The Skeletal System: Bone Structure and Cells

• The skeletal system offers support, movement, protection, blood cell production, calcium storage, and endocrine regulation.
• An adult human skeleton has 206 bones; children have more because some bones fuse during growth.

Bone Structure

• Bones consist of connective tissue reinforced with calcium and specialized bone cells.
• Compact bone is dense and strong and forms the rigid outer structure.
• Spongy bone is lighter and more flexible than compact bone.
• Red bone marrow within spongy bone produces blood cells, while yellow bone marrow stores fat.
• Periosteum, a fibrous membrane, covers the outer bone surface, providing nutrients and facilitating growth and repair.

Bone Cells

• Osteoblasts form bone by secreting the bone matrix.
• Osteocytes are mature bone cells embedded in the bone matrix that maintain bone tissue.
• Osteoclasts break down bone tissue, which aids bone remodeling and calcium release.

Bone Development

• Ossification is bone formation that starts from cartilage models in embryos.
• Intramembranous ossification directly forms flat bones, like those in the skull, from mesenchymal tissue.
• Endochondral ossification forms long bones by replacing cartilage with bone tissue.
• Growth plates (epiphyseal plates) at the ends of long bones allow bones to lengthen during childhood and adolescence.

Axial Skeleton

• The axial skeleton, composed of the skull, vertebral column, and rib cage, provides central support and protection.
• The skull includes cranial bones to protect the brain and facial bones that form the face.
• The vertebral column consists of vertebrae separated by intervertebral discs, providing flexibility and protecting the spinal cord.
• The rib cage, made of ribs and the sternum, protects the heart and lungs.

Appendicular Skeleton

• The appendicular skeleton includes the bones of the limbs and their attachments to the axial skeleton.
• The pectoral girdle (shoulder girdle) connects the upper limbs to the axial skeleton, which allows for a wide range of motion.
• The pelvic girdle (hip girdle) connects the lower limbs to the axial skeleton, bearing weight and enabling locomotion.
• The upper limb is composed of the humerus, radius, ulna, carpals, metacarpals, and phalanges.
• The lower limb consists of the femur, tibia, fibula, tarsals, metatarsals, and phalanges.

Joints

• Joints (articulations) are locations where two or more bones meet, which facilitates movement and provides mechanical support.
• Fibrous joints, like sutures in the skull, are immovable or slightly movable.
• Cartilaginous joints, like intervertebral discs, permit limited movement.
• Synovial joints have high mobility, featuring a joint cavity filled with synovial fluid for lubrication and shock absorption.
• Synovial joints include hinge joints (e.g., elbow), ball-and-socket joints (e.g., hip), pivot joints (e.g., neck), and gliding joints (e.g., wrist).

Bone Remodeling

• Bone remodeling is a continuous process of replacing old bone tissue with new bone tissue.
• Bone remodeling includes bone resorption by osteoclasts and bone formation by osteoblasts.
• This process helps maintain bone strength, repair damage, and regulate calcium levels in the blood.

Calcium Homeostasis

• Bones act as a major calcium reservoir, which is essential for various physiological processes.
• Calcium homeostasis is regulated by hormones like parathyroid hormone (PTH) and calcitonin.
• PTH increases blood calcium levels by stimulating bone resorption, while calcitonin decreases blood calcium levels by promoting bone deposition.

Common Bone Disorders

• Osteoporosis is characterized by decreased bone density and an increased risk of fractures.
• Osteoarthritis is a degenerative joint disease caused by cartilage breakdown in joints.
• Rheumatoid arthritis is an autoimmune disease causing joint inflammation.
• Fractures are breaks in bones from trauma or conditions like osteoporosis.

Explore the functions and components of the skeletal system. Learn about bone structure, including compact and spongy bone, and the roles of osteoblasts, osteocytes, and osteoclasts. Understand how bones support the body, enable movement, and protect vital organs.