Arias-Stella Reaction: Definition

The Arias-Stella reaction is a noncancerous change most often seen in the tissue that lines the uterus, called the endometrium, and sometimes in the fallopian tube. It develops in response to pregnancy or certain hormone medications, causing the affected cells to become larger and appear unusual under the microscope. It is named after Javier Arias-Stella, the pathologist who first described it. This article explains what the Arias-Stella reaction is, what causes it, and why it may appear in your pathology report.

What causes the Arias-Stella reaction?

The Arias-Stella reaction is caused by the hormone progesterone, which rises during pregnancy and prepares the lining of the uterus to support a developing pregnancy. High progesterone levels signal cells to grow and change, and these changes can make them appear unusual under the microscope. Most often, the reaction is linked to a normal pregnancy, but it can occur in any situation where progesterone is high, including:

• Ectopic pregnancy — A pregnancy that develops outside the uterus, most commonly in a fallopian tube.
• Molar pregnancy — An abnormal pregnancy in which placental tissue grows in a disorganized way.
• Hormone treatments — Medications that contain progesterone or progesterone-like hormones (progestins), such as some forms of contraception or hormone therapy.

Is the Arias-Stella reaction normal?

Yes. The Arias-Stella reaction is a normal response to the hormonal changes of pregnancy, and it is not a disease or a sign of cancer. Because the cells enlarge and may appear unusual, the reaction can resemble cancer under the microscope. Pathologists are familiar with this pattern and recognize it as benign by considering the unusual appearance in the context of the clinical situation, such as a recent or current pregnancy. When the reaction is found outside of pregnancy, it may prompt the care team to look for a cause of elevated hormones, such as a hormone medication or, rarely, an unsuspected pregnancy.

Where is the Arias-Stella reaction seen?

The Arias-Stella reaction is most often seen in the lining of the uterus (endometrium). It can also appear in other tissues exposed to high progesterone levels, such as the fallopian tubes in an ectopic pregnancy, and occasionally in the glands of the cervix.

How is the Arias-Stella reaction diagnosed?

The Arias-Stella reaction is identified by examining a tissue sample under the microscope. The sample is usually obtained during an endometrial biopsy or a curettage, a procedure that gently removes tissue from the lining of the uterus. A pathologist looks for the characteristic enlarged cells and considers the clinical context, such as a recent or current pregnancy or hormone treatment, to confirm that the changes represent an Arias-Stella reaction.

What are the microscopic features of the Arias-Stella reaction?

Under the microscope, the Arias-Stella reaction shows cells that are larger than normal with abundant cytoplasm (the material that fills the cell around the nucleus). The nuclei are often enlarged and irregular and may have a clear or “cleared-out” appearance, and some cells bulge into the gland space in a pattern pathologists call hobnail. Although these features can look worrying on their own, pathologists recognize them as part of a benign, hormone-driven change rather than cancer.

Questions to ask your doctor

• What does “Arias-Stella reaction” mean in my pathology report?
• Is this finding related to a current or recent pregnancy?
• Could it be related to a hormone medication I am taking?
• Does the Arias-Stella reaction itself need any treatment or follow-up?
• Were there any other findings in my sample that need attention?
• If I am not pregnant, what could be causing this change?
• Does this finding affect my current or future pregnancies in any way?

Related articles on MyPathologyReport.com

• Endometrium (the tissue lining the uterus, where this change is most often seen)
• Secretory endometrium (a normal progesterone-driven change in the uterine lining)
• Atypical (what it means when cells look unusual but are not necessarily cancer)
• Biopsy (the procedure used to obtain the tissue sample)