Functions of Bone

• Functions of bone

Bone Functions

• Provide support, without bones the body would essentially be a bag of water
• Provide protection for the brain, spinal cord, heart, and lungs
• Bones act as levers to facilitate movement
• Bones store minerals like calcium and phosphorus
• Bones are the site of hematopoiesis, or blood cell formation

Divisions of the Skeletal System

• Axial Skeleton forms the vertical axis of the body
• The axial skeleton includes the skull
• The axial skeleton includes the vertebral column
• The axial skeleton includes the thoracic cage, which is comprised of the ribs and sternum
• Appendicular Skeleton consists of the bones of the upper and lower limbs
• Appendicular Skeleton connects to the axial skeleton

Types of Bone: Compact Bone (dense)

• Makes up 80% of bone mass
• Has a nonporous appearance
• Strong and provides support and protection

Types of Bone: Spongy Bone (Cancellous or Trabecular)

• Accounts for 20% of bone mass
• Located deep to compact bone
• Has a porous appearance due to the presence of trabeculae
• Strong for multidirectional support and weight distribution

Types of Cartilage: Hyaline

• The most abundant type of cartilage
• Attaches the ribs to the sternum
• Covers the ends of bones
• Is present in growth plates
• Acts as a model for bone information

Types of Cartilage: Fibrocartilage

• Withstands pressure and stress
• Is present in intervertebral discs
• Part of the pubic symphysis
• Is part of the menisci of the knee

Ligaments

• Connect bone-to-bone at joints
• Composed of dense regular connective tissue

Tendons

• Connect muscle-to-bone that they act on
• Composed of dense regular connective tissue

Bone Classification: Long Bones

• Length is greater than width

Bone Classification: Short Bones

• Length is equal to width

Bone Classification: Flat Bones

• Have flat, thin surfaces that can be curved

Bone Classification: Irregular Bones

• Complex shapes

Bone Classification: Sesamoid Bones

• Small, round bones suspended within tendon or ligament

Regions of Long Bone: Diaphysis

• The shaft of the bone, elongated, and typically cylindrical
• Provides leverage and weight support

Regions of Long Bone: Epiphysis

• The end of the bone, described as "knobby"
• Cartilage covers its external joint surface

Regions of Long Bone: Metaphysis

• The region located between the diaphysis and epiphysis

• Contains the epiphyseal plate (growth plate)

• Epiphyseal line in adults

• The medullary cavity is the hollowed portion within the diaphysis, and is filled with bone marrow

Bone Linings

• Osteo = bone

Bone Linings: Periosteum (peri = around)

• Tough sheath that covers the outer surface of the bone, excluding areas covered by cartilage
• Is responsible for the growth of bone width by nourishing compact bone
• Allows for the anchoring of tendons through weaving with collagen fibers

Bone Linings: Endosteum (endo = inside)

• Very thin layer that covers the internal surfaces of the bone and the medullary cavity
• Is very active role during bone growth, repair, and remodeling

Bone Marrow: Red Bone Marrow

• Functions to produce new blood cells (hematopoiesis)
• More distributed in spongy bone in children
• In adults, distribution decreases due to an increase in adipocytes
• Only found in the axial skeleton and proximal epiphyses of the humerus and femur

Bone Marrow: Yellow Bone Marrow

• Adipose tissue
• Serves as an energy source with triglycerides
• Replaces red bone marrow in the medullary cavities of long bones

Bone Cells: Osteoprogenitor Cells (Osteogenic)

• Stem cells located in both the periosteum and endosteum
• Highly mitotic, only bone cells that can replicate
• Matures and becomes an osteoblast

Bone Cells: Osteoblasts

• Synthesize and secrete osteoid (semi-solid matrix of calcium and collagen)
• Produce new bone during development, injuries, and remodeling
• Matures into osteocytes, becoming entrapped by mineral deposits in the matrix

Bone Cells: Osteocytes

• Maintains the bone matrix and detect mechanical stress on bone
• Trigger new bone formation during stress by signaling osteoblasts
• Fully mature, most common cells in bone tissue and do not replicate or mature
• Found within lacuna

Bone Cells: Osteoclasts

• Large phagocytic cells that reabsorb bone (bone breakdown)

• Found on bony surfaces

• Derived from leukocytes (WBCs) and not osteogenic cells

• Osteoblasts form new bone, while osteoclasts break down old bone; both reshape bones throughout life

Bone Matrix: Organic Components

• Osteoid (formed by osteoblasts)
• Made up of collage, proteoglycans and glycoproteins
• Gives bone tensile strength and flexibility

Bone Matrix: Inorganic Components

• Salt Crystals (calcium phosphate)
• Interacts with calcium hydroxide to form hydroxyapatite crystals
• Calcium carbonate, sodium, magnesium, sulfate, fluoride are also incorporated

Bone Matrix

• Crystals are deposited along collagen fibers that harden the matrix
• Rigidity and inflexibility of bones

