Four types of lobules are found in the human breast. type I lobules are the least differentiated and consist of bud-like structures; type II lobules have a larger number of ducts; and type III and IV lobules may develop further during pregnancy and lactation. type I lobules are seen at the onset of menstruation and consist of clusters of alveolar buds surrounding a terminal duct. types II and III consist of ducts that gradually increase in size around the duct. Type IV lobules consist of fully developed glandular vesicles. The average number of components per lobule increases progressively from type I to type IV. After maternal weaning, the mammary glands are enriched with type III lobules, which are better differentiated and have low estrogen receptor content and low proliferative activity. However, type I lobules are most common in infertile women of any age, with high estrogen receptor content and high cell proliferation. type I lobules are thought to be the site of ductal carcinoma in situ, type II lobules are the site of lobular carcinoma, and type III lobules are the site of adenoma, fibroadenoma, and cysts. Type I and type II lobules are more reactive to chemical carcinogens in vitro compared to type III lobules.