1. Stages of Intramembranous Ossification 1. An ossification center appears in the fibrous membrane 2. Bone matrix is secreted within the fibrous membrane 3. Spongy bone and periosteum form 4. Bone collar of compact bone forms 5. Red marrow appears

2. Stages of Intramembranous Ossification Figure 6.7.1

3. Stages of Intramembranous Ossification Figure 6.7.2

4. Stages of Intramembranous Ossification Figure 6.7.3

5. Stages of Intramembranous Ossification Figure 6.7.4

6. Endochondral Ossification Begins in the second month of development Uses hyaline cartilage “bones” as patterns for bone construction Requires breakdown of hyaline cartilage prior to ossification

7. Stages of Endochondral Ossification 1. Formation of bone collar 2. Cavitation of the hyaline cartilage 3. Invasion of internal cavities by the periosteal bud, and spongy bone formation 4. Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses 5. Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates

8. Stages of Endochondral Ossification Figure 6.8 Formation of bone collar around hyaline cartilage model. Hyaline cartilage Cavitation of the hyaline carti- lage within the cartilage model. Invasion of internal cavities by the periosteal bud and spongy bone formation. Formation of the medullary cavity as ossification continues; appearance of sec- ondary ossification centers in the epiphy- ses in preparation for stage 5. Ossification of the epiphyses; when completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. Deteriorating cartilage matrix Epiphyseal blood vessel Spongy bone formation Epiphyseal plate cartilage Secondary ossificaton center Blood vessel of periosteal bud Medullary cavity Articular cartilage Spongy bone Primary ossification center Bone collar 1 2 3 4 5

9. Postnatal Bone Growth Growth in length of long bones Cartilage closest to the epiphysis is inactive Cartilage closest to diaphysis can grow Cells of the epiphyseal plate form three zones: 1. Growth zone 2. Transformation zone 3. Osteogenic zone

10. Functional Zones in Long Bone Growth 1. Growth zone – cartilage cells undergo mitosis 2. Transformation zone – has 2 parts Hypertrophic zone - older cells enlarge Calcification zone – cells die; the matrix becomes calcified and deteriorates 3. Osteogenic zone – new bone formation occurs

11. Growth in Length of Long Bone Figure 6.9

12. Long Bone Growth and Remodeling Growth in length – cartilage continually grows and is replaced by bone Remodeling – bone is resorbed and added by appositional growth

13. Long Bone Growth and Remodeling Figure 6.10

14. Hormonal Regulation of Bone Growth During Youth During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone During puberty, testosterone and estrogens: Promote adolescent growth spurts Cause masculinization and feminization Later induce epiphyseal plate closure

15. Bone Remodeling Remodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces For bone deposit to occur, many things are required (e.g. calcium, phosphorous, vitamin D, and protein)

16. Bone Resorption Accomplished by osteoclasts Resorption bays – grooves formed by osteoclasts Resorption involves digestion of matrix by osteoclasts Dissolved matrix is transcytosed across the osteoclast and into the bloodstream

17. Importance of Ionic Calcium in the Body Calcium is necessary for: Transmission of nerve impulses Muscle contraction Blood coagulation Secretion by glands and nerve cells Cell division

18. Control of Remodeling Two control loops regulate bone remodeling Hormonal mechanism Mechanical and gravitational forces

19. Hormonal Mechanism Rising blood Ca 2+ levels trigger the thyroid to release calcitonin Calcitonin stimulates calcium salt deposit in bone Falling blood Ca 2+ levels signal the parathyroid glands to release PTH PTH signals osteoclasts to degrade bone matrix and release Ca 2+ into the blood

20. Hormonal Control of Blood Ca 2+ Figure 6.11 Calcitonin secreted Calcitonin stimulates calcium salt deposit in bone Parathyroid glands release parathyroid hormone (PTH) Thyroid gland Thyroid gland Parathyroid glands Osteoclasts degrade bone matrix and release Ca 2+ into blood Falling blood Ca 2+ levels Rising blood Ca 2+ levels Calcium homeostasis of blood: 9–11 mg/100 ml PTH Imbalance

21. Response to Mechanical Stress Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it Observations supporting Wolff’s law include Long bones are thickest midway along the shaft (where bending stress is greatest) Curved bones are thickest where they are most likely to buckle

22. Response to Mechanical Stress Trabeculae form along lines of stress Large, bony projections occur where heavy, active muscles attach

23. Response to Mechanical Stress Figure 6.12

24. Types of Bone Fractures – Bone Position Nondisplaced – bone ends retain their normal position Displaced – bone ends are out of normal alignment

25. Types of Bone Fractures – Completeness of Break Complete – bone is broken all the way through Incomplete – bone is not broken all the way through

26. Types of Bone Fractures – Orientation of Break to Long Axis Linear – the fracture is parallel to the long axis of the bone Transverse – the fracture is perpendicular to the long axis of the bone

27. Types of Bone Fractures – Penetration of Skin Compound (open) – bone ends penetrate the skin Simple (closed) – bone ends do not penetrate the skin

28. Common Types of Fractures Comminuted – bone fragments into three or more pieces; common in the elderly Spiral – ragged break when bone is excessively twisted; common sports injury Depressed – broken bone portion pressed inward; typical skull fracture

29. Common Types of Fractures Compression – bone is crushed; common in porous bones Epiphyseal – epiphysis separates from diaphysis along epiphyseal line Greenstick – incomplete fracture where one side of the bone breaks and the other side bends; common in children

30. Common Types of Fractures Table 6.2.1

31. Common Types of Fractures Table 6.2.2

32. Common Types of Fractures Table 6.2.3

33. Stages in the Healing of a Bone Fracture Hematoma formation Torn blood vessels hemorrhage A hematoma forms at the fracture site Site becomes swollen, painful, and inflamed Figure 6.13.1

34. Stages in the Healing of a Bone Fracture Granulation tissue (soft callus) forms a few days after the fracture Fibrocartilaginous callus forms Capillaries grow into granulation tissue and phagocytic cells begin cleaning debris Figure 6.13.2

35. Stages in the Healing of a Bone Fracture The fibrocartilaginous callus forms when: Osteoblasts and fibroblasts begin reconstructing the bone Fibroblasts secrete collagen fibers that connect broken bone ends Osteoblasts begin forming spongy bone

36. Stages in the Healing of a Bone Fracture Bony callus formation Bone trabeculae form Fibrocartilaginous callus converts into a bony (hard) callus Bone callus begins 3-4 weeks after injury Bones are rejoined 2-3 months later Figure 6.13.3

37. Stages in the Healing of a Bone Fracture Bone remodeling Excess material on the bone shaft exterior and in the medullary canal is removed Compact bone is laid down to reconstruct shaft walls Figure 6.13.4

38. Homeostatic Imbalances Osteomalacia Bones are inadequately mineralized causing softened, weakened bones Main symptom is pain when weight is put on the affected bone Caused by insufficient calcium in the diet, or by vitamin D deficiency

39. Homeostatic Imbalances Rickets Childhood form of osteomalacia Bowed legs and deformities of the pelvis, skull, and rib cage are common

40. Homeostatic Imbalances Osteoporosis Group of diseases in which bone reabsorption outpaces bone deposit Spongy bone of the spine is most vulnerable Occurs most often in postmenopausal women Bones become very fragile

41. Paget’s Disease Characterized by excessive bone formation and breakdown Much more spongy than compact bone is formed This causes weakening of bone in certain spots Osteoclast activity wanes, but osteoblast activity continues to work Deposits are called Pagetic bone Usually localized in the spine, pelvis, femur, and skull

42. Fetal Primary Ossification Centers Figure 6.15