The Evolution of the Breast and the Origins of Breast Diseases

Approximately 150 to 200 million years ago, the common ancestor of all mammals—the cynodont, which lived in the Triassic period—appeared on Earth. Mammals evolved from reptile-like animals; they are the most structurally advanced and physiologically complete animals in the animal kingdom.

Most mammals are covered in fur, which allows them to maintain a constant body temperature and adapt more readily to their environment. In the warm, humid Mesozoic climate, mammals flourished; to date, more than 4,000 species of mammals exist worldwide.

Mammals share certain characteristics: the most obvious are lactation and viviparity. Both female and male mammals possess mammary glands; in females these are highly developed and nourish the young. Nearly all mammals are warm-blooded, with a basically constant body temperature and fur protecting the body from heat and cold. Mammals have more developed brains, with imitative and learning ability, making their behavior more complex and their reproductive and survival abilities stronger.

These features already existed 150 million years ago and have changed only slightly over the long course of evolution. This shows that these widespread molecular and biochemical features already existed before the various mammals appeared, and had been optimized through natural evolution. Today, when humans conduct animal experiments for medical research, the final stage uses various mammals; treatments that work for other mammals basically also work for humans.

The breast is an ectodermal structure of mammals, originating from the skin through local thickening of the epidermis. Its structure resembles that of sebaceous glands, and its functional activity resembles that of sweat glands—it is a modified sweat gland. In female mammals, breast development passes through embryonic, infantile, pubertal, menstrual, gestational, lactational, weaning, menopausal, and senile stages. Endocrine hormones play important roles in breast development and in each stage.

Except for a few mammals such as humans, most mammals have more than one pair of breasts. In fact, human ancestors also had more than one pair—6 to 8 pairs. During our embryonic development, 6 to 8 mammary ridges (milk line primordia) appear, distributed along a line called the mammary ridge (milk line), which runs along the trunk from the axilla to the groin on both ventral sides of the chest and abdomen.

In the sixth week of human embryonic development, ectodermal cells locally proliferate and thicken along the milk line to form 6-8 pairs of mammary primordia. Normally, by the ninth week, only the pair located at the fifth intercostal space on the midclavicular line survives and develops into the later breast; all others regress. This process of human embryonic development is a microcosm of human evolution, proving that humans evolved from lower to higher organisms.

However, in disease states, some people develop mammary tissue at other points along the milk line; these are called accessory (ectopic) breasts—a form of atavism. Some accessory breasts develop only glandular tissue; others also develop nipples and areolae. In a very few women with vigorous estrogen secretion, after mastectomy a complete pair of breasts may redevelop at other points along the milk line. Breast tumors can also arise in accessory breasts and are no different from ordinary breast tumors. Tumors in accessory breasts are often misdiagnosed or missed, becoming clinically difficult cases.

The breast is a target organ of many endocrine hormones: ovarian hormones, adenohypophyseal hormones, adrenal cortical hormones, thyroxine, testosterone, and insulin all influence breast growth and development. Normal breast growth, development, and secretion are regulated by the cerebral cortex and hypothalamus. The most important endocrine hormones for the breast are estrogen and progesterone secreted by the ovaries: estrogen promotes growth of the mammary ducts, progesterone stimulates alveolar development and lobule formation. Estrogen and progesterone, coordinated by pituitary hormones, act together so that the breast can fully develop.

Therefore most breast-related diseases are associated with endocrine hormone imbalance. Imbalance of ovarian estrogen and progesterone causes breast disease. Ovarian hormones are not only the chief culprits behind some breast cancers; even common conditions such as mammary hyperplasia and lobular hyperplasia are related to them. Drugs such as tamoxifen, which suppress estrogen secretion, treat not only breast cancer but also mammary hyperplasia due to estrogen-progesterone imbalance.

Adrenal hormones are a source of estrogen, androgens, and progesterone; when adrenal secretion is abnormal, breast pathology can also occur. The reason postmenopausal women and men can develop breast tissue changes from excessive estrogen lies in adrenal estrogen secretion. Some breast cancer patients who relapse after castration therapy also have adrenal estrogen to thank. Abnormal secretion of pituitary hormones, thyroxine, insulin, placental lactogen, and prolactin can also cause a series of breast diseases.

Endocrine imbalance usually also brings a series of emotional changes: pituitary hormones, estrogen, progesterone, and thyroxine can cause irritability and depression. Thus clinicians since ancient times have observed that breast disease patients have emotional disturbance; regulating the patient's endocrinology can also relieve emotional problems.

Dopamine, closely tied to neural excitation, and the hypothalamic prolactin-inhibiting factor (PIF) may even be the same substance; prolactin is also influenced by thyrotropin-releasing hormone and serotonin, all of which are closely linked to human emotion. This gives us a biological basis for the link between breast disease and emotion.

Traditional TCM treats breast diseases mainly by soothing the liver and resolving depression, which also has a scientific rationale. Liver-soothing, depression-resolving herbs may act on the human endocrine system, regulating endocrine function and thereby resolving both the patient's emotional problems and the breast disease.

Patients with endocrine imbalance often also show features inconsistent with warm-blooded homeostasis: low body temperature, or tendency to hot flashes and profuse sweating. TCM usually attributes these to kidney-yang deficiency or kidney-yin deficiency. Why some TCM formulas for kidney-yang or kidney-yin deficiency (such as Yanghe Tang, or Erzhi Wan, Zhibai Dihuang Wan) work for some breast disease patients may also be related to this.

Endocrine hormones are closely tied to human homeostasis; they are biochemical self-regulatory features produced as humans adapt to environmental change. Thus the occurrence and development of breast disease are closely tied to human evolution itself. As human science advances, our understanding of breast disease will deepen, and treatments will become more numerous and effective.