2. The Skeletal System (6-1)
Components include:
Bones of the skeleton
Cartilages, joints, ligaments, connective tissue that stabilize or connect bones

3. Functions of the Skeletal System (6-1)
Support
Provides structural support for the body
Bones provide attachments for soft tissues and organs
Storage of minerals and lipids
Calcium and phosphorus ions held in bone
Yellow bone marrow stores lipids
Blood cell production
Red blood cells, white blood cells, and other blood elements produced in the red bone marrow

4. Functions of the Skeletal System cont. (6-1)
Protection
Soft tissues and organs are surrounded by skeletal structures
Examples
The skull encloses the brain
The ribs protect the heart and lungs
Leverage
Bones function as levers
Result is body movement

5. Bone Tissue Characteristics (6-2)
Bone or osseous tissue
Connective tissue containing cells in a matrix
Cells are osteocytes
Matrix contains:
Calcium salts in the form of calcium phosphate, Ca3(PO4)2
Accounts for 2/3 of the weight of bone
Extracellular protein fibers like collagen

6. Classifying Bones by Shape (6-2)
Bones of the skeleton are categorized into four general shapes
Long bones
Longer than they are wide
Short bones
About as wide as they are long
Flat bones
Thin and relatively broad
Irregular bones
Complex shapes that don’t fit another category

7. Figure 6-1 A Classification of Bones by Shape.

8. Features of a Long Bone (6-2)
The diaphysis is the central shaft
Surrounds a marrow cavity, or medullary cavity, in the center filled with bone marrow
The epiphyses are the expanded portions at each end
Covered with articular cartilage
Articulate with an adjacent bone at a joint

9. Types of Bone Tissue in a Long Bone (6-2)
Compact bone,
Relatively solid
Forms the wall of the diaphysis
Spongy bone
Interlacing network of bony rods separated by spaces
Fills the epiphyses
Lines the marrow cavity

10. Coverings of a Long Bone (6-2)
Outer surface covered by periosteum
Inner cellular layer
Outer fibrous layer
Isolates bone from surrounding tissue
Forms attachments with fibers of tendons and ligaments
Inner surfaces and spongy bone of marrow cavity covered by endosteum
Functions during bone growth and repair

11. Figure 6-2 The Structure of a Long Bone.

12. Microscopic Features of Bone (6-2)
Bone cells are osteocytes
Located in pockets called lacunae
Found between sheets of calcified matrix called lamellae
Canaliculi are small channels
Contain cytoplasmic extensions of the osteocytes
Radiate through the matrix
Connect the lacunae to blood vessels for nutrient and waste exchange

13. Histology of Compact Bone (6-2)
Basic functional unit is the osteon
Osteocytes arranged in concentric layers (lamellae)
Layers surround a central canal
Central canals run parallel to surface of bone and contain blood vessels
Perforating canals
Link blood vessels of central canal with blood vessels of periosteum and marrow cavity

14. Figure 6-3b The Microscopic Structure of a Typical Bone.
Central
canal
Osteon
Canaliculi
Lacunae
Lamellae
Osteon
LM × 343 b
In this thin section through compact bone, the intact
matrix making up the lamellae appears white, and
the central canal, lacunae, and canaliculi appear
black due to the presence of bone dust.
Figure 6-3b The Microscopic Structure of a Typical Bone.

15. Characteristics of Compact Bone (6-2)
Covers all bone surfaces except inside joint capsules
Articular cartilage protects opposing bone surfaces here
Parallel arrangement of osteons resists stress in specific direction
Withstands forces applied on either end of a long bone
Cannot tolerate moderate stress applied to the side of the shaft

16. Structural Features of Spongy Bone (6-2)
Has no osteons
Lamellae form rods or plates called trabeculae
Still contains osteocytes, lacunae, and canaliculi
Contains red bone marrow
Found in spaces between trabeculae

17. Functional Features of Spongy Bone (6-2)
Found in:
Locations not heavily stressed
Locations with stresses arriving from many different directions
Example: epiphyses of long bones where stresses transferred across joints
Much lighter than compact bone
Reduces weight of the skeleton
Easier for muscles to move bones

18. Figure 6-3a The Microscopic Structure of a Typical Bone.
Capillary
Small vein
Lamellae
Endosteum
Osteon
Trabeculae
of spongy bone
Vein
Perforating
canal
Central
canal
Artery a
This diagrammatic view depicts the parallel osteons of compact bone
and the trabecular network of spongy bone.
Figure 6-3a The Microscopic Structure of a Typical Bone.

