Skeletal System

 

Anatomy & Physiology \n Summer SEMESTER 2022

 

Chapter 7.1: Introduction to the Skeletal System:

        Components of the skeletal system:

  • Bones:

  • Primary organs of the skeletal system

  • Lightweight, but strong

  • Provide support for the body:

  • Keeps us upright

  • Provide protection:

  • Protects vital organs

  • Act as levers for movement:

  • It helps to make a coordinated effort to walk

  • Hematopoiesis: formation of blood cells

  • In red bone marrow : the center of the bone contains stem cells (progenitor cells) that help to produce WBC, RBC, and Platelets

  • Mineral Storage:

  • Calcium, phosphate

  • Cartilage:

  • A somewhat flexible, semi-rigid connective tissue

  • Covers the joint surface

  • Covers the growth plates

  • The growing tissue at the ends of long bones

  • Eventually, the cartilage is replaced by bone, and the lengthening of bones stops

  • Reduces Friction: allows bones to move against each other

  • Absorbs shock in moveable joints: cushions the force of the bones coming together

  • Ligaments: connects bone to bone

  • Help to attach one bone to the next helping to keep the bones in the right place

  • Tendons: connects muscle to bone

  • Allows for normal movement to occur

  • Joints:

  • Places of contact between:

  • Bones and Bones:

  • Ex: Vertebral disc are joints that allow the spine to move

  • For knees: there is a synovial cavity that contains synovial fluid (which allowed shock absorption and lubes the bones

  • Bones and cartilage:

  • An inverse relationship between mobility and stability

  • With joints that have more stability (skull and the spine), there is not a lot of mobility

  • With joints that have more mobility (knee, elbow) there is not a lot of stability

Chapter 7.2e: Microscopic Anatomy - Bone Connective Tissue:

        Cells of bone:

  • Osteoprogenitor cells: beginning (come from stem cells in the bone marrow)

  • Bone stem cells

  • Can differentiate into osteocytes and osteoblasts

  • Osteoblasts: anabolic effect (building up)

  • Formed from osteoprogenitor stem cells

  • Synthesize and secrete semisolid organic bone matrix called osteoid

  • Become entrapped within the matrix

  • Osteoids and osteoblasts mix together to form the surface of the bone

  • After finishing secreting osteoid they Differentiate into osteocytes

  • Involved with building up bone

  • Osteocytes: cells

  • Mature bone cells derived from osteoblasts

  • Maintain bone matrix

  • Osteoclasts: catabolic effect (breaking down)

  • Large, multinuclear, phagocytic cells

  • Derived from fused bone marrow cells

  • A ruffled border increases the surface area exposed to bone

  • Involved in bone resorption

        Bone Formation:

  • Begins with the secretion of the osteoid by osteoblasts

  • Osteoid contains collagen, glycoproteins, etc.

  • Calcification occurs, deposition of hydroxyapatite

        Inorganic components of the bone matrix:

  • Calcium phosphate crystals (Ca3(PO4)2) and calcium hydroxide interact to form hydroxyapatite (Ca10(PO4)6(OH)2) crystals

  • Other substances incorporated into crystals

  • Calcium carbonate, sodium, magnesium, sulfate, fluoride

  • Hydroxyapatite hardens the matrix and accounts for the strength and rigidity of bones

        Bone Formation:

  • Process requires

  • Vitamin D - enhances calcium absorption from the GI tract

  • Vitamin C - is required for collagen formation

  • Calcium and phosphate for calcification

  • Eventually, cartilage is replaced by bone, and growth stops

        Bone resorption:

  • Bone matrix is destroyed by substance released from osteoclasts

  • Proteolytic enzymes released from lysosomes withing osteoclasts

  • Chemically digest organic matrix components

  • Freed calcium and phosphate ions enter the blood

  • Occurs when blood calcium levels are low

  • Increase bone resorption if blood calcium levels are low

  • This is not something you wish to speed up as low calcium levels will increase bone fragility

Chapter 7.5b: Bone Remodeling:

