1. CHAPTER 6
THE SKELETAL SYSTEM

2. FUN FACTS Babies have ~ 300 bones Adults have ~ 206 bones
Humans and Giraffes have the same # of bones in the neck
Longest bone = femur
Smallest bone = inner ear

3. Functions of the skeletal system
Support – structural framework
Protection of internal organs
Blood Cell Production (hematopoiesis)
Movement – skeletal muscle attaches to bone
Mineral Homeostasis (calcium and phosphorous)
Storage – calcium salts represent a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in body fluids. In addition, bones also store energy reserves as lipids in areas fillwed with yellow marrow.
Blood cell production – red blood cells, white blood cells and other blood elements are produced within ref marro which fills the internal cavities of many bones
Leverage – movements range from delicate motion of a fingertip to powerful changes in the position of the entire body.

4. Organization of the Skeletal System
2 main divisions of the skeletal system
Axial skeleton (head, neck, trunk) – 80 bones
Appendicular skeleton (limbs) – 126 bones

5. Macroscopic features of bone
Bone Types
Long bones (bones of arms and legs)
Short bones (bones of wrist)
Flat bones (skull)
Irregular bones (vertebrae)

6. Macroscopic features of bone

7. Structure of a Long Bone
Diaphysis
central shaft
Epiphysis
forms joint with another bone.
Articular Cartilage (Hyaline)
Medullary cavity
Hollow chamber filled with bone marrow
Periosteum
Covers outside of bone
Endosteum
Lines medullary cavity

8. Microscopic features of bone
Bone Cells
Osteocytes
mature bone cells
Exchange nutrients and wastes with blood
Osteoclasts – (clast = break)
Release enzymes to dissolve bone matrix
Resorption of bone helps regulate calcium and phosphate concentration in body fluids
Osteoblasts (blast = precursor)
Produce new bone

9. Microscopic features of bone
Bone Matrix (secreted by osteocytes)
Organic Components (33.3%)
Collagen fibers provide resilience against stretching and twisting
Inorganic components (66.7%)
Mg, F, Na, Ca, P
Hydroxyapatite (calcium phosphate and calcium hydroxide)
Provide hardness and resist compression

10. Macroscopic features of bone
Bone Tissues
Compact (cortical) bone
Solid, strong, forms walls of the diaphysis
Resists stresses produced by weight and movement
Spongy (cancellous) bone
Interior bone tissue

11. Microscopic features of bone
Define the following:
Osteocytes
Osteoblasts
Osteoclasts
Trabeculae
Lamellae
Lacunae
canaliculi

12. Microscopic features of bone
Organization of Compact Bone (OSTEON)
Central (haversion) canals run the length of the bone
Adjacent Haversion canals are connected via Volkmann’s canals
Around the central canals are concentric lamellae
Between the lamellae are spaces called lacunae
Lacunae contain osteocytes
Lacunae are connected in all direction via canaliculi

13. Microscopic features of bone
COMPACT BONE

14. Microscopic features of bone
COMPACT BONE

15. Microscopic features of bone
COMPACT BONE

16. Microscopic features of bone
Organization of Spongy Bone
Spongy bone lacks osteons
Lamellae arranged in a lattice of thin columns called trabeculae
Spaces make bone lighter
Support and protect red marrow
Blood cell production occurs here
Within each trabeculae are lacunae that contain osteocytes

17. Microscopic features of bone
SPONGY BONE

18. Bone Formation

19. Bone Formation (osteogenesis)
Ossification = the process of replacing other tissues with bone
Calcification = the deposition of calcium salts
Occurs during ossification but can also occur in other tissues besides bone
Bone Formation occurs in 4 situations
Formation of bone in an embryo
Growth of bones into adulthood
Remodeling of bones
Repair of fractures

20. 1. Formation of Bone in an Embryo
Begins during the 6th week of development
Intramembranous ossification – continues to age 2
Bone develops from fibrous connective tissue
Growth is outward from the center
Flat bones of the skull and jaw
Endochondral ossification – continues to age 25
Bone develops from pre-existing hyaline cartilage
Most bones of the body are formed this way

21. 1. Formation of Bone in an Embryo
Intramembranous ossification

22. 1. Formation of Bone in an Embryo
Endochondral ossification

23. 2. Growth of bone into adulthood
Endochondral ossification (cartilage replaced)
Primary ossification center
Located in the diaphysis
Lengthening of bone – causes epiphyseal (growth) plate to close
Secondary ossification center
Located in epiphysis
Allows bone to grow in width

24. 3. Remodeling of bone Ongoing replacement of old bone with new bone
Due to the action of osteoclasts (breakdown of bone) and osteoblasts (building of bone)

25. 4. Fracture and Repair
Fracture hematoma – blood leaks from broken blood vessels in bone
Callus Formation – fibroblasts invade the tissue and produce collagen fibers
Bony Callus Formation – osteoblasts begin to build spongy bone
Bone remodeling – compact bone replaces spongy bone

26. 4. Repair of Fractures 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

27. 4. Repair of Fractures

28. Requirements for normal bone growth
Minerals, especially calcium and phosphate
Vitamin D3 stimulates absorption of calcium
Rickets – vitamin D3 deficiency – causes bone to soften
Vitamin A stimulates osteoblasts
Vitamin C for synthesis of collagen
Scurvy – vitamin - C deficiency

29. Hormones and Bone Growth
Growth Hormone increases bone growth
Thyroxine increases bone growth
Sex Hormones (estrogens and androgens)
Cause growth spurt during puberty; epiphyseal plate closure

30. The skeleton as a calcium reserve
Levels of calcium in the blood are maintained by controlling the rates of calcium resorption and deposition
Nerve and muscle cells depend on calcium
Blood clotting requires calcium
30% increase causes neurons and muscle cells to become unresponsive
35% decrease causes convulsions
50% reduction in calcium causes death

31. Calcium Homeostasis Actions that elevate blood Ca2+
When blood calcium is low
Parathyroid hormone (PTH)
increases activity of osteoclasts (bone breakers)
decreases activity of osteoblasts (bone builders)
stimulates calcitrol, a hormone that promotes absorption of calcium from food

32. Calcium Homeostasis Actions that decrease blood Ca2+
When blood calcium is high
Calcitonin (CT)
stimulates osteoblasts (bone builders)
inhibits osteoclasts (bone breakers)
promotes bone formation
decreases blood Ca2+

33. Negative Feedback
Calcium Regulation

34. BONE MARKINGS

35. Elevations and Projections
Process = bony projection or bump
Ramus = curved portion of a bone (ram’s horn)

36. Processes formed where tendons and ligaments attach
Trochanter = large rough projection
Tuberosity = small rough projection
Tubercle = small rounded projection
Crest = prominent ridge
Line = low ridge
Spine = pointed process

37. Processes formed for articulation with other bones
Head = epiphysis
Neck = connection between epiphysis and diaphysis
Condyle = smooth, rounded bump that forms joint with another bone
Trochlea = small, grooved pulley shaped process
Facet = small flat articular surface

38. Depressions
Fossa = shallow groove or depression
Sulcus = narrow grove

39. Openings Foramen = rounded passageway for blood vessels and nerves
Canal or Meatus = passageway through bone
Fissure = elongated cleft
Sinus = chamber within bone usually filled with air