19. Types of Bone Cells (6-2) Osteoblasts Osteocytes Osteoclasts
Produce new bone through a process called ossification
Osteocytes
Most abundant cells in bone
Mature cells that maintain bone structure by recycling calcium salts
Osteoclasts
Secrete acid and enzymes that dissolve the matrix
Process releases minerals through osteolysis, or resorption

20. Figure 6-3c The Microscopic Structure of a Typical Bone.

21. Bone Formation (6-3) Embryonic development of bone Two types
Begins at week 6 as a cartilaginous formation
Replaced with bone, a process called ossification
Two types
Intramembranous ossification
Endochondral ossification
Calcification occurs during ossification
Can also occur in other tissues besides bone

22. Intramembranous Ossification (6-3)
Occurs during fetal development
Bone develops within sheets of connective tissue
Begins in an ossification center
Osteoblasts differentiate from connective tissue stem cells and form new bone matrix
Osteoblasts change into osteocytes
Blood vessels grow into area and are trapped within developing bone
Bone remodeling produces osteons of compact bone

23. Endochondral Ossification (6-3)
Process of formation for most bones of the skeleton
Begins with hyaline cartilage models
Cartilage replaced by true bone in five steps

24. Figure 6-4 Bone Formation in a 16-Week-Old Fetus.
Intramembranous
bones
Endochondral bones

25. Steps 1–3 of Endochondral Ossification (6-3)
Chondrocytes enlarge and surrounding matrix begins to calcify
Bone formation starts at the shaft surface
Blood vessels grow around edges
Osteoblasts produce bone matrix around shaft surface
Blood vessels invade inner region of cartilage
Migrating fibroblasts differentiate into osteoblasts
New osteoblasts form spongy bone in center of shaft at the primary ossification center
Bone develops toward each end filling shaft with spongy bone

26. Steps 4–5 of Endochondral Ossification (6-3)
Osteoclasts begin to break down spongy bone in center of bone
Forms marrow cavity
Epiphyseal cartilages, or epiphyseal plates, on the ends of the bone continue to enlarge
Increases length of the bone
Centers of the epiphyses begin to calcify
Secondary ossification centers form as blood vessels and osteoblasts enter
Epiphyses fill with spongy bone
Thin layer of original cartilage covers joint surfaces as the articular cartilage
Bone of shaft and epiphysis separated by epiphyseal cartilage

27. Epiphyseal Line (6-3)
At puberty, bone growth accelerates due to increased sex hormone production
Osteoblasts produce bone faster than the epiphyseal cartilage can expand
Epiphyseal cartilages get narrower until disappear
Called epiphyseal closure
X-rays of adult bones show former location of epiphyseal cartilage as epiphyseal line

28. Figure 6-5 Endochondral Ossification.

29. Appositional Growth (6-3)
Process of growing in diameter is appositional growth
Periosteum cells develop into osteoblasts
Produce more matrix on the outer surface of the bone
Osteoclasts erode the inner surface
Marrow cavity enlarges as a result

30. Figure 6-6 Appositional Bone Growth.

31. Requirements for Bone Growth (6-3)
Mineral supply
Calcium salts (calcium and phosphate)
Absorbed from mother’s bloodstream during prenatal development
Vitamin D3
Plays role in calcium metabolism
Liver and kidney process into calcitriol that stimulates calcium and phosphate absorption
Manufactured by epidermal cells exposed to UV radiation
Also obtained from dietary supplements
Deficiency leads to softening of bones
Condition called osteomalacia in adults and rickets in children

