Bone

• 1. Introduction

  ›

  A bone is a rigid organ, and inside it something strange is happening every single second. Over 2.5 billion red blood cells and platelets pour out of bone marrow each day, along with 50 to 100 billion granulocytes. The skeleton most people picture as a dry, dead frame is in fact one of the busiest tissues in the body. It is alive, it is wet, and it is constantly tearing itself down and building itself back up.

  A newborn human carries roughly 300 bones. By adulthood that number drops to 206, as many fuse together during development, not counting the numerous small sesamoid bones. Within that count sits the femur, the largest bone in the body, and the stapes, the smallest, hidden deep in the middle ear. Between these two extremes lies a world of architecture, chemistry, and cell biology.

  How can a tissue be hard enough to protect the brain yet light enough to let a bird fly? How does a structure that feels solid manage to store minerals, buffer the blood, and even help regulate blood sugar? Why does the body bother rebuilding about 10 percent of its skeleton every year? The answers reach from the columns inside a single bone to the oracle bones of ancient China.

• 2. Five Shapes Of Strength

  ›

  Long bones declare themselves by proportion. A shaft, the diaphysis, runs much longer than its width, capped at each end by a rounded head called an epiphysis. Most bones of the limbs, including those of the fingers and toes, are long bones. The clavicle, with its differently shaped shaft, earns the label modified long bone.

  Short bones break that rule entirely. Roughly cube-shaped, they wrap only a thin layer of compact bone around a spongy interior, offering stability with limited motion. The eight carpal bones of the wrist and the seven articulating tarsal bones of the ankle belong to this group.

  Flat bones are thin and generally curved, two parallel layers of compact bone sandwiching a layer of spongy bone between them. Most of the bones of the skull are flat bones, and so is the sternum.

  Sesamoid bones hide inside tendons. By holding a tendon further from a joint, they increase the angle of the tendon and so increase the leverage of the muscle. The patella, the kneecap, is one example, and the pisiform is another.

  Irregular bones refuse every other category. Thin layers of compact bone surround a spongy interior, but their shapes are complicated, often because they carry many centers of ossification or contain bony sinuses. The ethmoid and sphenoid bones of the skull, along with the bones of the spine and pelvis, fall here. Each of these five forms emerges from a single material woven in two fundamentally different patterns.

• 3. Cortex And Lattice

  ›

  Cortical bone is the hard outer layer, denser than the spongy tissue within, and it accounts for 80 percent of the total bone mass of an adult human skeleton. It gives bone its smooth, white, solid appearance. Look closer and it dissolves into microscopic columns, each one an osteon, or Haversian system, built from layers of cells wrapped around a central osteonic canal. Volkmann's canals run at right angles to link the osteons together.

  Cancellous bone tells the opposite story. Also called spongy or trabecular bone, it forms an open-cell porous network that follows the material properties of biofoams. It is less dense, weaker, and more flexible than the cortex, and it is found at the ends of long bones, near joints, and inside vertebrae. Its primary unit is the trabecula, a tiny rod- or plate-like strut.

  Those struts are not arranged at random. The trabeculae align toward the mechanical load a bone experiences, as seen within long bones such as the femur. Trabecular bone accounts for the remaining 20 percent of total bone mass, yet it carries nearly ten times the surface area of compact bone. That vast surface makes it ideal for metabolic work such as the exchange of calcium ions.

  The lattice was first illustrated accurately in the engravings of Crisóstomo Martinez. Highly vascular, it cradles red bone marrow, where the production of blood cells unfolds.

• 4. The Living Crew

  ›

  Osteoblasts build by trapping themselves. These mononucleate cells sit on bone surfaces and lay down a protein mixture called osteoid, primarily Type I collagen. The osteoblast first puts up collagen fibers as a framework, then deposits calcium phosphate hardened by hydroxide and bicarbonate ions. Once the new bone hardens around it, the osteoblast is caught inside, and at that moment it becomes an osteocyte.

  Osteocytes live in tiny chambers. The spaces their cell bodies occupy within the mineralized collagen matrix are called lacunae, while their slender processes run through channels called canaliculi. Reaching out through gap junctions, they stay in contact with osteoblasts, osteoclasts, lining cells, and one another, probably for communication.

  Osteoclasts demolish. These very large multinucleate cells break bone down through resorption, carving out pits known as Howship's lacunae. Unlike the bone-formers, they descend from the same monocyte stem-cell lineage that gives rise to macrophages, and they come armed with phagocytic-like mechanisms. On arrival they secrete active enzymes such as tartrate-resistant acid phosphatase against the mineral substrate.

  These cells answer to chemical signals. Calcitonin, produced by parafollicular cells in the thyroid gland, binds receptors on osteoclasts to directly inhibit them. Osteoprotegerin, secreted by osteoblasts, binds RANK-L to keep osteoclast stimulation in check, one node in a web of paracrine signaling between the crew.

• 5. Collagen And Crystal

  ›

  Bone is about 30 percent flexible matrix and about 70 percent bound minerals, woven together and continuously remodeled. The matrix is 90 to 95 percent elastic collagen fibers, also known as ossein, with the rest a ground substance of proteoglycans such as hyaluronic acid and chondroitin sulfate, plus non-collagenous proteins like osteocalcin, osteopontin, and bone sialoprotein.

