The Skeletal System

1. Chapter 7 The Skeletal System Anatomy & Physiology Ivyanatomy.com

2. Overview Osteology is the science of bones Bones are a living dynamic tissue: constantly remodeled through life • Functions of Bone: • Protection – brain is protected by the skull & heart and lungs by the thoracic cage • Support & Movement – muscles attach to the skeleton • Calcium and Phosphate storage • Blood Cell Production – blood cells are produced by red bone marrow

3. Bone Marrow • Red Bone Marrow – site of blood cell production • Locations of Red Bone Marrow: • Infant – all bones are filled with red bone marrow • Adult – red bone marrow is limited to • flat bones (sternum, ribs, skull, hips) and • irregular bones (vertebrae) Yellow Bone Marrow – Adipose tissue

4. Bone is Connective Tissue: Cells & Extracellular Matrix • The cells associated with bones include: • Osteocytes = cells that maintain bone. • Osteocytes reside within chambers, called lacunae • Osteoblasts = cells that deposit new bone. Once mature, osteoblasts become osteocytes. • Osteoclasts= cells that dissolve bone. • Osteoclasts originate from white blood cells • and they secrete an acid that dissolves the inorganic salts of bone.

5. Bone is Connective Tissue: Cells & Extracellular Matrix • The extracellular matrix of bones is composed of • hydroxyapatite– a calcium phosphate salt that provides the hardness of bones • collagen fibers – provides bone with some pliability

7. Bone Classification by Shape • Long bones = elongated diaphysis • Humerus radius ulna metacarpals phalanges • Femur tibia fibula metatarsals phalanges • Short Bones = cube-shaped • carpals • tarsals • Flat Bones = plate-like • Sternum, ribs, scapula • parietal and frontal bones

8. Bone Classification by Shape • Irregular bones = variety of shapes • vertebrae • mandible • maxilla • ethmoidbone • sphenoid bone • sesamoid (or round) bone = develops within tendons • patella

9. Parts of a long bone • Diaphysis = shaft of long bone • Lined with compact bone • Epiphysis = expanded end of bone • Filled with spongy (cancellous) bone • Proximal epiphysis & distal epiphysis • Sites of articulation (joint) • Articular cartilage = covers epiphyses • Composed of Hyaline cartilage Epiphyseal line = remnant of bone growth (epiphyseal plate)

10. Parts of a long bone • Medullary Cavity = chamber within diaphysis • Filled with bone marrow, blood vessels and nerves • Yellow Bone Marrow in adults • Red Bone Marrow in children • Endosteum= Membrane that lines medullary cavity • Contains dense connective tissue • Blood vessels and nerves • osteoblasts • Periosteum = Tough membrane covering the bone • Composed of Dense Connective Tissue • Blood Vessels, Nerves, Osteoblasts • Continuous with tendons and ligaments

11. Parts of a long bone • Compact bone • Lines the Diaphysis and a thin layer surrounds the epiphyses • Composed of osteons • Spongy bone • Fills the epiphyses and a thin layer lines the medullary cavity • Trabiculae= thin bony plates • Osteocytes lie within trabiculae

12. Compact Bone Osteon = Structural & functional unit of compact bone • Lamella= concentric rings of bone around a central canal • Central Canal (Haversian Canal) • Contains blood supply and nerve • Aligned parallel to diaphysis • Lacunae = bony chamber that contains an osteocyte • Osteocyte = maintains the bone • Canaliculi = canals connecting osteocytes to the central canal • Canaliculi are filled with cellular processes • Pathway for nutrient and waste diffusion

13. Osteon continued • Perforating Canal(Volkmann’s Canal) • conveys blood from periosteum towards the central canal • Aligned perpendicular to diaphysis

14. Compact bone is composed of osteons cemented together by bone matrix.

15. Bone Development and Growth • Parts of the skeletal system begin to develop during the first • few weeks of prenatal development • Bone formation = ossification • Bones replace existing connective tissue in one of two ways: • As intramembranous bones • As endchondral bones

16. Intramembranous Ossification • Intramembranous Ossification • Forms broad, flat bones of the skull • Formed by replacing layers of embryonic connective tissue (mesenchyme) with bone • Osteoblasts within mesenchyme • deposit bony matrix in all directions • Osteoblasts become osteocytes • once surrounded by bone

17. Intramembranous Ossification Intramembranous ossification follows four steps. Mesenchymal cells group into clusters, and ossification centers form. Secreted osteoid traps osteoblasts, which then become osteocytes. Trabecular matrix and periosteum form. Compact bone develops superficial to the trabecular bone, and crowded blood vessels condense into red marrow.

