Biology 211 Anatomy & Physiology I Dr. Thompson Histology of Bone

Recall: SYSTEMS are composed of one or more organs, all serving a common function ORGANS are composed of one or more types of tissues, all serving a common function TISSUES are composed of one or more types of cells and their products, all serving a common function

Organs of skeletal system = Bones (e.g. femur, ulna, vertebra, mandible) Like all organs, these contain four types of tissue: Epithelium Connective tissue Nervous tissue Muscular tissue However: Two types of specialized connective tissues predominate: Cartilage and Bone Tissue each of which is surrounded by dense irregular connective tissue: Perichondrium and Periosteum

Cartilage: Function = Flexible Support Found in skeletal system: Costal cartilages Intervertebral disks Articular cartilages at ends of bones Nasal septum Also found in External ear Larynx Trachea & bronchi

Cartilage: Function = Flexible Support In the embryo, forms the "model" for many bones and: a) Remains on ends of long bones throughout life b) Remains at growth plates (epiphyseal plates) of immature growing bones.

Cartilage: Young cells = chondroblasts Actively forming new extracellular matrix Mature cells = chondrocytes Maintain and repair extracellular matrix Extracellular Matrix Ground substance = very firm gel Fibers = collagen, elastic Chondrocytes located in lacunae

Cartilage: Three types: Hyaline Elastic Fibrous All three types: Cells are the same Differences due to fibers

Hyaline Cartilage: Extracellular matrix appears smooth, No fibers evident by light microscopy Cells (in lacunae) form isogenous groups or nests Extracellular matrix often stains more darkly around cells

Elastic Cartilage: Many elastic fibers visible in extracellular matrix Cells (in lacunae) form isogenous groups or nests Extracellular matrix often stains more darkly around cells

Fibrous Cartilage: Many large collagen fibers visible in extracellular matrix Cells (in lacunae) usually individual (no isogenous groups) Extracellular matrix usually stains more darkly around cells

Bone: Young cells = osteoblasts Actively forming new extracellular matrix Mature cells = osteocytes Maintain and repair extracellular matrix Located in lacunae Third cell type = osteoclast Reabsorb extracellular matrix

Bone: Extracellular matrix: Fibers: Collagen Ground substance contains precipitated calcium phosphate called calcium hydroxyapatite Bone tissue constantly remodeling itself: Old extracellular matrix removed by osteclasts New extracellular matrix produced by osteocytes

Two forms of bone tissue: Trabecular ("woven", "spongy") Shelves or ridges ("trabeculae") of extracellular bone matrix with osteocytes embedded within it. Lamellar ("compact", "Haversian") Concentric rings ("lamellae") of extracellular matrix and ostocytes, surrounding a central canal containing nerves and capillaries

Lamellar Bone:

Osteocytes in lacunae, between lamellae

Both trabecular and lamellar bone: Osteocytes have lost ability to divide so Growth can occur only by differentiation of new osteoblasts, which then form extracellular matrix around themselves As the osteoblasts become trapped in their matrix, they mature into osteocytes located in lacunae Osteoblasts at edges

Both trabecular and lamellar bone: Osteocytes have long cellular extensions to retain connections with each other. These extensions of osteocytes run through little canals called canaliculi in the rigid extracellular matrix

Two types of bone formation ("osteogenesis") for growth, remodeling, and repair: - Endochondral osteogenesis: Model of cartilage formed first. Cartilage then replaced by bone. - Intramembranous osteogenesis: Bone formed within pre-existing ordinary connective tissue, often at edge of existing bone.

Bones (organs) grow by both intramembranous and endochondral osteogenesis: Average age (years) at completion of ossification: Scapula Clavicle Os coxa Vertebrae - 25 Sacrum Sternum - body manubrium Humerus, radius, ulna Femur, tibia, fibula

Bones repair fractures by both intramembranous and endochondral osteogenesis:

Bone tissue is constantly remodelling itself to adjust to different physical and biochemical demands placed upon it. Old bone is destroyed by osteoclasts and new bone is produced by osteoblasts and osteocytes. Example: If you have a cast on your arm, the bones will become smaller since they are not being stressed and more bone tissue is removed than is synthesized. But: When the cast is removed and the bones again subjected to stress, they will become larger again.

Bone remodelling regulated by many hormones: Growth hormone stimulated osteoblast activity & collagen synthesis Thyroid hormone stimulates osteoblast activity & collagen synthesis; stimulates formation of ossification centers Testosterone stimulates osteoblast activity & bone growth Progesterone stimulates osteoclast activity & bone loss Estrogen stimulates osteoblast activity & bone growth

Two hormones primarily responsible for day-to-day remodelling of bone to regulate concentration of calcium in blood: Calcitonin Secreted by thyroid gland Stimulates osteoblasts to produce more matrix Inhibits osteoclasts from breaking down matrix Thus: Calcium removed from blood & stored in bone Parathyroid Hormone Secreted by parathyroid glands Inhibits osteoblasts from producing more matrix Stimulates osteoclasts to break down matrix Thus: Calcium released from bone into blood

Blood Ca ++ level returns to normal Blood Ca ++ level continues to increase Blood Ca ++ level continues to decrease Blood Ca ++ level returns to normal Thyroid secretes calcitonin Parathyroid glands secrete PTH Ca ++ moved from blood to bone Ca ++ moved from bone to blood