6.1 Tissues and Organs of the Skeletal System (p. 130) The skeletal system is a framework composed of bones, cartilages, and ligaments. The study of this system is called osteology.
The functions of this system include support, movement, protection of soft tissues, blood formation, electrolyte balance, acid–base balance, and detoxification.
Osseous tissue (bone) is a connective tissue in which the matrix is hardened by calcium phosphate and other minerals. Other tissues found in a bone include blood, bone marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue.
Bones are classified by shape as long, short, flat, and irregular bones.
A bone has an outer shell of compact bone, which usually encloses more loosely organized spongy bone.
A long bone has a relatively narrow, long diaphysis (shaft) with an expanded epiphysis (head) at each end. The epiphysis is filled with spongy bone. Bone marrow occupies the diaphysis and the spaces amid the spongy bone of the epiphysis.
A cartilaginous epiphyseal plate separates the marrow spaces of the epiphysis and diaphysis in children and adolescents. It is the site of bone elongation.
A bone is externally covered with a fibrous periosteum, which is bound to the bone by collagenous perforating fibers. The medullary cavity is lined with a fibrous endosteum.
At most joints, the ends of a bone have no periosteum but are covered with hyaline articular cartilage.
Flat bones consist of a sandwichlike arrangement of spongy bone enclosed between two layers of compact bone. The spongy bone layer of the skull is called the diploe.
6.2 Histology of Osseous Tissue (p. 133) Osseous tissue has four kinds of cells: osteogenic cells, osteoblasts, osteocytes, and osteoclasts.
Osteogenic cells are stem cells found in the endosteum, periosteum, and central canals. They arise from embryonic mesenchyme and give rise to osteoblasts.
Osteoblasts are bone-depositing cells found on the bone surfaces. They produce the organic components of the bone matrix and promote its mineralization.
Osteocytes are bone cells found within the lacunae and surrounded by bone matrix. They communicate with each other and with surface osteoblasts by way of cytoplasmic processes in the canaliculi of the matrix. They both deposit and absorb bone matrix, and function as strain detectors that stimulate osteoblasts to deposit bone.
Osteoclasts are bone-dissolving cells found on the bone surfaces. On the side of the cell facing the bone, that have a ruffled border where they secrete bone-dissolving enzymes and hydrochloric acid.
The matrix of bone is about one-third organic and two-thirds inorganic matter by dry weight.
The inorganic part of the matrix is about 85% hydroxyapatite (crystalline calcium phosphate), 10% calcium carbonate, and 5% other minerals.
The organic part of the matrix consists of collagen and large protein–carbohydrate complexes called glycosaminoglycans, proteoglycans, and glycoproteins.
The mineral component of the bone renders it resistant to compression, so it does not crumble under the body’s weight, whereas the protein component renders it resistant to tension, so it can bend slightly without breaking.
Compact bone is composed largely of cylindrical units called osteons, in which the matrix is arranged in concentric lamellae around a central canal. Osteocytes in their lacunae lie between the lamellae of matrix and their canaliculi cross from one lamella to the next.
Collagen fibers wind helically along the length of each lamella, with the helices coiling in alternating directions in adjacent lamellae to give the matrix added strength.
Blood vessels enter the bone matrix through nutrient foramina on the surface and pass by way of perforating canals to reach the central canals.
In addition to the concentric lamellae of osteons, compact bone exhibits circumferential lamellae that travel parallel to the inner and outer bone surfaces, and interstitial lamellae located between osteons, representing the remains of older osteons that have partially broken down.
Spongy bone consists of thin trabeculae and spiculesof osseous tissue, with spaces between the trabeculae occupied by bone marrow. The matrix is arranged in lamellae but shows few osteons. Spongy bone provides a bone with maximal strength in proportion to its light weight.
There are two kinds of bone marrow: blood-forming (hemopoietic) red bone marrow and fatty yellow bone marrow. Red marrow occupies the medullary spaces of nearly all bones in children and adolescents. By adulthood, red marrow is limited to the vertebrae, ribs, sternum, pectoral and pelvic girdles, and proximal heads of the humerus and femur; it is replaced by yellow marrow elsewhere.
