Histological characteristics of the uterine endometrium and corpus luteum during early embryogenesis and the relationship to embryonic mortality in the domestic cat

Pregnancy rates are low and litter sizes generally small when assisted reproduction techniques are used in gonadotropin-treated felid (cat) species. A prerequisite to determining whether or not abnormal morphological changes in the uterine lumen or corpus luteum (CL) are related to this reproductive failure is the documentation of normal histological kinetics during natural embryogenesis. This study characterized the histological changes of the endometrium and ovarian CL during the early stages of preimplantation embryonic development in the naturally estrous, mated queen. The purpose was to 1) develop a system for dating the cat endometrium and CL of early pregnancy; 2) document the frequency of abnormal uterine and CL histology under natural mating conditions; and 3) compare histological traits of queens producing good- vs. poor-quality embryos. Naturally estrous, mated queens were ovariohysterectomized at 64 h (n = 8), 76 h (n = 11), 100 h (n = 8), 124 h (n = 7), 148 h (n = 6), or 480 h (n = 8) after first copulation. Embryos collected from oviductal and uterine flushings were graded for quality, and uteri and ovaries were fixed in formalin. Fixed tissue sections were stained and multiple histological traits described for each uterine (endometrial height, endometrial vacuolation, percentage of glandular cells with subnuclear vacuoles, number of mitoses, nuclear-to-cytoplasmic ratio) and ovarian (presence of tertiary follicles, vacuolation of luteal cell cytoplasm, luteal cell shape) sample. Modest histological changes were observed at each time point, and these were documented in detail. The most prominent modifications occurred at 124 h after first copulation and included thickening of the endometrium, straightening of the glands, increased cytoplasmic vacuolation, and increased epithelial height. Of 15 queens failing to produce good-quality embryos, only 4 expressed unusual histological characteristics; and 3 of 25 queens producing only high-quality embryos exhibited abnormal uterine or CL cellular integrity. Therefore, aberrant histological changes are not primarily responsible for failure of the naturally estrous, mated queen to produce good-quality embryos. Furthermore, a normative database now is available to date the endometrium and CL of early pregnancy and to examine the impact of exogenous gonadotropins and assisted techniques on uterine/CL structure and function.