1. 5
The Skeletal System 1

2. Skeletal System Consists of Connective Tissue
Three types of connective tissue:
1. Bone
Hard elements of the skeleton
2. Ligaments:
Dense fibrous connective tissue
Attach bones to other bones
3. Cartilage:
Specialized connective tissue, fibers of collagen and elastic in a gel-like ground substance

3. Bones Are the Hard Elements of the Skeleton
Bones: hard elements of the skeleton
Five important functions
1. Support
2. Protection
3. Movement
4. Blood cell formation
5. Mineral storage
Calcium
Phosphate

4. Bone Contains Living Cells
Bone: hard inorganic matrix of calcium salts
Compact bone: forms shaft and ends, contains marrow space
Yellow bone marrow (mostly fat) in marrow space
Spongy bone: trabeculae form lattice-like support
Spaces may contain red bone marrow
Cells: osteoblast, osteocytes, osteoclasts
Types: long, flat, irregular
Osteons/haversian system: cellular arrangement
Periosteum: connective tissue covering

5. lacuna. Osteocytes remain in contact with each other by
Figure 5.1
Central canal
Osteon
Osteocytes
A photograph of an
osteon of compact bone
showing osteocytes
embeded within the
solid structure.
Osteocyte
Lacuna
Osteon
Spongy bone
(spaces contain
red bone
marrow)
Epiphysis
Canalicula
A single osteocyte in a
lacuna. Osteocytes remain
in contact with each other by
cytoplasmic extensions into
the canaliculi between cells.
Compact
bone
Yellow
bone
marrow
Blood
vessel
Diaphysis
Figure 5.1 Structure of bone.
Periosteum
Central cavity
(contains
yellow bone
marrow)
Spongy bone
Compact bone
Epiphysis
Blood vessels and
nerve in central canal
Osteoblasts
A closer view of a section of bone.
Compact bone is a nearly solid structure with
central canals for the blood vessels and nerves.
A partial cut through a long bone. 5

6. Ligaments Hold Bones Together
Function: attach bone to bone
Made of dense fibrous connective tissue
Provide strength in joints yet enable movement

7. Cartilage Lends Support
Function: support under pressure
Types
Fibrocartilage
Intervertebral disks between vertebrae
Menisci in knee joints
Hyaline
Forms embryonic structure which later form bone
Covers and protects ends of long bones in joints, providing protection, reducing friction
Elastic cartilage
Flexible, outer ear, epiglottis

8. Bone Development Begins in the Embryo
Early fetal development: cartilage model forms
Formed by chondroblasts (cartilage-forming cells)
Later fetal development: osteoblasts replace cartilage with bone
Childhood: primary and secondary ossification sites formed
Adolescence: elongation at growth plates

9. Bone Development Is Regulated by Hormones
Preadolescence
Growth hormone stimulates bone lengthening
Early Adolescence
Estrogen and testosterone stimulate bone lengthening
Late Adolescence
Estrogen and testosterone cause replacement of cartilage growth plates with bone

10. long bones move farther apart and osteoblast activity continues just
Figure 5.2
Adolescence
Cartilage
growth
plate
Childhood
Fetus: At
2–3 months
Fetus: First
2 months
Developing
periosteum
Compact
bone
containing
osteocytes
Blood
vessel
Chondroblasts
form hyaline
cartilage, creating
a rudimentary
model of future
bone.
The periosteum
begins to develop
and cartilage starts
to dissolve. Newly
developing blood vessels
transport osteoblasts
into the area from the
periosteum.
Osteoblasts secrete
osteoid and enzymes,
facilitating the
deposition of hard
hydroxyapatite
crystals.
Figure 5.2 How bone develops.
Cartilage
growth
plate
The growth plates in
long bones move farther
apart and osteoblast
activity continues just
below the periosteum.
The bone lengthens and
widens. 10

11. Osteoblasts convert cartilage to bone at the inner surface
Figure 5.3
Osteoblasts
convert cartilage
to bone at the
inner surface
Chondroblasts
deposit new
cartilage at the
outer surface
Joint
cartilage
Figure 5.3 How long bones increase in length.
Growth
plate 11

12. Mature Bone Undergoes Remodeling and Repair
Remodeling: changes in shape, size, strength
Dependent on diet, exercise, age
Weight-bearing exercise increases overall bone mass and strength
Bone homeostasis depends upon balance of osteoblast and osteoclast activity
Osteoporosis: loss of bone mass due to prolonged imbalance of osteoblast and osteoclast activity

13. Cells Involved in the Development and Maintenance of Bone
Chondroblasts: cartilage-forming cells
Osteoblasts: young bone-forming cells
Osteocytes: mature bone cells
Osteoclasts: bone-dissolving cells

14. Bone Cells Are Regulated by Hormones
Parathyroid hormone (PTH):
PTH secretion increases when blood levels of Ca fall
Stimulates osteoclasts to secrete more bone-dissolving enzymes
Calcitonin:
Calcitonin secretion increases when blood levels of Ca are high
Stimulates osteoblasts to add calcium and phosphate to bone

