Uterine leiomyomas are monoclonal smooth muscle tumors. Early research on isoform analysis of glucose-6-phosphate dehydrogenase in the smooth muscle cells of uterine leiomyomas pointed to the monoclonal nature of this tumor. Each leiomyoma lesion in the uterus may have a distinct isoform and are presumed to arise independently. Cytogenetic abnormalities of several chromosomes have been identified within these smooth muscle tumors with normal karyotype in the adjacent non-tumorous regions. These cytogenetic mutations have been identified in about 40% of uterine leiomyomas. Some of the mutations involve genes involved in cellular growth regulation. Correlation between the genotype of the leiomyomas and the phenotype has not led to conclusive observations.

Gross morphology

Uterine leiomyomas are usually well-circumscribed tumors. They can occur in any part of the uterus, including the cervix. They may also occur in the round ligaments. Generally they are divided into subserosal, intramural, and submucosal. The subserosal and submucosal uterine leiomyomas can become pedunculated. The submucosal uterine leiomyomas can protrude into the uterine cavity or become pedunculated, and protrude through the cervix. Uterine leiomyomas can become separated from the uterus, and can be found in different areas such as the retroperitoneal space between the leaves of the broad ligament of the uterus.