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What are the steps involved in bone growth?

The embryonic primordiae of the appendicular skeleton are the limb buds, which are mesodermal structures covered by ectoderm. The first visible outline of the embryonic limb follows a condensation of mesenchymal cells which subsequently differentiate into cartilage cells, the chondrocytes. These cells secrete a matrix and so produce cartilaginous models of the future bones. Surrounding this cartilage is the perichondrium, the outer layer of which becomes a connective tissue sheath while the inner cells remain pluripotential. This cartilage rudiment grows by interstitial and appositional growth, and a vascular system develops to invade the perichondrium. A collar of bone is then laid down around the mid-shaft of the bone. This ossification is a result of the inner perichondrial cells differentiating into bone forming cells, the osteoblasts. At the same time the osteoblasts, together with capillaries, invade the centre of the shaft to form a primary, or diaphyseal ossification centre, at a site where the cartilage cells and matrix have begun to disintegrate. Trabecular bone is then deposited on cartilaginous remnants. The embryonic bone increases in width by appositional growth, and the central cancellous bone core gradually becomes resorbed to form a marrow cavity.

In long bones, another secondary centre of ossification appears at the growing cartilaginous ends, the epiphyseal ossification centre (Fig. 1). (This ossification does not replace the cartilage at the articular end of the model; this remains as articular cartilage.) In addition, a transverse plate of cartilage extends across the bone separating the epiphyseal from the diaphyseal ossification centre. This is the epiphyseal growth plate that persists until an individual stops growing. Growth of cartilage in the epiphyseal plate is continuous, but the plate does not become thickened because on its diaphyseal side the cartilage matures, is calcified, resorbed and replaced by bone. This is endochondral ossification, the mechanism responsible for increasing the length of the bone. In the growing child this is a site of many complex cellular events; namely cartilage growth, maturation, resorption and bone formation. Disturbance of any one of these processes may be reflected in growth retardation.

As an individual's height increases, the bone must increase its diameter, and this is achieved by new bone being laid down by the osteogenic layer of the periosteum. This is intramembranous ossification that does not involve prior cartilage formation. However, the shafts of long bones do not increase in width significantly, as this would increase skeletal mass excessively, because there is resorption of bone on the inner (endosteal) surface by bone resorbing cells, the osteoclasts. This leads to an increase in the size of the marrow cavity with age, and means that the cortical bone of an adult's femur, for example, is not the same bone that existed in childhood. The cycle of bone resorption and formation is bone remodelling, and in the growing skeleton this is often described as 'structural modelling'. Remodelling of bone is a dynamic process that continues throughout life with losses from osteoclastic bone resorption made good by bone formation. Histomorphometric studies of bone have shown that in the remodelling cycle osteoclasts resorb bone surfaces to form an erosion cavity. Mononuclear cells then fill in the cavity, differentiate into osteoblasts and begin to lay down matrix (Eriksen, 1986). It has been estimated that this process can take up to three months with mature osteoblasts secreting matrix for up to 100 days. This balanced process is described as coupling (Frost, 1964), and it is the uncoupling of formation from resorption that leads to skeletal diseases such as osteoporosis where net resorption is greater than formation.

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