Adults keep 206 bones; newborns start near 300
A newborn skeleton has about three hundred bones that fuse down to two hundred six in most adults, not counting tiny sesamoids. The femur is the largest; the stapes in the middle ear is the smallest. Bone is living tissue that stores minerals, makes blood cells, and remolds itself for life.
Bone is a rigid organ in vertebrate skeletons: it supports the body, protects organs, enables movement and hearing via middle-ear ossicles, stores minerals, buffers acid-base balance, and houses marrow that produces blood cells. Osseous tissue is mineralized connective tissue built by osteoblasts and osteocytes, cleared by osteoclasts, with an organic collagen matrix and inorganic bone mineral. Compact cortical bone forms the dense shell; cancellous bone is the spongy interior that often holds marrow.
Humans classify bones as long, short, flat, irregular, or sesamoid. Long bones such as limb shafts have a diaphysis and epiphyses; wrist and ankle bones are short cubes; skull plates and the sternum are flat; vertebrae and pelvis are irregular; the patella is a classic sesamoid embedded in tendon. Mechanically, bone compresses well—about one hundred seventy megapascals—but handles tension less well and shear poorly. Collagen supplies useful elasticity even though bone is essentially brittle.
Cortical bone is about eighty percent of adult skeletal mass and is organized into osteons around Haversian canals. Overall, bone is roughly thirty percent flexible matrix and seventy percent bound minerals; the matrix itself is ninety to ninety-five percent collagen. Marrow hematopoiesis turns out over two point five billion red cells and platelets daily, plus tens of billions of granulocytes. Bone also acts as an endocrine organ, releasing FGF-23 to tune phosphate handling and osteocalcin that influences glucose and fat metabolism.
Source: Bone