15. The application of force to a slightly bent bone produces
Figure 5.4
Compressive force
Bone removed
here
New bone
added here
Figure 5.4 Bone remodeling.
The application of force to a
slightly bent bone produces
a greater compressive force
on the inside curvature.
Compressive force produces
weak electrical currents, which
stimulate osteoblasts.
Over time, bone is
deposited on the inside
curvature and removed
from the outside
curvature.
The final result is a bone
matched to the compressive
force to which it is
exposed. 15

16. Bones Undergo Repair Bone fracture Hematoma forms at site of fracture
Fibroblasts migrate to area to begin repair process
Tough fibrocartilage callus is formed at site of fracture by chondroblasts
Osteoclasts remove dead damaged bone and remnants of hematoma
Osteoblasts deposit new bone in area of fracture
Repair process takes from weeks to months

17. The Skeleton Protects, Supports, and Permits Movement
206 bones
Four types of bones
Long (bones of limbs and fingers)
Short (wrist bones)
Flat (cranial bones, sternum, ribs)
Irregular (coxal bone, verterbrae)

18. The Skeleton Protects, Supports, and Permits Movement
Provides support for soft organs
Protection of many organs
Joints provide flexible movement of many parts of the body
Mineral (calcium, phosphorus) storage
Bone marrow: synthesis of blood cells

19. Appendicular skeleton
Figure 5.5
Axial skeleton
Appendicular skeleton
Cranium (skull)
Maxilla
Mandible
Clavicle
Scapula
Sternum
Ribs
Humerus
Vertebrae
Ulna
Radius
Carpals
Metacarpals
Sacrum
Phalanges
Figure 5.5 The human skeleton.
Coxal bone
Femur
Patella
Tibia
Fibula
Tarsals
Metatarsals
Phalanges 19

20. Skeleton Organized into Two Groupings
Axial skeleton—midline of body
Skull, sternum, ribs, vertebral column, sternum
Appendicular skeleton—bones of the appendages and their attachment structures
Pectoral girdle, arms, pelvic girdle, legs

21. Axial Skeleton Forms Midline of the Body
Skull:
Cranial bones
Facial bones
Hyoid bone
Does not articulate with any other bone
Attachment for muscles of tongue, larynx, pharynx
Vertebral column
Ribs and sternum

22. The Skull: Cranial Bones
Function: protect the brain
Cranial bones:
Frontal bone
Parietal bones
Temporal bones
Sphenoid bone
Ethmoid bone
Nasal bone
Lacrimal bones
Occipital bone

23. The Skull: Facial Bones
Maxilla
Palatine bones
Vomer bone
Zygomatic bones
Sinuses—air spaces within several cranial and facial bones

24. Temporal bone Parietal bone Frontal bone Sphenoid bone Ethmoid bone
Figure 5.6
Temporal bone
Parietal bone
Frontal bone
Sphenoid bone
Ethmoid bone
Lacrimal bone
Nasal bone
Zygomatic bone
Maxilla
Occipital
bone
Mandible
External auditory
meatus
Maxilla
Zygomatic bone
Figure 5.6 The human skull.
Palatine bone
Sphenoid bone
Vomer bone
Foramen magnum
Occipital bone 24

25. Figure 5.7
Larynx
Figure 5.7 The hyoid bone.
Hyoid bone 25

26. Axial Skeleton: Vertebral Column
Protects spinal cord
Regions:
Cervical (neck): seven vertebrae
Thoracic (chest): 12 vertebrae
Lumbar (small of the back): five vertebrae
Sacral (sacrum): five fused vertebrae
Coccygeal (coccyx): four fused vertebrae
Intervertebral disks: cushion vertebrae; assist in movement and flexibility

27. Cervical vertebrae (7) Thoracic vertebrae (12) Lumbar vertebrae (5)
Figure 5.8 1 2 3
Cervical
vertebrae
(7) 4 5 6 7 1 2 3 4 5 6
Thoracic
vertebrae
(12) 7 8 9 10 11 12 1 2
Figure 5.8 The vertebral column.
Lumbar
vertebrae
(5) 3 4 5
Sacrum
(5 fused)
Coccyx
(4 fused) 27

28. Healthy disks. A herniated disk. Spinal cord Articulations
Figure 5.9
Spinal cord
Articulations
with another
vertebra
Intervertebral
disk
Spinal nerve
Articulation
with ribs
Main bodies
of vertebrae
Healthy disks.
Herniated area
pressing against
a nerve
Figure 5.9 Vertebrae.
A herniated disk. 28

29. Ribs and Sternum: Protecting the Chest Cavity
Protect lungs, heart
12 pair
10 pair are attached to vertebrae in back, sternum in front
Lowest two pair are “floating,” attached to vertebrae but NOT attached to sternum
Sternum
Protects heart
Breastbone

30. C7 T1 Sternum (breastbone) Ribs T11 Cartilage T12 Vertebral column L1
Figure 5.10 C7 T1 1 2
Sternum
(breastbone) 3 4
Figure 5.10 Ribs.
Ribs 5 6
T11
Cartilage 7
T12 8 9
Vertebral column L1 12 10 L2 11
Floating ribs 30

