Bone Histology PDF

## BLOCK (1): MUSCOLSKELETAL & INTEGUMENT ### GENERAL CHARACTERS OF BONE - A type of connective tissue consists of cells, fibers and ground substances (matrix) - Hard matrix (calcified with mineral salts) - The cells are present in lacunae - Bone is a highly vascular organ - Dynamic tissue (continuo...

## BLOCK (1): MUSCOLSKELETAL & INTEGUMENT ### GENERAL CHARACTERS OF BONE - A type of connective tissue consists of cells, fibers and ground substances (matrix) - Hard matrix (calcified with mineral salts) - The cells are present in lacunae - Bone is a highly vascular organ - Dynamic tissue (continuously formed and remodeled) under hormonal control and stresses placed on it - For example, pressures applied to bone lead to its resorption, whereas tension applied to it results in development of new bone. - Dentist is able to remodel the bone of the dental arches by moving and straightening the teeth to correct malocclusion, thus providing the patient with a more natural and pleasing smile ### BONE AS AN ORGAN - Bones are the organs of the skeletal system - Bone tissue is the structural component of bones - Bone consists of bone tissue and other connective tissues, including hemopoietic tissue, fat tissue, blood vessels, and nerves. - If bone forms movable joint, (synovial joint), hyaline cartilage is present. - The ability of the bone to perform its skeletal function is attributable to the (1) bone tissue, (2) ligaments and, the (3) articular (hyaline) cartilage ### CLASSIFICATION OF BONE TISSUE - **Compact bone:** A compact, dense layer forms the exterior of the bone - **Spongy bone:** a sponge-like meshwork consisting of trabeculae (thin, anastomosing spicules of bone tissue) forms the interior of the bone - The spaces are continuous and, occupied by marrow and blood vessels. ### ANATOMICAL CLASSIFICATION OF BONE - **Long bones** are longer in one dimension than other bones and consist of a shaft and two ends (tibia and the metacarpals) - **Short bones** are nearly equal in length and diameter (carpal bones). - **Flat bones** are thin and plate-like (bones of skull vault and sternum) - **Irregular bones** have a shape that does not fit into any one of the three groups just described (vertebra and ethmoid bone). ### OUTER SURFACE OF BONES - Bones are covered by **periosteum,** a sheath of dense fibrous connective tissue containing osteoprogenitor cells. - Bones are covered by a periosteum **EXCEPT** in areas where they articulate with another bone (covered by cartilage). - The periosteum that covers an actively growing bone consists of: - **Outer fibrous layer** that resembles other dense connective tissues - **Inner cellular layer** that contains the osteoprogenitor cells. - If active bone formation is not in progress on the bone surface, the fibrous layer is the main component of the periosteum, and the inner layer is not well defined. - Few cells are present, (periosteal cells), are capable of undergoing division and becoming osteoblasts under appropriate stimulus. - Collagen fibers are arranged parallel to surface as a capsule. - Collagen fibers from ligaments and tendons extend and continuous with the collagen fibers of the extracellular matrix. (perforating or Sharpey's fibers) - They extend into outer circumferential and interstitial lamellae but do not enter osteons. ### BONE CAVITIES - Bone cavities are lined by **endosteum**, a layer of connective tissue cells that contain osteoprogenitor cells. - Lining tissue of both compact bone facing the marrow cavity and the trabeculae of spongy bone within the cavity is referred to as endosteum. - Endosteum is often only one cell layer thick and consists of osteoprogenitor cells that can differentiate into osteoblasts, and bone-lining cells. - Because of their location within bone cavities, they are called **endosteal cells**. ### BONE MARROW - Red bone marrow consists of blood cells in different stages of development and a network of reticular cells and fibers that serve as a supporting framework for the developing blood cells and vessels. - In adults, the tissue in the marrow cavity consists mostly of fat cells (**yellow marrow**). ### TYPES OF BONE TISSUE: Mature Bone and Immature Bone - **Mature bone** is composed of structural units called **osteons** (**Haversian systems**). - Mature bone is largely composed of cylindrical units called osteons of Haversian systems. - It consists of concentric lamellae of bone matrix surrounding osteonal (Haversian) canal which contains vascular and nerve supply of osteon. - Canaliculi containing the processes of osteocytes are arranged in a radial pattern with respect to the canal. - Canaliculi that open to the osteonal canal serves for the passage of substances between the osteocytes and blood vessels. - Between osteons are remnants of previous concentric lamellae (interstitial lamellae) - Because of this organization, mature bone is called **lamellar bone**. - The long axis of an osteon is usually parallel to the long axis of the bone. - The collagen fibers in the concentric lamellae in an osteon are laid down parallel to one another but in different directions in adjacent lamellae. - This arrangement gives the cut surface of lamellar bone the appearance of plywood and imparts great strength to the osteon. - Circumferential lamellae follow the entire inner and outer circumferences of shaft of a long bone, appearing as growth rings of tree - Perforating (Volkmann's) canals channels in lamellar bone through which blood vessels and nerves - Travel in right angles to the long axis of osteons from periosteum and endosteum to Haversian canal and connect osteonal canals - Volkmann's canals ARE NOT surrounded by concentric lamellae, a key feature in their histologic identification. - Mature spongy bone is similar in structure to mature compact bone except that the tissue is arranged as trabeculae or spicules; numerous marrow spaces of various sizes are present among the bone tissue. The matrix of the bone is lamellated - **IMMATURE BONE** - Immature bone: Bone tissue initially formed in the skeleton of a developing fetus - Immature bone does not display an organized lamellated appearance - On the basis of its collagen fiber arrangement, such bone is designated nonlamellar. - Nonlamellar bone is also referred to as **bundle bone** or **woven bone** because of the interlacing arrangement of the collagen fibers. - It differs from mature bone in: - Immature bone contains more cells per unit area than does mature bone. - Cells in immature bone randomly arranged, but cells in mature bone are usually arranged with their long axes in same direction as lamellae. - Matrix of immature bone has more ground substance than mature bone. - Matrix in immature bone stains more intensely with hematoxylin, whereas the matrix of mature bone stains more intensely with eosin. - Immature bone forms more rapidly than mature bone. - Mature bone is major bone type in adult and immature bone is the major in fetus - Areas of immature bone are common in the alveolar sockets of the adult oral cavity (possible to make orthodontic corrections) and where tendons insert into bones.