IN THE NAME OF GOD THE COMPASSIONATE THE MERCIFUL

Histology Lecture 8 Bone Formation and Joints

2 Types of Bone Tissue  Bone histogenesis may be of two types intramembranous or endochondral  First bone that is formed by either of the two methods is the same and is called primary bone which is later replaced by secondary bone  Immature, primary or woven bone where collagen bundles have random orientation  Mature, secondary or lamellar bone where collagen bundles are oriented in bone lamellae

Primary Bone  First to be formed during embryonic period and during bone repair in adults  Replaced by secondary except near sutures in flat bones of skull, tooth sockets and insertion areas of some tendons  Collagen bundles are irregular and random in orientation  Decreased mineral content and more osteocytes per unit volume than secondary bone

Secondary Bone  Secondary bone is mature form of primary bone  It is composed of parallel or concentric lamellae  Osteocytes are dispersed at regular intervals  Canaliculi connect neighboring lacunae  Matrix is more calcified than primary bone  Collagen fibers are arranged so that they parallel each other within a given lamellae

2 Types of Bone Histogenesis  Intramembranous ossification occurs in a layer of condensed mesenchymal connective tissue  Endochondral ossification occurs within a cartilaginous template, cartilage precursor is gradually replaced by ossified tissue differentiating from the surrounding periosteum

Intramembranous Ossification  It is the source of most flattened bones such as skull, mandible and maxilla  Intramembranous ossification starts with a mesenchymal connective tissue  Primary ossification centers form when mesenchymal cells differentiate into osteoblasts around growing vessels  Islands of developing bone are spicules and trabeculae of primary bone whose surface is lined by these cells  Following fusion of spicules the mesenchymal connective tissue in their interstices transformed into bone marrow  Uncalcified mesenchymal tissue on the surface of trabeculae differentiate into periosteum  In skull, internal and external plates of compact bone are separated by a layer of spongy bone form in this way

Endochondral Ossification  Endochondral Ossification Occurs in hyaline cartilage template  It forms most of the long and short bones of the body  Endochondral Ossification has 2 Phases Formation of a hyaline cartilage template Growing of cartilage followed by resorbtion and replacement by bone

Primary Center Events  Formation and development of a hyaline cartilage model in the region where bone should be formed  In the middle of cartilage chondrocytes accumulate glycogen, hypertrophied, and vacuolated  The lacunae enlarge and intervening matrix reduced and calcified  Perichondrium at the middle of the diaphysis of cartilage model becomes vacularized and chondrogenic cells replaced by osteoprogenitor cells  Osteoprogenitor cells forming chondroblasts and perichondrium becomes periosteum  By intramembranous ossification osteoblasts forming subperiosteal bone collar on the surface of the cartilage model

Primary Center Events  Bone collar stops diffusion of nutrients to chondrocytes in cartilage template and causing them to die  The lacunae becomes empty, confluents and forming large cavities  Osteoclasts makes holes in bone collar and permit osteogenic bud enter the cavities in cartilage model  Osteogenic bud is composed of osteoprogenitor cells, hemopoietic cells and blood vessels  Osteoblasts secrete bone matrix on the surface of calcified cartilage forming calcified cartilage – bone complex  Calcified cartilage is basophilic (blue) and new bone is acidophilic (pink)

Primary Center Events  Collar bone becomes thicker and grows in each direction toward epiphysis  As this happen osteoclasts resorbs the calcified cartilage bone complex enlarging bone marrow  In this way the cartilage of the diaphysis replaced by bone except for the epiphysial plate  Later secondary ossification centers form at each epiphysis

Secondary Centers Events  Secondary center forms epiphysis  The process of bone formation is identical to diaphysis except that a bone collar is not present  The cartilage of epiphysis replaced by bone except at the articular surface and epiphysial plate  As epiphyseal plate growth continues, matrix produced is processed into bone by expanding diaphyseal ossification center

Zones of Epiphyseal Plate  Resting zone (Reserve zone): chondrocytes show mitotic figure and randomly distributed in hyaline cartilage  Proliferative zone: rapidly dividing chondrocytes form columns of stacked cells  Hypertrophied cartilaginous zone: chondrocytes mature accumulate glycogen in cytoplasm, and hypertrophied  Calcified cartilage zone: chondrocytes dying, deposition of hydroxy apatite in matrix  Ossification zone: endochondral bone appears, vessels invade from periosteum, osteoblasts deposit calcified bone matrix on calcified cartilage followed by resorption of this complex

Mechanisms of Calcification  Not completely understood  Alkaline phosphatase causes accumulation of phosphate for precipitation of hydroxy apatite crystals  Calcium binding glycoproteins secreted  Vesicles in osteoblasts accumulate calcium and phosphate for calcification

Remodeling and Growth of Bone *Resorption of bone matrix and deposition of new bone remain active throughout life *As bone matrix is added to outer bone surfaces marrow cavity enlarges leading to increase in bone diameter *Bone length increases due to epiphyseal plate growth *Bone diameter increases as periosteal cells differentiate into new osteoblasts and deposit new bone on the surface (Appositional growth)

Joints  Bones are connected to each other by joints  Joints classification: - Permit limited movement - Permit free movement  Some joints are formed by fibrocollagenous (Syndesmoses in skull) or cartilaginous tissue (Synchondroses between ribs and sternum )  In old age will replace by bone (Synostosis )

Synovial joints  Allow free movement  The bone held together by collagenous tissue (Joint capsule and Ligaments)  Bone ends are coated by a smooth, friction-free layer of hyaline cartilage (articular cartilage)  Joint capsule is composed of two layers an outer fibrous layer, and an inner cellular synovial layer  The internal lining of the joint capsule is a specialized secretory epithelium, the synovium

 Synovium produce synovial fluid and composed of 1-4 layers of cells  Cells vary from flat to cuboidal or polyhedral in shape  Two cell types defined: -Type A are phagocytic and have numerous lysosomes - Type B have abundant RER and produce protein  These layers of cells supported by a loose fibrocollagenous tissue containing adipocytes, mast cells, fibroblasts and macrophages Synovial joints

 The synovial membrae has an abundant blood, lymph, and nerve supply running in the loose fibrocollagenous tissue  Synovial fluid contains of hyaluronic acid and glycoprotein(Lubricin) combined with a filtrate of plasma  The synovial membrane supported by dense fibrocollagenous tissue ( Fibrous layer)  The joint supported by some internal and external ligaments Synovial joints

SOMATIC CONNECTIVE TISSUES Dermis Hypodermis Joint capsule Articular cartilage Bone Marrow Periosteum Tendon Ligament & Muscle

Department of Histology F. Rajaei