Fig.21.2(A)Activeosteoblastssynthesizingbonematrix.Thesurroundingspindlecellsrepresentosteoprogenitorcells.(B)Twoosteoclastsresorbingbone.
Osteocytes, located within the bone, are interconnected by an intricate network of cytoplasmic processes through tunnels known as canaliculi. Osteocytes help to control calcium and phosphate levels, detect mechanical forces, and translate them into biologic activity—a process called mechanotransduction.
Osteoclasts, located on the surface of bone, are specialized multinucleated macrophages, derived from circulating monocytes, which are responsible for bone resorption (
Fig. 21.2B ). By means of cell surface integrins, osteoclasts attach to bone matrix and create a sealed extracellular trench (resorption pit). The cells secrete acid and neutral proteases (predominantly matrix metalloproteases [MMPs]) into the pit, and these enzymes resorb the bone.
During embryogenesis, long bones develop from a cartilage mold by the process of endochondral ossification.
A cartilage mold (anlagen) is synthesized by mesenchymal precursor cells. At approximately 8 weeks’ gestation, the central portion of the anlagen is resorbed, creating the medullary canal. Simultaneously, at midshaft (diaphysis), osteoblasts begin to deposit the cortex beneath the periosteum producing the primary center of ossification and growing the bone radially. At each longitudinal end (epiphysis), endochondral ossification proceeds in a centrifugal fashion (secondary center of ossification). Eventually, a plate of the cartilage becomes entrapped between the two expanding centers of ossification, forming the physis or growth plate (
Fig. 21.3 ). The chondrocytes within the growth plate undergo sequential proliferation, hypertrophy, and apoptosis. In the region of apoptosis the matrix mineralizes and is invaded by capillaries, providing the nutrients for osteoblasts, which synthesize osteoid. This process produces longitudinal bone growth.
Intramembranous ossification, by contrast, is responsible for the development of flat bones. Bones of the cranium, for example, are formed by osteoblasts directly http://ebooksmedicine.net
Fig.21.3Activegrowthplatewithongoingendochondralossification.1,Reservezone.2,Zoneofproliferation.3,Zoneofhypertrophy.4,Zoneofmineralization.5,Primaryspongiosa.
from a fibrous layer of tissue, without cartilage anlagen. The enlargement of flat bones is achieved by deposition of new bone on a preexisting surface.
The adult skeleton appears static but is actually constantly turning over in a tightly regulated process known as remodeling. Remodeling takes place at a microscopic locus known as the bone (or basic) multicellular unit (BMU), which consists of a unit of coupled osteoblast and osteoclast activity on the bone surface.
The events at the BMU are regulated by cell–cell interactions and cytokines (