Bones are the foundation of our skeletal system, providing structure and support for our bodies. They're not just rigid structures, but dynamic organs that constantly change and adapt. Let's explore the key features that make bones so incredible.

From the dense shaft to the spongy ends, bones have unique structures that serve specific purposes. We'll look at how bones are built, from tiny cells to complex systems, and how they work together to keep us moving and healthy.

Bone Anatomy and Physiology

Key anatomical features of bones

• Diaphysis forms the shaft or main portion of long bones (femur, humerus) and is composed primarily of dense compact bone
• Epiphysis forms the ends of long bones, is covered by smooth articular cartilage, and is composed primarily of porous spongy bone
• Metaphysis is the region between the diaphysis and epiphysis where bone growth and elongation occur during development (growth plates)
• Medullary cavity is the hollow space within the diaphysis that contains red bone marrow, a site of blood cell production
• Periosteum is a fibrous connective tissue covering the outer surface of bone that contains blood vessels, nerves, and osteoblasts for bone formation and repair
• Endosteum is a thin connective tissue lining the medullary cavity that contains blood vessels, nerves, and osteoblasts for bone remodeling

Common bone markings and functions

• Foramen is a hole or opening in bone that allows passage of blood vessels (nutrient foramen) and nerves
• Fossa is a shallow or deep depression in bone that provides space for muscle attachment (mandibular fossa) or articulation with other bones
• Condyle is a rounded projection at the end of a bone that articulates with another bone to form a joint (femoral condyles)
• Tuberosity is a rounded or roughened projection on a bone that serves as an attachment point for muscles (tibial tuberosity), tendons, or ligaments
• Crest is a prominent ridge or border on a bone that provides attachment points for muscles (iliac crest) and ligaments
• Spine is a sharp, slender projection on a bone that serves as an attachment point for muscles and ligaments (vertebral spine)

Cellular composition of bone tissue

• Osteoblasts are bone-forming cells that secrete organic matrix (osteoid) and initiate mineralization of bone tissue
• Osteocytes are mature bone cells that maintain bone matrix and regulate mineral homeostasis, housed in lacunae and connected via canaliculi
• Osteoclasts are bone-resorbing cells that break down and remodel bone tissue in response to hormonal and mechanical signals
• Extracellular matrix consists of an organic component (osteoid) primarily composed of type I collagen that provides flexibility and tensile strength, and an inorganic component (mineral) primarily composed of hydroxyapatite $Ca_{10}(PO_4)_6(OH)_2$ that provides hardness and compressive strength
• Bone matrix is the composite material formed by the organic and inorganic components, providing the structural and functional properties of bone tissue

Compact vs spongy bone structure

• Compact bone is dense and solid, found in the diaphysis of long bones and outer layer of all bones, and is organized into osteons (Haversian systems) with concentric lamellae surrounding a central Haversian canal containing blood vessels and nerves, providing strength and protection
• Spongy bone is porous and trabecular, found in the epiphyses of long bones and interior of flat (skull) and irregular bones (vertebrae), and is composed of interconnected bony trabeculae that align along lines of stress with spaces filled with bone marrow, providing lightweight support and metabolic functions
• Bone density refers to the amount of mineral content in a given volume of bone, which is an important factor in determining bone strength and resistance to fracture

Blood supply and nerves in bones

• Blood supply includes nutrient arteries that enter bone through nutrient foramen and supply the medullary cavity and inner two-thirds of compact bone, periosteal arteries that supply the outer one-third of compact bone, and metaphyseal and epiphyseal arteries that supply the ends of long bones, with venous drainage following a similar pattern
• Nerves provide sensory innervation to the highly innervated periosteum allowing sensation of pain, pressure, and proprioception, and autonomic innervation controlling blood flow and influencing bone metabolism
• Blood supply and nerves are important for providing oxygen and nutrients for bone cells, removing waste products, enabling bone remodeling and repair, allowing sensory feedback, and regulating bone metabolism

Bone Function and Regulation

• The skeletal system provides structural support, protection for vital organs, and serves as a mineral reservoir for calcium homeostasis
• Osteogenesis is the process of bone formation, which occurs during development and continues throughout life during bone remodeling and repair
• Biomechanics of bone involves the study of how bones respond to mechanical forces, influencing their structure and strength