Compact Bone Structure

• Osteon (Haversian System)
• Small, cylindrical structures that resemble bull's eye targets or tree rings
• Functional unit of compact bone
• Oriented parallel to the diaphysis of long bones, in the direction of weight-bearing stress

Components of Osteon: Central Canal (Haversian)

• Vertical
• Houses blood/lymph vessels & nerves

Components of Osteon: Perforating Canals (Volkmann's)

• Perpendicular connections between central canals
• Houses blood vessels & nerves

Components of Osteon: Lamellae

• Concentric rings that surround the central canal

Components of Osteon: Lacuna

• Small spaces that contain osteocytes

Components of Osteon: Canaliculi

• Interconnecting channels between lacunae and central canal
• Allows for nutrients, minerals, gases, and waste to travel to-and-from osteocytes

Spongy Bone Components

• Trabeculae
• Narrow rods and plates of bone creating a meshwork of bars
• Bone marrow fills the spaces
• Allows resistance to stress in multiple directions

Spongy Bone Components: Lamellae

• Bone matrix

• Osteocytes in lacuna between lamellae

• Canaliculi

• Open spaces in spongy bone make the bone lighter and contain red bone marrow

Osteogenesis/Ossification

• Formation and development of bone
• Begins during embryonic stages (six or seventh week) and continues through adulthood
• Two types: Intramembranous and Endochondral ossification

Intramembranous Ossification

• Bone source is mesenchyme (embryonic origination for connective tissue)
• Produces flat bones of the skull, facial bones, mandible, and central clavicle

Summary of Intramembranous Ossification

1. Ossification Centers (cluster of osteoblasts) form within thickened regions of mesenchyme
2. Osteoid is secreted by osteoblasts and undergoes calcification
3. Trabeculae and the surrounding periosteum form
4. Compact bone develops surrounding trabecular bone

• Flat bones can resist injury well due to a double wall of compact bone with spongy bone between

Endochondral Ossification

• Bone source is hyaline cartilage
• Produces most bones of skeleton, upper and lower limbs, pelvis, vertebrae and clavicle ends

Summary of Endochondral Ossification

1. Fetal hyaline cartilage model of bone forms
2. Periosteal bone collar and Primary ossification center (cluster of osteoblasts) forms in diaphysis
3. Vascular invasion of primary center and Secondary ossification center forms in epiphysis
4. Bone replaces almost all of the cartilage EXCEPT articular cartilage and epiphyseal plate
5. Epiphyseal plates ossify to form lines by person's mid-late 20s and the clavicle is last

Bone Growth: Endochondral (Interstitial) growth

• Lengthening of bone
• Occurs at the epiphyseal plate
• Chondrocytes divide and increase in number before ossification
• At maturity, cartilage production slows
• How bones grow taller

Bone Growth: Appositional Growth

• Increasing the width of bone
• Occurs within the periosteum
• Bone matrix deposited within layers parallel to the surface
• Osteoclasts reabsorb bone matrix along the medullary cavity

Bone Remodeling

• Replacement of old bone tissue with new bone tissue
• 20% of the skeleton is replaced yearly
• Occurs throughout life
• Replacement of worn and injured bone
• Allows bone to act as a calcium stroage place
• Influenced by hormones and mechanical stress
• Coordination of osteoblasts, osteocytes, and osteoclasts

Mechanical Stress

• Weight-bearing movement/exercise

• Detected by osteocytes

• Fluctuations in bone mass

• Wolff's Law: Bone in a healthy animal will adapt to the loads under which it is placed

Hormones Involved with Remodeling

• Human growth hormone (GH) increases mitosis of chondrocytes

• T3 and T4 increase osteoblast activity

• Estrogen and testosterone aid in osteoblasts producing matrix and removing cartilage in the epiphyseal plate (highly active during puberty)

• Calcitriol increases absorption of calcium and phosphate from the digestive tract

• Parathyroid hormone (PTH) increases osteoclasts and increases blood calcium levels

• Calcitonin increases osteoblasts and decreases osteoclasts, decreases blood calcium levels

• Minerals for bone remodelling include Calcium and Phosphorus

• Vitamin D is required for the absorption of Calcium

• Vitamin C is required to maintain the bone matrix

• Vitamin A can regulate the activity of osteoblasts and osteoclasts

Parathyroid Hormone (PTH)

• Calcium regulation will increase the number and activity of osteoclasts, which breaks down bone and releases calcium and phosphates into the bloodstream.
• PTH slows the amount of calcium absorbed by the kidneys in urine being produced
• The kidneys will convert vitamin D into calcitriol

Calcitonin

• Released in response to high blood calcium levels and during exercise
• Inhibits osteoclast activity
• Stimulates kidneys to increase loss of calcium in urine
• Overall effect is to lead to reduced blood calcium levels