        Bone Remodeling:

  • Continuous removal of old bone and replacement with new bone
  • Continues throughout adulthood
  • Occurs at surfaces of a bone
  • Occurs at different rates
  • 20% of the skeleton is replaced yearly
  • Dependent upon coordinated activities of osteoblasts, osteocytes, and osteoclasts
  • Influenced by hormones

Chapter 7.5c: Hormones That Influence Bone Growth and Remodeling

        Hormones:

  • Molecules released from one cell into the blood
  • Travel throughout the body to affect other cells
  • Initiate specific cellular changes
  • Some alter rates of osteoblast and osteoclast activity
  • Affect bone composition and growth patterns

        Growth Hormone:

  • Somatotropin - Growth hormone

  • Increase the growth of every organ/ bone in the body

  • Produced by the anterior pituitary gland

  • Increase at puberty

  • Stimulates the liver to produce the hormone, somatomedin

  • Both directly stimulate the growth of cartilage at the epiphyseal plate and bone elongation

        Thyroid Hormone:

  • Secreted by the thyroid gland
  • Stimulates metabolic rate of osteoblasts: if there is not enough thyroid function then osteoblast creation is decreased (leading to soft bones)

        Sex Hormones:

  • Estrogen and testosterone

  • Secreted in large amount at puberty

  • Dramatically accelerate bone growth

  • Stimulate osteoblasts and increase the rate of cartilage growth and bone formation in the epiphyseal plate

        Glucocorticoids:

  • Group of steroid hormones

  • Released from the adrenal cortex

  • Reduce bone growth and bone thickness. Increase in bone fractures and bone fragility

  • High amounts increase bone loss

  • Impairs growth at the epiphyseal plate in children

  • Must monitor if the child receiving high doses of glucocorticoids.

Chapter 7.6: Regulating Blood Calcium Levels:

        Regulating calcium concentration in blood is essential Calcium is required for:

  • Initiation of muscle contraction
  • Exocytosis of molecules from cells, including neurons
  • Stimulation of the heart by pacemaker cells
  • Blood clotting

        Two primary hormones raise blood calcium:

  • Calcitriol
  • Parathyroid hormone

Chapter 7.6a: Activation of Vitamin D to Calcitriol:

  1. UV light converts 7-dehydrocholesterol to vitamin D3 (cholecalciferol)

  2. Released into the blood

  3. Absorbed in the small intestine from diet

  4. Vitamin D3 circulates throughout the body

  5. Converted to calcidiol by liver enzymes

  6. Both steps 1 & 2 occur continuously

  7. Calcidiol circulates in the blood

  8. Converted to calcitriol (the active form of Vitamin D3) by kidney enzymes

  9. Parathyroid hormone (PTH) increases rate, so more calcitriol formed

  10. Calcitriol stimulates the absorption of calcium ions from the small intestine into the blood and increased renal calcium reabsorption.

  11. The more PTH there is the faster calcitriol is formed

Chapter 7.6b: Parathyroid Hormone and Calcitriol:

        Parathyroid hormone (PTH):

  • Secreted and released by parathyroid glands in response to reduced blood calcium levels
  • Accelerates formation of calcitriol by kidney

PTH and calcitriol interact with major organs:

        Bone:

  • Act synergistically to increase osteoclasts activity and thus the resorption of bone matrix, releasing calcium into the blood
  • PTH will digest our bones to raise calcium levels

        Kidneys:

  • Increase calcium reabsorption by the kidney

        Small Intestine:

  • Only calcitriol increases the absorption of calcium from the small intestine into the blood

Chapter 7.6c: Calcitonin:

        Calcitonin:

  • Aids in regulating blood calcium levels

  • Less significant role than PTH or calcitriol

  • Released from the thyroid gland in response to high blood calcium levels

  • Inhibits osteoclast activity, thus less calcium is released into the blood (Promotes deposition of Ca2+ into bones)

  • Stimulates kidneys to increase the loss of calcium in the urine

  • Reducing blood calcium levels