NOTES: Skeletal System

(Ch 5, part 1)

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

active tissues.

BONE STRUCTURE:

*Bone structure reflects its function.

Parts of a long bone:

• EPIPHYSES:

-enlarged portions at ends of a long bone;

-covered with 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

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

**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 (MARROW)

Bone Markings

• Bones not smooth but can have bumps, holes, ridges– These are called bone markings

• Purpose:– Muscles, tendon, ligament attachments– Allow blood vessels and nerves to pass

through

• Two categories of bone markings– Projections/processes

• Grow out from bone surfaces• Example trochanter

– Depressions/cavities• Indentations into bone• Example fissure

Microscopic Structure of Bone:

• bone cells = OSTEOCYTES; located in small chambers (LACUNAE) which form concentric circles around central canals

• intercellular material = mostly COLLAGEN (gives bone its strength and elasaticity) and inorganic salts (make bone hard and resistant to crushing).

Microscopic Structure of Bone:

• compact bone contains OSTEONS cemented together

• central canals (HAVERSIAN CANALS) contain blood vessels that nourish the cells of osteons

• diffusion from the surface of the thin, bony plates nourishes the cells of spongy bone

Bone Development and Growth Intramembranous Bones:

• layers of connective tissue form membranes at the site of future bones;

• some cells mature and differentiate into bone-producing cells (OSTEOBLASTS);

• osteoblasts deposit bony matrix around themselves and SPONGY BONE forms in all directions within the layers of connective tissues;

Intramembranous Bones

• 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 Bones:• most bones in the body develop in this way• develop as hyaline cartilage “models”; later replaced by bone tissue• primary ossification center appears first in the diaphysis• later, secondary ossification center appears in the epiphyses

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)

• a developing long bone thickens as compact bone is deposited beneath the periosteum

Endochondral Bones• 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 on the ends of the epiphyses persists throughout life as articular cartilage

**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**

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

• 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

• blood calcium levels vary; when blood calcium is:

LOW: osteoclasts break down bone, releasing calcium salts

HIGH: osteoblasts form bone tissue and store calcium salts

Storage of Inorganic Salts

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