SKELETAL SYSTEM Unit 3

The skeletal system acts as support, mineral storage, produces blood

cells, and protects the body.

Are bones living?

• Bones have a very unique structure, as they are made of both living and non-living material

• Bone is a connective tissue that consists of:– Bone Matrix (foundation)– Cells (bone cells, blood

cells)

• Skeletal system components– Bones (206 total)• Divisions

1. Axial skeleton (126 bones)» Bones of skull, thorax, and vertebral column» Form longitudinal axis of body

2. Appendicular skeleton (80 bones)» Bones of the limbs and girdles that attach them to the

axial skeleton

– Cartilages– Ligaments and other connective tissues

An Introduction to Bones

Figure 6 Section 1 1

Axial Skeleton(126 Bones)

The axial skeletonconsists of thebones of the skull,thorax, and vertebralcolumn. Theseelements form thelongitudinal axis ofthe body.

The adult skeletalsystem, which can bedivided into the axialskeleton and theappendicular skeleton

The appendicularskeleton includesthe bones of thelimbs and thepectoral and pelvicgirdles that attachthe limbs to theaxial skeleton.

AppendicularSkeleton (80 Bones)

• Functions of the skeletal system– Support (support for body, attachment for soft tissues)– Storage of minerals (calcium and phosphate)

• Calcium most abundant mineral in body (~2–4 lb)– 98% stored in bones

– Blood cell production (all formed elements of blood)– Protection (delicate tissues and organs surrounded by

bone)– Leverage (act as levers with skeletal muscles to move

body)

An Introduction to Bones

Bone classification

• Six categories based on shape1. Flat bones• Thin, roughly parallel surfaces• Examples: cranial bones, sternum

2. Sutural bones• Irregular bones formed between cranial bones• Number, size, and shape vary

3. Long bones• Relatively long and slender• Examples: various bones of the limbs

4. Irregular bones• Complex shapes• Examples: vertebrae, bones of pelvis, facial bones

5. Sesamoid bones• Small, flat, and somewhat shaped like sesame seed• Develop in tendons of knee, hands, and feet– Individual variation in location and number

6. Short bones• Small and boxy• Examples: bones of the wrist (carpals) and ankles

(tarsals)

Bone classification Cont.

Bone Cells• Bone tissue is a type of

connective tissue, so it must consist of a lot of bone matrix

• But where does the bone matrix all come from?

• The 3 Bone cells:– Osteoblasts– Osteocytes– Osteoclasts

Bone Structure

1. Osteoblasts• Bone-building cells.– Called

ossification…• Synthesize and

secrete collagen fibers & other organic parts of bone matrix. – Collagen is a

protein– It is made by the ER

and Golgi– **Remember how

the ER/Golgi make proteins…from way back in freshman biology?

The blue arrows indicate the osteoblasts. The yellow arrows indicate the bone matrix they’ve just secreted.

Bone Structure

2. OSTEOCYTES– Mature bone cells.– Osteoblasts that have

become trapped by making too much bone matrix.

– No longer secrete matrix, but continue to live…

– Responsible for maintaining the inner compact and spongy bone

Yellow arrows indicate osteocytes – notice how they are surrounded by the pinkish bone matrix.

Blue arrow shows an osteoblast in the process of becoming an osteocyte.

On the right, notice how the “osteocyte” is trapped within the matrix

3. Osteoclasts• Huge cells derived from the fusion of as many as 50

monocytes (a type of white blood cell).• Cells that break down bone • They digest bone matrix – this process is called bone

resorption and is part of normal bone growth, maintenance, and repair.

• The osteoclast secretes digestive enzymes to break down the bone matrix.

Why do we want a cell that eats away at bone?

Bone Matrix

• Bone matrix resembles reinforced concrete rebar.

• The rebar is like collagen fibers and the cement is like hydroxyapatite– Collagen (1/3)• Composed of fibers, and organic

protein• Collagen is the most abundant organic

substance– Hydroxyapatite (2/3)• Primarily calcium phosphate (called hydroxyapatite)• This gives bone its hardness; resists

compression and twisting

Bone Matrix

• If hydroxyapatite removed, bone is too bendable• If collagen removed, bone is too brittle