Estrogen, the female sex hormone, plays an important role in the formation of new bone at the ends of long bones in both males and females. The present study was an attempt to determine the results of various doses of estrogen on the complicated process of bone formation. The effects of ovariectomy (low estrogens), sham surgery (normal estrogens) and ovariectomy with estrogen injections (high dose estrogens) were studied in the long bones of rabbits.
Growth of long bones depends on the production of a cartilage matrix and the growth of chondrocytes (cartilage cells) in spaces in the matrix. The growth plates are at the ends of the long bones. Chondrocytes proliferate, secrete bone matrix and proliferate into large chondrocytes to fill spaces in the bony matrix. Osteoblasts migrate to the area to lay down more bone.
Both, ovariectomy, which reduced of estrogen levels, and ovariectomy with estrogen injections, which elevated estrogen levels, resulted in a reduction in the number of chondrocytes at the end plates. Both of these two groups showed deceased proliferation and the latter group showed increased apoptosis (programmed cell death). Ovariectomy led to more rapid growth of the growth plate and to delayed plate fusion. High dose estrogen led to growth and early growth plate fusion in long bones.
The femurs of ovariectomized rabbits were longer than those of normal rabbits. The femurs of ovariectomized rabbits given high doses of estrogens were shorter than normal femurs because of premature end plate closure.
There are two kinds of estrogen receptors: ER-alpha, which accelerated endochondral bone formation and slowed apoptosis, and ER-beta, which represses long bone growth and increases apoptosis. The type of receptors stimulated depends on the sex, species and the dose of estrogen given.
CONCLUSION: “…estrogen has to be maintained within a narrow range for optimal bone growth.” Bone growth is altered by the animal receiving either too little or too much estrogen.
NOTE: A chondrocyte is a mature cartilage cell in a hole (lacuna) in the cartilage matrix.
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