Surgical exposure of the mammary gland for GFP imaging does not inhibit subsequent gland development. A 4-week-old GFP mouse was anesthetized and one #4 mammary gland exposed in vivo and imaged. The epithelial tree is clearly visible, branching out from the nipple (^). (A) The lymph node (*) is visible at the junction of the blood vessels (open arrowheads). The dark crescent below the lymph node is caused by the mammary fat pad blocking the fluorescence of the underlying skin. (B) Two terminal end buds (marked with a box in panel A), at high magnification. After images were acquired the skin was closed and the mouse was allowed to recover. At 8 weeks of age the mouse was sacrificed and both #4 mammary glands were imaged for GFP fluorescence (C-D). Both glands show a similar degree of mammary development. Arrows: mammary ducts, open arrowheads: blood vessels, closed arrowheads: terminal end buds, *: lymph node, ^: nipple. Magnifications are as indicated.