Impaired mammary development in tamoxifen-treated prepubertal heifers is associated with altered development and morphology of myoepithelial cells1

Prepubertal mammary development involves elongation and branching of ducts and stromal tissue remodeling. This process is closely linked with ovarian and pituitary hormones, growth factors, and local regulators. Accumulating evidence suggests that the myoepithelial cells also play a role in ductal development in addition to their well-recognized importance in the milk ejection reflex. Following reports that myoepithelial cells changed in correspondence with decreased mammary growth after ovariectomy of prepubertal heifers, we evaluated myoepithelial cells in mammary tissue collected from prepubertal heifers treated with the antiestrogen tamoxifen. Briefly, heifers were given placebo (n = 7) or tamoxifen (n = 8; 0.3 mg/kg per day) beginning on d 28 of life until the animals were euthanized on d 120. Tissues were collected from each of 3 zones (near the gland cistern, midway between the gland cistern and mammary fat pad, and at the interface of the parenchyma and mammary fat pad). Samples were processed to measure expression of transformation-related protein 63 (p63), smooth muscle actin, and common acute lymphoblastic leukemia antigen. We found that smooth muscle actin and common acute lymphoblastic leukemia antigen were expressed in the cytoplasm and p63 in the nuclei of myoepithelial cells. In concert with a 50% impairment in mammary growth after tamoxifen, we found that the number of myoepithelial cells around developing mammary ducts was reduced. But the average intensity of p63 expression per nucleus was not affected. We used the very distinct and exclusive staining of p63 in myoepithelial cell nuclei to capture hundreds of nuclear images for subsequent analysis using CellProfiler software. From this image analysis, we found that the area of myoepithelial cell nuclei and perimeter distances were reduced by tamoxifen. When nuclei were classified based on nuclear shape (eccentricity), we found differences in area, perimeter, and patterns of p63 expression based on Zernike number evaluations as well as treatment differences within each shape classification. These data provide support to the concept that myoepithelial cells are also the involved in mammary development in the prepubertal bovine mammary gland and that use of multispectral imaging combined with image analysis software can provide quantitative data to better understand the complex cellular interactions that ultimately regulate mammary morphogenesis in the bovine.