1. The Skeletal System
(Chp 6)

2. Skeletal System Overview
Parts of the skeletal system:
Bones (skeleton)
Joints
Cartilages
Ligaments (bone to bone)
Tendon* (bone to muscle)
Two divisions:
Axial skeleton
Appendicular skeleton – limbs and girdle

3. Skeletal Cartilage
Our skeleton is initially made up of cartilage, but most of it is replaced by bone. The cartilages that remain in adults are found mainly in region where more flexible skeletal tissues are needed.
Skeletal cartilage is made of cartilage tissue that consists primarily of water. The high water content accounts for its resilience.
Cartilage contains no blood vessels or nerves and is surrounded by a layer of dense, irregular connective tissue called the perichondrium
The perichondrium helps resist outward expansion during compression an is the source of blood vessels from which nutrients diffuse

4. Skeletal Cartilage Locations
Hyaline: support with flexibility and resilience; articular cartilage at the ends of bones at most movable joints, costal cartilages which connect ribs to the sternum, respiratory cartilage & nasal cartilages
Elastic: flexible & able to withstand repeated bending; supports external ear and forms the epiglottis
Fibrocartilage: highly compressible, great tensile strength (intermediate); at sights of both heavy pressure and stretch such as the menisci of the knee & the discs between vertebrate

5. The bones and cartilages of the human skeleton.
Epiglottis
Larynx
Thyroid
cartilage
Cartilage in
external ear
Cartilages in
nose
Trachea
Cricoid
cartilage
Lung
Articular
Cartilage
of a joint
Cartilage in
Intervertebral disc
Costal
cartilage
Respiratory tube cartilages
in neck and thorax
Pubic
symphysis
Bones of skeleton
Axial skeleton
Meniscus (padlike cartilage in knee joint)
Appendicular skeleton
Cartilages
Articular cartilage
of a joint
Hyaline cartilages
Elastic cartilages
Fibrocartilages

6. Growth of Cartilage
Cartilage consists of cells called chondrocytes encased in small cavities (lacunae) within an extracellular matrix containing jellylike ground substance and fibers.
Appositional Growth: cartilage cells in the surrounding perichondrium secrete new matrix against the external face of existing cartilage
Interstitial Growth: the lacunae bound chondrocytes inside the cartilage divide and secrete new matrix expanding the cartilage within
During normal bone growth and aging, calcium salts may be deposited in cartilage. Calcified cartilage is not bone

7. Functions of Bones Support of the body Protection of soft organs
Movement due to attached skeletal muscles
Storage of minerals and fats
Blood cell formation

8. Bones of the Human Body The adult human skeleton has 206 bones
Two basic types of bone tissue
Compact bone
Homogeneous
Spongy bone
Small needle-like pieces of bone
Many open spaces

9. Microscopic Anatomy of Bone
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

10. Diagram of a Single Osteon
Artery with
capillaries
Structures in the
Central canal
Vein
Nerve fiber
Lamellae
Collagen fibers
run in different
directions
Twisting
force

11. Classification of Bones Based on Shape

12. Long Bones Typically longer than wide
Diaphysis – central shaft, composed of compact bone
Epiphysis – Ends of the bone, composed mostly of spongy bone
Ex) Femur, humerus

13. Structures of a Long Bone
Periosteum
Outside covering of the diaphysis
Fibrous connective tissue membrane
Sharpey’s fibers
Secure periosteum to underlying bone
Arteries
Supply bone cells with nutrients

15. Short Bones, Flat Bones & Irregular Bones
Generally cube-shape, contain mostly spongy bone
Examples: Carpals, tarsals
Flat bones
Thin and flattened, usually curved
Thin layers of compact bone around a layer of spongy bone
Examples: Skull, ribs, sternum
Irregular bones
Irregular shape, do not fit into other bone classification categories
Example: Vertebrae and hip

16. Flat bones consist of a layer of spongy bone sandwiched between two thin layers of compact bone.
Trabeculae

17. Changes in the Human Skeleton
In embryos, the skeleton is primarily hyaline cartilage
During development, much of this cartilage is replaced by bone
Cartilage remains in isolated areas
Bridge of the nose
Parts of ribs
Joints

18. Types of Bone Cells
Osteogenic Cell: stem cell in the inner layer of the periosteum; that gives rise to osteoblasts
Osteoblasts: Bone-forming cells
Osteocytes: Mature bone cells
Osteoclasts: Bone-destroying cells; Break down bone matrix for remodeling and release of calcium
Bone remodeling is a process executed by both osteoblasts and osteoclasts
(a) Osteogenic cell
(b) Osteoblast
(c) Osteocyte
(d) Osteoclast
Stem cell
Matrix-synthesizing
cell responsible
for bone growth
Mature bone cell
that maintains the
bone matrix
Bone-resorbing cell

19. Bone Development
Osteogensis & ossification both refer to the process of bone tissue formation
In embryos this leads to the formation of bony skeleton
Bone growth is also a form of ossification that occurs from birth into early adulthood
Although bones are capable of thickening throughout our lives, most ossification in adults is for remodeling and repair of bones.

20. Formation of the Bony Skeleton
Before week 8 – fibrous membranes & hyaline cartilage
When bone develops from a fibrous membrane it is called intramembranous ossification (membrane bone forms)
Bone formation from the replacement of hyaline cartilage is called endochondral ossification (cartilage or endochondral bone forms)

21. Intramembranous Ossification I
Mesenchymal
cell
Collagen
fiber
Ossification
center
Osteoid
Osteoblast
Ossification centers appear in the fibrous connective tissue
membrane.
• Selected centrally located mesenchymal cells cluster
and differentiate into osteoblasts, forming an
ossification center. 1

22. Intramembranous Ossification II
Osteoblast
Osteoid
Osteocyte
Newly calcified
bone matrix
Bone matrix (osteoid) is secreted within the fibrous membrane
and calcifies.
• Osteoblasts begin to secrete osteoid, which is calcified
within a few days.
• Trapped osteoblasts become osteocytes. 2

23. Intramembranous Ossification III
Mesenchyme
condensing to form the periosteum
Trabeculae of
woven bone
Blood vessel 3
Woven bone and periosteum form.
• Accumulating osteoid is laid down between embryonic
blood vessels in a random manner. The result is a network
(instead of lamellae) of trabeculae called woven bone.
• Vascularized mesenchyme condenses on the external face
of the woven bone and becomes the periosteum.

