Abstract
Nineteen rats and 30 guinea pigs having regular cycles were used. Daily smears were taken in the rats for 3 weeks and in the guinea pigs for 2 months. The animals were killed at various times during the oestrus cycle. The height of the epithelium was determined by averaging 10 cell-counts taken from sections near the center of the nipple.
In the rat the difference of the cell-count at oestrus and dioestrus is slight, but the shape of the epithelial cells changes. At dioestrum the cell is flat and the nucleus occupies most of the cell. During oestrum the cell is oblong. The change in shape increases the distance from the basement membrane to the periphery, while the nucleus appears to occupy a smaller part of the cell.
In the guinea pig the change in the shape of the superficial epithelial cells of the nipple is marked and there is an increase in number. The epithelium of the nipple of the guinea pig has rete pegs. During dioestrum the epithelium averages 10.5 cells for the height of the papilla and 4.2 cells for the intervening epithelium. The proliferation begins on the first day of the oestrus cycle. It then averages 16.1 cells for the height of the rete pegs and 7.3 cells for the height of the lower epithelium. It remains high until the fifth day after the onset of oestrum.
The epithelium from the nipples of 2 adult males, counting the lowest and highest part of the epithelium averaged 5.6 and 6.0 cells respectively. Females spayed for periods of 28, 33, and 46 days have a low nipple epithelium, averaging 9 and 3 cells in height, for rete pegs and lower epithelium respectively.
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