NOTES: Skeletal System

(Ch 6, part 3)

BONE FUNCTION: Support and Protection

• bones shape and form body structures

• bones support and protect softer, underlying tissues

BONE FUNCTION:

Body Movement

• bones and muscles function together as LEVERS

• a lever consists of: a rod, a pivot (fulcrum), a resistance, and a force that supplies the energy

BONE FUNCTION:

Blood Cell Formation (Hematopoeisis)

• depending on stage of life, blood cell formation occurs in the :

-yolk sac (embryo)

-liver and spleen

-bone marrow

• Marrow can be:

-RED: stores RBCs, WBCs, and platelets

-YELLOW: stores fat

BONE FUNCTION:

Storage of Inorganic Salts

• matrix of bone tissue contains large quantities of calcium phosphate

Throughout life, osteoclasts continually break down bone matrix and

osteoblasts replace it; these opposing processes of resorption and deposition

of calcium help to maintain calcium levels in the body**

Blood calcium levels vary; when blood calcium is:

LOW: parathyroid hormone is released which causes osteoclasts to break down bone, releasing calcium salts

HIGH: calcitonin is released which causes osteoblasts

to form bone

tissue and

store calcium

salts

Storage of Inorganic Salts

• bone also stores small amounts of magnesium, sodium, potassium, and carbonate

Individual bones are the organs of the skeletal system. A bone contains very

active tissues.

Some of the cells that make up these tissues:

1. Osteoprogenitor cells: unspecialized; can undergo mitosis & develop into osteoblasts

2. Osteoblasts: cells that form bone matrix (no longer dividing); secrete collagen

3. Osteocytes: mature bone cells; exchange nutrients/waste with blood

4. Osteoclasts: break down bone matrix

Bones are classified by SHAPE:

1. Long bones

2. Flat

3. Irregular

4. Short

BONE STRUCTURE:

*Bone structure reflects its function.

Parts of a long bone:

• EPIPHYSES:

-enlarged portions at ends of a long bone;

-covered with articular cartilage;

-articulate (form JOINTS) with other bones.

Parts of a long bone:• DIAPHYSIS:

-shaft of bone (located between the epiphyses)

• PERIOSTEUM:

-tough, vascular covering that

encloses the entire bone

except where the articulate

cartilage is

**the center of the diaphysis is a hollow chamber (MEDULLARY CAVITY),

lined with a thin layer of cells (ENDOSTEUM)

and filled with soft connective tissue (YELLOW MARROW)

Types of Bone Tissue:

COMPACT BONE: has a continuous matrix with no gaps; found in the wall of the diaphysis

Types of Bone Tissue:

SPONGY BONE (a.k.a. cancellous bone): has irregular interconnecting spaces between bony plates that reduce the weight of bone.

**both compact and spongy bone are strong and resist bending

NOTES- Skeletal System Part 4: Microscopic Structure of Bone, Bone Development & Growth, &

Bone Remodeling

Microscopic Structure of Bone:• Intercellular material = mostly COLLAGEN (gives

bone its strength and elasticity) and inorganic salts (make bone hard and resistant to crushing).

• The structural unit of compact bone is called the Osteon or Haversian System

• Compact bone contains OSTEONS running in cylindrical rings cemented together which give long bones strength

Transverse cut

• In an osteon you would find…– Inorganic matrix deposited in concentric layers

called LAMELLAE– Lamellae form concentric rings around a

central or Haversian Canal• Contain nerves and blood vessels that supply

osteocytes

– Volkmann’s Canals run at a right angle to

central canals and

connect the vascular

and nerve supplies to

the periosteum

In each lamella there are cavities called Lacunae which hold Osteocytes (mature bone cells)

• Lacunae also have tiny canals radiating out from them called CANALICULI– Nutrients diffuse through these to reach

osteocyte

Bone Development and Growth:• Process by which bone forms is called

ossification• Bone starts out as hyaline cartilage in the

right shape & then becomes calcified & gains a blood supply

• 2 types: – 1) Intramembranous Ossification(will become flat bones)

– 2) Endochondral Ossification

Intramembranous Ossification

• osteoblasts completely surrounded by matrix are mature OSTEOCYTES

EXAMPLE: the broad, flat bones of the skull form in this way and fuse together at sutures.

Bone Development and Growth

Endochondral Ossification:• Most bones in the body develop in this way• Bones develop in early embryos as hyaline cartilage “models”; later replaced by bone tissue

Endochondral Ossification

• In the middle of the diaphysis, OSTEOCLASTS break down bone tissue and the resulting space becomes the medullary cavity, which later fills with marrow

• The bone in the central regions of the epiphyses and diaphysis remain spongy bone

• The hyaline cartilage that remains on the ends of the epiphyses persists throughout life as articular cartilage

Endochondral Bones

• An epiphyseal plate remains between the primary and secondary ossification centers

• Long bones continue to lengthen until the epiphyseal plate are ossified (hardened) in adolescence• A developing long bone thickens as compact bone is deposited beneath the periosteum (appositional growth)

Growth occurs at the epiphyseal plate

Factors Necessary for Bone Growth:

1. Growth hormone

2. Thyroid hormone

3. Sex hormones

4. Vitamins A & C

**Genetics & nutrition play a role

Bone Remodeling:• Bone continually renews

itself• Takes place at different

rates in various body regions– Distal portion of femur

replaced every 4 months• Osteoclasts destroy matrix

(resorption)• Osteoblasts deposit new

bone

Approximately 20% of bone tissue is replaced annually by this process on a cyclical basis throughout the skeleton. The entire remodeling process occurs over approximately 4 to 8 months, with a range of 3 months to 2 years.

Osteoclasts are transported to the bone. The osteoclast cells release acids and enzymes that resorb the existing bone structure. After the osteoclasts resorb the bone, osteoblasts appear and coat the resorbed area with adhesive substances. Moreover, osteoblast cells produce bone proteins, such as collagen, to help calcium attach to the bone proteins. Only by attaching calcium to bone proteins, the new-bone can be formed.