Skeletal System 206 named bones make up the skeleton.It is divided into two main divisions: 1. Axial skeleton :

Forms the long axis of the bodyComprised of the skull, vertebral

column, and bony thorax The orange bones

2. Appendicular skeleton :

Upper and lower limbs (appendages) and their girdles

Tan bones

Bone classification via shape…1. Long bones – Description:

Example- All bones of the limbs (except patella, wrist and ankle bones)

The bones of the fingers are long bones2. Short bones – Description:

Example –wrist and ankle bonesSpecial type of cubed bones are sesamoid

bones --bones embedded within tendons (such as the patella) and work to strengthen tendons where they cross joints with lots of tension.

3. Flat bones – Description:

Example-Skull bones, scapulae, sternum, ribs4. Irregular bones – Description:

Examples – vertebrae and bones of the pelvis

Bone Characteristics Bone Functions:

1. Support – bones provide a framework that supports the body and cradles the

soft organs 2. Protection – fused skull bones protect the _________, the

vertebral column protects the _________, the bony thorax protects the

____________.3. Movement – skeletal muscles use bones as levers to move the

body4. Mineral storage – mainly Calcium and Phosphate, we are

constantly depositing and withdrawing these minerals from bone…(more on

this during the endocrine system.)5. Hematopoiesis – the cells found within the blood form in the

bone marrow

Gross Anatomy of bones: Every bone has a different “anatomy” depending upon its

specific job. Found on the external surface of bones are features such as: Projections and depressions – serve as sites of muscle, ligament,

and tendon attachmentsJoint surfacesOpenings and grooves for the passage of blood vessels and nerves. Specific bone markings will be covered in more detail in the six

bones/muscles/regional terms modules.

Bone textures: A. Compact bone Dense smooth solid bone (at least to the

naked eye)Makes up the external surface of all bonesB. Spongy boneTwo other names:1.2.

A sponge like arrangement of trabeculae (small needle like pieces

of bone) and spaces which are filled with bone marrow

Found on the inside of all bones except the “shaft” of long bones

Long BoneCoverings

1. Periosteum – covers the entire external surface of the bone

except the joint surfaceMade up of two layers:

A. Outer fibrous layerMade of DICTRichly supplied with blood supply and nerves which enter the

bone via a nutrient foramenWhat happens when you bang your anterior tibia against

something?Where tendons and ligaments insert and attach to the boneSharpey’s fibers secure the periosteum to bone Sharpey's fibers are very dense in areas where the pull of tendons

and ligaments produce a great deal of tensionB. Inner osteogenic layer Articulates with the bone surface

2 cell types found hereosteoblast – make new boneosteoclast – break down bone 2. endosteum – lines the inside of the marrow cavity and

covers every single piece of bone trabeculae found within spongy bone.

only made of the osteogenic layer (osteoclast and osteoblast)

Short, Irregular or Flat Bone Structure

• Like a sandwich of 2 thin plates of compact bone (i.e. bread) with a thin spongy bone layer in the middle (i.e. peanut butter)

• All marrow in the spongy bone spaces is active red marrow in everyone… children and adults

• This is the reason bone marrow samples are usually taken from the sternum or pelvic bone in adults

• The coverings are much the same as with long bones…there is periosteum covering the external surface of both layers of compact bone and endosteum covering each little piece of spongy bone. Each covering has the same layers as discussed earlier.

Microscopic Anatomy of Compact bone:

• Although compact bone appears solid to the naked eye it is riddled with passageways for blood vessels and nerves. The matrix of bone is so dense that blood vessels and nerves can’t directly penetrate the tissues. Therefore canals must be made to allow these vital components to pass to all bone cells.

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

• This unit is defined by a central blood supply and the amount of bone surrounding it that can be supported by the blood supply.

• Osteons are circular columns• The central canal houses the blood supply and nerves

for the osteon. • It contains a branch of the blood vessels that came in

through the periosteum and penetrated the bone via the nutrient foramen.

• Encircling the central canal are the lacunae (little lakes) which are depressions where osteocytes are located.

