Skeletal System

Exoskeleton (Exo in Greek is outside or external)

Skeleton is outside of the lady bug.

EndoSkeletons (Endo in Greek is within, inner) – Vertebrates

1. Network of bones located on the interior(inside)
2. Supports our body and provides a framework for movement
3. It protects our most vital organs Ex. Skull, houses our brain. Ribcage protects our Heart and Lung and other organs
4. It performs a variety of physiological roles. Ex. Storage of calcium, (Hematopoeisis – Production of all cellular components(plasma, proteins, RBC, WBC, platelets) within our blood. These are all formed in bone marrows
5. Axial
   1. Axial skeleton is made up of skull and ribcage and our vertebral column(right in the center) that is the axial skeleton. It forms sort of axis to our body
6. Appendicular
   1. Appendicular skeleton is made up of bones of the forelimbs, pelvis(Four appendages)
   2. Hands, Legs
7. Flat
   1. Make up your skull, ribs, pelvis. Flat bones describes the shape of the bone. These bones are made up of inner spongy or cancellous bone. Outer shell is made up of compact bone.
   2. Flat bones serve primarily to protect our organ and serve as a site for the organ(Hematopoeisis)
8. Long
   1. Humerus in your upper arm, The femur in your lower leg
   2. Made up of inner spongy or cancellous bone, with an outer shell of compact bone
   3. Long bones provide a framework for movement
   4. They also serve as a site of Hematopoeisis
   5. DiaPhysis
      1. Lengthy middle portion of the long bone is called Diathesis
   6. Epiphysis
      1. End of the long bone is called epiphysis
   7. MetaPhysis
      1. Joining portion of Diathesis and Epiphysis
      2. Contains the growth plate which is present in the long bones of children

Bone Marrow

1. Place where Hematopoeisis takes place
2. There are 2 different types
   1. Red Bone Marrow(For Blood)
      1. Serves as the primary site for Hematopoeisis
      2. Typically we can find red bone marrow within flat bones and then in epiphysis of long bone
   2. Yellow Bone Marrow
      1. Primarily a site for flat storage, made up of fat cells called Adipocytes
      2. Generally we can find yellow bone marrow within the diaphysis of long bones

Structure of Bone

Haversian System

1. Spongy, Cancellous, Trabecular Bone – Inner most part of the bone is made up of spongy bone(Cancellous or Trabecular Bone). Surface layer of spongy bone is 10 times that of the outer layer of compact bone. Basically spongy bone is just the porous network of spikes surrounding the innermost portion of the bone marrow. Overall effect of spongy bone is, it makes the bone lighter
2. Compact Bone – At the periphery of the bone we have what is the harder, denser layer that surrounds the spongy bone.
   1. It has a specific type of organisation made up of these osteons, repeating functional units
   2. Another word for theses osteons is the Haversian System.
3. Osteon
   1. Each of these osteons looks like a cylinder
   2. It has multiple concentric layers of bone that wrap around each other to form this osteon
   3. Each of these layers is called lamellae
   4. In the center of these layers there is a canal. This canal is called the Haversian canal or central canal.
   5. In this canal travels
      1. Blood Vessels
      2. Lymph Vessels
      3. Nerves
   6. In between these layers there are tiny channels that are called canaliculi
      1. They branch out from the central haversian canal to these empty spaces that are called lacunae. Lacuna or Lacunae means empty space. or Osteocytes or Bone cells.
      2. Osteocytes – have these long cellular process that branch through the canaliculi to contact other osteocytes via gap junctions. Which allow these cells to communicate with each other and exchange nutrients and signals with each other
4. Volkman Canals
   1. Canals that run perpendicular to the haversian canals
   2. These connects osteons to one another and carry their own set of small blood vessels
5. Periosteum – Very outer most superficial layer of bone

Cellular Structure of Bone

1. Bone Matrix
   1. osteoid – This forms the organic portion of the matrix
      1. Soft but highly ordered structure of proteins and collage type I
      2. Tensile Strength – Elastic Strength – Ex. Rubber
   2. Hydroxyapatite – This forms the inorganic portion of the matrix
      1. Calcium Phosphate and Water(Calcium Phosphate Crystals)
      2. These are mineral portion of the bone
      3. Gives bone its rigid strength and density
2. Cells
3. Osteo means bone.
   1. Osteoprogenitor Cells
      1. Just a precursor to Osteoblasts. Immature version of osteoblasts
   2. Osteoblasts cells
      1. Responsible for synthesizing proteins, collagen. specifically couple or proteins called Osteoid. Osteoid made of
      2. Osteocalcin

