Glandular tissue and milk ducts can be present in the armpit, a phenomenon known as accessory breast tissue. Medically termed polymastia or ectopic breast tissue, this tissue is functionally identical to the tissue of the main breast. The presence of this extra tissue is a common anatomical variation resulting from embryonic development. Though often unnoticed for years, it can become a source of discomfort when hormone levels change throughout life.
Understanding the Mammary Ridge
Breast tissue development begins during the fourth to sixth week of gestation with the formation of the mammary ridge, often called the “milk line.” This embryonic structure is a thickened strip of ectoderm that runs bilaterally from the axilla down the chest and abdomen to the groin area. In typical human development, the majority of this ridge regresses, leaving only the two segments in the pectoral region to form the normal breasts.
The persistence of any portion of this embryonic ridge outside of the chest leads to the formation of accessory breast tissue. Since the milk line extends far into the armpit, this location is the most common site for this extra tissue to appear.
The Presence of Accessory Breast Tissue
The most frequent location for accessory breast tissue is the axilla, accounting for an estimated 60 to 70 percent of all cases. This extra tissue is composed of fibroglandular elements, containing the same ducts, lobules, and connective tissue found in a typical breast. This is considered a normal developmental variation, not a disease.
In some individuals, the accessory tissue is an extension of the main breast tissue, known as the Axillary Tail of Spence, which is connected to the primary duct system. True accessory breast tissue (polymastia) is a separate mass of glandular material, often presenting as a subcutaneous lump in the armpit. This condition is distinct from polythelia, which is the presence of an extra nipple or areola without any underlying glandular tissue. Accessory breast tissue occurs in approximately 0.4 to 6 percent of women.
Functional Changes During Hormonal Cycles
Because accessory breast tissue contains functional ducts and glandular components, it responds to the same hormonal signals as the main breasts. This response is why the tissue is often unnoticed until periods of significant hormonal fluctuation, such as puberty or pregnancy. The tissue will often swell and become tender during the days leading up to menstruation, mirroring changes in the pectoral breasts.
This hormonal responsiveness is most pronounced during pregnancy and lactation. High levels of hormones, including prolactin and estrogen, can cause the accessory tissue to increase noticeably in size and firmness. This engorgement is a common cause of pain and swelling in the armpit during the third trimester or immediately postpartum. In rare instances, if the accessory tissue contains a complete ductal system, it may produce and secrete milk, a phenomenon called axillary lactation.
The symptoms of tenderness and restricted arm movement are directly related to the volume of the accessory tissue and the degree of hormonal stimulation. Comfort measures like cold compresses can help manage the temporary swelling and discomfort.
When to Seek Medical Guidance
While accessory breast tissue is benign, any new or changing lump in the armpit should be evaluated by a healthcare provider. The axilla contains numerous structures, and a lump could be caused by a variety of conditions that require diagnosis. A lump might be a swollen lymph node reacting to a local infection or systemic illness, a benign fatty tumor called a lipoma, or a sebaceous cyst.
Though rare, accessory breast tissue can develop the same pathologies as normal breast tissue, including benign tumors like fibroadenomas or breast cancer. Ectopic breast cancers account for an estimated 0.3 to 0.6 percent of all breast cancers. A healthcare provider will typically perform a physical examination and may order imaging, such as an ultrasound, to determine the nature of the lump. Ultrasound can confirm if the mass is glandular breast tissue separate from other structures.