What Are Milk Lines?

Milk lines, also known by the technical term ventral epidermal ridges, are precursors to the mammary glands and nipples.

An embryo's breasts begin to develop during the fourth week of the gestation period. The milk lines themselves appear by the sixth week. They arch down from the armpit to the groin on both sides. As the breasts continue to develop along the ridge over the chest, these lines eventually disintegrate, usually by about week nine of fetal life. Occasionally, milk lines will persist and may be connected to extra breast tissue and extra nipples.

All mammals, male and female, have a pair of symmetrical milk lines along which breast tissue, nipples, and areolas may develop.

Do My Milk Lines Play a Role in the Possibility of Breast Cancer?

As mentioned above, the development of the milk lines is the first evidence of mammary gland development.

In normal human development, these ridges, for the most part, disappear. In some cases, however, there may be supernumerary breast tissue—extra nipples and breasts, also known as polythelia and polymastia, respectively—which can be an indication that the milk lines did not entirely disintegrate.

For the most part, this extra breast tissue has no physiologic significance but, sometimes, it can enlarge with the onset of puberty, pregnancy, or lactation, and can be the site of breast carcinoma. This is a type of cancer that forms in the cells of the breasts.

One case study published in a 2014 issue of the Journal of Clinical Oncology explored an instance of what was referred to as ectopic milk line breast cancer, a breast cancer occurring in the chest wall, but it was acknowledged to be a rare case. The treatment for ectopic breast cancer is the same as for similar-stage cancer located in the breast, and includes surgery with or without lymph node dissection, radiation, chemotherapy, and hormonal therapy.

Further Reading on Breast Development

Breast Anatomy and Development. The development of the breasts from conception all the way through puberty.

What Is Breast Cancer? Breast cancer is a malignant (cancerous) growth that begins in the tissues of the breast. Cancer is a disease in which abnormal cells grow in an uncontrolled way. Breast cancer is the most common cancer in women, but it can also appear in men.

Signs of Breast Cancer to Look Out for in Women of All Ages. Signs of breast cancer may vary in different women, but the disease does yield some common symptoms.

How Your Breasts Change To Prepare For Breastfeeding. When you're pregnant, and your body is preparing to breastfeed, your breasts go through many different changes. The shifts in major hormones that takes place during pregnancy influence the growth and development of your breasts.