Additional Influences on Remodeling: Glucocorticoids

• High amounts increase bone loss

Additional Influences on Remodeling: Aging

• Causes demineralization of bones as the level of the sex hormones drops
• Leads to osteopenia and then osteoporosis
• Causes a decrease in collagen formation, so bones become brittle and easily broken
• Hip fractures are more frequent and prevalent among those >65 years of age

Osteomalacia:

• Softening of bones due to impaired bone metabolism

• Results from inadequate levels of phosphate, calcium and vit D

• Risk to people in places like nursing homes and homebound geriatric patients

• Risk increased in people with celiac's disease, due to poor absorption of nutrients

• In children, this is known as Rickets

Rickets:

• Vitamin D deficiency during childhood

• Exclusive breastfeeding with vit D deficient mother

• little sunlight exposure and little vitamin d diet in adolescents

• Osteoid has insufficient calcification, that can cause Hypocalcemia and lead to:

• Bowlegged apperance

Osteoporosis

• The process that allows the bone to regulate itself becomes conflicted

• Bone creation can not keep up with old bone being removed, leading a disparity in structure

• Risk factors of this occurence are

• Age: Post-Menopausal Women, poor nutrition and exercise and constant breastfeeding

• Secondary risks related to : Alcoholism, and Cushing's Syndrome

• Hyperthyroidism and Hyperparathyroidism

Paget's Disease

• Chronic Bone disorder where bone Becomes Larger
• New bone forms at a faster rate than at which old bone is removed
• Activity of bone cells increase which include Osteoclast and Osteoblast
• Thickened structure of Lamellae and Trabeculae
• Occurence of this is most often higher in males 55 yrs plus
• Common Symptoms are of the Skull, often leading to Hat's not fitting

Osteogenesis Imperfecta

• Also known as Brittle Bone Disease
• Caused by mutations in the Structure of collagen leading to fractures and impaired healing.
• Bones are thin, poorly developed and fracture easily
• Signs and Symptoms will often not be noted until child begins to move around
• Preventative Treatments focus on Fractures and stabilization of bones and its surrounding areas

Types of Fractures: Stress

• Fatigue-induced (caused by repetitive stress

Types of Fractures: Pathologic

• Occurs in bone weakened by disease

Types of Fractures: Simple

• Broken bone not pentetrating skin

Types of Fractures: Compound

• When bone breaks AND pierces the skin

Fracture Repair

• A fracture hemotoma forms

• A fibrocartillaginous (soft) callus forms

• A hand(bony ) calls forms

• and FINALLY the bone is remodeled

Articulations

• Is place of the bone where two bones are connected, the joining is the point and its contact
• The term of articulation is used to describe this
  • There are 3 Types of joints

Types of Joints: Fibrous:

• No cavity space for the bones held together that are connected with dense connective tissue

Types of Joints: Cartilaginous

• Again, like the fibrous joints, it will not have a gap or void/no joint cavity however there are bones joined here WITH and BY cartilage

Types of Joints: Synovial

• There is a lubricating fluid in a "joint" area AND a capsule in that area and the capsule area will then be in "ligaments"

Subcategories of Fibrous Joints: Gomphosis:

• When Periodontal "like gums" membranes Hold a tooth/teeth

Subcategories of Fibrous Joints: Suture:

= Connects Skull of the bones

Subcategories of Fibrous Joints: Syndesmosis:

• Inerossous space/membranes exists bones

Cartilaginous joint: Synchondrosis:

• Cartilage are in Hyaline for (bone cells)

Types of Synovial Joints: Plane-

• Sliding/glide across smooth surfaces

Types of Synovial Joints: Pivot-

• Is Rotation, or the movement of the Bone

Types of Synovial Joints: Hinge-

• Involves Flexion and Extension

Types of Synovial Joints: Saddle and Condyloid-

• Biaxial in Two Directions

Synovial Joints: Ball and Socket-

• It is multiaxial in Nature

Osteoarthritis (OA)-

Most common type of Arthritis

• Can lead to Cartilage that breaks apart , then Leading to bone with bones colliding

• Cause by Wear and tear from life and previous injuries, often linked with weight and nutrition/diet

• Can lead to a joint replacement procedures

• Often involving the knee and hip

Rheumatoid Arthritis (RA)

• Is immune-mediated inflammatory joint PAIN
• Immune system attacks synoriam , causing swollen and red capsular tissues
• Is bilateral
• Is genetic

The Skeleton: Scoliosis:

• Is genetic and environmental-related (postural)

• The Spine Is not symettrical in Shape, appearing "S' and "C" (curved)

• The Cause is idiopathic as it is often seen during the adolescent-age groups, with higher risk in young females

• Treatments are surgery for that involves the Spine

Body Issues : Strain and Sprain:

• The Body will endure different problems and issues such as, in Sprain

• Stretching and/or ligament tearing

• While Strain is the stretching AND tearing of Muscle Tissue and Tendons