Research Paper Volume 17, Issue 1 pp 161—169

Figure 3. Estrous cycle patterns in young (3 momths, A), middle-aged (10 months, B) and old (26 months, C) female rats. The Y axis represents the estrous cycle phases of the animal on the indicated day of a 31-day window (X-axis) as determined by vaginal cytology. Four estrous phases were defined according to the proportion of epithelial and cornified cells or leukocytes; Proestrus (P) was characterized by abundance of epithelial cells and the presence of leukocytes. Estrus (E), showed abundance of cornified cells and to a lesser extent epithelial cells. Diestrus 1 (D1), was defined by the presence of an increasing number of leukocytes accompanied by substantial numbers of cornified cells. Diestrus 2 (D2) was characterized by high abundance of leukocyes which constituted the vast majority of cells present in the vaginal smears at D2.