many collagen fibers parallel to other fibers in the same layer (these parallel columns are called osteons). in cross-section, the fibers run in opposite directions in alternating layers, much like in plywood, assisting in the bone's ability to resist torsion forces. after a fracture, woven bone forms initially and is gradually replaced by lamellar bone during a process known as "bony substitution." compared to woven bone, lamellar bone formation takes place more slowly. the orderly deposition of collagen fibers restricts the formation of osteoid to about 1 to 2 µm per day. lamellar bone also requires a relatively flat surface to lay the collagen fibers in parallel or concentric layers. deposition the extracellular matrix of bone is laid down by osteoblasts, which secrete both collagen and ground substance. these synthesise collagen within the cell and then secrete collagen fibrils. the collagen fibers rapidly polymerise to form collagen strands. at this stage, they are not yet mineralised, and are called "osteoid". around the strands calcium and phosphate precipitate on the surface of these strands, within days to weeks becoming crystals of hydroxyapatite. in order to mineralise the bone, the osteoblasts secrete vesicles containing alkaline phosphatase. this cleaves the phosphate groups and acts as the foci for calcium and phosphate deposition. the vesicles then rupture and act as a centre for crystals to grow on. more particularly, bone mineral is formed from globular and plate structures. types structure of a long bone one way to classify bones is by their shape or appearance. there are five types of bones in the human body: long, short, flat, irregular, and sesamoid. skeletal system of human bodylong bones are characterized by a shaft, the diaphysis, that is much longer than its width; and by an epiphysis, a rounded head at each end of the shaft. they are made up mostly of compact bone, with lesser amounts of marrow, located within the medullary cavity, and areas of spongy, cancellous bone at the ends of the bones. most bones of the limbs, including those of the fingers and toes, are long bones. the exceptions are the eight carpal bones of the wrist, the seven articulating tarsal bones of the ankle and the sesamoid bone of the kneecap. long bones such as the clavicle, that have a differently shaped shaft or ends are also called modified long bones. short bones are roughly cube-shaped, and have only a thin layer of compact bone surrounding a spongy interior. the bones of the wrist and ankle are short bones. flat bones are thin and generally curved, with two parallel layers of compact bone sandwiching a layer of spongy bone. most of the bones of the skull are flat bones, as is the sternum. sesamoid bones are bones embedded in tendons. since they act to hold the tendon further away from the joint, the angle of the tendon is increased and thus the leverage of the muscle is increased. examples of sesamoid bones are the patella and the pisiform. irregular bones do not fit into the above categories. they consist of thin layers of compact bone surrounding a spongy interior. as implied by the name, their shapes are irregular and complicated. often this irregular shape is due to their many centers of ossification or because they contain bony sinuses. the bones of the spine, pelvis, and some bones of the skull are irregular bones. examples include the ethmoid and sphenoid bones. terminology main article: anatomical terms of bone in the study of anatomy, anatomists use a number of anatomical terms to describe the appearance, shape and function of bones. other anatomical terms are also used to describe the location of bones. like other anatomical terms, many of these derive from latin and greek. some anatomists still use latin to refer to bones. the term "osseous", and the prefix "osteo-", referring to things related to bone, are still used commonly today. some examples of terms used to describe bones include the term "foramen" to describe a hole through which something passes, and a "canal" or "meatus" to describe a tunnel-like structure. a protrusion from a bone can be called a number of terms, including a "condyle", "crest", "spine", "eminence", "tubercle" or "tuberosity", depending on the protrusion's shape and location. in general, long bones are said to have a "head", "neck", and "body". when two bones join, they are said to "articulate". if the two bones have a fibrous connection and are relatively immobile, then the joint is called a "suture". development endochondral ossification light micrograph of a section through a juvenile knee joint (rat) showing the cartilagineous growth plates the formation of bone is called ossification. during the fetal stage of development this occurs by two processes: intramembranous ossification and endochondral ossification. intramembranous ossification involves the formation of bone from connective tissue whereas endochondral ossification involves the formation of bone from cartilage. intramembranous ossification mainly occurs during formation of the flat bones of the skull but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as mesenchyme tissue rather than from cartilage. the process includes: the development of the ossification center, calcification, trabeculae formation and the development of the periosteum. endochondral ossification occurs in long bones and most other bones in the body; it involves the development of bone from cartilage. this process includes the development of a cartilage model, its growth and development, development of the primary and secondary ossification centers, and the formation of articular cartilage and the epiphyseal plates. endochondral ossification begins with points in the cartilage called "primary ossification centers." they mostly appear during fetal development, though a few short bones begin their primary ossification after birth. they are responsible for the formation of the diaphyses of long bones, short bones and certain parts of irregular bones. secondary ossification occurs after birth and forms the epiphyses of long bones and the extremities of irregular and flat bones. the diaphysis and both epiphyses of a long bone are separated by a growing zone of cartilage (the epiphyseal plate). at skeletal maturity (18 to 25 years of age), all of the cartilage is replaced by bone, fusing the diaphysis and both epiphyses together (epiphyseal closure). in the upper limbs, only the diaphyses of the long bones and scapula are ossified. the epiphyses, carpal bones, coracoid process, medial border of the scapula, and acromion are still cartilaginous. the following steps are followed in the conversion of cartilage to bone: zone of reserve cartilage. this region, farthest from the marrow cavity, consists of typical hyaline cartilage that as yet shows no sign of transforming into bone. zone of cell proliferation. a little closer to the marrow cavity, chondrocytes multiply and arrange themselves into longitudinal columns of flattened lacunae. zone of cell hypertrophy. next, the chondrocytes cease to divide and begin to hypertrophy (enlarge), much like they do in the primary ossification center of the fetus. the walls of the matrix between lacunae become very thin. zone of calcification. minerals are deposited in the matrix between the columns of lacunae and calcify the cartilage. these are not the permanent mineral deposits of bone, but only a temporary support for the cartilage that would otherwise soon be weakened by the breakdown of the enlarged lacunae. zone of bone deposition. within each column, the walls between the lacunae break down and the chondrocytes die. this converts each column into a longitudinal channel, which is immediately invaded by blood vessels and marrow from the marrow cavity. osteoblasts line up along the walls of these channels and begin depositing concentric lamellae of matrix, while osteoclasts dissolve the temporarily calcified cartilage. functions functions of bone mechanical protection gives structure facilitates movement facilitates hearing synthetic contains bone marrow metabolic stores calcium helps regulate the acid-base balance bones have a variety of functions: mechanical