Bone, the skeletal organ, is classified as being either intramembranous or endochondral in origin. Intramem-branous bone is formed by the in situ differentiation of mesenchymal progenitor cells into osteoblasts that secrete osteoid; this matrix then undergoes calcification with hydroxyapatite crystal deposition. In contrast, endo-chondral bones are embryonically formed as a cartilage anlage. Mesenchymal progenitor cells differentiate into chondroblasts and mature to chondrocytes that coalesce into a model representing a miniature version of the future bone. Chondrocytes proliferate and then mature, a process that includes hypertrophy with secretion and mineralization of matrix. Through this maturation process, the local milieu changes, inducing the death of the most centrally located chondrocytes, thereby permitting blood vessel invasion through the nutrient foramen (Fig. 1). Accompanying the vasculature are marrow stem cells, osteoblast progenitor cells, and osteoclast and chondroclast precursors. Chondroclasts locally degrade mineralized cartilage matrix, whereas osteoblasts utilize the cartilage matrix remnants as a substrate for osteoid deposition. This central invasion forms the primary center of ossification. At each of the two ends of the anlage, the vascular invasion is reiterated, generating three distinct centers of ossification: the primary and two secondary centers. The ossification centers remain distinct due to a retained cartilage disk between the primary and each secondary center. The cartilage disk, or growth plate, offers growth potential (discussed later in this article). Additional cartilage remains at the extreme ends of the anlage to become the articular cartilage essential in joint function. The bone is now defined into anatomical regions relative to the growth plates. The central region enveloping the primary center of ossification is the diaphysis. The regions encompassing the growth plate are the metaphyses, whereas the two ends of the bone are the epiphyses.
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