Effects of neonatal estrogenization on rat bone development: a histomorphometric study.

The effects of a single dose of 500 micrograms of estradiol benzoate, administered on the first day of life, on rat bone development have been histomorphometrically studied at 15 days of age. Estrogenized animals presented decreased total tibial length (16.55 +/- 0.50 vs. 17.84 +/- 0.73 mm, P less than 0.05) and increased thickness of the cartilage growth plate (528.92 +/- 13.30 vs. 382.77 +/- 37.85 micron, P less than 0.01). This increase was mostly due to the presence of a wider (P less than 0.01) layer of hypertrophic cartilage in the estrogenized rats than in control ones. It might be related to the decreased number of chondroclasts (0.20 +/- 0.01 vs. 0.36 +/- 0.06 mm-1, P less than 0.05) found in the resorption zone. Two metaphyseal zones have been considered. In the upper metaphyseal zone there was an increase in the surface density of the cartilaginous trabeculae (49.20 +/- 1.80 vs. 40.72 +/- 1.95 mm2/mm3, P less than 0.05), without changes in the volume density. It was related to the presence of thinner and more irregular trabeculae in the estrogenized animals. In the lower metaphyseal zone both the volume (0.19 +/- 0.01 vs. 0.14 +/- 0.01 mm3/mm3, P less than 0.01) and surface (34.83 +/- 3.01 vs. 26.52 +/- 2.46 mm2/mm3, P less than 0.05) densities of the osseous trabecular tissue were increased in estrogenized rats. No significant differences were found either in the number of osteocytes per area unit of osseous tissue or in the number of osteoclasts per unit length of trabecular osseous tissue.