Structural and functional comparison of mast cells in the pregnant versus nonpregnant human uterus

Objective: To characterize uterine mast cells and investigate uterine muscle strips contractile responses to challenge with an allergen in nonpregnant and pregnant women. Study design: A double-antibody labeling technique was used to identify tryptase positive or chymase-tryptase-positive mast cells in uterine samples from term pregnant and nonpregnant women. Longitudinal myometrial strips were prepared from biopsies of hysterectomy specimens from patients undergoing procedures for benign indications and women who had elective cesarean sections. Responses to ragweed antigen were compared in uterine tissues from allergic and nonallergic patients and with passively sensitized tissues in the absence or presence of H₁ receptor antagonist and a cyclooxygenase inhibitor. Results: Tryptase-chymase-positive mast cells were predominant in nonpregnant myometrium, whereas tryptase-positive cells were dominant in pregnant myometrium. Mast cell density was significantly higher in tissue from pregnant women than those of nonpregnant women. Studies in tissues from patients with known allergy to ragweed, as well as in passively sensitized tissues, showed an immediate and substantial increase in the frequency and force of myometrial contraction after a challenge with ragweed antigen. A similar response was observed to histamine. There was no contractile response to antigen challenge in tissues from nonallergic patients. An H₁ receptor antagonist partially inhibited the response to antigen, whereas a cyclooxygenase inhibitor had no effect. Conclusion: Sensitized isolated pregnant and nonpregnant human uterine tissue is capable of responding to antigen challenge with strong myometrial contractions. This ability, along with the increased density of mast cells in pregnant tissues as compared with nonpregnant tissues, indicates a possible role for mast cells in mediating uterine contractility in pregnancy.