Human Anatomy Chapter 6: Skeletal System

The Skeletal System

• Consists of 206 named bones, divided into two principal divisions: axial skeleton and appendicular skeleton
• Axial skeleton: skull bones, auditory ossicles, hyoid bone, ribs, sternum, and bones of the vertebral column
• Appendicular skeleton: bones of upper and lower limbs, plus bones forming the girdles that connect limbs to axial skeleton

Functions of the Skeletal System

• Support: provides hard framework that supports and anchors soft organs of the body
• Protection: surrounds organs such as brain and spinal cord
• Movement: allows for muscle attachment, using bones as levers
• Storage: stores minerals and lipids within bone material
• Blood cell formation: bone marrow is responsible for blood cell production

Structure of Bone Tissue

• Two types of bone tissue: compact and spongy, differing in density
• Three types of cells contributing to bone homeostasis: osteoblasts, osteoclasts, and osteocytes
• Bone tissue is a specialized form of connective tissue characterized by a mineralized extracellular matrix
• Mineral composition: calcium phosphate (hydroxyapatite crystals), calcium carbonate, magnesium hydroxide, fluoride, and sulfate
• Matrix composition: mainly collagen (type I, VI) along with other matrix proteins, with collagen molecules making up ~90% of total weight of bone matrix

Compact Bone

• Composed of two types of osseous tissue, making up the outer cortex of all bones
• In immediate contact with periosteum
• Solid, strong bone located along diaphysis of long bones, providing solid structure to skeletal frame
• Forms a hollow tube called the medullary cavity

Microscopic Structure of Compact Bone

• Structural unit is the osteon (haversian system), an elongated cylinder oriented parallel to the long axis of the bone

Spongy Bone (Cancellous Bone)

• Sponge-like meshwork consisting of trabeculae, with spaces occupied by marrow and blood vessels
• Does not contain osteons, consisting of trabeculae surrounding many red marrow-filled spaces
• Forms most of the structure of short, flat, and irregular bones, and the epiphyses of long bones
• Light and supports and protects red bone marrow, reducing weight of the bone and providing strength against forces of compression

Classification of Bones by Shape

• Long bones: longer in one dimension, with an elongated shaft (diaphysis) and two expanded ends (epiphyses)
• Short bones: nearly equal in length and diameter, can be of any shape