The study of bones is called [1] osteology . Although a bone has several tissues, the hard, mineralized tissue that preserves best is called [2] osseous tissue . Bones specialized for leverage are in a class called [3] long bones. Those specialized to enclose and protect soft organs are [4] flat bones. The first type has a long shaft or [5] diaphysis enclosing a [6] medullary cavity, and has an expanded head or [7] epiphysis enclosing a layer of spongy osseous tissue called the [8] diploe . A bone is covered with a connective tissue membrane called the [9] periosteum . Between the shaft and the head, children have a cartilaginous growth zone called the [10] epiphyseal plate .

[11] Osteogenic cells arise from embryonic fibroblasts and multiply continually. They divide and differentiate into [12] osteoblasts , which deposit the organic matrix of bone and become osteocytes as they become entrapped in it. The cells that dissolve bone, called [13] osteoclasts , develop from the same bone marrow cells that produce [14] monocytes of the blood. Osteocytes are in touch with each other by slender cellular processes passing through narrow channels called [15] canaliculi .

The organic matter of osseous tissue includes fibers of the protein [16] collagen and a [17] GAG (glycosaminoglycan) called chondroitin sulfate. The inorganic matter is largely made up of crystals of calcium phosphate salt called [18] hydroxyapatite . In [19] compact, or dense bone, most of the matrix is organized into osteons, which consist of multiple layers of [20] lamellae organized around a central canal. Spongy bone consists of slender plates and rods called [21] trabeculae . Of the kinds of bone marrow, [22] red bone marrow (myeloid tissue) performs a hemopoietic role, whereas [23] yellow bone marrow is mostly adipose tissue.

Most bones form by a process called [24] endochondral ossification , which begins with a cartilage model. The [25] primary ossification center in this bone begins as a region of hypertrophied chondrocytes. As the matrix between these breaks down, the chondrocytes die and the center is invaded by a bud of vascular connective tissue. This forms a hollow space called the [26] primary marrow space . At each end of this is a region called the [27] metaphysis where cartilage of the heads of the bone continues to be converted to osseous tissue. The 27 has five zones including the zone of [28] cell proliferation , in which chondrocytes begin to multiply rapidly but are not yet enlarged, the zone of [29] cell hypertrophy , a region in which they enlarge, and the zone of [30] calcification , where the matrix between cell columns begins to calcify. Secondary ossification centers form in the [31] epiphyses of the bone around the time of [32] birth .

Even after a bone is formed, it continues to undergo [33] remodeling , meaning changes in size and shape in response to mechanical stress, gravity, hormones, and other factors. To increase in length, a bone can only undergo [34] appositional growth, in which new osteons are added at the surface. Achondroplastic [35] dwarfism results from a genetic anomaly in which the chondrocytes of this growth zone fail to multiply.

The process of taking minerals from the blood and incorporating them into osseous tissue is called mineral [36] deposition, or mineralization . The crystals formed in this process only develop if the product of blood calcium and phosphate concentrations reaches a threshold value called the [37] solubility product . The process of dissolving bone and releasing minerals into the blood is called mineral [38] resorption . In part, this involves an osteoclast enzyme called [39] acid phosphatase , which digests collagen.

Calcium balance in the blood must be carefully controlled because a calcium deficiency, called [40] hypocalcemia , can cause tetany and suffocation, whereas a calcium excess, called [41] hypercalcemia , can depress neuromuscular function. The latter condition is normally prevented in children by the hormone [42] calcitonin , which inhibits osteoclasts and stimulates osteoblasts. In both children and adults, calcium deficiency is normally prevented by [43] parathyroid hormone, which has the opposite effects as well as promoting calcium absorption and reduced calcium excretion. Calcium absorption also depends on an active form of vitamin D called [44] calcitriol , produced by the coordinated action of the skin, [45] liver , and [46] kidneys .

Bone Disorders

Bone fractures are classified in several ways. For example, a/an [47] open (compound) fracture is one in which a bone breaks through the skin, and a/an [48] closed (simple) fracture is one in which it does not. Any 47 fracture is also a/an [49] displaced fracture, meaning the two broken ends of the bone are no longer in anatomical alignment with each other. A fracture that goes only part way across a bone, leaving the two pieces joined, is a/an [50] incomplete fracture. A twisting motion may produce a/an [51] spiral fracture, which coils around a bone. The healing of a fracture begins with a blood clot called a fracture [52] hematoma , which becomes infiltrated with blood vessels, fibroblasts, and osteogenic cells to form [53] granulation tissue. As cartilage and then minerals form in this tissue, it becomes a soft and then a hard [54] callus . Most fractures can be set by [55] closed reduction , a procedure that does not require surgical exposure of the bone. The most common bone disease is not fractures, but a demineralization disease called [56] osteoporosis , seen most frequently in postmenopausal white women. This condition leads to bone weakness and susceptibility to fractures and spinal deformities such as "widow’s hump."