BONE The most amazing story of bone histology!

Functions of Bone Supports soft tissue Protects vital organs (cranium, thoracic cavity) Contains bone marrow Reservoir of Ca++, PO4 to maintain constant concentrations in body fluids Allows body to move

Specialized CT Cells –Osteoblasts –Osteocytes –Osteoclasts Bone matrix –Calcified material, lacunae And more…. –Canaliculi –Periosteum –Endosteum

Ground substance Protoglycans Glycoprotein Mineral ( 75% of the bone mass) calcium, citrate, and bicrobante ions. Water Fibers type I collagen to provide strength.

Anatomy of a Long Bone

Osteoblasts Synthesize organic components of matrix (collagen type I, proteoglycans, glycoproteins.) Collagen forms osteoids: strands of spiral fibers that form matrix Influence deposit of Ca++, PO4. Active vs inactive osteoblasts Estrogen, PTH stimulate activity

Spongy and compact bone

Bone structure The articular ends of the long bones are covered with cartilage; each bone (including the patella) is composed of a hard "shell" of mature, fully developed bone: i.e., lamellar bone, so called because of the organization of its compact substance. Inside the hollow ends of each of the long bones is the trabecular network of spongy or cancellous bone. The greater part of a long bone is the shaft, or diaphysis.The outer surface of the shaft is covered with a tough collagenous CT, the periosteum. Some of the cells in it have osteogenic capabilities, and are derived from the same stem line as the fibroblasts. The periosteum is analogous to the perichondrium, and it's tightly adherent to the surface of the bone. If the bone is from a young animal that's still growing, there should be a cartilaginous growth plate at one or both ends. You can see such a bone at right: the growth plate is the place where replacement of cartilage by bone occurs in the process endochondral ossification. Anatomically this is the epiphysis of the bone. Elongation of the anatomic bone occurs at the epiphyses (there's a second epiphysis at the other end)

Cells Osteoprogenrator cells diffrencaite into osteoblasts during osteogensis and bone repair Osteoblasts they secret bone matrix, and secret IL-1 which inhibt ostoclast activity

Osteoblasts

Osteocytes Mature bone cells that sit in lacunae Gap junctions between osteocytes provide nutrition (15 cells in a row) Maintain bony matrix; long lived cells Stimulated by calcitonin; inhibited by PTH

Osteocyte with Cytoplasmic Extensions

Osteocytes with Canaliculi Photomicrograph of dried bone ground very thin. The lacunae and canaliculi filled with air deflect the light and appear dark, showing the communication between these structures through which nutrients derived from blood vessels flow.

Osteoclasts Derived from monocytes; engulf bony material Active osteoblasts stimulate osteoclast activity Large, branched, motile cells Secrete enzymes that digest matrix

Ostoclasts Removal of existing bone is the job of the osteoclast The osteoclast is much larger than the osteoblasts, and is usually multinucleated. Look for these cells on the ends of spicules of the trabeculae The osteoclasts secrete hydrogen ions into the extracellular space to solubilize the mineralized matrix, and al so produce hydrolytic enzymes osteoclast has been identified as a blood-borne element: (monocytes, and macrophages all have a common precursor cell)

Osteoclasts

Bone Replacing Cartilage

Remodeling Bone

Bone Remodeling Changes in the size and shape of bones during the period of growth imply some bone reorganisation. Osteoblast and osteoclast constantly deposit and remove bone to adjust its properties to growth-related demands on size and/or changes of tensile and compressive forces

Endochondral Ossification

Epiphyseal Plate

The 5 zones of bone growth Resting zone Zone of proliferation zone of hypertrophy and cell death zone of cacification Zone of ossification

Periosteum Mesenchyme Fibroblasts Osteoprogenitor cells

Osteon Long cylinder parallel to long axis of diaphysis Consists of: –Haversian canal with nerves, blood vessels; lamellae with osteocytes Haversian canals communicate with marrow cavity, periosteum, other canals through Volkmann’s canals

Figure 6.4 Figure 6.4 The Structure of Osseus Tissue

Compact Bone

Canaliculi between Osteocytes