Skeletal Tissues

Compact vs. Spongy Compact bone is dense and solid in appearance. Cancellous, or spongy, bone is characterized by open space partially filled by an assemblage of needle-like structures.

Compact and Spongy Bone

Four Bone Types Long bones – extended longitudinal axis and expanded and often uniquely shaped articular ends. Examples are the femur and humerus Short Bones – cube- or box-shaped structure, which are about as broad as they are long. Examples include the wrist (carpals) and ankle (tarsal) bones

Flat bones – generally broad and thin with a flattened and often curved surface. Examples include some skull bones, scapulae, ribs, and sternum Irregular Bones – often clustered in groups and come in various shapes and sizes. Examples include the vertebral bones that form the spine and facial bones

Parts of a Long Bone Diaphysis – main shaftlike portion Epiphyses – both ends of a long bone have a bulbous shape that provides generous space near the joints for stability and muscle attachment. Articular Cartilage – thin layer of hyaline cartilage that covers joint epiphyses and acts as a cushion.

Periosteum – Dense, white fibrous membrane that covers bone except at joint surfaces Medullary (marrow) cavity – tubelike hollow space in the diaphysis of a long bone. In adults it is filled with fatty yellow marrow. Endosteum – thin epithelial membrane that lines the medullary cavity of long bones.

Bone (Osseous) Tissue Connective tissue Consists of cells, fibers, and extracellular matrix Extracellular components are hard and calcified Matrix is more abundant that the bone cells

Composition of Bone Matrix Inorganic Salt are responsible for the hardness of bone. Hydroxyapatite – composed of calcium and phosphate Organic Matrix – composite of collagenous fibers and a mixture of protein and polysaccharides called ground substance.

Osteoporosis Age related skeletal disease that is characterized by loss of bone mineral density, increased bone fragility, and susceptibility to fractures More common in women May lose 4% - 8% of their bone density on a yearly basis

Microscopic Structure of Bone Compact bone contains many cylinder-shaped structural units called osteons, or Haversian systems. Each osteon surrounds a canal that runs lengthwise through the bone.

Main Structures of the Osteon Lamellae – concentric, cylinder shaped layers of calcified matrix. Lacunae – small spaces containing fluid and bone cells Canaliculi – ultrasmall canals radiating in all directions from the lacunae that connects them to each other and the Haversian canal

Haversian canal – Extends lengthwise through the center of each Haversian system; contains blood vessels, lymph vessels and nerves Volkmann’s canals – run perpendicular to Haversian canals

Structure of Compact and Cancellous Bone

Section of Flat Bone

Cancellous Bone There are no osteons in cancellous bone Consists of needle-like bony spicules called trabeculae. Bone cells are found within the trabaculae Nutrients are delivered and wastes removed by diffusion

Orientation of Trabeculae

Types of Bone Cells Osteoblasts – synthesize and secrete a specialized organic matrix called osteoid, that is an important part of the ground substance of bone. (bone forming cells) Osteoclasts – giant multinucleate cells that are responsible for the active erosion of bone minerals. (bone reabsorbing cells)

Osteocytes – mature, non-dividing osteoblasts that have become surrounded by matrix and now within a lacunae.

Osteoblasts and Osteocytes

Osteoclast

Bone Marrow Soft, diffuse connective tissue called myeloid tissue. Site of blood cell production Found in medullary cavities of certain long bones and in the spaces of some spongy bones.

Red Marrow & Yellow Marrow In a child’s body, virtually all of the bones contain red marrow. Red marrow produces red blood cells. As an individual ages red marrow is replaced by yellow marrow. Yellow marrow has become saturated with fat and is no longer active in blood cell production.

Active Marrow in Adults

Functions of Bone Support – supporting framework of body Protection – protect delicate structures such as the brain Movement – joints act as levers and allow movement in conjuction with muscular system Mineral Storage – major reservoir for calcium, phosphorus, and certain other minerals Hematopoiesis – blood cell formation

Regulation of Blood Calcium Levels 98% of body calcium is stored in bone Osteoblasts remove calcium from blood Osteoclasts break down bone and calcium levels in the blood increases Calcium is important for normal blood clotting, nerve impulses, and muscle contraction

Calcium in the Body Calcium that is consumed is absorbed through the intestines Calcium is stored and released from bone tissue The kidneys eliminate extra calcium and absorb calcium from urine if blood calcium levels get too low.

Parathyroid Hormone When calcium levels fall below their “set point,” osteoclasts are stimulated to increase the rate of bone matrix breakdown. Calcium is released into the blood until the level returns to normal.

Vitamin D (Calcitriol) Acts to increase blood calcium levels Facilitates the absorption of calcium in the small intestines

Calcitonin Functions to reduce blood calcium levels Enhances excretion of calcium in urine Inhibits osteoclasts

Calcium Deprivation

Development of Bone Before birth, the skeleton consists of cartilage and fibrous structures shaped like bones. Cartilage is replaced with calcified bone matrix The combined action of osteoblast and osteoclasts to make bone is known as osteogenesis

Intramembranous Ossification Takes place within a connective tissue membrane

Endochondral Ossification Bone forms from cartilage models Forms from the center to the ends

Endochondral Ossification

Epiphyseal Fracture

Bone Growth and Resorption Bone growth is from the combined action of osteoclasts and osteoblasts Osteoclasts break down bone to enlarge the diameter of the medullary cavity. Osteoblasts from the periosteum build new bone around the outside of the bone

Bone Fracture Repair