Skeletal System: Divisions and Bones

Bone Microanatomy

• Lacunae: cavities that contain osteocytes (bone cells), arranged in concentric rings
• Lamellae: rings around the central canal, where lacunae are found
• Canaliculi: tiny canals that radiate from the central canal to lacunae, transport nutrients to bone cells

Osteon (Haversian System)

• A "unit" of bone
• Central (Haversian) canal: opening in the center of osteon, contains blood vessels
• Perforating (Volkman's) canal: perpendicular to the central canal, contains blood vessels and nerves

Divisions of the Skeletal System

• Axial skeleton: includes skull bones, auditory ossicles, hyoid bone, ribs, sternum, and vertebral column
• Appendicular skeleton: includes bones of the upper and lower limbs, and bones forming the shoulder and pelvic girdles

Skull

• Formed by two sets of bones: cranium and facial bones
• Cranium: consists of 8 large flat bones, including frontal, parietal, temporal, occipital, ethmoid, and sphenoid bones
• Frontal bone: forms the anterior part of the skull, articulates with parietal bone at the coronal suture

Structure of Bones

• Diaphysis: shaft of long bone, made of compact bone
• Epiphyses: ends of long bone, made of compact bone surrounding spongy bone, covered with articular cartilage
• Epiphyseal line: line of bone tissue that looks different from the rest of the bone, remnant of epiphyseal plate
• Periosteum: outer covering of shaft, like "skin" on chicken bone
• Sharpey's fibers: attach periosteum to bone
• Arteries: supply nutrients to bone cells
• Medullary cavity: large cavity within the diaphysis, contains yellow marrow (adipose) in adults, contains red marrow in infants (for blood cell formation)

Bone Formation

• Epiphyseal plates allow for growth of long bone during childhood
• New cartilage is continuously formed, older cartilage becomes ossified, and bone replaces cartilage
• Bones are remodeled, lengthened until growth stops, and change shape somewhat

Functions of the Skeletal System

• Support: serves as the structural framework for the body, supporting soft tissues and providing attachment points for tendons of most skeletal muscles
• Protection: protects the most important internal organs from injury
• Movement: most skeletal muscles attach to bones, when they contract, they pull on bones to produce movement
• Storage: bone tissue stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone
• Blood cell formation: within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis