1. The Skeletal System
Chapter 5
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2. Bones: An Overview
No other animal has an s-shaped spine, or long legs in comparison to their arms/forelimbs
Subdivided into:
Axial Skeleton
Appendicular Skeleton
Skeletal system includes bones, joints, cartilages, and ligaments

3. Functions of the Bones
Support – “steel girders” and “reinforced concrete” of the body
Protection
Movement – works in conjunction with the muscles and tendons of the skeletal system
Storage – fat, phosphorus, calcium
Calcium is mostly stored as calcium salts, but some is always in ion form and available for nervous system use
Available Ca2+ is regulated by hormones according to the needs of the body. Calcium ions are continuously being shuffled between the blood circulation and storage in the bones
Blood cell formation

4. Classification of Bones
Adult skeleton: 206 bones
Two basic types of osseous tissue:
Compact bone
Spongy bone
Bones come in a variety of four basic shapes based on their functions

5. Classification of Bones Cont.
Four shapes:
Long bones – mostly compact, found in limbs
Short bones – mostly spongy and cube-shaped
Sesamoid bones are unique type of short bones that form within tendons. Ex: patella
Flat bones- thin, flattened, often curved, two layers of compact bone sandwiching a thin spongy layer
Irregular bones – any bone that does not fit a preceding category
Ex: Vertebrae

6. Did you get it?
What is the relationship between muscle function and bones?
What are two functions of a bone’s marrow cavities?
Where are most long bones found in the body?

7. Structure of a Bone: Long Bones
Diaphysis- the shaft, most of the length and made of compact bone.
Covered by fibrous CT called the Periosteum
Held in place by perforating (or) Sharpey’s fibers
Epiphyses – are the ends of the bone mostly spongy bone covered by a thin layer of compact bone
The very ends are covered in Articular (hyaline) cartilage in stead of periosteum
Provides a smooth, slippery surface to decrease friction
The epiphyses is separated from the diaphysis by the Epiphyseal line, formerly the epiphyseal plate

8. Structure of a Bone: Long Bones Cont.
In adults, the shaft is mainly a storage site for adipose tissue
Called yellow marrow, or medullary, cavity
In infants, this is the site of blood cells formation and Red marrow is found here
Adults have red marrow in their spongy bone

9. Structure of a Bone: Long Bones Cont.
Bone markings are where muscles, tendons and ligaments attach
Two categories:
Projections or processes which grow out of the bone
All terms beginning with a T are projections
Depressions or cavities which indent into the bone
All terms beginning with an F (except facet) are depressions

10. Osteocytes are mature bone cells found within tiny cavities of the matrix called lacunae.
Lacunae are arranged in concentric circles called lamellae around the Haversian (central) canals.
Each complex of central canal and lamellae is called an osteon, or Haversian system
Blood vessels and nerves are found in each canal running the length of the bony matrix
Along the canals, canaliculi (tiny canals) radiate out to all the lacunae
This complex system of canals allows bone cells to be well nourished despite the calcified matrix
The outside of the bone is connected to the inside via perforating (Volkmann’s) canals which come in at right angles to the shaft
Microanatomy of Bones

11. Amazing bone concepts
Bone is one of the hardest substances in the body
Light in weight and high in tensile strength
Calcium salts give bone its incredible hardness and durability where as the collagen fibers and organic materials allow for flexibility and tensile strength (ability to stretch without breaking)

12. Did you get it?
What is the anatomical name for the shaft of a long bone? For its ends?
How does the structure of compact bone differ from the structure of spongy bone when viewed with the naked eye?
What is the importance of canaliculi?

13. Bone Formation, Growth, and Remodeling
Skeleton comprised of cartilage and bone
Embryo/Fetal skeleton is mostly hyaline cartilage but is mostly replaced by bone as a young child
Cartilage still found in nose, parts of the ribs, joints and a few other places

14. Long bone formation in an embryo, fetus and young child
The process of bone formation, ossification occurs in two stages:
The hyaline cartilage is covered in a bone collar (calcified matrix) formed by osteoblasts
The enclosed hyaline cartilage is digested away, opening up a medullary cavity within the diaphysis
By birth, most of the cartilage models have been replaced by bone except for the articular cartilage and epiphyseal plates

15. Epiphyseal Plates and Bone Growth
New cartilage is formed continuously on the external face of the articular cartilage (in the joint cavity) and on the epiphyseal plate surface that faces the bone end
The old cartilage on the medullary cavity side is broken down and replaced by bony matrix
Bones widen as the grow long
Osteoblasts in the periosteum at bone tissue to the external bone face
Osteoclasts in the endosteum breakdown and remove bone from the inner face
Increase in diameter = appositional growth

16. Bone Growth Diagram

17. Regulators of Bone growth
Hormones controlled long- bone growth
Growth Hormone
Sex hormones (during puberty)
When bones finalize growing, the epiphyseal plates are converted entirely to bone
Occurs during adolescence

18. Bone Remodeling Bones are remodeled based on:
Calcium levels in the blood
Gravity and the pull of muscles on the skeleton
Low blood Ca2+ stimulates release of parathyroid hormone (PTH) from the parathyroid gland in the neck
PTH causes osteoclasts to break down bone matrix and release calcium ions into the blood
When blood calcium is too high (hypercalcemia), extra calcium is deposited into the matrix as calcium salts

19. Bone Remodeling Continued
Bone Remodeling is necessary for bones to retain the proper tensile strength and proportions through life
As osteoblasts lay down new matrix and become trapped, they become osteocytes
Bones that are subject to higher stress tend to be thicker and stronger
Bones of bedridden people tend to atrophy
PTH determines when (if) the bones need to be broken down in response to blood calcium
Stressed from muscle and gravity determine where the bone matrix is to be broken down or formed

20. Rickets Disease in children where bones fail to calcify
Bones soften and the stress of gravity and muscles cause weight- bearing bones to bow
Caused by lack of Vit D in the diet which is required for intestinal absorption of calcium into the bloodstream
Not common in the US due to fortification of foods
Can occur in nursed children whose mothers are Vit D deficient and in some parts of the world

21. Did you get it? Bones don’t begin as bones. What do they begin as?
Which stimulus – PTH (a hormone) or mechanical forces acting on the skeleton – is more important in maintaining blood calcium levels than in maintaining bone strength?
If osteoclasts in a long bone are more active than osteoblasts, what change in bone mass is likely to occur?

22. Bone Fractures
More likely to occur with age as the bones thin and weaken
Closed fracture – the bone does not penetrate through skin
Open fracture – broken bone ends penetrate the skin
Fractures are treated with reduction, which is realignment of the broken ends
Closed reduction – the bones are manipulated back in place
Open reduction - surgery is performed and bones are secured with pins or wires
Both are followed by immobilization either by a cast or traction
Healing time is 6-8 weeks based on circulation in the tissue

23. Repair of Bone Fracture – 4 events
Formation of a hematoma – blood vessels break when a bone breaks, forming a blood-swelling called a hemotoma. Bone cells are nutritionally deprived and die
Fibrocartilage callus forms- growth of new capillaries into the clotted blood allows disposal of dead cells by phagocytes
CT of various types for a mass of repair tissue at the same time forming a fibrocartilage callus. Made of cartilage matrix, bony matrix and collagen fibers
Bony callus forms – osteoblasts and osteoclasts migrate in, multiply and replace the fibrocartilage callus with bony callus made of spongy bone
Bone remodeling occurs – over next few weeks to months depending on the size and site of the break
Remodeling occurs based on response to mechanical stress