• Osteocytes are bone cells• Small canals where the blood vessels branch from the

central canal and go to nourish each osteocyte are called cannaliculi

• The hard matrix of bone is called lamellae. Notice how collagen fibers are aligned in different directions through out the lamellae. This design gives the bone a large potential for resisting stretch and twisting

Typical Long BoneDiaphysis – the shaft of the bone• Has a thick collar of compact bone on the outside• No spongy bone on the inside. • Instead has a well developed marrow cavity• In young children this cavity is filled with red bone marrow which

is active in hematopoiesis.• In adults the cavity is filled with yellow marrow which is mainly

fat and is not active in hematopoiesis• This intramedullary fat can become a “fat emboli” when adults

have severe breaks of long bones• If adults are in a crisis and need more blood cells produced, the

inactive yellow marrow can convert back to active red marrow again

Epiphyses – the two rounded bone ends (proximal and distal epiphysis)

• Thin rim of compact bone on the outside and spongy bone in the center

• Active red bone marrow in adults and children• Joint surfaces are here• Epiphyseal line or plate – found between the diaphysis and

epiphysis• Plate – an active strip of hyaline cartilage that allows for bone

growth in length throughout puberty. (This is called endochondrial bone formation and will be discussed in more detail later.)

• If this cartilage strip is damaged, bone growth in length is compromised, slowed, or stopped… creating a shorter limb.

• Line – an inactive remnant found post puberty. It can no longer produce new bone growth. “The plates are closed in adults” therefore we cannot grow any taller.

Chemical Composition of BoneHas both organic and inorganic components

A. Organic componentsCells (osteoblast, osteoclast, osteocytes)Osteoid - makes up 1/3 of matrix and consists of proteoglycans, glycoproteins, and collagen fibers. This part of the osteoid is

secreted by osteocytes.

B. Inorganic components – other 2/3s of the matrixHydroxyapatites “mineral salts” which are tiny crystals of calcium phosphates within the matrixGives bone its characteristic hardness and ability to resist compression.These salts do not break down after death which is the reason why bones can be found from very old specimens.Impressive point: **Healthy bone is half as strong as steel in resisting compression, and fully as strong as steel in resisting tension**

Bone Development (also called ossification or osteogenesis)

Embryonic formation of the skeleton…Our first skeleton is formed very quickly but it is made of fibrous connective tissue membranes and hyaline cartilage. It does not

start to ossify until the 8th week of gestation.Bone that forms from fibrous connective tissue membranes through a process known as Intramembranous Ossification leads to the

formation of flat bones.

A. Simplified Steps of Intramembranous Ossification 1. Found within the fibrous connective tissue membranes are mesenchymal cells. (stem cells that can give rise to many different

types of connective tissue) 2. A foci of mesenchymal cells ossify into bone….Newly formed osteoblasts start secreting osteoid (bone matrix)3. Osteoid mineralizes and the trapped osteoblasts stop secreting matrix and become osteocytes4. Bone begins to take the shape of flat bones---associated membranes are formed and blood vessels penetrate the spongy bone

center(Left over areas of fibrous connective tissue membranes that do not ossify prior to birth are called : _______________)

B. Endochondral Ossification

Bones form from a hyaline cartilage model through a process known as Endochondral Ossification (Every other bone besides those of the skull and clavicle form this way)

Simplified steps of Endochondral bone formation:1. The perichondrium covering of the original hyaline cartilage skeleton increases in number of blood vessels and becomes a

vascular periosteum. 2,As a result of the increase in nutrition coming to the area the mesenchymal cells in the center of the hyaline cartilage turn into

osteoblast and start making bone matrix3. The osteoblast of the newly formed peritosteum located on the external surface of the bone start secreting bone on the

external surface of the diaphysis4. The hyaline cartilage in the center of the diaphysis calcifies but then dies because the blood supply can no longer get to these

cells due to the thick collar of bone formed on the outside as discussed above. This leaves a cavity in the middle of the bone.

5. Blood vessels (part of the periosteal bud) penetrate the cavity and bring with it osteoblasts and osteoclasts which then form an initial spongy bone center of the long bone.

6. Osteoclasts break down the spongy bone and leave a medullary cavity.7. Shortly before birth (or soon after) the epiphyses ossify in almost the exact way as the diaphysis did except …a spongy bone

center is maintainedHyaline cartilage remnants are left at two places…a. epiphyseal plates – for continued bone growth in lengthb. at the joint surfaces to become articular cartilage which allows for smooth frictionless joint movement.

Postnatal Bone Growth• Bones continue to grow until we reach our geneticly-determined height at puberty. • What primary environmental factor also influences height?

