Bones

• Bone is a living tissue, which makes up the body skeleton and is one of the hardest structures of the animal body.
• Bone or osseous tissue represents the highest differentiation among supporting tissues.
• Classification of bones according to shape: Long bones, short bones, flat bones, irregular bones, sesamoid bones.
• Development of bones (Classification): Endochondral and Intramembranous.
• Microscopic structure of the bone: Mature bone (Compact bone), otherwise known as (Cortical/Lamellar). Cancellous bone (Spongy), Immature bone (Woven bone).
• Dense cortical bone

  
• Dense to porous cortical bone surrounding trabecular bone

  
• Thin porous cortical bone surrounding fine trabecular bone

  
• Fine trabecular bone

  
• Anatomy of a bone: Articular cartilage, cancellous bone, Epiphyseal plate, Marrow cavity, Periosteum, and Compact bone.

  
• Diaphysis is the shaft or main portion of the bone.
• Epiphysis is the extremities or ends of the bone.
• Metaphysis is the region in a mature bone where diaphysis joins epiphysis.
• Articular Cartilage is a thin layer of hyaline cartilage covering the epiphysis where bone forms a joint with another bone.
• Periosteum is the tissue covering the outer surface of bone. It consists of two layers. The outer fibrous layer is rich in blood vessels, lymphatic vessels, and nerves that pass into the bone and inner layer is composed of osteoblasts surrounded by osteoprogenitor cells.
• Medullary or Marrow cavity is the space within the diaphysis that contains the fatty yellow marrow in adults. Yellow marrow consists primarily of fat cells and a few scattered blood cells. Thus, yellow marrow functions in fat storage.
• Endosteum is a layer of osteoprogenitor cells and osteoblasts that lines medullary cavity and also contains scattered osteoclasts
• Characteristic of all bones are a dense outer sheet of compact bone and a central, medullary cavity.
• In living bone the cavity is filled with red or yellow bone marrow that is interrupted, particularly at the extremities of long bones, by a network of bone trabeculae (trabecular, cancellous, or spongy bone are the terms used to describe this network ).
• Compact bone consists of microscopic layers or lamellae
• Three distinct types of layering are recognized: circumferential, concentric, and interstitial.
• Circumferential lamellae enclose the entire adult bone, forming its outer and inner perimeters.
• Concentric lamellae make up the bulk of compact bone and form the basic metabolic unit of bone, the osteon (also called the haversian system).
• The osteon is a cylinder of bone, generally oriented parallel to the long axis of the bone. In the center of each is a canal, the haversian canal, which is lined by a single layer of bone cells that cover the bone surface; each canal houses a capillary. Adjacent haversian canals are interconnected by Volkmann canals, channels that, like haversian canals, contain blood vessels, thus creating a rich vascular network throughout compact bone.
• Interstitial lamellae are interspersed between adjacent concentric lamellae and fill the spaces between them.
• The outer layer of the periosteum consists of a dense, irregular connective tissue termed the fibrous layer. The inner layer of the periosteum, next to the bone surface, consists of bone cells, their precursors, and a rich microvascular supply.
• The internal surfaces of compact and cancellous bone are covered by Endosteum
• The periosteal surface of bone is more active in bone formation than the endosteal one.
• Cancellous/Trabecular/Spongy bone looks like poorly organized tissue in contrast to compact bone. It also does not contain any true osteon.
• Consists of long, slender spicules called as trabeculae.
• It makes up most of the bone tissue of short, flat and irregular bones and most of epiphysis of long bones. (Cancellous bone)
• Bone cell types: osteoprogenitor cells, osteoclasts, osteoblasts, osteocytes.
• It possesses a certain degree of hardness and elasticity. (Bone)