Bones and Skeletal Tissues

Bones and Skeletal Tissues Histology of cartilage and bone Function of the skeletal system Bone terminology Bone development and growth

Cartilage: Hyaline cartilage (glassy) Resilient tissue—it springs back to original shape Hyaline cartilage (glassy) Most abundant Support through flexibility Elastic cartilage—contains many elastic fibers Tolerate repeated bending Fibrocartilage—resists compression and tension Intermediate between hyaline and elastic

Cartilage: Connective tissue Chondroblasts lay down matrix, becoming chondrocytes when surrounded by EC matrix Chondrocytes do not mitose resulting in poor healing On aging, cartilage becomes calcified and less flexible

Tissues in Bone Dominated by bone CT Bones contain several types of tissues Dominated by bone CT Contain nervous tissue and blood CT Contain cartilage in articular cartilages Contain ET lining blood vessels

Function of Bones Support Movement Protection Mineral storage Blood-cell formation

Bone Tissue Bone tissue Organic components: Inorganic components: cells & fibers Inorganic components: mineral salts that invade bony matrix

Extracellular Matrix Gives bone exceptional properties: 35%—organic components Contributes to flexibility and tensile strength 65%—inorganic components Provide exceptional harness, resists compression

Three Types of Cells Osteogenic cells: stem cells that differentiate into osteoblasts Osteoblasts: actively produce and secrete bone matrix Bone matrix is osteoid Osteocytes: keep bone matrix healthy Osteoclasts Responsible for resorption of bone Secrete hydrochloric acid and lysosomal enzymes

Classification of Bones Long bones—longer than wide; a shaft plus ends Short bones—roughly cube-shaped Flat bones—thin and flattened, usually curved Irregular bones—various shapes, do not fit into other categories

Classification of Bones

Compact bone—dense outer layer of bone Gross Anatomy of Bones Compact bone—dense outer layer of bone Spongy (cancellous) bone—internal network of bone

Structure of Short, Irregular, and Flat Bones Flat bones, short bones, and irregular bones Contain bone marrow but no marrow cavity Diploë Internal spongy bone of flat bones Spongy bone (diploë) Compact bone Trabeculae igure 6.5

Structure of a Typical Long Bone Diaphysis—“shaft” of a bone Epiphysis—ends of a bone Blood vessels—well vascularized Medullary cavity—hollow cavity filled with yellow marrow Membranes Periosteum, perforating fibers, and endosteum

Structure of a Long Bone Articular cartilage Compact bone Proximal epiphysis Spongy bone Endosteum Epiphyseal line Endosteum Periosteum Compact bone Medullary cavity (lined by endosteum) (b) Diaphysis Yellow bone marrow Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries Distal epiphysis (a) (c) Figure 6.4a–c

1. Compact Bone Osteoclasts break down bone Osteon = functional unit of bone Osteoblasts lay down the matrix in layers (lamellae) Become osteocytes when surrounded by EC matrix Lacunae Osteoclasts break down bone

Microscopic Structure of Compact Bones Contains passage ways for blood vessels, lymph vessels, and nerves Osteons—long cylindrical structures Function in support Structurally—resembles rings of a tree in cross-section

Microscopic Structure of Compact Bones Spongy bone Perforating (Volkmann’s) canal Central (Haversian) canal Endosteum lining bony canals and covering trabeculae Osteon (Haversian system) Circumferential lamellae (a) Perforating (Sharpey’s) fibers Periosteal blood vessel Lamellae Periosteum () Nerve Vein Artery Canaliculi Osteocyte in a lacuna Lacunae Lamellae Central canal () Interstitial lamellae Lacunae Lamellae Central Canal Lacuna (with osteocyte)

2. Spongy Bone Lacy network of struts called trabeculae reinforce the bone Covered by endosteum

Microscopic Structure of Spongy Bones Is less complex than compact bone Trabeculae contain layers of lamellae and osteocytes Are too small to contain osteons

Microscopic Structure of Spongy Bones Marrow space Trabecula Osteocytes Endosteum (a) Osteoblasts (b) Figure 6.9

3. Marrow Red Marrow Yellow Marrow Active Blood Precursors Inactive Mostly fat

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

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

Bone Remodeling Bone is dynamic living tissue 500 mg of calcium may enter or leave the adult skeleton each day Spongy bone of the skeleton is replaced every 3–4 years Compact bone is replaced every 10 years

Osteoclast—A Bone-Degrading Cell Crawls along bone surfaces Breaks down bone tissue Secretes concentrated HCl Lysosomal enzymes released Derived from hematopoietic stem cells

Fractures Young vs. old patients Simple vs. open (compound) fractures Healing: Reduction and stabilization Formation of hematoma Callus— fibrocartilagenous and bony Remodeling—months later

Common Types of Fractures Table 6.2 (1 of 3)

Common Types of Fractures

Common Types of Fractures Table 6.2 (3 of 3)

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

Osteoporosis Figure 6.16

The End