The human endometrium contains a substantial population of NK cells or in other words -uterine NK cells which vary in number and in proportion to the total number of endometrial stromal cells during the menstrual cycle. Although present in proliferative endometrium, uterine NK cells increase in number substantially in the mid-secretory phase and are the major endometrial lymphocyte population in the late secretory phase and the first trimester of pregnancy.
Their role included cytotoxicity, regulatory features, chemokine and cytokine production and they facilitate normal trophoblast invasion. Although measurements of uterine NK cell cytotoxicity in vitro are difficult, there are several reports that demonstrate increased cytotoxic activity in cases of obstetric pathology.
