Adenomyosis and fibrosis define the morphological memory of the postpartum uterus of dairy cows previously exposed to metritis

Optimal fertility after calving in lactating dairy cows is dependent upon the successful completion of uterine involution. Invasion of pathogenic bacteria into the uterine environment within the first week postpartum can lead to uterine disease (metritis). Metritis is associated with decreased fertility and a failure or delay in establishing pregnancy. We hypothesized that early postpartum metritis would be associated with long-term changes in uterine morphology that begin within the first 30 d postpartum (DPP) and are present during a typical breeding window (2-6 mo postpartum). First parity Holstein cows were diagnosed with metritis (M) or deemed healthy (H) at 7 to 10 DPP and uterine tissues were collected and analyzed postmortem at 30 (experiment [Exp.] 1; M = 10, H = 10), or 80 and 165 (Exp. 2; M = 9, H = 10) DPP for the presence of abnormal morphology, including abnormal invasion of endometrial glands and stroma into the myometrium (adenomyosis) and endometrial fibrosis. Glands were identified using immunohistochemistry for FOXA2 (uterine gland specific marker) and fibrosis was identified using Masson's trichrome stain (MTS). The severity of adenomyosis was assessed by the number and size of adenomyotic foci and the distance of foci from the endometrium-myometrium interface (EMI). The degree of fibrosis was defined by MTS intensity. Data were analyzed using a 2-way ANOVA procedure including the effect of metritis and DPP on the dependent variables foci size, distance from EMI, and MTS intensity. A negative binomial regression model was used for the dependent variable of foci number. The presence, size, and distance from the EMI of adenomyotic foci were greater for later postpartum (Exp. 2, 80 and 165 DPP) and early postpartum cows (Exp. 1, 30 DPP) that were previously diagnosed with metritis, suggesting greater severity of adenomyosis. Endometrial fibrosis was greater at the stratum basalis (near EMI) compared with the stratum compactum endometrium (near uterine lumen) for all Exp. 2 (80 and 165 DPP) cows. Greater fibrosis (regardless of endometrial region) was observed in cows diagnosed with metritis compared with healthy controls. Taken together, these data indicate that early postpartum metritis is associated with long-term changes to postpartum uterine morphology, including increased pathological fibrogenesis, leading to the presence of late postpartum endometrial fibrosis (scar tissue), and aberrant endometrial invasion into the myometrium (adenomyosis). Additionally, increased collagen fiber at the EMI suggests a correlation between the development of adenomyosis and fibrosis, which could result from sustained endometrial inflammation caused by uterine disease.