Growth Hormone Stimulates Cartilage and Bone Growth
Although growth hormone stimulates increased deposition of protein and increased growth in almost all tissues of the body, its most obvious effect is to increase growth of the skeletal frame. This results from multiple effects of growth hormone on bone, including
1. increased deposition of protein by the chondro-cytic and osteogenic cells that cause bone growth,
2. increased rate of reproduction of these cells, and (3) a specific effect of converting chondrocytes into osteogenic cells, thus causing deposition of new bone.
There are two principal mechanisms of bone growth. First, in response to growth hormone stimula-tion, the long bones grow in length at the epiphyseal cartilages, where the epiphyses at the ends of the bone are separated from the shaft. This growth first causes deposition of new cartilage, followed by its conversion into new bone, thus elongating the shaft and pushing the epiphyses farther and farther apart. At the same time, the epiphyseal cartilage itself is progressively used up, so that by late adolescence, no additional epi-physeal cartilage remains to provide for further long bone growth. At this time, bony fusion occurs between the shaft and the epiphysis at each end, so that no further lengthening of the long bone can occur.
Second, osteoblasts in the bone periosteum and in some bone cavities deposit new bone on the surfaces of older bone. Simultaneously, osteoclasts in the bone remove old bone. When the rate of deposition is greater than that of resorption, the thickness of the bone increases. Growthhormone strongly stimulates osteoblasts. Therefore, thebones can continue to become thicker throughout life under the influence of growth hormone; this is espe-cially true for the membranous bones. For instance, the jaw bones can be stimulated to grow even after ado-lescence, causing forward protrusion of the chin and lower teeth. Likewise, the bones of the skull can grow in thickness and give rise to bony protrusions over the eyes.
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