vessels breaking in
the periosteum and
in osteons.

60. Fracture and Repair
The second and third steps involve the formation of a callus (takes a few weeks, to as many as six months).
Phagocytes remove cellular debris and fibroblasts
deposit collagen to
form a fibro-
cartilaginous callus...

61. Fracture and Repair
... which is followed by osteoblasts forming a bony
callus of spongy bone.

62. Fracture and Repair
The final step takes several months and is called remodeling :
Spongy bone is replaced by
compact bone.
The fracture line
disappears, but
evidence of the break
remains.

63. Exercise and Bone Tissue
Bone tissue alters its strength in response to changes in mechanical stress
Under stress, bone tissue becomes stronger through deposition of mineral salts and production of collagen fibers by osteoblasts and fibroblasts
Unstressed bones diminishes because of the loss of bone minerals and decreased numbers of collagen fibers
The main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of gravity
Weight-bearing activities help build and retain bone mass

64. Aging and Bone Tissue
The level of sex hormones diminishes during middle age, especially in women after menopause
A decrease in bone mass occurs
Bone resorption by osteoclasts outpaces bone deposition by osteoblasts
Female bones generally are smaller and less massive than males
Loss of bone mass in old age has a greater adverse effect in females

65. Aging and Bone Tissue
There are two principal effects of aging on bone tissue:
1) Loss of bone mass
Results from the loss of calcium from bone matrix
The loss of calcium from bones is one of the symptoms in osteoporosis
2) Brittleness
Results from a decreased rate of protein synthesis
Collagen fibers gives bone its tensile strength
The loss of tensile strength causes the bones to become very brittle and susceptible to fracture

66. Compact Bone

67. Spongy Bone (Cancellous)

68. Hyaline Cartilage

69. Adipose Tissue