24. Intramembranous Ossification IV
Fibrous
periosteum
Osteoblast
Plate of
compact bone
Spongy Bone cavities contain red marrow 4
Lamellar bone replaces woven bone, just deep to the periosteum.
Red marrow appears.
• Trabeculae just deep to the periosteum thicken, and are later
replaced with mature lamellar bone, forming compact bone plates.
• Spongy bone (diploë), consisting of distinct trabeculae, persists
internally and its vascular tissue becomes red marrow.

25. Endochondral Ossification in Long Bone
Week 9
Month 3
Birth
Childhood to
adolescence
Articular
cartilage
Secondary
ossification
center
Spongy
bone
Epiphyseal
blood vessel
Area of
deteriorating
cartilage matrix
Epiphyseal
plate
cartilage
Hyaline
cartilage
Spongy
bone
formation
Medullary
cavity
Bone
collar
Blood
vessel of
periosteal
bud
Primary
ossification
center
Bone collar
forms around
hyaline cartilage
model. 1
Cartilage in the
center of the
diaphysis calcifies
and then develops
cavities. 2
The periosteal
bud invades the
internal cavities
and spongy bone
begins to form. 3
The diaphysis elongates
and a medullary cavity
forms as ossification
continues. Secondary
ossification centers appear
in the epiphyses in
preparation for stage 5. 4
The epiphyses
ossify. When
completed, hyaline
cartilage remains only
in the epiphyseal
plates and articular
cartilages. 5

26. Post-Natal Bone Growth

27. Bone Growth & Remodeling
Epiphyseal plates allow for growth of long bone during childhood
Endochondral ossification occurs at the articular cartilages and epiphyseal plates as the bone grows in length.
Remodeling occurs to maintain proper bone proportions.

28. Homeostatic Controls of Ca2+ Levels in Blood

29. Importance of Ionic Calcium
Necessary for numerous physiological processes:
The human body contains g of calcium, 99% as bone minerals and the balance mostly within cells; only ~ 1.5g in blood.
Concentration of calcium in blood ~ 9-11mg/100mL
Calcium is absorbed from the intestine under the control of vitamin D metabolites

30. Bone Response to Mechanical Stress I
Wolff’s Law states that a bone grows or remodels in repsone to the forces or demands placed on it
A bone’s anatomy reflects the stresses that are placed upon it
Bones are stressed when weight bears down on them or muscles pull on them.
This loading is usually off center causing the bone to bend
This results in compression on one side and tension on the other

31. Controls of Bone Remodeling
The mechanism by which bones respond to mechanical stimulation is unclear
Deforming a bone causes an electrical current, it is hypothesized that electrical signals may direct remodeling
It is also believed that the hormonal loop determines whether & when remodeling will occur in response to blood calcium levels
Mechanical stress determines where remodeling occurs

32. Bone Markings I

33. Bone Markings II

34. Bone Fractures A break in a bone Types of bone fractures
Closed (simple) fracture – break that does not penetrate the skin
Open (compound) fracture – broken bone penetrates through the skin
Bone fractures are treated by reduction and immobilization
Realignment of the bone

35. Some Common Types of Fractures

36. Fractures – Closer Look

37. Fractures – Closer Look

38. Fractures – Closer Look

39. Stages in the Healing of a Bone Fracture

40. Osteomalacia & Rickets
Osteomalacia: “soft bones”; bones are not mineralized sufficiently; often due to lack of or inability to process vitamin D
Osteoid is produced, but calcium salts are not deposited
Bones soften & weaken
Pain is felt when weight is put on the affected bones
Rickets: analogous disease in children; much more severe than osteomalacia because bones are in a period of rapid growth
Often results in bowed legs and deformities of the pelvis, skull and ribcage
Epiphyseal plates cannot be calcified so they continue to enlarge causing the ends of the bones to become visibly enlarged and abnormally long

41. Osteoporosis
Group of diseases in which bone reabsorption occurs faster than bone deposit.
Matrix composition remains normal, but bone mass is reduced and the bones become porous and lighter.
Most common in elderly; as sex hormone production declines; insufficient exercise, diet, abnormal vitamin D receptors, smoking & hormonal conditions also contribute.
Usually treated with calcium & vitamin D supplements, weight-bearing exercises & hormone replacement therapy (HRT) & other drugs.

42. Paget’s Disease
Excessive bone formation & breakdown (usually localized)
Rarely occurs before age 40, mostly in elderly.
The newly formed bone is made rapidly; it has a high spongy to compact bone ratio & reduced mineralization leading to spot weakening
Eventually osteoclast activity wanes, but osteoblasts stay active and bones thicken irregularly or marrow fills
There are drug therapies.

43. Developmental Aspects
Most bones begin ossifying by 8 weeks, by
12 weeks, primary ossification centers
are obvious
At birth, most long bones have ossified
diaphyses
After birth secondary ossification centers
form and the epiphyses start to ossify
Epiphyseal plates remain and allow for long bone growth in childhood and adolescence
In childhood and adolesence, bone formation occurs more than reabsorption, in young adults the processes are in balance and in old age, reabsorption dominates formation