Fluorescence-based immunodetection of CD31 (a vascular endothelial cell marker) in the parabiosis model. Six to twelve ovaries were obtained from three to six wild-type mice in their natural estrous cycle. Immunostaining was evaluated on three to four tissue sections in each ovary in each developmental stage of the follicles. The developmental stages are defined in Materials and Methods. Arrows indicate CD31 and GFP double-positive cells. T: theca cell layer, G: granulosa cell layer. Scale bars; 20 μm.