Membrane bone, such as that which makes up the upper portion of the skull, forms after cells of vertebrate embryo connective tissue gather together in the area where such bone is to form. This aggregation is followed by the development of small blood vessels in the area and the differentiation of the connective tissue cells into osteoblast cells, which make collagen, intracellular material, and cause the deposition of calcium.
In contrast, cartilage bone, in the long bones, for example, forms where cartilage was initially laid down in the vertebrate embryo. This kind of bone, also called the endochondrial bone, results via osteoblast formation from chondrocytes, followed by ossification. The differentiation between membrane bone and cartilage bone, which are indistin guishable after ossification, is made by the careful examination of appropriate tissue structures during the course of embryogenesis.
The conversion of cartilage into endochondrial bone is not complete until adulthood. At the ends of the immature cartilage bones, regions of actively growing cartilage (epiphysial plates) occur. In these regions, continued longitudinal cartilage growth followed by ossification leads to the lengthening of bones required for the development of newborn vertebrates into full-sized adults. At adulthood, the cells of the epiphysial plates stop reproducing. It is believed that imbalances between chondrocyte numbers and bone matrix material levels in articular cartilage probably play major roles in the genesis of arthritis.
All the bones in the bodies of the vertebrates change their sizes and shapes as these organisms pass through their lives. The processes involved are collectively called remodeling. An example of such change is the growth of the long bones in circumference as the limbs grow from puberty to adulthood. In the course of such bone growth the periosteum provides the osteoblasts required to deposit bone matrix around the bone exterior and to calcify it. At the same time the endosteum-derived osteoclasts often dissolve bone in the interior, thus enlarging the marrow cavity.
Remodeling in such cases occurs in response to biosignals including those caused by increases in the need for bone to bear additional weight or to anchor increased muscle mass. Conversely, inactivity and the lack of exercise can result in remodeling which produces diminished bone mass. The complex changes involved in bone remodeling are also controlled by vitamin D and hormones originating in the pituitary gland, the thyroid gland, and the parathyroid glands. Abnormalities in bone growth and remodeling are associated with a great many bone diseases, ranging from rickets to bone cancer.
—Sanford S. Singer See also: Anatomy; Cell types; Endoskeletons; Exoskeletons; Growth; Horns and antlers; Hydrostatic skeletons; Locomotion; Muscles in vertebrates; Nutrient requirements; Vertebrates.
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