Lecture 28 – Bone Formation

1 – Intramembranous – develops in the connective tissue in embryo.  (Abbreviation for connective tissue is CT).  For example, the bones of the calvarium develop in connective tissue. Calvarial bones form by direct ossification of mesenchyme, which requires condensation of mesenchymal cells which then proliferate and differentiate into osteoblasts.

Embryology is a branch of science that pertains to the development of am embryo from fertilization of the ovum to the fetus stage.  The more you understand embryology, the more you will understand anatomy.  Much later we will delve into embryology.  Whether you are a 4th year medical student, intern, or attending, a deeper understanding of anatomy will assist in practicing and teaching the art of medicine.

2 – Endochondrial bone formation – cartilage model of skeleton insists that you have to remove cartilage to lay down bone.  Please note:  BONE IS NOT CALCIFIED CARTILAGE.  So, how do we grow bones?  Ossification centers form bone by removing cartilage and replacing with bone.  Bone growth occurs at the epiphyseal disc.  The disc remains the same width, but increases in length.  In the late teens or early 20’s, no new cartilage will form.  It can only be destroyed.  Once the diaphysis meets the epiphysis no disc remains and bone growth will cease.  The text books state that this age is 18 for girls and 20 for boys.

Structural and Functional Unit of Compact Bone

Bone is dynamic.  Internally, the spongy bone is called hemopoietic tissue.  It makes blood cells.  (Note the root hem- in the word.)  Compact bone exists on the surface and blood vessels are parallel to the surface.  The matrix of compact bone is very dense.  If you need a bone marrow sample, a convenient place is the crest of the ilium.  Thin people can feel the prominence easily if they place their hands on their hips.

If we cut a cross section of bone, we have a Haversion System.  Many of these systems exist in the shaft of the bone.  A Haversion canal has an artery and a nerve.  Lamella are concentric rings of bone.  Lacunae are little lakes that have osteocytes inside to maintain the bone.  A canaliculi is a structure that radiates from the vessel to a lacuna.  Volkmann’s Canal is parallel to the surface of the bone and feeds into the artery in the Haversion Canal.  Volkmann’s Canal brings blood into the Haversion System.  We could go much deeper into the Haversion Canal, but will keep it brief for now.  We are very close to our survey of blood.  I enjoy teaching the blood system after the skeletal system in a systems review.  Medical students learn regional anatomy, but that is not possible in a virtual environment.  My medical students, especially those from liberal arts backgrounds appreciate these practical applications.

Basic Arthritis Review

Osteoarthritis –

Destruction of articular cartilage

Triggered by trauma or injury

Extreme pain and immobility

Reduction in viscosity of synovial fluid (stickiness –viscosity – root word is viscous).  I think of how a car will not function properly once the engine oil is old.  The engine oil lubricates the metal parts of a car.  The synovial fluid lubricates our joints.

Rheumatoid Arthritis –

Default in the immune system caused by emotional stress.  (As Mary J. Blige would say, “No more drama!”)  Uncontrolled stress can cause many physical symptoms, such as headaches and fatigue.  Exercise, laugh, pace yourself with your work, and limit negative situations.  I am still learning how to keep a balance between work and regular play time.

Inflammation of synovial membrane, causing an increase in synovial fluid

Extreme pain and immobility

Osteoporosis Review

Loss of bone mass

More frequent in females than males; more frequent in whites than blacks

Prevention – Calcium intake and exercise


Also a type of connective tissue-

Bone is a CT with cells, fibers, calcified matrix, and lots of blood vessels.

The matrix for bone is a hydroxyapatite. (Ca++)10(PO4)6(OH)2

Cartilage has cells, fibers, and a firm, yet pliable matrix.  Cartilage has no blood vessels and take a while to heal when damaged.  Cartilage gets its nutrients by diffusion.

Definition – http://en.wikipedia.org/wiki/Diffusion

Diffusion describes the spread of particles through random motion from regions of higher concentration to regions of lower concentration. The time dependence of the statistical distribution in space is given by the diffusion equation. The concept of diffusion is tied to that of mass transfer driven by a concentration gradient, but diffusion can still occur when there is no concentration gradient (but there will be no net flux). Diffusion is invoked in the social sciences to describe the spread of ideas.

You may also seek information from an old chemistry book if further clarification of diffusion is needed.

The matrix of cartilage is glycoaminoglycans.  Let’s break the word down.  Do you remember our beginning lectures about etymology or word origins?  Glyco- refers to sugar,  amino- refers to protein, and –glycans also refers to sugar.  Basically, this is a protein surrounded by sugar.  (Silly joke!)  These collagenous fibers are strong and are embedded in the matrix.  Supposedly, a steel fiber of the same dimension is not as strong as a collagenous fiber.  These fibers also have a cross-linking, which helps provide strength.

Types and Locations of Cartilage

(You can let out a sigh of relief!  This is the last segment before moving on to our fascinating blood.)

1 – Hyaline cartilage – glassy, shiny appearance (Seen in the butcher shop.  Soup bones have shiny, white cartilage.)  I am left-handed, and quite skilled with a knife in the professional setting, but I am not a carnivore.  (I grew up in the cattle rustling state, and was forced to sit at the table many days because I did not want to eat the animals.)  The whole meat in my kitchen sink idea is too much for me, but I had to let you know that you have seen hyaline cartilage many times while preparing your steak.


articular surfaces of joints

costal cartilages (attach ribs)

embryonic skeleton – We were all once hyaline cartilage.  Hyaline cartilage keeps the nose open for breathing.  The nose, larynx, trachea, and bronchi are all held open by hyaline cartilage.

2- Elastic cartilage – matrix has elastic fibers

External ear

Eustacian tube  – connects nasopharynx with the middle ear.  The nasopharynx is posterior to the nasal cavity.

Epiglottis is the opening between vocal cords.  (epi – upon; glottis – vocal cords)

Air must pass through vocal cords to talk.  The leaf-like elastic cartilage (epiglottis) closes to force food into the esophagus.

3 – Fibrocartilage – Lots of collagenous fibers in the matrix of fibrocartilage

Locations –

Intervertebral discs and pubic symphysis (bringing together).


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