Functions of Bone Rigid skeleton supports the body Provides sites for attachment of muscles and organs Protective cover for soft organs Hemopoiesis (marrow cavities) Reservoir for body’s calcium supply Calcium used for muscle contraction, blood clotting, cell membrane permeability, transmission of nerve impulses

Bone Connective tissue with a mineralized matrix (calcium phosphate) Matrix is dense and rigid, contains fibers of mainly Type I collagen. Both the collagen and the ground substance become mineralized in bone.

Bone Matrix Calcified matrix of fibers (Type I collagen) and ground substance (proteoglycans with chondroitin sulfate and keratan sulfate side chains, glycoproteins) Organic component (osteoid)= type I collagen, highly cross-linked Inorganic (mineral) component = crystals of calcium hydroxyapatite (calcium and phosphorus) Hard – nutrients and metabolites cannot diffuse through the calcified matrix as done in cartilage. Highly vascularized: Canaliculi –small channels or canals between lacunae that allow osteocytes to exchange wastes and bring metabolites in; osteocytes send out cytoplasmic processes into the canaliculi and contact neighboring cells through gap junctions.

Structure of Bones Diaphysis – shaft Epiphysis – articular ends Periosteum – covers external surfaces; - consists of an outer layer of dense, irregular collagenous (fibrous) CT and an inner layer of osteoprogenitor and osteoblast cells Marrow – in central cavity of bone, hemopoietic tissue, red or yellow

Epiphysis of an adult long bone Spongy (cancellous) bone Compact bone Compact bone

Spongy (Cancellous) Bone - composed of spicules or trabeculae of bone; - forms network around bone marrow.

Compact Bone dense bone no cavities; found along the outer surface of shafts of long bone.;

Compact bone has a lamellar structure circumferential lamellae (on surfaces) Haversian lamellae interstitial lamellae.

Compact Bone – Low Power highly vascularized Haversian canal - the space running through an osteon; - contains blood vessels, nerves, osteoblasts and osteoprogenitor cells. Haversian canal Volkmann’s canal Volkmann’s canals – connect Haversian canals of adjacent osteons

Ground Bone – higher power canaliculus lacuna

Bone Marrow fat cells Blood-forming cells Osteogenic (osteoprogenitor ) cells – line inner layer of endosteum periosteum, and haversian canals; - undifferentiated stem cells that give rise to osteoblasts

Bone Formation – 2 types Intramembranous ossification – formation of bone directly from mesenchyme (embryonic CT) with no cartilage precursor Endochondral – formation of bone on cartilage scaffolding; - bone eventually replaces cartilage.

Occurs in flat bones of skull, mandible, maxilla. Intramembranous Ossification Occurs in flat bones of skull, mandible, maxilla. Osteoblasts develop from CT mesenchyme cells. Osteoblasts secrete osteoid (uncalcified bone matrix).

Intramembranous Ossification Bone matrix is first formed as a network of spicules or trabeculae. Osteoblasts – immature bone cells; - - secrete osteoid (uncalcified bone matrix Osteocytes – mature bone cells; trapped within lacunae of bone matrix osteocytes osteoblasts

Bones are dynamic structures! Continually being remodeled Osteoclasts – large, multinucleate cells; at the surfaces of bone - resorb & remodel bone in response to hormone and calcium levels Howships lacunae – depressions made by osteoclasts as they etch away the bone.

Osteoclast activity is regulated by hormones Parathyroid hormone – increases blood calcium levels; stimulates activity of osteoclasts and increases their numbers. Calcitonin (from thyroid gland) – decreases blood calcium levels; reduces the numbers and activity of osteoclasts.

Endochondral Ossification Occurs in long and short bones, vertebrae Bone is proceeded by temporary hyaline cartilage model that serves as a structural scaffold. Bone tissue REPLACES the cartilage Cartilage begins to calcify and causes chondrocytes to hypertrophy. Cartilage calcifies so diffusion of nutrients to chondrocytes stops and they die. Fragmented pieces of calcified cartilage serve as the framework to deposit the bone. Osetoprogenitor cells and blood vessels from surrounding connective tissue penetrate and invade degenerating cartilage model. Cartilage of diaphysis replaced by bone except for at epiphyseal plate.

Endochondral Ossification – bone replaces a cartilage model

Endochondral Ossification cartilage bone replacing cartilage

Endochondral Ossification bone replacing cartilage

Summary of Bone Cell Types

Primary vs. Secondary Bone Primary – immature or “woven” bone First bone formed in fetal development or bone repair Abundant osteocytes Irregular bundles of collagen (Type I) Secondary bone – mature or lamellar bone

Methods to Make Slides of Bone Decalcified bone sections Acid solution removes calcium salts and softens bone. Bone can then be embedded and sectioned. Osteocytes are distorted. Ground bone sections Saw bone in slices. Grind piece down until thin enough to put on a slide. Cells are destroyed and lacunae and canaliculi filled with bone debris.