Mucinous borderline tumors - endocervical type show complex papillary architecture and are morphologically similar to serous borderline tumors as seen here. They are lined by non-ciliated tall columnar mucinous cells resembling endocervical epithelium. Note the abdundant mucin in the lumen of irregular gland-like spaces. Mild to moderate nuclear atypia may be present. Mitotic activity is not increased. There is no destructive stromal invasion. Given their morphologic and immunohistochemical similarities to serous borderline tumors, some authors have advocated the use of the term "seromucinous tumors" for this group.