1. Osteopontin
2. Responsible for producing Alkaline Phosphatase. This enzyme is responsible for forming Hydroxyapatite(mineral portion of the bone)
3. OsteoBlasts matures(Once its all done synthesizing bony matrix) and become osteocytes
4. Osteocytes
   1. Spaces that occupied by Osteocytes are called Lacunae(Empty spaces within bone, looks like a little lake in microscope )
   2. have little arms or branches to communicate with other osteocytes, which gives osteocytes a star like shape
   3. These little branches acts as sensors that picks up new information, sends out signals basically allowing osteocytes to communicate with other cells to maintain bone
5. Osteoclasts
   1. Osteoclasts are derived from cell line called monocytes
   2. Responsible for bone resorption (Resorption is the breakdown and assimilation of old bone in the cycle of bone growth)
   3. These break down bones using an enzyme called Tartrate–resistant acid phosphatase
   4. Osteoblasts help build up bone with an enzyme called alkaline phosphatase
   5. Osteoclasts occupies empty spaces called Howship’s Lacunae – little resorption pits formed by osteoclasts as they break down.

Osteoblasts Build Bone and OsteoClasts crushes the bone

Skeletal Endocrine Control

1. Calcium Homeostasis(in blood stream and bone) under Endocrine or Hormonal Control
2. Endocrin Hormones alter the ratio of osteoclast activity: osteoblast activity
3. Osteoclast Activity
   1. liberation of calcium, Phosphate from bone into the blood stream
4. OsteoBlast Activity

Calcium and Phosphate going from the blood stream into the bone

Hormones involed for Calcium, Phosphate Homeostasis

1. Parathyroid Hormone(PTH)
   1. Increases Calcium and Phosphate in the blood
2. Calcitonin
   1. Decreases the amount of calacium and Phosphate in the blood
3. Calcitriol(An active form of Vitamin D)

Increases Calcium and Phospahe in the blood

These Hormones help us to regulate the amount of calcium that is either absorbed from the gut or reabsorbed from the kidneys and they controls the activity of Osteoclast and Osteoblast.

Calcium and Phosphate levels are proportional to each other in blood.If calcium and phosphate increases/decreases in blood, calcium and phosphate in bone decreases/increases.

Concentration of calcium ions in the blood actually has very important effect physiological effects

1. Too much calcium in the blood leads to Hypoexcitable cell membranes which results in lethargy, fatigue, and memory loss.

Too little calcium in the blood leads to muscle cramps and convulsions

Cartilage

1. Cartilage is a connective tissue found throughout the entire human body
2. Cartilage is an extracellular tissue secreted by cells called Chondrocytes which are deried from the same precursor cells as bone, FibroBlast
3. Chondrocytes secrete fibrous materials such as
   1. collagen which is a string fibrous protein
      1. Collagen is one of the building blocks of Cartilage
   2. elastin  – Ealstic Protein
      1. Elastin is one of the building blocks of cartilage
4. Collagen and Elastin provides the strength and the flexibility to Cartilage
5. Cartilage is not innervated(It doesn’t have nerve cells)
6. Cartilage is also avascular(It doesn’t have any vasculature that runs through it). It doesn’t have arteries and veins and blood vessels that run through.
7. Cartilage receives nutrition and immune protection through surrounding fluids.
8. 3 types of cartilage found in our body
   1. Hyaline Cartilage(Articular Cartilage)
      1. in the larynx and trachea in the throat portion of the body and also in the joints
      2. Main function is to reduce friction and absorb shocks
   2. Elastic Cartilage
      1. Found in the shape of the outer ear and also the epiglottis(Structure that protects your airway whenever you are swallowing food)
      2. Main function is to provide shape and support
   3. Fibrous Cartilage
      1. Found in the intervertebral discs in the spine and also in the are of the pelvis called the pubic symphysis

Main function is to provide rigidity and to absorb the shock

Ligaments

Ligaments connects bone to other bone

Tendons

Tendons connects muscle to bone

Joint

Point where one bone articulates or meets up with the other another is called a joint

Different types of joints found in our body

1. Synarthroses
   1. Immovable joints where two bones are fused together. We can find these type of joints in the skull
2. Amphiarthroses
   1. Amphi means stiff but also slightly movable. Ex. verterabal joints
3. Synovial (Diarthroses) – Synovial joints are named so because they are lubricated by a fluid called Synovial Fluid, which is contained in Synovial Capsule that surrounds the whole joint.
   1. Ball and Socket Synovial Joint – Ex Shoulders, Hips. These joints have many degrees of motion

Hinge Joint – Ex. elbow, knee, where that joint pretty much moves in just one plane, like the hinge of the door

Arthritis – Arth – Joint ; itis – Inflammation