1. 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Human Biology Sylvia S. Mader Michael Windelspecht Chapter 11 Skeletal System Lecture Outline Part 4

2. 2 How does endochondral ossification occur? 1.The cartilage model – chondrocytes lay down hyaline cartilage in the shape of the future bones 2.The bone collar – osteoblasts secrete bone matrix and results in a collar made of compact bone 3.The primary ossification center – osteoblasts are brought interiorly by blood and lay down spongy bone 11.5 Bone Growth and Homeostasis

3. 3 How does endochondral ossification occur? 4.The medullary cavity and secondary ossification sites – spongy bone of the diaphysis is absorbed by osteoclasts becoming the medullary cavity and secondary ossification centers form in the epiphyses after birth 5.The epiphyseal (growth) plate – a cartilage band that acts as a growth plate that allows bones to lengthen 11.5 Bone Growth and Homeostasis

4. 4 Visualizing endochondral ossification Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. A cartilaginous model develops and becomes calcified during fetal development. b. Newly derived osteoblasts cover the diaphysis with a compact-bone collar. c. Blood vessels bring osteoblasts into the cartilage. At a primary ossification center, osteoblasts form spongy bone. d. The medullary cavity forms, the compact bone collar thickens, and secondary ossification centers appear in the epiphyses. e. During childhood, growth is still possible as long as cartilage remains at the growth plates. secondary ossification centers articular cartilage medullary cavity epiphyseal growth plate secondary ossification centers epiphysis primary ossification center spongy bone blood vessel chondrocyte in lacuna compact- bone collar cartilage diaphysis epiphysis Figure 11.11 Bone growth by endochondral ossification. 11.5 Bone Growth and Homeostasis

5. 5 How do bones lengthen? Figure 11.12 Increasing bone length. 11.5 Bone Growth and Homeostasis epiphysis articular cartilage epiphyseal growth plate a. diaphysis b. epiphysis resting zone proliferating zone degenerating zone ossification zone Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

6. 6 How do hormones affect bone growth? Vitamin D is converted to a hormone that allows calcium absorption in the intestine. Growth hormone (GH) stimulates general bone growth and the epiphyseal plates Sex hormones increase growth during adolescence. 11.5 Bone Growth and Homeostasis

7. 7 What is bone remodeling and what is its role in homeostasis? The process of bone renewal, often called bone remodeling, recycles as much as 18% of bone each year. Remodeling allows bones to respond to stress. It regulates the calcium in the blood through hormones. –Parathyroid hormone (PTH) increases blood calcium by accelerating bone recycling. –Calcitonin decreases blood calcium. 11.5 Bone Growth and Homeostasis

8. 8 Bone remodeling Diameter increases as bone absorption inside the shaft is matched by bone formation outside the shaft. Figure 11.13 Bone remodeling. 11.5 Bone Growth and Homeostasis bone formation bone absorption Child Infant medullary cavity compact bone Adult Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

9. 9 Osteoporosis Osteoporosis is a condition where bones are weakened due to a decreased bone mass. Bone reabsorption exceeds absorption usually by age 40. Risk factors include: women, white or Asian, thin, family history, early menopause, smoking, diet low in calcium, excessive caffeine or alcohol consumption, and a sedentary lifestyle. 11.5 Bone Growth and Homeostasis

10. 10 Osteoporosis Osteoporosis can lead to fractures and other complications. It can be treated with drugs, hormones, and lifestyle change. 11.5 Bone Growth and Homeostasis

11. 11 Steps in bone repair Hematoma (6-8 hours) – blood clot formed between broken bones Fibrocartilaginous callus (3 weeks) – cartilaginous callus forms between broken bones Bony callus (3-4 months) – cartilaginous callus is turned to bone Remodeling – old bone tissue is replaced by new bone tissue 11.5 Bone Growth and Homeostasis

12. 12 Bone repair Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. periosteumhematoma compact bone fibrocartilaginous callus 1. Hematoma 3. Bony callus4. Remodeling healed fracture spongy bone 2. Fibrocartilaginous callus medullary cavity a.b. bony callus b: © Tony Freeman/PhotoEdit Figure 11.14 Bone repair following a fracture. 11.5 Bone Growth and Homeostasis