18. Endochondral Bones • Endochondral Bones • Most of the bones in the skeleton are endochondral • Bone formation begins with a hyaline cartilage model • Cartilage decomposes and is replaced by bone.

19. Endochondral Ossification Hyaline cartilage forms model of future bone Cartilage degenerates and periosteum surrounds bone Osteoblasts from periosteum invade the degenerating tissue Osteoblasts beneath periosteum form compact bone at diaphysis = primary ossification center Later, Osteoblasts form spongy bone at epiphyses = secondary ossification center

20. Endochondral Ossification Intramembranous ossification follows four steps. Hyaline cartilage forms model of future bone Cartilage degenerates and periosteum forms around bone Primary ossification center forms compact bone within the diaphysis (e) Secondary ossification centers form spongy bone within the epiphyses Some cartilage remains at the epiphyseal plates & as articular cartilage at the end of bones

21. Endochondral Ossification • Two areas of endochondral bone retain cartilage after ossification. • Articular cartilage • surrounds the epiphyses for joints • Epiphyseal plates • retain cartilage for bone growth

22. Growth at the Epiphyseal Plate • Epiphyseal Plate • Band of hyaline cartilage that remains • between the two ossification centers • Bone growth continues at epiphyseal • plates until adulthood. • New cartilage is added towards the epiphysis • and cartilage is ossified towards diaphysis • Once the epiphyseal plates ossify the • bones can no longer be lengthened

23. 4 Layers (zones) of growth at epiphyseal Plate • Zone of resting cartilage (reserve zone) • Cartilage cells near epiphysis • Do not participate in bone growth • Anchor epiphyseal plate to epiphysis • Zone of proliferating cartilage • Young chondrocytes undergoing mitosis • Adds new cartilage to plate

24. 4 Layers (zones) of growth at epiphyseal Plate • Zone of hypertrophic cartilage • Older cells enlarge and thicken the epiphyseal plate • Osteoblasts invade and calcify the cartilaginous matrix. • Zone of calcified cartilage • Dead cells & calcium matrix • Ossified bone (zone of ossification) • Osteoclasts dissolve and phagocytize the matrix • Osteoblasts invade the region and deposit new bone. (b)

26. Bone Remodeling Calcium is constantly exchanged between the blood and bone. • Bone resorption = Osteoclasts breakdown bone releasing calcium into the blood. • Bone resorption occurs when blood [Ca2+] is low and it’s stimulated by parathyroid hormone (PTH). • Bone deposition = Osteoblasts deposit new bone from calcium in the blood stream. • Bone deposition occurs when blood [Ca2+] is high and it’s stimulated by the hormone calcitonin.

27. Nutrients that effect bone homeostasis • Vitamin D – promotes Ca2+ absorption in small intestine • Vitamin D deficiency = softened and deformed bones • Osteomalacia in adults • Rickets in children • Vitamin A – balances bone resorption and deposition • Vitamin A deficiency = retards bone development • Vitamin C – is required for collagen synthesis. • Vitamin C deficiency = results in fragile bones

28. Hormones that affect bone homeostasis • Growth Hormone (GH) • Secreted from pituitary gland • Promotes bone growth at epiphyseal plates Pituitary Gigantism over secretion of GH during childhood Pituitary Dwarfism insufficient GH during childhood • Acromegaly • Over secretion of GH as an adult • Occurs after epiphyseal plates have sealed • Enlargement of hands, feet, nose