6.3 Bone Development (p. 137) Bones are produced by two developmental processes: intramembranous ossification, which produces mainly flat bones, and endochondral ossification, which produces most of the skeleton.
Intramembranous ossification begins when mesenchyme condenses into a sheet of soft tissue populated with osteogenic cells. Osteogenic cells gather along soft trabeculae of mesenchyme and differentiate into osteoblasts. The osteoblasts deposit soft osteoid tissue and then calcify it. Calcified trabeculae become spongy bone, while compact surface bone is formed by filling in the spaces between trabeculae with osseous tissue.
Endochondral ossification is a process in which a hyaline cartilage model is replaced by bone. It begins when chondrocytes in the middle of the cartilage model enlarge and form a primary ossification center. As the cartilage lacunae break down, they merge into a primary marrow cavity and the chondrocytes die.
Stem cells enter the primary marrow cavity by way of blood vessels and differentiate into osteoblasts and osteoclasts. Osteoblasts deposit osteoid tissue and then calcify it to form temporary bony trabeculae. Osteoclasts dissolve cartilage remnants and enlarge the marrow cavity, which grows toward the ends of the bone.
Near the time of birth, a secondary ossification center appears in the middle of the epiphysis. Ossification proceeds from here outward, creating a secondary marrow cavity and leaving a layer of articular cartilage over the end of the bone. The primary and secondary marrow cavities remain separated for a time by a cartilaginous epiphyseal plate.
Long bones increase in length by the interstitial growth of cartilage in a transitional zone called the metaphysis, which lies between the primary marrow cavity and the cartilage of an epiphysis or epiphyseal plate.
The metaphysis has five zones: the zone of reserve cartilage farthest from the marrow space; the zone of cell proliferation, where chondrocytes multiply and form longitudinal columns of cells; the zone of cell hypertrophy, where these chondrocytes enlarge; the zone of calcification, where the matrix becomes temporarily calcified; and nearest the marrow space, the zone of bone deposition, where lacunae break down, chondrocytes die, and bone is deposited.
The epiphyseal plates are depleted from the end of adolescence into the 20s and do not appear in adults older than 30. A bone cannot grow longer, and one cannot grow taller, once these plates are depleted and the gaps between the epiphyses and diaphyses close. The plates leave surface markings called epiphyseal lines on a bone.
Bones grow in thickness and width by appositional growth, the deposition of new osseous tissue on the bone surface by a process similar to intramembranous ossification.
Bones are remodeled throughout life to accommodate bodily growth and changes in force applied to the skeleton. According to Wolff’s law of bone, osteoblasts and osteoclasts reshape bones throughout life to adapt to the stresses placed on them.
Some nutrients required for bone development include calcium, phosphate, and vitamins A, C, and D. Hormones that stimulate bone growth include calcitonin, growth hormone, estrogen, testosterone, thyroid hormone, and insulin. Parathyroid hormone promotes bone resorption by osteoclasts.
With age, osteoblast activity begins to lag behind osteoclast activity, resulting in a loss of bone mass (osteopenia). This can lead to tooth loss and bone brittleness, and if severe enough, to osteoporosis.
Bones can break because of trauma (stress fracture) or diseases that weaken a bone and make it unable to withstand normal levels of stress (pathologic fracture). Fractures are set by either closed reduction, which does not involve surgical exposure of the bone, or open reduction, which involves the surgical use of plates, screws, or pins to align bone fragments.
Fractures are classified according to completeness of the break, number of pieces into which a bone is broken, alignment or misalignment of the fragments, and whether or not the skin is broken.
The most common bone disease is osteoporosis, a loss of bone mass (especially spongy bone) causing increasing susceptibility to pathologic fractures. Fractures of the vertebrae, wrist, and hip, and a spinal deformity called kyphosis (exaggerated thoracic curvature) commonly result from osteoporosis.
Osteoporosis can occur in either sex, any race, and a wide range of ages, but risk factors that increase its incidence include being white, female, of light build, and of postmenopausal age, as well as inadequate exercise, low calcium intake, smoking, vitamin C deficiency, and diabetes mellitus.