Comprised of the skull, vertebral column, and bony thorax
The orange bones
2. Appendicular skeleton :
Upper and lower limbs ( appendages ) and their girdles
Bone classification via shape…
Long bones –
Example- All bones of the limbs (except patella, wrist and ankle bones)
The bones of the fingers are long bones
2. Short bones –
Example –wrist and ankle bones
Special 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 –
Example-Skull bones, scapulae, sternum, ribs
4. Irregular bones –
Examples – vertebrae and bones of the pelvis
Bone Characterist ics
1. Support – bones provide a framework that supports the body and cradles the
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 body
4 . 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 attachments
Openings 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.
A. Compact bone
Dense smooth solid bone (at least to the naked eye)
Makes up the external surface of all bones
B. Spongy bone
Two other names:
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 Bone Coverings
1. Periosteum – covers the entire external surface of the bone except the joint surface
Made up of two layers:
A. Outer fibrous layer
Made of DICT
Richly supplied with blood supply and nerves which enter the bone via a nutrient foramen
What happens when you bang your anterior tibia against something?
Where tendons and ligaments insert and attach to the bone
Sharpey’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 tension
B. Inner osteogenic layer
Articulates with the bone surface
2 cell types found here
osteoblast – make new bone
osteoclast – 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 Bone
Diaphysis – 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 Bone
Has both organic and inorganic components
A. Organic components
Cells (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 matrix
Hydroxyapatites “mineral salts” which are tiny crystals of calcium phosphates within the matrix
Gives 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.
**Healthy bone is half as strong as steel in resisting compression, and fully as strong as steel in resisting tension**
(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 osteocytes
4. 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 matrix
3. The osteoblast of the newly formed peritosteum located on the external surface of the bone start secreting bone on the external surface of the diaphysis
4. 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 maintained
Hyaline cartilage remnants are left at two places…
a. epiphyseal plates – for continued bone growth in length
b. 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.
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, painful
2. Fibrocartilaginous callus formation – “soft callus” occurs within a few days
Capillaries reestablish blood flow---Cells that migrate in via the newly established blood flow include:
a. Immune cells come in and begin to cleanup area
b. Fibroblasts come in and begin to secret collagen fibers to span the break
c. 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 months
The soft callus is slowly converted to a bony callus of spongy bone and then to compact bone
4. 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.
Our 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 mechanisms
Because 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.
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 Stress
Our bones remodel in response to mechanical stresses such as muscle pull and gravity.
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?
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 protein
4. Abnormal Vitamin D metabolism
5. Smoking – decreases sex hormone levels
6. Primary hormonal abnormalities of PTH and calcitonin regulation