Skeletal system includes bones of the skeleton, and the cartilage, ligaments, and other connective tissues that stabilize and/or connect the bones

SUPPORT› Provides structural support and framework for

entire body

STORAGE OF MINERALS AND LIPIDS› Mainly calcium salts and fats (in yellow marrow)

BLOOD CELL PRODUCTION› RBCs, WBCs, and other blood elements are

made in red marrow

PROTECTION› Many soft tissues are surrounded by bone

LEVERAGE› Bones function as levers that change the

magnitude and direction of the forces generated by skeletal muscles

There are 206 bones in the human body, which are categorized into 6 shapes

1) Long bones› Long and slender› Located in arm, thigh, leg, palms, soles,

fingers and toes.› The femur (thigh bone) is largest and

heaviest bone in the body

2) Flat bones› Thin, relatively parallel surfaces› Found in roof of skull, sternum, ribs, scapula› Provide protection of underlying tissue› Large surface area for attachment of

muscles

3) Sutural bones (Wormian bones)› Small, flat, irregularly shaped› Found between flat bones of the skull

4) Irregular bones› Complex shapes with short, flat, notched or

ridged surfaces› Vertebrae, pelvis, some skull bones

5) Short bones› Small and boxy› Wrist bones and ankle bones

6) Sesamoid bones› Small, flat, shaped like sesame seed› Kneecaps, some bones in hands and feet

Bones have external and internal features

There are different parts to bone› Diaphysis – tubular shaft› Epiphysys – expanded area at each end of

the diaphysis› Metaphysis – thin area that connects the

epiphysis to the diaphysis

There are different parts to bone› Compact bone – outer portion of diaphysis

Solid, sturdy layer that surrounds the marrow cavity

› Spongy bone (cancellous) – mainly found in the epiphysis Open network of struts and plates with a thin

covering of compact bone Marrow is present here, but no marrow

cavity

Bone tissue is a supporting connective tissue

Contains specialized cells and matrix› Matrix is solid, due to calcium salts around

protein fibers

4 characteristics of bone1. Matrix dense and contains calcium salts2. Matrix contains bone cells (osteocytes),

within pockets (lacunae), which are organized around blood vessels

3. Canaliculi, canals from lacunae to the blood vessels, allow for the transfer of materials

4. Except at joints, the outer surface of bone is covered by a periosteum

Calcium phosphate (Ca3(PO4)2) makes up about 2/3 of bone

About 1/3 of bone consists of collagen fibers

About 2% of bone mass is cells

Calcium phosphate is very hard, but inflexible and brittle› Can withstand compression, but will

shatter when exposed to bending, twisting, or sudden impacts.

Collagen fibers are strong and flexible› When subjected to tension, they are

stronger than steel› Easily tolerate bending and twisting, but

when compressed, they just bend out of the way

This mixture of calcium crystals and protein fibers give bone properties between the two› Strong, somewhat flexible, resistant to

shattering› As good as steel reinforced concrete

Bone tissue contains 4 types of cells1) Osteocytes – mature bone cells

Most bone cells are these Occupy a lacuna, between layers of matrix

(lamella) Osteocytes can’t divide Canaliculi connect lacuna to each other and

blood vessels

Bone tissue contains 4 types of cells Osteocytes have two major functions

1) Maintain mineral and protein content of surrounding matrix

2) Participate in the repair of bone

2) Osteoblasts – make new bone matrix› Process is called osteogenesis› Eventually turn into osteocytes

Bone tissue contains 4 types of cells3) Osteoprogenitor cells – stem cells that

give rise to osteoblasts

4) Osteoclasts – remove and recycle bone matrix› Process is called osteolysis

Osteon (Haversion System) – basic functional unit of mature compact bone› Osteocytes are arranged in layers around a

central canal (Haversian canal) Canal contains blood vessels Canals run parallel to surface of bone

› Perforating canals run perpindicular to surface of bone Contain blood vessels that take blood to

deeper osteons and to marrow cavity

Compact bone is thickest where stresses arrive from limited directions

Osteons in diaphysis are parallel to long axis› Shaft doesn’t bend.

