Here is a quick revision of the main hormones of the mammary endocrine system and their roles. We don't need too much detail on this topic to be highly effective in the clinic, but remembering the following key hormones and their functions is still useful.

It's also worth noting that in the field of clinical breastfeeding and lactation support, the roles of the hormones oxytocin and prolactin are often interpreted through reductionist (causative) lenses, rather than through the lens of complexity science and biological systems. An example would be the way prolactin is simplistically viewed as switching on milk production, which is actually not the case - prolactin levels do not directly relate to milk yields, and gradually decrease postbirth in exclusively breastfeeding women. You'll often see the same simplistic interpretations of oxytocin's actions.

Oestrogen

• Stimulates ductal growth and branching.

• Important during puberty for the initial development of breast tissue.

• Works synergistically with progesterone.

Progesterone

• Promotes alveolar (lobule) development within the mammary gland.

• Essential for preparing the breast for milk production.

Prolactin

• Secreted from the anterior pituitary.

• Stimulates milk synthesis after childbirth.

• Requires the withdrawal of placental hormones (mainly progesterone) to act fully.

Oxytocin

• Secreted by the posterior pituitary.

• Responsible for the milk ejection reflex ("let-down"), triggered by suckling.

  • Causes contraction of myoepithelial cells around alveoli (resulting in scrunching up or contraction of the alveoli).

  • Causes contraction of myoepithelial cells around the ducts (resulting in ductal dilation).

Human Placental Lactogen (hPL)

• Produced during pregnancy by the placenta.

• Prepares the breast for lactation alongside prolactin and progesterone.

Cortisol, Insulin, and Growth Hormone

• Support mammary development and milk production by modulating metabolism and tissue growth.