32. Requirements for Bone Growth cont. (6-3)
Vitamin A and vitamin C
Provide support for osteoblasts
Various hormones
Growth hormone
Thyroid hormone
Sex hormones
Calcium-balancing hormones

33. Bone Remodeling Process (6-4)
Remodeling process recycles and renews components of bone matrix
Osteocytes maintain matrix, continually removing and replacing calcium salts
Osteoclasts continually remove matrix
Osteoblasts continually build matrix
Normally activity is balanced

34. Role of Bone Remodeling (6-4)
Turnover of minerals gives bone ability to adapt to new stresses
Heavily stressed bones are thicker and stronger
Regular exercise important to maintaining bone structure
Bones receiving lower than normal stress are thinner and more brittle
Inactivity (such as using crutches) results in loss of bone mass

35. Skeleton as a Calcium Reserve (6-4)
Calcium is the most abundant mineral in the body
99% of calcium is deposited in the skeleton
Very close regulation required
Calcium balance is regulated by:
Parathyroid hormone (PTH) and calcitriol to raise calcium levels
Calcitonin to lower calcium levels in body fluids

36. Fracture Naming (6-4) Fracture is a crack or break in a bone
Two broad categories based on external appearance
Closed (simple) fractures
Completely internal (no break in the skin)
Can be seen only on x-rays
Open (compound) fractures
Project through the skin
More dangerous due to risk of infection
Fractures also named by nature or location of the break

37. Unnumbered Figure, Page 150

38. Unnumbered Figure, Page 150

39. Unnumbered Figure, Page 150

40. Unnumbered Figure, Page 150

41. Four Steps to Repair Fractures (6-4)
Large blood clot, or fracture hematoma, forms.
Closes off the injured blood vessels
Lack of blood flow kills osteocytes, resulting in dead bone extending on either side of the fracture
Cells of periosteum and endosteum divide and migrate into fracture area
Osteoblasts replace cartilage with spongy bone
Spongy bone is replaced by compact bone
Leaves slightly thicker spot at the fracture site

42. Unnumbered Figure pages 150-151 Types of Fractures and Steps in Repair.

43. Osteopenia and Aging (6-5)
Bones become thinner and weaker as normal part of aging
Osteopenia
Inadequate ossification that naturally occurs as part of the aging process
Begins between ages 30 and 40
Osteoblast activity slows
Osteoclast activity remains constant
Women lose about 8% of skeletal mass each decade
Men lose about 3% of skeletal mass each decade

44. Osteoporosis (6-5) Osteoporosis
Loss of bone mass that impairs normal function and can lead to more fractures
Condition is more common in women
Process accelerates after menopause due to a decline in estrogen
Causes increased bone fractures and decreased ability to repair

45. Bone Markings (6-6)
Landmark features on the surfaces of bones are called bone markings
Elevations or projections
Where tendons and ligaments attach
Where bones articulate
Depressions, grooves, and openings
Where blood vessels and nerves pass through the bone

46. Figure 6-7a An Introduction to Bone Markings.

47. Figure 6-7b An Introduction to Bone Markings.

48. Skeletal Divisions (6-6)
Skeletal system consists of 206 bones
Can be divided into two divisions
Axial skeleton
Appendicular skeleton

49. Axial Skeleton (6-6) Forms longitudinal axis of body
Contains 80 bones subdivided into:
Skull bones (22)
The bones associated with the skull (6 auditory ossicles and the hyoid bone)
Bones of the thoracic cage (25)
Bones of the vertebral column (26)

50. Appendicular Skeleton (6-6)
Includes:
Bones of the pectoral girdle and upper limbs
Bones of the pelvic girdle and lower limbs

51. Figure 6-9 The Axial and Appendicular Divisions of the Skeleton.

52. Functions of the Axial Skeleton (6-7)
Creates framework for support and protection of the brain, spinal cord, and organs in the ventral body cavity
Provides surface area for attachment of muscles that:
Move the head, neck, and trunk
Perform respiration
Stabilize elements of the appendicular skeleton