Connective Tissue and Bone
Peter Takizawa [email protected] What we will talk about…
• Types and functions of connective tissue
• Cells of connective tissue
• Composition and structure of bone
• Cells that control the shape and integrity of bone
• Development of bone Connective tissue serves a variety of functions throughout the body.
Resist stress Organize tissues Connective Tissue
Immunity Metabolic
Bacterium Fat cell
Macrophage Connective tissue can generate a range of mechanical strengths.
Organ Blood Cartilage Tendon Bone Support Vessels Connective tissue resists tension and compression.
Collagen Elastin
Glycosamino- glycans Connective tissue can be classified based on the amount and organization of collagen.
Dense Regular Dense Irregular Loose
Glycoproteins
Collagen Collagen Reticular fibers are composed of type III collagen and organize cells in organs.
Cells
Reticular Fiber Adipose tissue contains adipocytes that store large amounts of lipid and triglycerides.
Cytoplasm Lipids
Nucleus Brown fat stores lipids and triglycerides and generates heat.
Lipid Droplets
Nucleus Cells of connective tissue Fibroblasts are prominent in connective tissue and synthesize most of the protein components.
Areolar Tissue
Collagen Fiber
Collagen
Fibroblast Mast cells store and release histamine during an immune response
Mast Cell Macrophage engulf foreign particles and cellular debris.
Macrophage Cartilage Chondrocytes synthesize cartilage which resists compression.
Nucleus Matrix
Lipid Droplets
Chondrocyte Hyaline cartilage contains type II cartilage and glycosaminoglycans.
Toluidine Blue H&E
Perichondrium
Matrix
Chondrocytes Fibrocartilage contains a large amount of type I collagen.
Collagen Fibers
Matrix Chondrocytes Elastic cartilage contains elastic fibers and type II collagen.
Elastic Fiber
Chondrocyte Bone Bone serves mechanical, metabolic and cellular functions.
Ca2+ The major structure components of bone are calcium-phosphate crystals and type I collagen.
Collagen Composition of bone Ca2+ cyrstals Fibril Mineralized collagen fibril Bone can be arranged in lamellar or woven forms.
Lamellar
Woven Architecture of bone Compact bone and trabecular bone are two structures in most bones.
Trabecular Bone Epiphysis
Endosteal Surface Periosteal Surface
Diaphysis
Compact Bone Compact bone is organized into circumferential lamellae and Haversian systems.
Circumferential bone
Haversian system
Periosteal Surface
Haversian Canal
Volkmann’s Canal Volkmann’s Canal
Haversian Canal Bone Cells Osteoblasts secrete collagen and catalyze the crystallization of calcium on collagen fibers. Osteoblasts synthesize osteoid that mineralizes into bone.
Osteoblast
Osteoid
Bone Osteoid
Bone Osteocytes are osteoblasts that become trapped in bone matrix.
Osteoblast
Osteocyte Ground section of Haverisan System showing osteocytes and their filopodia
Haversian Canal
Filopodia Osteocyte Osteocytes communicate via gap junctions on filopodia.
Osteocyte Gap junction Bone
Filopodia
Canalicullus Osteoclasts Osteoclasts secrete acid on bone to dissolve calcium-phosphate crystals.
Cl-
CO2 H2O CA HCO3- CO2
ADP Pi H+ Cl- ATP
H+ H+ Cl- Cl- Osteoclasts secrete cathepsin K onto bone to digest collagen. Transcytosis delivers digested bone matrix components to basal side of osteoclasts. Histological image and electron micrograph showing an osteoclast
Osteoclast
Bone
Bone Dynamics of bone turnover Bone is a dynamic material that undergoes synthesis and resorption.
Synthesis Increase length or store Ca2+
Resorption Increase Ca2+ levels
Resorption Maintain integrity Synthesis Osteoblasts and osteoclasts reshape bone and replace old bone.
Reshape Bone modeling bone
Replace Bone remodeling bone
Osteoblast
Osteoclast Bone modeling Bone modeling shapes bone with osteoblasts and osteoclasts working on different surfaces. Trabecular bone aligns along lines of stress. Periosteal apposition and endocortical resorption increase diameter of bone.
Seeman (2008) J Bone Miner Metab 26 1-8 Osteocytes use a primary cilium to detect mechanical stress in bone.
Primary cilium
Fluid flow Ca2+ Bone remodeling Bone remodeling removes old bone and replaces it with new bone.
Compact bone Trabecular bone Remodeling repairs bone with osteoclasts and osteoblasts working on the same surface.
Activation/Resorption
Formation
Reversal Basic multicellular unit consists of osteoclasts, osteoblasts that remodel compact bone.
Osteoblast
Osteoclast
Kerr Atlas of Functional Histology 1st edition Remodeling erodes old Haversian Systems while forming new canals.
Resorption
Formation Development of osteoclasts Pre-osteoclasts are activated by M-CSF and RANK ligand on the surface of osteoblasts.
Preosteoclast Monocyte RANK receptor Osteoclast
M-CSF
RANK-L
Osteoblast Osteoprotegerin is a decoy receptor for RANKL that prevents activation of preosteoclasts.
Monocyte Preosteoclast Osteoclast
RANK receptor
M-CSF
OPG
Osteoblast Parathyroid hormone stimulates osteoblasts to increases RANKL and decrease OPGs.
RANK receptor
Ca2+
PTH RANK-L
Osteoblast Estrogen has direct and indirect effects on the development and lifespan of osteoclasts.
Estrogen Apoptosis
Estrogen
Osteoblast
OPG synthesis Bone development Intramembraneous ossification involves bone formation on mesenchymal tissue.
Bone
Mesenchyme
Osteoblast
Osteocyte Endochondrial ossification is bone formation on cartilage.
Cartilage
Chondrocyte
Cartilage
Growth Plate Bone Osteoid Bone Marrow The developmental sequence of chondrocytes drives bone formation in growth plates.
Resting Chondrocytes
Proliferating Chondrocytes
Hypertrophic Chondrocytes
Calcified Cartilage Take home points...
• The properties of connective tissue is determined by the and arrangement type of ECM components and the cellular composition.
• Bones contain of compact bone and trabecular bone.
• Osteoblasts synthesize bone and osteoclasts dissolve bone.
• Osteocytes are mechanical sensors that regulate bone synthesis and absorption.
• Bone modeling reshapes bone and bone remodeling replaces old bone.
• Increased activity and numbers of osteoclasts leads to osteoporosis.