Bones and Skeletal Tissues

1. 6 Bones andSkeletal Tissues

2. Bone Development • Ossification (osteogenesis)—bone-tissue formation • Membrane bones—formed directly from mesenchyme • Intramembranous ossification • Other bones—develop initially from hyaline cartilage • Endochondral ossification

3. Intramembranous Ossification Mesenchymal cell Osteoblast Osteoid Collagen fiber Osteocyte Ossification center Newly calcified bone matrix Osteoid Osteoblast 2 1 Bone matrix (osteoid) is secreted within the fibrous membrane and calcifies. • Osteoblasts begin to secrete osteoid, which is calcified within a few days. • Trapped osteoblasts become osteocytes. Ossification centers appear in the fibrous connective tissue membrane. • Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center. Figure 6.10, steps 1–2

4. Intramembranous Ossification Fibrous periosteum Mesenchyme condensing to form the periosteum Osteoblast Plate of compact bone Trabeculae of woven bone Diploë (spongy bone) cavities contain red marrow Blood vessel 4 Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears. • Trabeculae just deep to the periosteum thicken and are later replaced with mature lamellar bone, forming compact bone plates. • Spongy bone (diploë), consisting of distinct trabeculae, persists internally, and its vascular tissue becomes red marrow. 3 Woven bone and periosteum form. • Accumulating osteoid is laid down between embryonic blood vessels in a random manner. The result is a network (instead of lamellae) of trabeculae called woven bone. • Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum. Figure 6.10, steps 3–4

5. Endochondral Ossification • All bones except some bones of the skull and clavicles • Bones are modeled in hyaline cartilage • Begins forming late in the second month of embryonic development • Continues forming until early adulthood

6. Stages in Endochondral Ossification Month 3 Week 9 Birth Childhood to adolescence Articular cartilage Secondary ossification center Spongy bone Epiphyseal blood vessel Area of deteriorating cartilage matrix Epiphyseal plate cartilage Hyaline cartilage Medullary cavity Spongy bone formation Bone collar Blood vessel of periosteal bud Primary ossification center 4 3 5 1 2 Bone collar forms around hyaline cartilage model. Cartilage in the center of the diaphysis calcifies and then develops cavities. The periosteal bud invades the internal cavities, and spongy bone begins to form. The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5. The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. Figure 6.11

7. Anatomy of Epiphyseal Growth Areas • In epiphyseal plates of growing bones: • Cartilage is organized for quick, efficient growth • Cartilage cells form tall stacks • Chondroblasts at the top of stacks divide quickly • Pushes the epiphysis away from the diaphysis • Lengthens entire long bone

8. Anatomy of Epiphyseal Growth Areas • Older chondrocytes signal surrounding matrix to calcify • Older chondrocytes then die and disintegrate • Leaves long trabeculae (spicules) of calcified cartilage on diaphysis side • Trabeculae are partly eroded by osteoclasts • Osteoblasts then cover trabeculae with bone tissue • Trabeculae finally eaten away from their tips by osteoclasts

9. Organization of Cartilage within Epiphyseal Plate of Growing Long Bone Resting zone Proliferation zone Cartilage cells undergo mitosis. 1 Hypertrophic zone Older cartilage cells enlarge. 2 Calcification zone Matrix becomes calcified; cartilage cells die; matrix begins deteriorating. 3 Calcified cartilage spicule Osseous tissue Ossification zone New bone formation is occurring. 4 Figure 6.12

10. Postnatal Growth of Endochondral Bones • During childhood and adolescence: • Bones lengthen entirely by growth of the epiphyseal plates • Cartilage is replaced with bone CT as quickly as it grows • Epiphyseal plate maintains constant thickness • Whole bone lengthens

11. Hormonal Regulation of Bone Growth • Growth hormone—produced by the pituitary gland • Stimulates epiphyseal plates • Thyroid hormone—ensures that the skeleton retains proper proportions • Sex hormones (estrogen and testosterone) • Promote bone growth • Later induces closure of epiphyseal plates

12. Postnatal Growth of Endochondral Bones • As adolescence draws to an end: • Chondroblasts divide less often • Epiphyseal plates become thinner • Cartilage stops growing • Replaced by bone tissue • Long bones stop lengthening when diaphysis and epiphysis fuse

13. Bone Remodeling • Bone is dynamic living tissue • 500 mg of calcium may enter or leave the adult skeleton each day • Cancellous bone of the skeleton is replaced every3–4 years • Compact bone is replaced every 10 years

14. Postnatal Growth of Endochondral Bones • Growing bones widen as they lengthen • Osteoblasts—add bone tissue to the external surface of the diaphysis • Osteoclasts—remove bone from the internal surface of the diaphysis • Appositional growth—growth of a bone by addition of bone tissue to its surface

15. Bone Remodeling • Bone deposit and removal • Occurs at periosteal and endosteal surfaces • Bone remodeling • Bone deposition—accomplished by osteoblasts • Bone reabsorption—accomplished by osteoclasts

16. Remodeling, Spongy Bone Spongy bone Compact bone Osteoclast Osteoblast Osteoblast New bone Trabeculae of spongy bone Resorption of bone matrix by osteoclasts Deposition of new bone by osteoblasts Figure 6.13

17. Osteoclast—A Bone-Degrading Cell • A giant cell with many nuclei • Crawls along bone surfaces • Breaks down bone tissue • Secretes concentrated HCl • Lysosomal enzymes are released • Derived from hematopoietic stem cells Bone matrix Osteocyte within a lacuna Ruffled border of osteoclast Nuclei Figure 6.14

18. Repair of Bone Fractures • Simple and compound fractures • Treatment by reduction • Closed reduction • Open reduction

19. Stages of Healing a Fracture External callus Hematoma Bony callus of spongy bone New blood vessels Internal callus (fibrous tissue and cartilage) Healed fracture Spongy bone trabecula A hematoma forms. Fibrocartilaginous callus forms. 1 2 3 4 Bony callus forms. Bone remodeling occurs. Figure 6.15

20. Common Types of Fractures Table 6.2 (1 of 3)

21. Common Types of Fractures Table 6.2 (2 of 3)

22. Common Types of Fractures Table 6.2 (3 of 3)

23. Disorders of Bones • Osteoporosis • Characterized by low bone mass • Bone reabsorption outpaces bone deposition • Occurs most often in women after menopause

24. Osteoporosis Figure 6.16

25. Disorders of Bones • Osteomalacia • Occurs in adults—bones are inadequately mineralized • Rickets • Occurs in children—analogous to osteomalacia

26. Disorders of Bones • Paget’s disease • Characterized by excessive rate of bone deposition • Osteosarcoma • A form of bone cancer

27. The Skeleton Throughout Life • Cartilage grows quickly in youth • Skeleton shows fewer chondrocytes in the elderly • Bones are a timetable • Mesoderm • Gives rise to embryonic mesenchyme cells • Mesenchyme • Produces membranes and cartilage • Membranes and cartilage ossify

28. The Skeleton Throughout Life • Skeleton grows until the age of 18–21 years • In children and adolescents, bone formation exceeds rate of bone reabsorption • In young adults, bone formation and bone reabsorption are in balance • In old age, reabsorption predominates • Bone mass declines with age