Apoptosis in the uterotubal junction and oviductal isthmus during the estrous cycle of the bitch.

Apoptotic cells were evaluated in the lamina epithelialis (LE) and lamina propria (LP) of the canine uterotubal junction and oviductal isthmus by means of the TUNEL assay and Caspase-3 detection regarding cyclic changes. Oviducts of bitches in anestrus, early and late follicular phase, 2 and 4 days after ovulation, mid-luteal phase, and endometrial reparation were available. The rate of apoptosis was generally low (min = 0; max = 33 apoptotic cells in 500 cells). Regardless of the stage of the estrous cycle, the number of apoptotic cells was significantly higher in the LE compared to the LP (P < 0.05). In anoestrus, early and late follicular phase as well as during endometrial reparation, the number of apoptotic cells was on a constantly low level. A significant increase was found 4 days after ovulation by caspase-3 detection coinciding with early luteal development and in mid-luteal phase. The results demonstrate that the number of apoptotic cells changes depending on the stage of the ovarian cycle. The coincidence of the increase in the number of apoptotic cells and the time of achievement of oocyte competence, which is known to occur in the bitch 3-4 days after ovulation, suggests a causal relationship between the postovulatory maturation stage of oocytes and the rate of apoptosis in the caudal oviduct. Furthermore, a specific relevance of apoptosis may exist in the process of fertilization regarding sperm storage and detachment of spermatozoa bound to the epithelium of the caudal oviduct, which has been identified as a functional sperm reservoir in other species.