AbstractA total of 45 prepubertal beagles 6 to 8 weeks of age were used to study the cytological changes that accompany regression of the oviductal epithelium. The oviductal epithelium in untreated pups consisted of undifferentiated low cuboidal cells that measured 10.3 ± 2.0 μm in height. In response to estradiol (E2), low cuboidal cells underwent hypertrophy, hyperplasia, and cytodifferentiation and gave rise to columnar ciliated and secretory cells. After 12 days of E2treatment the epithelium was fully differentiated and measured 29.4 ± 2.6 μm in height with 56% of the cells possessing cilia. When E2treatment was continued for an additional 12 days, the epithelium was maintained in a differentiated state. However, if E2treatment was terminated or progesterone (P) given alone or in conjunction with E2, the oviductal epithelium regressed and after 6 days was composed of low cuboidal cells that ranged in height from 9 to 14 μm with approximately 25% of the cells possessing cilia. A variety of cytological changes characterized the process of regression. The most immediate signs that regression was underway were a reduction in the height of the epithelium and the presence of cells with shrunken, pleomorphic nuclei that lacked prominent nucleoli. Degenerative events included: (1) pinching off and shedding of the apical cytoplasm of cells comprising the epithelium, (2) extrusion of whole cells and/or nuclei, and (3) resorption of cilia and basal bodies. During the first 6 days following E2withdrawal or P treatment, macrophages and cellular debris were frequently present within the lumen of the oviduct. The process of regression did not proceed synchronously throughout the ampulla of the oviduct, nor did all cells appear to degenerate in the same manner. The cytological changes that accompanied oviductal regression following P treatment were identical to those observed following E2withdrawal. Results from experiments conducted in the present study show that: (1) E2induces the oviductal epithelium to differentiate and is required to maintain the epithelium in a differentiated state, (2) E2withdrawal or P treatment causes the oviductal epithelium to regress, (3) at least three distinct degenerative processes are involved in the transition of columnar ciliated and secretory cells into low cuboidal cells, and (4) regression does not occur synchronously throughout the ampulla region of the ovi