Alveolar bone

Alveolar Bone in HealthPart - BDr. Ibrahim Shaikh

Dept. of Periodontology & ImplantologySeminar No. 610/08/2015Guide : Dr. Varsha Rathod

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Cell types in boneMatrix componentsUltrastructural organizationPhysiologic remodelling of alveolar boneThe implant - bone interfaceConclusionReferences2CONTENTS

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CELL TYPES

Several cell types are responsible for the synthesis, maintenance and resorption of bone. These can be regarded as belonging to two main families, one mesenchymal and the other haemopoietic. The osteoblasts, osteocytes and bone lining cells are derived from a mesenchymal (or ectomesenchymal) stem cell. These stem cells reside in the bone marrow and in a region of proliferating cells adjacent to the osteoblast layer in the periosteum. In the periodontal ligament and other bone-forming tissues, the osteogenic precursors may be associated with small blood vessels. The osteoclasts, however, belong to a different lineage. They form part of the haemopoietic system, being derived from the mononuclear/phagocyte system (including monocytes and macrophages).3

The cells that eventually give rise to osteoblasts are termed osteoprogenitor cells.Reside in the layer of cells beneath the osteoblast layer in the periosteal region, in the periodontal ligament, or in the marrow spaces.Fibroblast-like cells, with an elongated nucleus and few organelles.Life cycle-up to about eight cell divisions.OSTEOPROGENITOR CELLS

In order to generate the osteoblasts throughout the life, a stem-cell population is required. The stem cells have the ability to maintain their numbers throughout the life. When a stem cell divides, one of the daughter cells remains as a stem cell, while the other can differentiate into another cell type. In case of alveolar bone, the cells that eventually4

Friedenstein (1973) divided osteoprogenitor cells into; Determined osteogenic precursor cells are present in bone marrow, in the endosteum and in the periosteum that cover the surfaces of the bone. These cells possess an intrinsic capacity to proliferate and differentiate into osteoblasts.Inducible osteogenic precursor cells represent mesenchymal cells present in the other organs and tissues (e.g. muscles) that may become bone forming cells when exposed to specific stimuli.OSTEOPROGENITOR CELLS

Determined capacity to proliferate and differentiate into osteoblasts.

Inducible May become bone forming cells when exposed to specific stimuli.5

OSTEOBLASTSThese are specialized fibroblast-like cells of mesenchymal origin.Basophilic, plump cuboidal or slightly flattened, mononucleated cells. Contain a cytoplasm rich in synthetic and secretory organelles as rough ER, Golgi apparatus, secretory granules and microtubules6

Osteoblasts are mononucleated cells responsible for the synthesis and secretion of the macromolecular organic constituents of bone matrix.

Is prominent on the bone surface where there is active bone formation. Unlike cartilage which grows interstitially, bone can be deposited only at the surfaces.6

Secretes- Osteoid unmineralised bone matrixthickness 5-10 before reaching a level of maturity conducive to mineralisation.consists of type 1 collagen fibres, more or less parallel to bone.There is a lag phase of about 10 days before the deeper layer of osteoid has matured sufficiently to undergo mineralisation

7OSTEOBLASTS

Secretes the organic matrix of bone, which initially is represented by an unmineralised layer known as osteoid. Whose thickness is 5-10 um.Intrinsic collagen fibers lie parallel to bone surface.7

Functions of osteoblastsSecretion of osteoid and control of mineralization of bone.Production of paracrine and autocrine factors.Production of proteases, which are involved in matrix degradation.Expression of RANKL, whose presence is vital in osteoclast differentiation.OSTEOBLASTS

The main function of osteoblasts is formation of new bone via synthesis of various proteins and polysachharides.

In addition to its obvious involvement in bone formation, the osteoblast has a controlling influence in activating the bone-resorbing cells.A growth factor produced by one cell and acting on another is termed as paracrine regulation; whereas the process of cell that recaptures its own product is known as autocrine regulation.

