bone tissue

• what are the functions of skeletal system?

  1. protection: skeleton protects vital organs
  2. mineral storage and acid-base homeostasis: bone stores minerals such as Ca+ and PO4^3-, which are necessary for electrolyte and acid-base balance
  3. blood cell formation: red bone marrow is the site of blood cell formation; blood cells in red bone marrow
  4. fat storage: yellow bone marrow stores triglyceride; fat in yellow bone marrow
  5. movement: muscles produce movement via their attachment to bones
  6. support: the skeleton supports the weight of the body
• there are 206 bones in average adult; range from 206-208
• long bones: length > width

  • e.g. humerus

  short bones: length = width

  • e.g. carpals and tarsals

  flat bones: thin & broad

  • e.g. skull bones, sternum, pelvis ribs

  irregular bones: don't fit in above categories

  • e.g. vertebra

  sesamoid bones: bones found within tendons

  • e.g. patella

  wormian (sutural bones): found within skull sutures
• long bones:

  • articular cartilage (hylaine): covers surfaces that form joints along with lubricating fluid, allow ease of movement
  • periosteum: outer dense irregular collagenous layer + inner osteogenic later. Collagen fibers extend into bone matrix as perforating (Sharpey's) fibers
  • nutrient foramina: minute holes in bone tissue that allow blood vessels to penetrate
  • endosteum: reticular connective tissue, lines marrow cavities & covers trabecular surfaces of spongy bone
  • epiphyseal plate/line: hyaline cartilage, zone of bone elongation
• • compact: dense osseous tissue, encloses medullary cavity
  • spongy (cancellous) bone: within ends of long bones, inner portion of short, flat and irregular bones
  • contains bone marrow
  • always surrounded by compact bone
  • bony struts = trabeculae
• periosteum --> perforating fibers --> compact bones --> spongy bone or diploe (with red bone marrow) --> compact bone
• inner compact bone: covered with periosteum layer
• diploe: layer of spongy bone sandwiched between 2 compact bone layers. Trabeculae lined with endosteum house
• what is the difference between red and yellow bone marrow?

  red marrow

  • hematopoietic cells: produce blood cells

  yellow marrow

  • stores triglycerides in adults
• Osseous tissue: made up of cells and extracellular matrix
• cells in osseous tissue:

  1. osteogenic cells: undifferentiated stem cells
  2. osteoblasts: build bone
  3. osteocytes: mature osteoblasts that maintain the ECM
  4. osteoclasts: dissolve bone ECM
• Extracellular matrix in osseous tissue:

  • inorganic matrix
  • 65% of ECM
  • hydroxyapatite crystals (Ca & P) Bicarbonate, K, Mg, Na
  • organic matrix (osteoid)
  • 35% of ECM (osteoid)
  • collagen fibers, proteoglycans, GAGs, glycoproteins
• osteogenic cells: highly mitotically active, found in periosteum and endosteum, some differentiate into osteoblasts (some remain osteogenic stern cells)
• osteoblasts: don't undergo cell division, secrete collagen fibers that form bone matrix

  • become encrusted with minerals like calcium
  • trapped in hardening matrix and become osteocytes
• osteocytes: mature bone cells, trapped in lacunae, maintain bone matrix and signal for bone repair or remodeling
• osteoclasts: dissolve bone tissue when signaled (resorption)

  • formed from blood cell lineages
  • found in resorption bays that they themselves etch into the bone surface
  • secrete H- & enzymes
  • side of osteoclast facing bone surface has a ruffled border
  • increases cell surface area and ability to resorb bone tissue
• compact bone

  • spongy bone: blood vessels continue into spongy bone and bone marrow
  • osteon: with concentric lamella structural cylinders
  • central canals (haversian): innermost osteocytes receive nutrients from blood vessels pass them through gap junctions and canaliculi to neighboring osteocytes
  • nutrient foramina: openings on bone surface of perforating canals that allow capillaries and nerves passage through compact bone
  • interstitial lamella
  • circumferential lamella
  • periosteum with perforating fibers
• osteon

