Bones: Structure and Types

Skeletal cartilage, a type of connective tissue, is found in various parts of the skeleton. It contains a lot of water, allowing it to withstand tension and compression. Chondrocytes, located in lacunae, are embedded in an extracellular matrix. There are three types: hyaline, elastic, and fibrocartilage. Hyaline cartilage is the most common, found in the nose, ribs, larynx, and bone ends. Elastic cartilage is flexible, found in the ears and epiglottis. Fibrocartilage, highly compressible, is found in areas like the intervertebral discs.

Bones are categorized into axial (head and torso) and appendicular (limbs, pelvis, shoulders) skeletons. They are also classified by shape: long (limbs), short (ankles, wrists), flat (sternum, shoulder blades), and irregular (vertebrae, hip bones). Bones serve multiple functions, including support for the body, protection of organs, providing levers for movement, mineral storage (calcium, phosphate), fat storage, hormone production, and blood cell formation.

A complete bone is an organ made of several tissue types, not just bone tissue (connective, nervous, cartilage, blood vessels). Gross anatomy shows compact bone as the dense outer layer and spongy bone (honeycomb-like) inside, often filled with bone marrow. Short, irregular, and flat bones have spongy bone covered by compact bone, with hyaline cartilage at joint surfaces. Long bones feature a tubular diaphysis (shaft) made of compact bone surrounding a medullary cavity (marrow cavity) that contains yellow marrow in adults. The epiphyses (ends) contain spongy bone and are covered by cartilage. Red marrow, found in spongy bone cavities, produces blood cells. The epiphyseal line is a remnant of the epiphyseal plate, responsible for bone lengthening. The periosteum, a white membrane, covers the bone's exterior, while the endosteum covers internal spongy bone. Bones also have surface markings like projections, depressions, and openings.

Bones contain several cell types. Osteogenic cells are stem cells in the periosteum and endosteum that divide and can differentiate. Osteoblasts secrete bone matrix (collagen and proteins) and are responsible for bone growth. Osteocytes are mature bone cells that maintain the matrix. Bone lining cells on the bone surface also help maintain the matrix. Osteoclasts are large, multinucleated cells that break down bone (resorption), releasing minerals into the blood.

Compact bone is composed of cylindrical units called osteons. Each osteon consists of concentric lamellae (hollow tubes) arranged like tree rings, with collagen fibers in alternating directions to resist torsion. The central canal within each osteon contains blood vessels and nerve fibers. Shorter canals run perpendicularly, connecting the periosteum to central canals and the medullary cavity. Lacunae, tiny gaps where osteocytes reside, are connected by even tinier canals called canaliculi. Interstitial lamellae fill gaps between osteons, and circumferential lamellae form the outer circumference of the diaphysis.

Bone is composed of organic and inorganic components. Organic components include all bone cells and osteoid (ground substance and collagen fibers secreted by osteoblasts), which provide structure, flexibility, and tensile strength. Inorganic components, mainly hydroxyapatites (needle-like calcium phosphate crystals), surround collagen fibers and largely account for bone hardness. The combination of these components makes bones durable, strong, and resistant to tension and compression, similar to steel.