Scanning electron microscopic changes of the canine uterine luminal surface during oestrus and late metoestrus

Endometrial receptivity and embryo implantation in carnivores-commonalities and differences with other mammalian species

Endometrial receptivity and embryo implantation processes are a major point of pregnancy failure in many mammalian species, including humans. Although reproductive biology in many carnivore species remains enigmatic, the few that have been studied so far are invaluable comparative models. The goals of this review are to (1) summarize current data on the mechanisms involved in uterine receptivity and embryo implantation in carnivores, including commonalities and differences with other mammalian species and (2) identify research priorities to better understand a key phenomenon in a critical group of mammals. Besides unique reproductive traits in some carnivores (induced vs. spontaneous ovulation in cats, ovulation at the germinal vesicle stage in dogs), preimplantation embryo development is comparable with other orders. However, the timing of implantation varies, especially in species having an embryonic diapause. Mechanisms involved in endometrial receptivity and decidualization still remain to be fully understood, but specific markers have already been identified. Importantly, the use of endogenous hormones to control the ovarian activity may impact endometrial receptivity and subsequent embryo implantation. Next, research efforts should take advantage of advanced technologies to further study embryo implantation in carnivores and to provide more relevant models to reproductive medicine or for the conservation of rare and endangered species.

Frequency of Different Estrous Stages in Purpose-bred Beagles: A Retrospective Study

The beagle is a monoestric, nonseasonal breeder with a long estrous cycle. Owing to lengthy stages in individual phases of the estrous cycle, limited group size, and typical group assignment focused on homogenized body weight, dogs in the same stage of the cycle can be inadvertently assigned to one treatment group in toxicity studies potentially leading to erroneous interpretation. This study was conducted to better understand the frequency of the different stages of the cycle and review the associated histological features. Histologic sections of reproductive tissues were reviewed from 102 control dogs from thirty-two GLP studies. The average age of dogs at necropsy was 14.38 months, and the mean terminal body weight was 6.87 kg. Based on histological classification, fifty-five dogs were in anestrus, twenty-eight in diestrus, nine in estrus, five in proestrus, and five were classified as immature. Mean ovarian weights were higher in the estrus stage. This review indicates that more than 80% of the dogs in this study were in the anestrus-diestrus stage, and a small percentage of dogs were immature. Interpretation of drug-induced effects on the morphologic changes in the reproductive tract should be performed with due consideration given to the stage of the cycle and the potential for nonuniform assignment to drug treatment groups.

Proliferation patterns in the canine endometrium during the estrous cycle

The proliferative activity in the endometrium of 58 bitches in different stages of the estrous cycle was assessed by immunohistochemical detection of the Ki-67 proliferation associated nuclear antigen and by counting mitotic figures. The Ki-67 labelling index and the mitotic index were determined in the surface epithelium, the stroma, the crypts and the basal glands by calculating the percentage of Ki-67 positive cells and mitotic figures, respectively, on a total of 500 cells of each category. Endometrial vascular proliferation was also verified by counting the number of Ki-67 positive cells on a total of 100 endothelial cells. The present study showed two proliferation peaks involving different cell groups. In the surface epithelium, the stroma, the blood vessels and the crypts, the highest labelling and mitotic indexes were noticed during proestrus, whereas for the basal glands these indexes significantly increased (P < 0.05) during estrus compared to late metestrus and anestrus. Furthermore, a slightly positive correlation (P < 0.05) was found between the labelling index in the basal glands and the serum progesterone levels, whereas the labelling indexes in the other cell groups were positively correlated with the estradiol-17 beta levels, although not always significantly. These findings suggest that regulation of the proliferation in the surface epithelium, the stroma, the blood vessels and the crypts is different from the proliferation in the basal glands.

Apoptosis in the canine endometrium during the estrous cycle

Apoptotic cell death in the endometria of 58 female dogs in different stages of the estrous cycle was assessed (in formalin-fixed, paraffin-embedded sections) with both the terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate nick end labeling (TUNEL) assay and immunohistochemical detection of caspase-3 activity. For both techniques, the apoptotic index was determined in the surface epithelium, stroma, crypts, and basal glands by counting the percentage of stained cells in a total of 500 cells in each category. In the surface epithelium and stroma, TUNEL- and caspase-3-positive cells were rare (apoptotic index<1) throughout the estrous cycle. However, caspase-3 detection showed a significant increase in the apoptotic index in the stroma during anestrus as well as an increase in the index in both the stroma and surface epithelium in late metestrus. The apoptotic index increased during late metestrus and anestrus in the crypts and basal glands; in the crypts, this increase was significant only when caspase-3 detection was used, whereas in basal glands, significant differences were found for both techniques. In conclusion, apoptosis was present in canine endometrial cells during the estrous cycle, but caspase-3 detection showed more significant differences than the TUNEL assay. Furthermore, a high apoptotic index (suggestive of endometrial desquamation) was not detected in the surface epithelium and there was no significant correlation between the apoptotic index in any cell group and serum progesterone concentrations.

Cyclic changes of the canine endometrial surface: an electron-microscopic study

The endometrial surface morphology of 38 dogs during different stages of the estrous cycle was investigated with scanning electron microscopy. The cell surface altered from convex in proestrus and estrus to very variable in early metestrus, flattened in late metestrus and became completely plane in anestrus. Microvilli were numerous and long in proestrus and in estrus, became short and variable in number in early metestrus, decreased further in length in late metestrus and became very short and rare in anestrus. The variable appearance in early metestrus was not influenced by changing the osmolarity of the fixative and might be a physiological process. The number of glandular openings showed little variability throughout the estrous cycle. Ciliated cells were rare but present in all cycle stages except in late metestrus. However, in the latter cycle stage and in anestrus rare single strands were noted. Transmission electron microscopy was used to determine the inner structure of these strands. Microtubuli were detected in transversal and longitudinal sections but without the 9 + 2 arrangement which is characteristic for cilia. The nature and function of these structures remain unclear.