Lecture 23 - Bone Tissue: Part 1

Chapter 6 - Skeletal System: Bone Tissue

Skeletal System Functions

  • Bone, cartilage, dense connective tissue, epithelium, adipose tissue, nervous tissue, blood vessels

  1. Support - Bone is hard and rigid

    • Cartilage - Flexible & strong

      • Nose, external ear, trachea, joints

    • Ligaments - Bone to bone

      • Dense, regular collagenous connective tissue

    • Bone - Dynamic

      • constantly breaking down and rebuilding bone

        • Supports soft tissues of the body

  2. Protection

    • Skull protects brain

    • Ribs, sternum, vertebrae protect organs of thoracic cavity

  3. Movement

    • Produced by muscles on bones, via tendons

  4. Storage

    • For Calcium and Phosphorus

      • Calcium needed in many processes (Eg. Presynaptic terminals, muscle contraction)

      • Phosphorus needed for ATP, growth/maintanence of tissues

    • Stored then released as needed

    • Fat stored in marrow cavities (yellow colour)

  5. Blood Cell Production

    • Bone marrow gives rise to blood cells & platelets

    • Red blood marrow

Cartilage

  • Firm, smooth, resilient, non-vascular connective tissue

    • If you compress it, it bounces back to original shape

  • Cartilage cells & matrix (70-85% water, protein, ground substance)

    • Water gives resiliency

    • Ground substance made up of large portion of proteoglycans (molecules that trap water)

    • Protein - collagen fibers and some elastic fibers for one type of cartilage

  • 3 Types of Cartilage

    1. Hyaline

      • Most common, associated with bone

      • Found in any of the joints of the body on surface (Articular cartilage)

      • formed in embryological development → turned into skeletons

      • bridge of nose

      • rings of trachea

    2. Elastic

      • Found in areas with more flexibility

      • External part of ear

      • Has alot of elastic fibers

    3. Fibrocartilage

      • Strongest because of orientation and density of collagen fibers

      • found in between bodies of vertebrae (intervertebral disk)

      • also found in the knee

      • where we need more support/structure

  • Types differ by amount of protein, types of protein, organization of protein

  • during development, repair - bone develops from cartilage

  • 1 Main cell type: chondrocyte

  • Located in a cavity (lacuna)

  • Immature version of chondrocyte = Chondroblast

    • builds matrix, secrets matrix around themselves, eventually traps themselves within matrix → turn to chondrocyte cells

    • matrix = collagen, proteoglycans

    • chondroblast cells located towards surface of cartilage

  • Perichondrium: On surface of most cartilage - double layer of connective tissue

    • where blood supply/nerve supply is located

    • Outside Layer - Dense, irregular connective tissue (fibroblasts)

    • Inner Layer - layer of cells (chondroblast cells & osteochondral progenitor cells) → produces more cartilage

  • Articular Cartilage

    • No perichondrium

    • Due to constant friction in joints

    • Hard to repair because no blood supply (comes from the bone side)

    • Because of alot of water, its easy for nutrients to diffuse

  • Fibrocartilage

    • located in knee, in between vertebrae

    • Damage → hard to fix because no direct blood supply

  • 2 Ways to Grow cartilage:

    1. Appositional Growth

      • chondroblast cells

        • Multiply, secret matrix, trap themselves, mature into chondrocytes

      • Growth on outside of cartilage

    2. Interstitial Growth

      • Mature cartilage

      • Mature chondrocytes divide & secrete more matrix around themselves

      • Because there is so much water in matrix → easier for cells to move around

      • growth from within

  • When we are young - typically cartilage grows via interstitial growth (until puberty)

    • From then on appositional growth

Bone Histology

  • Cartilage & bone both supporting connective tissues

  • Extracellular bone matrix + bone cells

  • Bone cells produce matrix → trap themselves in it

    • Constantlybreak down and replace old bone matrix

Bone Matrix

  • 65% inorganic (crystallized mineral salts)

    • Hydroxyapatite: Calcium phosphate crystals → compressive/weight bearing strength

      • Bones very flexible without

    • “Caclifying/ossifying bone” = adding this mineral salt to matrix

    • makes it more solid/calcified

  • 35% organic

    • Collagen, proteoglycans, water → flexible strength

      • Bones very brittle without

    • less water and proteoglycans than cartilage

    • more minerals and crystals

Bone Cells - Osteoblasts

  • produce collagen & proteoglycans (vesicles)

    • makes both Organic & inorganic components

    • packages these into vesicles → vesicles move towards membrane of osteoblast cell

    • exocytosis → dumps contents out into extracellular space

  • form matrix vesicles of Ca2+ and PO4 3-

    • form via pinching off a portion of osteoblast membrane

    • Many Ca2+ and PO4 3- ions inside osteoblast →pinch/wrap some osteoblast membrane around it → concentrates Ca2+ & PO4 3- ions together → forms hydroxyapatite

      • migrates out to collagen framework and calcifies it (fills space & hardens by mineralizing it)

  • Responsible for ossification (bone formation)

  • Osteochondral Progenitor Cells: Stem cells that can become osteoblasts or chondroblasts

    • Osteochondral progenitor cell → osteoblast cell → osteocyte

Bone Cells - Osteocytes

  • Osteocyte = osteoblast surrounded by bone matrix

  • cells trapped in a cavity called Lacuna

  • Osteoblast cells connect to each other via little connections

    • Without connecting cell extensions, cells would be pushing against each other as extracellular matrix is being formed

    • Connecting cell extensions turn into connecting canals - Canaliculi

      • Nutrients & oxygen use canaliculi as pathway to get to cells that are trapped in the matrix

  • considered inactive

  • Maintain ability to produce components necessary to maintainbone matrix

Bone Cells - Osteoclasts

  • Large, multinuclear cells, found on surface of bones

  • Formed from the fusion of monocyte cells (WBC)

  • Resorption of bone

    • Resorption - breaking the bone down

  • Acid & enzymes dissolve calcium, phosphorus, & collagen

    • Secretes acid (calcium & phosphorus) & enzymes (protein)

  • Podosomes: Attachment of osteoclasts to surface of the bone

    • Creates sealed compartment

    • Part of membrane of osteoclasts become extendeed → makes ruffled boarder → releases acid & enzymes into sealed compartment

    • Broken down fragments taken up into osteoclast cells

      • Released on other side of cell into extracellular space → eventually into blood

      • Eg. calcium released into blood when calcium is low

    • Bone always being remodeled