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P & A Skeletal System

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P & A Notes Skeletal System

February 07, 2011

Skeletal System 

Microscopic structure:
-In bone tissue there's many longitudinal canals, called osteonic canals. Matrix is deposited in thin layers around these canals. Bone cells called osteocytes are located in chambers in this matrix. The chambers called lacunae form rings around the osteonic canal.
-One unit is called an osteon
-Each canal contains a blood vessel. The osteocytes have cytoplasmic extensions (canaliculi), which connect to other cells or to the blood vessel in the osteonic canal. Allows material to readily pass between blood and bone cells.

Bone production:
Bone growth involves both the destruction of it internal by bone reabsorbing cells called osteoclasts, and the laying down of tissue on the outside by bone-building cells, osteoblasts. Both processes occur at the same time -- so the marrow cavity inside grows while new bone is laid down outside by the osteoblasts.
-The osteoblasts secrete collagen and mineral salts that make up the bone matrix. While the osteoblasts are doing this they become trapped by putting matrix down all around it.
-It is trapped in a space called a lacunae.
-The osteoblast then becomes mature bone cell and is then called and osteocyte.

Macroscopic structure:
-Parts of a long bone --
Each end has an enlarged portion called the epiphysis. The epiphysis will attach to another bone forming a joint.
The outer surface of the epiphysis is coated with a layer of hyaline cartilage called articular cartilage. The shaft of the bone, located between the two epiphysis, is called the diaphysis. Except for the articular cartilage the bone is completely enclosed by a tough covering of dense fibrous tissue called periosteum.
This membrane is firmly attached to the bone and its fibers are continuous with ligaments and tendons that are connected to it.
-It is in the periosteum where the osteoblasts do their job to form and repair bone tissue.
-The wall of the diaphysis is composed of a tightly packed tissue called compact bone. This bone is solid, strong and resistant to bending.
-The epiphysis is composed of mostly spongy bone with a thin layer of compact bone at its surface. Spongy bone consists of an open interlacing framework of bony tissue that is very light in weight.
In the center of the bone, running through the diaphysis is a tube called the medullary cavity. This cavity contains yellow bone marrow which functions as a fat storage tissue.
What cells from the medullary cavity??

Growth and development of bone from birth:
-Parts of the skeleton begin to develop the first few weeks of life. Bones continue to grow and develop into adulthood
These bones either form as membranous bones or as cartilaginous bone.

Membranous bone: Some of the broad flat bones of the skull develop from sheets of connective tissue. Osteoblasts will form in this tissue and begin to form bone.

Cartilaginous bone:
-Most of the bones of the skeleton are cartilaginous bones
-They develop from masses of hyaline cartilage with shapes similar to the future bones.
These cartilage models will grow rapidly and then begin to undergo many changes.
-In a long bone, the changes begin in the diaphysis. The cartilage slowly breaks down and disappears, at the same time a periosteum forms on the outside of the developing diaphysis.
-Blood vessels and osteoblasts will invade the disintegrating cartilage, spongy bone is then formed in its place.
This region in the center of the shaft is called the primary ossification center. Bone will develop from this center toward the ends of he cartilage frame.
-Osteoblasts from the periosteum will begin to deposit a thin layer of compact bone around this primary ossification center.
-Later a secondary ossification center will appear in the epiphysis.
-The region of cartilage between the primary and secondary ossification centers is called the epiphyseal disk.
-The cartilage cells in this epiphyseal disk are undergoing mitosis -- this increases the length of the bone.
-Once the ossification centers of the diaphysis and epiphysis come together then growth is no longer possible.
-The actions of the osteoclasts and the osteoblasts will create the medullary cavity and produce more compact bone.

-Traumatic fracture - a break due to injury
-Spontaneous or pathologic fracture- break resulting from disease.

-Compound fracture- the broken bone is exposed to the outside by an opening in the skin. Here we have the added danger of infection by microbes.
-Simple fracture- if the broken bone doesn't damage the skin.

Repair of a bone fracture:
-When a bone is broke blood vessels will be ruptured and the periosteum will probably be torn.
Blood will escape and form a blood clot or a hematoma. Vessels in surrounding tissue will become dilated causing inflammation and swelling.
Eventually the hematoma is invaded by developing blood vessels and osteoblasts. The osteoblasts multiply rapidly in regions close to the new blood vessels. Further from the blood supply fibroblasts will produce fibrocartilage. In time the fibrocartilage will fill the gap between the ends of the broken bone forming a cartilaginous callus.
Phagocytic cells will begin to remove the blood clot as well as any dead or damaged cells in the area.
The cartilaginous callus will be replaced by bone the same way that hyaline cartilage is replaced in a developing bone.

Functions of bone:
1.) shape and support-
-shape head, face, thorax and limbs
-bones of feet, leg, pelvis, and backbone support the weight of the body
-bones of the skull protect eyes, ears and brain
-ribs and shoulder girdle protect heart and lungs
-pelvic girdle protects lower abdominal and reproductive organs
3.)Body movement-
-bones and muscles must work together for movement to take place.

Biceps brachii is attached to the radius bone on the lower end and the humerus at the lower end. When flexed it pulls the radius up bending the arm at the elbow.
The triceps brachii is attached to the back of the humerus and to the ulna. When it flexes the biceps relax and the arm straightens out.
4.) Blood cell formation-
Called hematopoiesis, it takes place in he red bone marrow. Red bone marrow occupies most bone cavities in infants. As you grow older some of the R.B.M is replaced by Y.B.M.
In adults R.B.M. is found mainly in spongy bones and in skull, ribs, sternum, clavicles, vertebrae and pelvis.
R.B.M. functions in the production of RBC, platelets and some WBC. Its red because of hemoglobin, an O2 carrying pigment in RBC.
-If blood cells get low then Y.B.M. can change back into R.B.M. and become active in blood formation.
5.) Storage of inorganic salts-
-most common is calcium phosphate
-calcium is needed by a number of vital body processes.
-when the blood calcium is low the parathyroid gland releases a parathyroid hormone stimulating the osteoclasts to break down bone and release calcium salts.
If too high the thyroid releases the hormone calcitonin and it inhibits osteoclasts and stimulates the osteoblasts - calcium salts are take out of blood.

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