Your femur can withstand 15 times your body weight through long axis

Much less force from the side will fracture your femur

Lamellae are not arranged like in osteons

Here, they form struts and plates called trabeculae

This creates an open network No blood supply in spongy bone

› Nutrients come from diffusion through canaliculi

Bone marrow is found in spongy bone› Red bone marrow makes blood cells› Yellow bone marrow is comprised of

adipose tissue Fat storage for energy reserve

Periosteum covers superficial layer of compact bone, except at joints› 3 functions

1. Isolates bone from surrounding tissue2. Provides route for circulatory and nervous

supply3. Actively participates in bone growth and

repair

Periosteum is interwoven with the synovial capsules and with tendons

Endosteum› Lines marrow cavity› Active during bone growth, repair and

remodeling› Single, incomplete layer of osteoprogenitor

cells Where layer is incomplete, osteoblasts and

osteoclasts can remodel bone tissue

Skeleton begins to form about 6 weeks after fertilization› Skeleton is cartilage

Portions of the skeleton don’t stop growing until the age of 25

Ossification - process of replacing other tissues with bone› 2 types

Endochondral ossification Intramembranous ossification

Calcification – deposition of calcium, occurs during ossification

Bones are made of hyaline cartilage in the embryo

Cartilage gradually converted to bone through endochondral ossification

6 steps to endochondral ossification1) Chondrocytes increase in size,

cartilage is reduced to struts that calcify. Chondrocytes die and disintegrate

2) Blood vessels grow into perichondrium. Cells in inner layer become osteoblasts and they produce a thin layer of bone along shaft (perichondrium become periosteum

6 steps to endochondral ossification3) Blood vessels make their way into the

center of the shaft. Fibroblasts become osteoblasts and spongy bone is created. Bone production begins at this site (primary ossification center) and spreads toward both ends of the bone

4) Osteoclasts appear and disintegrate the spongy bone in the diaphysis and create a marrow cavity. Diameter of bone enlarges

6 steps to endochondral ossification5) Centers of the epiphyses begin to calcify.

Capillaries and osteoblasts migrate in to create secondary ossification centers

6) Epiphyses fill in with spongy bone. Caps of cartilage (articular cartilage) remain on ends of bone exposed to the joint cavity. Narrow region of cartilage remains (epiphyseal cartilage) between epiphysis and diaphysis. This is your ‘growth plate’.

Endochondral Ossification Animation#2

Endochondral Ossification Animation #1

As long as cartilage is being produced on the epiphyseal side, and bone is replacing cartilage on the shaft side, the bone will continue to get longer.

At puberty, sex hormones cause dramatic bone growth.

Epiphyseal cartilage starts to disappear and become an epiphyseal line

Appositional Growth› Diameter of bone increases› Osteoblasts in inner layer of perisoteum

create matrix along outer surface of shaft.› Osteoclasts slowly remove bone matrix

along marrow cavity

Osteoblasts differentiate from mesenchymal stem cells in fibrous connective tissue› Result is dermal bones

Ex – flat bones of skull, jaw, calvicle 3 steps to this process1) Mesnechymal cells cluster together

and secrete components of matrix, then turn into osteoblasts (called ossification center)

2) Blood vessels grow into the initial spongy bone

3) Bone growth and remodeling produces compact bone

Bone is HIGHLY vascular 3 sets of blood vessels

› Nutrient Artery and Vein Supply diaphysis

› Metaphyseal vessels Supply epiphyses

› Periosteal vessels Supply superficial osteons of shaft

The matrix is constantly being recycled› Process is called remodeling› Occurs throughout life› Osteoclasts constantly make matrix› Osteoblasts constantly dissolve matrix

Osteoblasts are attracted to minute electrical fields, which are created when bone is stressed.› More matrix is produced where stress is high

Electrical stimulation is used in fracture healing

Bone that is not stressed will lose matrix

IF YOU DON’T USE IT, YOU WILL LOSE IT!!!!!!!

YOU NEED CALCIUM AND PHOSPHATE SALTS!!!!!!!

Vitamin D - helps make calcitrol, which helps body absorb calcium

Vitamin C – helps with collagen synthesis and with osteoblast differentiation

Vitamin A – osteoblast activity Vitamins K, B12 – protein fiber production

Growth hormone (in pituitary gland) – stimulates bone growth

Sex hormones – stimulate osteoblasts› Estrogen causes faster epiphyseal closure

than testosterone, which is why women are shorter than men

Damage to bone tissue is known as a fracture

4 steps to repair1) Extensive bleeding occurs and a fracture

hematoma forms2) An external callus forms

› Enlarged collar of cartilage and bone on surface of bone

› An internal callus forms in the marrow cavity

3) Osteoblasts replace the cartilage with spongy bone in external and internal callus› Broken ends of bone are united

4) Continued remodeling of bone by osteoblasts and osteoclasts.› Lasts from 4 months to over a year› Bone is thicker and stronger than original

Fracture Repair Animation

Fractures can be open (compound) or closed (simple)