In this work, the presence of estrogen receptor alpha (ER-alpha) was determined in different cell subpopulations in the left growing and right regressing ovaries of Gallus domesticus from 13-day-old chicken embryos to one-month-old chickens by immunohistochemistry. Results revealed positive ER-a immunostaining in both ovaries during development, but the percentage, staining intensity, and cellular distribution of ER-a immunostaining changes according to whether it is the left or right ovary and with the animal's age. In the left ovary, the ER-a was localized in the nuclei of the germinal epithelium and in germ cells of the ovarian cortex, as well as in the interstitial cells, undifferentiated cells, and epithelial cells of the lacunar channels of the ovarian medulla in all ages. In contrast, in the right ovary from 13-day-old chicken embryos to one-week-old chickens, only the epithelial cells of lacunar channels were ER-a immunoreactive, but in the right ovary of one-month-old chickens both the epithelial cells of lacunar channels and the interstitial cells presented ER-a. These results demonstrate differential expression of ER-a in both chicken ovaries during development in a cell type-specific distribution, suggesting that these differences may be regarded as an important cause in the process of asymmetric ovarian development in the chicken. (c) 2014 Elsevier Inc. All rights reserved.