31. Appendicular Skeleton: Pectoral Girdle and Upper Limbs
Pectoral girdle: shoulder
Clavicles (collarbones)
Scapulas (shoulder blades)
Arms
Upper arm: humerus
Forearm: radius, ulna
Wrist: carpal bones (8)
Hand (palm): metacarpal bones (5)
Fingers and thumb: phalanges (14)

32. 14 Phalanges (finger bones)
Figure 5.11
Clavicle
(collar bone)
Pectoral girdle
Scapula
(shoulder blade)
Humerus
(upper arm)
Ulna
Forearm
Figure 5.11 Bones of the right side of the pectoral girdle and the right arm and hand.
Radius
8 Carpals (wrist)
5 Metacarpals (hand)
14 Phalanges (finger bones) 32

33. Appendicular Skeleton: Pelvic Girdle and Lower Limbs
Pelvic girdle (hip):
Coxal bones (2), sacrum, pubic symphysis
Legs
Upper leg (thigh): femur
Knee cap: patella
Lower leg (calf): tibia, fibula
Ankle: tarsal bones (7)
Foot: metatarsals (5)
Toes: phalanges (14)

34. Coxal bones and sacrum (pelvis) Pubic symphysis Femur (upper leg)
Figure 5.12
Coxal bones and
sacrum (pelvis)
Pubic
symphysis
Femur (upper leg)
Patella (knee cap)
Tibia
Figure 5.12 Bones of the pelvic girdle and the left leg and foot.
Lower leg
Fibula
7 Tarsals (ankle)
5 Metatarsals (foot)
14 Phalanges (toe bones) 34

35. Comparison of Male and Female Pelvic Girdle
Pelvic girdle in women is:
Broader
Shallower
Wider opening

36. Figure 5.13
Figure 5.13 The pelvis.
Male
Female 36

37. Joints (Articulations) Form Connections Between Bones
Classified by degree of movement
Fibrous joint:
Immovable
Example: joints between flat bones of skull
Cartilaginous joint:
Slightly movable, cartilage connection
Example: joints between adjacent vertebrae
Synovial joint:
Freely movable
Bones separated by fluid-filled cavity

38. Synovial Joints Joint capsule: synovial membrane  hyaline cartilage
Synovial membrane secretes synovial fluid as a lubricant
Hyaline cartilage acts as a cushion
Types of synovial joints
Hinge joint
Ball and socket joint
Tendons—join bone to muscle

39. A cutaway anterior view of the right
Figure 5.14
Femur
Thigh
muscles
Hyaline
cartilage
Tendon
Posterior
cruciate
ligament
Ligament
Anterior
cruciate
ligament
Patella
Meniscus
Joint
capsule
Meniscus
Fibula
Ligaments
Tibia
Ligament
Ligaments
Figure 5.14 The knee joint is a hinged synovial joint.
Patella
Tendon
A cutaway anterior view of the right
knee with muscles, tendons, and the
joint capsule removed and the bones
pulled slightly apart so that the two
menisci are visible.
A view of the knee with muscles,
tendons, and ligaments in their normal
position surrounding the intact joint
capsule. The combination of ligaments,
tendons, and muscles holds the knee tightly
together. 39

40. Figure 5.15 Types of movements made possible by synovial joints.
Abduction:
Movement of a
limb away from
a body’s
midline
Adduction:
Movement of a limb
toward the body’s
midline
Circumduction:
Movement of a limb
so that it describes
a cone
Supination:
Rotation of the
forearm so palm
faces anteriorly
Pronation:
Rotation of the
forearm so palm
faces posteriorly
Extension
Flexion
Abduction
Flexion
Ulna
Flexion:
Decreases
the angle
of a joint
Adduction
Radius
Extension
Abduction
Imaginary
cone of
movement
Rotation:
Movement
of a body
part around
its own axis
Flexion
Adduction
Extension:
Increases
the angle of
a joint
Abduction
Figure 5.15 Types of movements made possible by synovial joints.
Adduction
Extension
Abduction and adduction.
Rotation and circumduction.
Flexion and
extension
Supination and
pronation. 40

41. Ligaments, Tendons, and Muscles Strengthen and Stabilize Joints
Ligaments: attach bone to bone in a synovial joint
Example: ACL (anterior cruciate ligament in the knee)
Tendon: attach muscle to bone
Example: Achilles tendon
Both tendons and ligaments are composed of tough connective tissue containing collagen (strong, flexible)
Muscles further stabilize joints

42. Diseases and Disorders of the Skeletal System
Sprains:
Stretched or torn ligaments
Heal slowly (few cells and poor blood supply)
Bursitis and tendinitis:
Inflammation of bursae or tendons
Arthritis: inflammation of joints
Osteoarthritis
Rheumatoid arthritis
Osteoporosis: excessive bone loss

43. A scanning electron micrograph (SEM) of normal bone.
Figure 5.16
A scanning electron micrograph (SEM) of
normal bone.
Figure 5.16 Bone loss in osteoporosis.
SEM of a bone showing osteoporosis. 43