Bone formation via Endochondral ossification
1. Most bones
2. Mesenchymal cells differentiate into chondroblasts and produce hyaline
a. Interstitial and appositional growth
3. Perichondrium – chondrogenic cells; C.T. surrounds hyaline
a. Perichondrium becomes periosteum (osteoblasts) due to increased blood supply
5 Steps of Endochondral Bone Development
8 weeks to birth
Step 1 – Bone collar forms around hyaline shaft (produced by osteogenic layer of periosteum)
Step 2 -chondrocytes hypertrophy and produce alkaline phosphatase
which causes cartilage calcification
-calcified hyaline matrix inhibits diffusion; chondrocytes die
and matrix breaks down
-cavity is formed when neighboring lacunae walls
break down
Step 3 -**Primary Ossification Center
-blood vessels enter bony collar and vascularize cavity
-periosteal bud – vessels, osteoprogenitor cells enter diaphysis
-osteoblasts lay down osteoid and form trabeculae
(spongy bone)
Around Birth
-Step 4 -osteoclasts continue to form medullary cavity
-ossification continues toward epiphyses
–Step 5
-Secondary Ossification Center
-capillaries and osteoblasts enter epiphyses
-Spongy bone formed; no cavity
-Hyaline remains as: Articular cartilage and Epiphyseal plates
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Examples of Calcium regulation via endocrine system
*2 Hormones involved
1. if Low [Ca2+] in blood stimulates the parathyroid glands to release
Parathyroid Hormone (PTH) which
• stimulates osteoclasts
• enhances action of calcitriol
• decrease Ca2+ excretion at kidney
RESULTS in increase in blood [Ca2+]
2. if High [Ca2+] in blood stimulates the thyroid gland to release calcitonin which
• inhibits osteoclasts
• increase rate of Ca2+ excretion at kidney
• decrease action of calcitriol
RESULTS in decrease in blood [Ca2+]
