Bone & Cartilage 2016 PDF

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Summary

Summary of Bone Cartilage Page 1 of 6 1. Understand the difference between interstitial and appositional cartilage growth. 2. Be able to compare and contrast the two processes through which bone can be formed intramembranous and endochondral ossification. 3. Describe the morphological steps of long ...

Description

Summary of Bone & Cartilage Page 1 of 6

1. Understand the difference between interstitial and appositional cartilage growth. 2. Be able to compare and contrast the two processes through which bone can be formed -intramembranous and endochondral ossification. 3. Describe the morphological steps of long bone development from initial formation of cartilage template to closure of the growth plate. 4. Be able to describe the steps that occur in secondary bone fracture healing. 5.

Understand why bone remodeling occurs and be able to describe the five steps that comprise the remodeling cycle.

Summary of Bone & Cartilage Page 2 of 6

Chondrogenesis -chondroblasts differentiate from mesenchyme -secrete matrix -once surrounded by matrix, are called chondrocytes -chondrocytes can undergo mitotic division (form isogenous groups) -matrix is very compliant and can be easily deformed -interstitial growth -production of matrix by internal chondrocytes -growth from “within” -lengthens bones -appositional growth -differentiation of cells from the perichondrium -forms new matrix at surface

Summary of Bone & Cartilage Page 3 of 6

Intramembranous bone formation -from within mesenchyme -osteoblasts differentiate from mesenchymal cells -flat bone development

Endochondral bone formation -Hyaline cartilage template -Two simultaneous events occur early on: -Bone collar (thin cortical bone region) forms by appositional bone formation -Chondrocytes within the region near the bone collar undergo hypertrophy and the cartilage matrix calcifies -Osteogenic bud penetrates bone collar; osteoclast-like cells remove the calcified cartilage and cells/vessels begin to populate the marrow cavity (called a primary ossification center).

Summary of Bone & Cartilage Page 4 of 6

-Epiphyseal growth plate forms at both ends of long bone. The growth plate has five histologically distinct regions (zones). -Resting zone – ‘non-active’ region of chondrocytes (hyaline cartilage) -Proliferative zone – dividing chondrocytes; aligned in vertical orientation -Hypertrophic zone – increased cell size; condensed matrix -Calcified cartilage zone – loss of chondrocytes; mineralization of cartilage -Ossification zone – bone is formed on calcified cartilage

-Secondary ossification center forms; epiphyseal plate separates -Closure of growth plate signifies end of long bone growth

Summary of Bone & Cartilage Page 5 of 6

Fracture healing Primary bone healing – when fracture is stabilized (via surgical plates or screws) healing can occur without any cartilage; bone remodeling occurs across the fracture site Secondary bone healing – when fracture is either not stabilized or minimally stabilized (via cast or pin) healing occurs with a combination of bone and cartilage. Steps in secondary bone healing 1) Hematoma forms -together with the periosteum the hematoma provides the cellular components for healing 2) Soft callus forms & vasculature regenerates -chondrocytes recapitulate the maturational process that occurs at growth plate -regeneration of vasculature is essential for healing 3) Hard (bony) callus forms - woven bone 4) Callus remodeling - lamellar bone

Bone remodeling

Summary of Bone & Cartilage Page 6 of 6

Mechanism for renewal of bone tissue A multi-step, localized process that involves coordinated action of osteoblasts and osteoclasts. -Activation – signals for osteoclasts due to osteocytes apoptosis, microdamage formation -Resorption – removal of bone by osteoclasts -Reversal – osteoclasts cease resorption; osteoblasts begin to arrive onsite -Formation – production of new matrix by osteoblasts -Quiescence – matrix formation stops and bone surface becomes covered by lining cells. The entire process takes ~ 6 months.

An increase in the rate of bone remodeling combined with a failure to form as much bone as is resorbed at each remodeling site is the underlying basis for bone loss in osteoporosis. Bone remodeling is affected by factors such as age, level of sex steroids, nutrition, physical activity, diseases, and medications. A patient’s bone ‘health’ is often determined by biomarkers (serum/urine) or bone density scans. Pharmaceutical agents used to improve bone health act by either slowing osteoclast formation/function (anti-resorptive) or by enhancing osteoblast function (anabolic): Anti-resorptive agents Bisphosphonates Estrogen / selective estrogen receptor modulators (SERMS) Calcitonin Denosumab: monoclonal antibody that targets RANK-L Anabolic agents Parathyroid hormone...