Mammary gland, 4 day-old mouse

The mammary gland is a fascinating organ in that the majority of its development occurs postnatally, unlike most other organs. At birth, only a basic structure of ducts are present, which remain relatively dormant until puberty when the ducts elongate and branch. The gland then expands again with pregnancy, whereby alveoli form to produce and store milk. This image shows the basic structure of the mammary gland of a 4-day old mouse, before puberty. The width of the image is approximately 1.5 mm.