Bone Meghan Barboza, PhD Online Course Bone: specialized form of connective tissue Support Attachment Leverage Protection Mineral Storage Principal Characteristics Vascularized Innervated Produces hydroxyapatite crystals [Ca10(PO4)6(OH)2] within its ECM Collagen type 1 Osteogenic Tissue Tissue that makes bone Osteoblasts, osteoprogenitor cells, and blood vessels Lined Externally: periosteum Internally: (the marrow cavity): endosteum www.ouhsc.edu Principal Cell Types of Bone Osteoblast Osteocyte (fully differentiated osteoblast) Osteoclast Osteoblast Initially forms osteoid (prebone) which continues to mineralize to become bone matrix Osteocyte Selected osteoblast that ‘remains behind’ Coated by thin layer of osteoid Connected through canaliculi Canaliculi Osteocyte Osteoclast Osteoclast - bone resorption (osteoclasia) and remodeling Originate from bone marrow, coalescing monocytes Lysosomally rich Ruffled border creates a resorption bay (Howslip’s lacuna) Vesicular zone Ruffled border creates a resorption bay (Howslip’s lacuna) Bone ECM Consists of collagen (type I), proteoglycans (PG), and minerals (Ca, P & variety of trace metals) Simple salts & hydroxyapatite crystals Reciprocal relationship between amount of PG and degree of mineralization Check your Understanding What types of cells/structures would you typically find near the endosteum? Periosteum? If your blood is low on calcium, which cell do you think would help solve the problem? What about when you take a calcium supplement, which cell would be activated in that scenario? If a canaliculi became blocked, what would happen? Bone Histogenesis: Ossification Intramembranous (within developing c.t.-mesenchyme) Endochondral (within developing cartilage) Primary bone formed first (remodeled later into secondary bone) Intramembranous Ossification Flat Bones of head Some bones of the skull Mandible and maxilla (jaw bones) Intramembranous Ossification blastema spicule spicule Primary center of ossification Located at area labeled blastema Intramembranous Ossification Spicules fuse into trabeculae Areas of surrounding mesenchyme become endosteum/periosteum http://www.lab.anhb.uwa.edu.au/mb140/corepages/bone/bone.htm Intramembranous Ossification Trabeculae fuse into a bone forming structure (flat bones) with periosteum and endosteum established (overall initially spongy) Eventually spongy appearance disappears w/ addition of layers (lamellae) by periosteum http://www.lab.anhb.uwa.edu.au/mb140/corepages/bone/bone.htm Endochondral Ossification Hyaline cartilage ‘model’ Long and Short Bones: weight bearing bones Develops appositionally and interstitially Endochondral Ossification Step 1 Chondrocytes in center enlarge (hypertrophy) and the matrix calcifies Step 2 Presence of Vasculature along the sides of the cartilage causes chondrogenic cells to become osteogenic 1 2 Endochondral Ossification Step 3 Osteoblast secrete bony matrix to create Subperiosteal Bone Collar Step 4 Bone collar prevents nutrients to hypertrophied chondrocytes-they die and leave cavity which will become…? Endochondral Ossification Step 5 Osteoprogenitor cells, hemaopoeitic cells, and blood vessels enter and line cavity (primary center of ossification) Step 6 Osteoblasts form internally secrete bony matrix onto the calcified cartilage (bone=acidophillic (pink), calcified cartilage = basophillic (blue)) Endochondral OssificationSecondary Center Step 7 Primary Center of Ossification of Ossification Osteoclasts resorb bone and enlarge cavity and replace cartilage with bone except at epiphyseal plates (growth plate). Secondary Center of Ossification plate plate Formation of epiphyseal plates: Chondrocytes become arranged in linear isogenous nests Secondary Centers of Ossification Organized interstitial growth of these chondrocytes leads to lengthwise growth of the cartilage model Organized interstitial growth consists of distinct zones Zone of Hypertrophy (Enlargement) Zone of (Provisional)Calcification Primary bone arising from the growth plate: Primary Spongiosa or Cancellous bone Length of long and short bones is due to interstitial development of the growth plates Width of long and short bones is due to periosteal activity along the collar (appositional growth) Bone Repair Blood clots and cell death result in an increased growth rate in periosteum and endosteum A new cartilage matrix is laid down The chondrocytes are replaced by osteoblasts and new bone is formed Osteoclasts come through and replace cartilage with bone. Check your Understanding Which step is the most important in endochondral ossification? What is the difference between a spicule and trabeculae? Bone Classification Based On: Development Immature Mature Organization Lamellar Osteonal Configuration (Shape) Spongy Compact Immature Bone Intramembranous & endochondral ossification initially form spongy bone Spongy bone: more interosseous space (area) than osseous (bone ECM) Mature Bone Mostly compact (more osseous than interosseous), resulting from continued activity of the periosteum during both forms of ossification Compact bone further classified on the basis of organization Lamellar Osteonal Compact bone further classified on the basis of organization Lamellar Osteonal (lamellar oriented in a circular way) Bone Classification: Configuration Spongy - comprising mostly immature bone; synonyms include: trabecular bone, cancellous bone, woven bone, primary bone Compact - comprising mostly mature bone; synonyms include: osteonal bone, lamellar bone, secondary bone, cortical bone Osteon/Haversian system (comprises most of compact bone) Osteon (Haversian system) Central/Haversian Canal Cement Line Osteocytes Lacuna Canaliculi Osteon Volksmann Canal Bone Preparation Decalcified Calcium salts removed through treatment of the tissue Then bone is cut and stained normally Trichrome is usually used-bone will stain blue, other tissues pink to red. Ground Bone is sawed into thin sections and then ground down until it can be observed using light microscopy. No Staining Check your Understanding What do you think would be a benefit of a bone having a lamellar structure? An osteonal structure? In the images below identify the stain and osteoblasts/cytes and/or chondrocytes-what type of bone development are these?