  Hydroxyapatite is the dominant bone mineral, with the nominal composition Ca10(PO4)6(OH)2. The collagen fibers give bone its tensile strength while the interspersed crystals of hydroxyapatite give it compressive strength, and these effects are synergistic. The ratio of calcium to phosphate can vary between 1.3 and 2.0 by weight, with trace minerals such as magnesium, sodium, potassium, and carbonate also present.

  Under a microscope, the collagen reveals two arrangements. Woven bone has a haphazard organization of fibers and is mechanically weak, produced when osteoblasts make osteoid rapidly. It appears first in all fetal bones, and in adults it forms after fractures or in Paget's disease. Lamellar bone aligns collagen into regular parallel sheets called lamellae and is mechanically strong.

  Lamellar bone makes its first appearance in humans in the fetus during the third trimester. In cross-section its fibers run in opposite directions in alternating layers, much like plywood, helping the bone resist torsion. It forms slowly, with the orderly deposition of collagen restricting osteoid to about 1 to 2 micrometers per day.

• 6. From Cartilage To Skeleton

  ›

  Ossification is the name for the formation of bone, and in the fetus it follows two routes. Intramembranous ossification builds bone from connective tissue such as mesenchyme, shaping the flat bones of the skull along with the mandible, maxilla, and clavicles. Endochondral ossification instead builds bone from cartilage, and it governs long bones and most others in the body.

  Primary ossification centers are where the second route begins. Appearing mostly during fetal development, they form the diaphyses of long bones, short bones, and parts of irregular bones. Secondary ossification arrives after birth to form the epiphyses, the ends of long bones, separated from the shaft by a growing zone of cartilage called the epiphyseal plate.

  The conversion of cartilage to bone moves through zones. Reserve cartilage shows no sign of change, then chondrocytes multiply into longitudinal columns, then they enlarge in the zone of hypertrophy. Minerals temporarily calcify the cartilage, and finally, in the zone of bone deposition, the cell walls break down, blood vessels and marrow invade, and osteoblasts deposit concentric lamellae while osteoclasts dissolve the calcified cartilage.

  At skeletal maturity, between 18 and 25 years of age, all the cartilage is replaced by bone and the diaphysis fuses with both epiphyses in epiphyseal closure. What happens before that closure matters for life. Weight-bearing and high-impact activity, in sports such as soccer, basketball, and tennis, stimulates bone growth in youth and lays groundwork that can reduce the risk of osteoporosis in adulthood.

• 7. The Hidden Factory

  ›

  Hematopoiesis is the production of blood cells, and it happens inside cancellous bone. Hematopoietic stem cells divide by mitosis to make precursor cells, which mature into red blood cells, white blood cells, and platelets before entering circulation. Platelets follow a different path, shed from very large cells called megakaryocytes. Bone marrow is also a major site where defective or aged red blood cells are destroyed.

  Marrow changes with age. In newborns, every bone holding cancellous tissue is filled exclusively with red, hematopoietic marrow. As a child grows, that fraction shrinks while the fatty yellow marrow adipose tissue increases. In adults, red marrow is mostly found in the femur, the ribs, the vertebrae, and the pelvic bones.

  The skeleton draws a striking share of the body's blood, receiving about 10 percent of cardiac output. Blood enters through the endosteum, flows through the marrow, and exits through small vessels in the cortex. In humans, blood oxygen tension in bone marrow runs about 6.6 percent, compared with about 12 percent in arterial blood and 5 percent in venous and capillary blood.

  Beyond making cells, bone runs a chemical economy. It stores calcium and phosphorus, buffers the blood against excessive pH changes with alkaline salts, and can lock away heavy metals to remove them from circulation. It even acts as an endocrine organ, releasing fibroblast growth factor 23 to reduce phosphate reabsorption in the kidneys, and osteocalcin, a hormone that raises insulin secretion and sensitivity while reducing stored fat.

• 8. Bones Beyond The Body

  ›

  Bird skeletons are very lightweight, their bones smaller and thinner than those of mammals to aid flight. Many bird bones hold little marrow because they are hollow, and a bird's beak is primarily bone, projections of the mandibles covered in keratin. Among mammals, bats come closest to birds in bone density, a hint that small dense bones are themselves a flight adaptation.

  The 80 percent share of cortical bone in the human skeleton can fall much lower elsewhere. Marine mammals, marine turtles, and Mesozoic marine reptiles such as ichthyosaurs show very different proportions. The figure can change quickly in evolution, often rising in early returns to aquatic life, as in early whales and pinnipeds, then falling in pelagic taxa that acquire spongy bone, while slow shallow-water animals like sea cows keep very thick, pachyostotic bone.

  Bone leaves the body and enters culture. Slaughtered animal bones become tools, buttons, beads, dice, and scrimshaw, or are boiled into bone glue still used in antiques restoration, or charred into bone char for filtration and black pigment. A deer's antlers are an unusual case of bone sitting outside the skin once the velvet is shed, and the extinct predatory fish Dunkleosteus carried sharp edges of hard exposed bone along its jaws.

  Oracle bone script was a writing system in ancient China based on inscriptions in bones, mainly ox clavicle. In the Shang dynasty, people wrote questions on the bone, burned it, and read the answer in where the bone cracked. The same skeleton that lets a bird fly once carried the questions of an entire civilization into the fire.