The Song of the Cell: An Exploration of Medicine and the New Human

by Siddhartha Mukherjee

Superficially, bone might look like a chunk of hardened calcium, but it is, in fact, made of a multiplicity of cells. The most familiar are cartilage cells—technically called chondrocytes—and there are two unfamiliar-sounding cell types. The second is the “osteoblast”—the cell that deposits calcium and other proteins to form a calcified matrix in layers, and then get trapped in its own deposit to form new bone. It is the bone-making, bone-depositing cell: typically, the osteoblasts thicken and lengthen the bone (my mnemonic for its name is the letter “b,” for “bone making”). The third is an osteoclast: these are large cells with multiple nuclei that are bone-eaters. They chew away on the matrix, or punch holes in it, removing and remodeling bone, like constantly pruning gardeners (I remember it because it contains the letter “c,” for “bone chewing”). The dynamic balance between osteoblasts and osteoclasts—bone makers and bone chewers—is one mechanism by which bone maintains homeostasis. Take away osteoblasts and new bone cannot be laid down. Make osteoclasts—the bone chewers—defective, and the bone turns thick—“stone bone,” as early pathologists called it—tough seeming, but hard to repair. The cavities inside contract, crowding out space for the marrow, precipitating a disease called osteopetrosis.