Elizabeth Robertson, FRS, Professor of Developmental Biology, Wellcome Trust Principal Research Fellow, Sir William Dunn School of Pathology, University of Oxford, UK
This is an image of a mouse mammary gland. The fluorescent staining highlights the grape-like alveolar units, many thousands of which bud out of the mammary tree in response to circulating steroid hormones of pregnancy. The alveoli contain specialized milk producing cells that secrete the milk into the lumen of the ductal system to feed the newborn. When the pups mature and start to eat solid food these alveoli regress. They are then completely rebuilt from resident stem cells at the next pregnancy.