  • elongated cylinder of concentric lamella rings
  • collagen fibers in adjacent lamella twist in opposite directions like plywood
  • osteoblasts secrete collagen, become osteocytes trapped on borders between lamellae after mineralization
• spongy bone

  • no osteons
  • blood supply obtained from compact bone
  • no central or perforating canals
• ossification or osteogenesis: formation of bone, creating mature, secondary lamellar bone

  1. intramembranous ossification
  2. flat bones of skull and clavicle
  3. starting material = membrane of embryonic connective tissue
  4. endochondral ossification
  5. rest of skeleton
  6. starting material = hyalin cartilage model
• primary (woven) bone

  • irregular arrangement of collagen fibers
  • abundant osteocytes, little inorganic matrix
  • present during embryonic development or fracture repair
  • absorbed by osteoclasts, replaced with secondary bone
• secondary (lamellar) bone

  • higher percentage of inorganic matrix (Stronger)
  • fully formed lamallae with regular arrangement of collagen fibers
• membrane of embryonic connective tissue --> intramembranous ossification --> primary, woven bone --> mature, secondary lamellar bone
• hyalin cartilage model --> endochondral ossification --> primary, woven bone --> mature, secondary lamellar bone
• intramembranous ossifiation process:

  1. osteoblasts develop in the primary ossification center from mesenchymal cells
  2. osteoblasts secrete organic matrix, which calcifies, and trapped osteoblasts become osteocytes
  3. osteoblasts lay down trabeculae of early spongy bone, and some of the surrounding mesenchyme differentiates into the periosteum
  4. osteoblasts in the periosteum lay down early compact bone
• Endochondral ossification process

  Process of bone growth that occurs in long bones
• endochondral ossification process:

  1. the chondroblasts in the perichondrium differentiate into osteoblasts
  2. osteoblasts build the bone collar on the bone's external surface as the bone begins to ossify from the outside
  3. simultaneously, the internal cartilage begins to calcify and the chondrocytes die
  4. in the primary ossification center, osteoblasts replace the calcified cartilage with early spongy bone; secondary ossification centers and medullary cavity develop
  5. medullary cavity enlarges and remaining cartilage is replaced by bone, epiphysis finish ossifying
• causes of osteoporosis:

  1. dietary
  2. female gender
  3. advanced age
  4. lack of exercise
  5. hormonal (lack of estrogen)
  6. genetic factors
  7. other diseases

  diagnosis: bone density measurement

  prevention: balanced diet, with supplementation as needed, weight-bearing exercise, and estrogen replacement if appropriate

  treatment: drugs that inhibit osteoclasts or stimulate osteoblasts
• longitudinal growth: increase in length

  • chondrocyte division in epiphyseal plates

  appositional growth: increase in width

  • osteoblasts underneath periosteum deposit new compact bone (intramembranous ossification)
  • with addition of osteoclasts and secondary bone formation (osteons) around blood vessels to form Haversion canals
• hormone: chemicals secreted into the blood by endocrine glands or cells that affect various aspects of target cell physiology

  • growth hormone: increase mitosis of chondrocytes in epiphyseal plate and activity of osteogenic cells and osteoblasts
  • testosterone: increases rate of appositional growth, accelerates closure of epiphyseal plate
  • estrogen: increases rate of longitudinal growth. potently accelerates closure of epiphyseal plate
• remodeling: continuous cycle of deposition & bone resorption

  • calcium ion (Ca 2+) homeostasis
  • bone repair
  • replacement of primary bone with secondary bone
  • bone adaptation to sustained tension & stresses
• which of the following structures persists for life?

  articular cartilage
• the canal that runs through the core of each osteon is the site of

  blood vessels and nerve fibers
• what produces new bone tissue by secreting matrix?

  osteoblasts
• what forms bone from within a model of hyaline cartilage?

  endochondral ossification
• if the inorganic matrix is destroyed, bone would become flexible and unable to resist compression
• blood cells are made in the red bone marrow of bones, a process known as hematopoiesis
• canaliculi: allow for the passage of oxygen, nutrients, and small substances to and from osteocytes trapped within lacunae
• the epiphysis is the enlarged, rounded end of a long bone