1. Endosteal new-bone formation was studied in estrogen-treated mice, including observations upon the gross, microscopic, and roentgenographic features of the skeletal reaction.
2. A quantitative study by means of combined roentgenographic and histological techniques gives accurate and uniform results in the measurement of the osteogenetic reaction of the skeleton.
3. The pattern, sequence, and rate of the bone changes in estrogen-treated mice are similar to those of the growth-rate differential of the skeleton.
4. Newborn mice, treated with estrogen from the date of birth, show microscopic evidence of changes within ten days, although no gross or roentgenographic changes are demonstrable until after twenty days of treatment. The skeletons of embryos of treated mothers show no endosteal bone in roentgenograms.
5. Mice from one to six months of age show no appreciable differences in their response to estrogens. Aged mice, one year of age or older, show some delay in the response, and the volume of endosteal bone is less than in similarly treated young mice. There was no variation in the bone reaction in the different hereditary strains studied.
6. The volume of new bone deposited is, within the limits of the tolerance of the animal, directly proportional to the dose and the period of administration.
7. Mice, treated with 2.0 milligrams of a-estratdiol benzoate over a period of four weeks, recovered from the effects of estrogen after treatment was discontinued. Normal endochondral ossification reappeared below the epiphyseal lines within six weeks. The endosteal bone was almost completely absorbed within eighteen weeks.
8. The serum calcium, inorganic phosphorus, and alkaline phosphatase are within the range of normal levels during administration of estrogen and endosteal new-bone formation.
9. Esterification of an estrogen is one of the most important factors in its osteogenetic potency.
10. Endosteal new-bone formation is not induced by lutoid, corticoid, or testoid hormones. There is neither inhibitory more synergistic action of progesterone, desoxycorticosterone acetate, or testosterone propionate upon the osteogenetic action of estrogens.
11. Osteogenetic action of estrogen is neither enhanced nor inhibited by simultaneous treatment of the animals with preparations of anterior-pituitary hormone. Adrenocortitotropic hormone induces no endosteal new-bone formation.
12. No evidence has been found that the mouse produces endosteal new bone, characteristic of the effects of estrogens, unnder strictly physiological conditions. During pregnancy, when large amounts of estrogen are being excreted in the urine, neither the foetal nor the maternal skeleton shows roentgenographic evidence of estrogen-induced bone formation. No production of endosteal bone has been observed in either immature or mature mice in which the ovaries have been under the influence of large doses of gonadotropic hormones, administered over a long period of time.
13. No specific local osteogenetic effects from implantation of free estrogens in bones have been observed. The skeleton showed the usual systemic osteogenetic reaction to absorption of implants of estrogen, but not to the control implants of cholesterol.
14. Evidence has been submitted which suggests that the estrogen-induced bone changes are a reaction to injury of the osteogenic connective tissues of the primitive bone marrow.
15. These findings constitute a demonstration of experimental marble-bone disease. The experimental condition in mice is like the disorder observed by Albers-Schönberg in the following ways: (1) The entire skeleton shows unabsorbed cartilage matrix in the bone tissue of the growing metaphyses; (2) the serum alkaline phosphatase, calcium, and inorganic phosphorus are within the normal range; (3) there is no appreciable osteoclast formation or bone resorption; (4) withdrawal of the causative factor reinstates the normal balance between bone formation and bone resorption, and results in a roentgenographic picture similar to marble bones during a period of remission of the disease.