Receptor activator nuclear kappa B - ligand

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Osteoblasts control the process of mineralization at three levels:Primary calcification, by production of an extracellular organelle called the matrix vesicleSecondary calcification, by modifying the matrix through the release of different enzymesBy regulating the amount of ions available for mineral deposition in the matrixOSTEOBLASTS

Cover most but not all inactive bone surfacesDecreased protein secretionRelative paucity of organelles

By covering the surface of bone, they protect it from any resorptive activity from osteoclasts.They may also be reactivated to form osteoblasts.Inactive surfaces are known to be a primary site of mineral ion exchange between blood and adult bone.

BONE LINING CELLS

Once osteoblasts have completed their function, they are either entrapped in bone matrix and become osteocytes or remain on the surface as lining cells.Osteoblasts flatten when bone is not forming and extend along the bone surface and hence the name.

They are regarded as post proliferative osteoblasts.10

Entrapped osteoblastsAbout 25000 osteocytes per cubic millimeter of bone Decreased quantity of secretory organellesSmaller size with large nucleus Formative and resorptive activity of these cells may vary under certain metabolic requirements-OSTEOCYTIC OSTEOLYSISNumerous cell processes from the osteocytes run in the canaliculi in all directions.Detect stresses induced in bone and are regarded as the main mechanoreceptors of bone.OSTEOCYTES

Cells lying within the bone itself and are entrapped osteoblasts. The number of osteoblasts that become osteocytes, depend on the rapidity of bone formation. Embryonic bone and repair bone show more osteocytes than lamellar bone as they are formed rapidly.

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Horizontal section of bone demonstrating a layer of osteoblasts (A) lining a surface where active bone formation is occurring (as indicated by the presence of a pale staining layer of osteoid), some large multinucleated osteoclasts (B) lying against Howship's lacunae in a region of bone undergoing resorption, and large numbers of osteocytes (C) lying embedded within the bone matrix itself. D - Bone-lining cells; E - pale-staining osteoid layer

They are derived from haemopoietic cells of the monocyte/ macrophage lineage by fusion of mononuclear precursors, giving rise to multinucleated cells.Osteoclasts are the cells responsible for bone resorption Howships lacunae : bony concavitiesOsteoclasts may be up to 100 um in diameter and have on average 10-20 nuclei.The lifespan of osteoclasts is thought to be about 10-14 days. OSTEOCLASTS

Howships lacunae resorbing surface of alveolar bone showing resortion concavities in which lie the multinucleated osteoclasts.

The cells can show considerable variation in size and shape, ranging from smaller mononuclear cells to very large cells.

There is evidence to indicate that large osteoclasts resorb more bone than small osteoclasts.13

Part that lies adjacent to bone foamy striated appearance (the so called ruffled border).Marker for osteoclasts Tartrate resistant acid phosphatase (TRAP)Osteoclasts are recruited only when required.OSTEOCLASTS

Increases surface area of resorption14

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The bone matrix is formed from a scaffold of interwoven collagen fibers within and between which small, uniform, plate-like crystals of carbonated hydroxyapatite (Ca10[PO4]6[OH]2) are deposited.

Other proteins, including proteoglycans, acidic glycosylated and non-glycosylated proteins associate with and regulate the formation of collagen fibrils and mineral crystals, or provide continuity between matrix components and between the matrix and cellular components.MATRIX COMPONENTS

The organic matrix of bone is about 90% collagen.16

Small amounts of carbohydrate and lipid contribute to the organic matrix - comprises 1/3rd of matrix.

Calcium and phosphate in the form of poorly crystalline, carbonated apatite, also described as dahllite, predominates the inorganic phase. MATRIX COMPONENTS

Collagen comprises the major ~8090% organic component.The collagen fibrils in bone are stabilized by intermolecular cross-linking involving lysines and modified lysines that form pyridinium ring structures (pyridinolines) - high tensile strength In rapidly forming (woven) bone that is produced during early development and in repair sites, the fibers are extensively interwoven, leaving a substantial volume of inter-fibrillar space that is largely occupied by mineral crystals and associated acidic proteins.COLLAGEN

Type I collagen (>95%) is the principal collagen Type III (