Bones grow in two directions:1. Length - achieved by the epiphyseal plates undergoing endochondral bone formation (text

page______)2. Width – achieved by the osteogenic layers of the periosteum and endosteum. As a bone gets

wider, its medullary cavity must also increase in size.

• At puberty the rapid spurt of growth is due to a spike in the sex hormones estrogen and testosterone.– The Plates usually close at 18 yrs of age for females and 21 for males. Ironically the very

same stimuli that turned them on also turn them off = high levels of sex hormones.– Once the plates close (or become inactive) they ossify and are termed epiphyseal lines.

These lines allow no additional growth in bone length.

Fracture Repair

Bone ends must come together and physicians usually help out with this by doing one of the following:A. Closed reduction – putting bone ends together through manipulation without making an incision.B. Open reduction – surgically putting bone ends together. Withoutmedical intervention, the bone will attempt to bridge the

gap often resulting in a shortened limb. 1. Hematoma formation – occurs immediately---Injury tears blood vessels in the area and those supplying the bone itself. Hemorrhage forms a hematoma--Bone cells deprived of blood die---Inflammation sets in, the area becomes swollen, painful2. Fibrocartilaginous callus formation – “soft callus” occurs within a few daysCapillaries reestablish blood flow---Cells that migrate in via the newly established blood flow include:

a. Immune cells come in and begin to cleanup areab. Fibroblasts come in and begin to secret collagen fibers to span the breakc. Osteoblasts come in and begin the secretion of spongy bone matrix

(b and c work to splint the two bone ends)3. Bony Callus formation – forms within a week and continues for next 2-3 monthsThe soft callus is slowly converted to a bony callus of spongy bone and then to compact bone4. Bone remodeling – reshapes new bone very close to that of the original bone. It is very important that the proper mechanical stresses be applied to the bone during this phase for proper remodeling to

occur. Therefore the need for:_____________________ Once completed, the new bone is just as strong as before the break.

Bone RemodelingOur skeleton is continually changing. Every week we recycle 5-7% of our bone mass and as

much as a half a gram of calcium may enter or leave the adult skeleton each day There are two stimuli for bone remodeling:A. Hormonal mechanismsBecause blood Ca+ levels must be maintained within a very narrow range_______for proper fxn. of muscle and the nervous system…2 hormones are Involved in reg Ca+ 1. PTH - secreted by the parathyroid gland when blood calcium levels drop.Turns on Osteoclasts which break down bone in order to free up calcium to enter the blood

stream. 2. Calcitonin - secreted by the thyroid gland in response to high blood Ca+ and turns on

Osteoblasts which work to deposit extra calcium into bone, building up bone.Important concepts:

We must keep Ca+ at proper blood levels we sacrifice bone to do this.Ca+ is needed for transmission of nerve impulses and muscle contraction. Without proper

Ca+ levels our brain and heart can’t function nor can we breathe.We normally have 1200-1400 grams of Ca+ in our body at any given time (99% in bone) Our body will break our skeletal system down to the point of creating very fragile bones that

break easily… if we need the Ca+. The need to contract our heart, breathe, and run our brain are much more important than our bone structure.

B. Mechanical StressOur bones remodel in response to mechanical stresses such as muscle pull and gravity. Examples:• Pregnant women tend to experience remodeling of the lumbar vertebrae due to the

new stress of the enlarged abdomen. • If a person started a job where he or she had to stand slightly bent over and do

repetitive work with their left forearm, the skeleton would remodel to these new stresses and after a short time they would be more comfortable in that position.

• Disuse of bones leads to bone thinning such as in paralysis or when in a cast or some type of immobilization device.

Remember---Hormones determine whether and when we break down bone but mechanical stresses determine where to take calcium from. Which has the greatest ultimate influence? _____________________

Would it be a good idea to take from the bone shown here as a 1 st or last resort? WHY?

Osteoporosis

Definition: a group of diseases in which bone reabsorption

out paces bone deposition and overall bone mass is reduced.

Known Causes:1. Decrease sex hormones in both males and females

(Sex hormones promote bone deposition and restrain osteoclasts)

2. Lack of weight-bearing exercise – decreased mechanical stress on bones

3. A diet poor in calcium and protein4. Abnormal Vitamin D metabolism5. Smoking – decreases sex hormone levels6. Primary hormonal abnormalities of PTH and

calcitonin regulation