29. Hormones that affect bone homeostasis • Calcitonin • Secreted from thyroid gland • Promotes bone deposition • Stimulates Osteoblast activity • Inhibits Osteoclast activity Thyroid Gland • Parathyroid Hormone (PTH) • Secreted from parathyroid glands • Promotes bone resorption • Stimulates Osteoclast activity • Inhibits Osteoblast activity

30. Thyroid Gland secretes Calcitonin Increased blood calcium detected by cells in thyroid gland Osteoblasts deposit calcium into bones Blood calcium level increases Blood Calcium Normal Range Blood calcium returns to normal Blood calcium level decreases Osteoclasts resorb bone to release Calcium into the blood Decreased blood calcium detected by cells in parathyroid gland Parathyroid glands secrete PTH

31. Hormones that affect bone homeostasis • Sex Hormones (testosterone & estrogen) • Promotes long bone growth at puberty • Sex hormones also stimulate ossification at epiphyseal plates* • *Estrogen has a stronger effect than testosterone on bone ossification, which is why ossification of the epiphyseal plates occurs earlier in development in females than in males.

32. Impacts of Exercise and Sunlight on Bone Homeostasis Exercise – especially resistance or strength exercise strengthens bones. Muscles pull on bones, and bones thicken in response. Sunlight – UV rays promote the release of Vitamin D from skin. Circulating vitamin D is activated in the liver, then it promotes the absorption of Calcium from the intestines.

33. Bone Fractures Greenstick fracture Fissured fracture Longitudinal break on the bone. One side of the bone bends, the other side breaks. Most common in children, because their bones are more pliable than adults.

34. Bone Fractures Transverse fracture Oblique fracture Complete break perpendicular to the diaphysis Complete break at any other angle not perpendicular to diaphysis

35. Bone Fractures Comminuted fracture Spiral fracture Results in several bony fragments Results from twisting the bone

36. Repair of a fracture Hematoma Formation Blood vessels rupture causing severe bleeding. The blood clots, forming a hematoma (c) Bony Callus Formation Osteoblasts replace the cartilaginous callus with a bony callus (b) Cartilaginous Callus Formation Fibroblasts deposit a mass of fibrocartilage (d) Bone Remodeling Osteoclasts and macrophages remove excess bone and debris

37. Disorders of Bone Over time, osteoclasts outnumber osteoblasts, and more bone is resorbed than can be deposited. Bone mass decreases as a result. Osteoporosis Bone Healthy Bone • Osteopenia “low bone mass” • Progresses towards osteoporosis • Osteoporosis “porous bone” • Bones develop spaces and canals • Bones are fragile and easily broken • Common in menopausal women • (from the low estrogen levels)

38. Ways to delay or prevent osteoporosis: • Exercise daily. • Consume enough calcium and vitamin D every day. • Do not smoke.

39. Attribution • Skeleton illustration Pixabay CC0 Public Domain https://pixabay.com/p-30160/?no_redirect • Classification of bone by shape By BruceBlaus (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/7/77/Blausen_0229_ClassificationofBones.png • Parts of a long bone By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/2/23/603_Anatomy_of_Long_Bone.jpg • Diagram of Compact Bone By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/5/58/624_Diagram_of_Compact_Bone-new.jpg • Diagram of Osteon By BDB [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/7/75/Transverse_Section_Of_Bone.png • Intramembranous ossification By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/a/a9/611_Intramembraneous_Ossification.jpg • Endochondral Ossification By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/9/97/608_Endochrondal_Ossification.jpg • Epiphyseal Plate By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/1/15/622_Longitudinal_Bone_Growth.jpg • Bone Remodeling By Cancer Research UK (Original email from CRUK) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/d/dd/Diagram_showing_bone_remodelling_Fig_CRUK_112.svg • Types of Fractures By OpenStax College [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/3/35/612_Types_of_Fractures.jpg • Repair of Bone Fracture By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/1/12/613_Stages_of_Fracture_Repair.jpg • Normal and Degraded Bone By Gtirouflet (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/8/8e/Bone_normal_and_degraded_micro_structure.jpg