Four Microstructures of Bone Cells Organic matrix Inorganic

Four Microstructures of Bone • • Cells Organic matrix Inorganic matrix Soluble signaling factors 1

Cell Phenotypes of Bone • Osteoblasts • Osteocytes • Osteoclasts 2

Osteoblasts • Determined Osteoprogenitor Cells • Inducible Osteoprogenitor Cells 3

Determined Osteoprogenitor Cells • Bone formation during embryogenesis. • Resident in the cambial layers of the periosteum, endosteum, dura. 4

Inducible Osteoprogenitor Cells • During fracture repair • Inducible population may be susceptible to soluble inductive morphogens. • Pericytes : arrive at the injury locus about 3 to 5 days after injury via transit in developing capillary sprouts 5

Osteoblasts and Its Functions • Express soluble signaling factors : BMPs, TGF- , IGF-I, Interleukin-I, PDGF • Express osteoid • Yield an organic insoluble substratum consisting mostly of type I collagen. 6

Osteoid Formation with Time • Produced at a rate of about 2 to 3 um per day during remodeling. • Mineralized at a rate of about 1 to 2 um per day under the aegis of a platoon of osteoblasts. 7

Osteocytes • Relatively inactive cells, yet their subdued metabolic activity is crucial to bone viability and to sustain homeostasis. • Bone viability : through a network of osteocytic cytoplasmic processes that transverse canaliculi. 8

Life Span of Human Bone Cells • Osteoblasts : from 1 to 10 weeks • Osteocytes : many years, perhaps even decades • Osteoclasts : 9

Osteoclasts • Multinucleated cell up to 100 um in diameter with an average of 10 to 12 nuclei. • Have a ruffled border, possess calcitonin receptors produce tartrate-resistant acid phosphatase, lack the Fc and C 3 receptors. • Modulators for osteoclasts : interleukins-1, -3, -6, -11, TNF- , TGF- 10

Organic Matrices • 35% of the dry weight of bone • 90% : type I collagen 10% : non-collagenous proteins - main subject of bone engineering, ex) BMPs 11

Inorganic Matrices • Ca, P, Na, Mg • Bone mineral crystal : apatite rather than hydroxyapatite , Ca 10(PO 4)6(OH)2 • Osteoid : template for calcium-phosphate deposition (mineralization) 12

Macrostructure of Bone • Cortical (compact) bone • Cancellous (trabecular) bone 13

Cortical (compact) Bone • Protect vital organs • Resist biofunctional chanllages • Enable locomotion by muscle-lever arm activities • Provide safe haven for hematopoiesis 14

Haversian System of Cortical Bone • Function as buttress to biofunctional forces. • 4 to 20 circumferential rings of concentrically arranged lamellae : osteocytes • Haversian canal contains : blood vessels, lymphatics, sometimes nerves 15

Volkmann’s Canals of Cortical Bone • Penetrate cortical bone in an oblique direction, anastomosing with the haversian systems. • Providing vascular-lymphatic channels for metabolic exchanges and trafficking of soluble signals, such as hormones. 16

Cancellous Bone • Three dimensional lattice of trabeculae : random orientation (isotropic) • Designed to respond rapidly to physiologic requirements rather than for loading. • 20 -fold increase in surface area and concomitant cell density per unit volume over 17

Soluble Factors Bone Morphogenic Proteins 18

Effect of fibroblasts in bone formation • Produce soluble factors which are inhibitory to bone cell differentiation and osteogenesis. • Co-culture with bone marrow cells : large reduction of bone nodule formation & inhibition of osteoblast differentiation and function 19