Mechanical pressures are the engine room of breastfeeding and lactation
It helps to understand how mechanical pressures work
Mechanical pressures occur everywhere in the natural world. Mechanobiology is a new field of science which investigates the effects of the physical forces which act on living tissues, either from within or from the outside.
The powerful effects of the physical forces which act upon or inside our body, our breasts, and our baby have been previously overlooked in the science of human lactation. This blind spot is not surprising, since our society's relationship with the bodies of women remains problematic.
Yet mechanical pressures are fundamental to lactation and breastfeeding when things are going well. It's especially important to understand the way mechanical pressures work when problems come up.
Ways in which mechanical pressures affect breastfeeding and your milk flow
Mechanical pressures act in many different ways when you're making milk. Here are some that are important to know about.
Milk ejection or letdown
-
Specialised cells wrap around your milk glands and contract when oxytocin surges through your blood stream in response to your baby's suckling. This mechanical pressure causes your milk glands, which are lined inside by milk-making cells (or lactocytes), to squash down and take on a random, scrunched-up shape. This mechanical pressure makes the milk that is waiting inside the alveoli flow out into your milk ducts. We call this your letdown, or milk ejection reflex.
-
Specialised cells also wrap around your milk ducts. Again, they contract when oxytocin surges through your blood stream in response to your baby's suckling. However, in response, the duct actually dilates or grows wider, and also shortens. This decreases pressure in the duct so that when you have a letdown, milk flows easily out of the milk glands into the ducts and then out of the openings on the face of your nipple.
The vacuum generated by baby's sucking
-
Your breastfeeding baby applies a mechanical force to your nipple and breast tissue, which is the vacuum. This vacuum or negative pressure acts to draw up the nipple and breast into your baby's mouth, and causes the tissues to expand.
-
Milk travels along pressure gradients, from areas of high pressure to areas of low pressure. The mechanical pressure of the vacuum (or the negative pressure of suckling) and the positive pressure of milk ejection in the milk duct system transfer milk from the breast, working in tandem. Sometimes you'll notice that your breasts leak milk in the absence of suckling, due to the positive mechanical pressure of milk ejection. If your baby isn't suckling and you have a letdown, there's some backflow of milk into the emptier ducts, because your milk follows the pressure gradients from high pressure to lower pressure.
Nipple and breast tissue drag
-
When the mechanical force of nipple and breast tissue drag pulls in a different direction to the mechanical force of the vacuum, your baby may fuss a lot, or back arch and pull off the breast.
-
Your directly breastfeeding baby also applies the mechanical force of the vacuum onto the skin of your nipple. This pressure stretches the outer layer of your nipple skin.
-
If there is nipple and breast tissue drag, this stretching force may not be properly spread out over the largest possible area of nipple, areolar and breast skin. When the stretching force focusses on a small area, or is consistently too high because it's not spread out, you may experience nipple pain.
-
If there is nipple and breast tissue drag, this stretching force may become so focussed on one small area at the tip of the nipple (or sometimes at the base of the nipple) that the epidermis breaks apart, resulting in a crack or ulcer.
-
If there is nipple and breast tissue drag, the mechanical force of suckling might also bend the nipple or place pressure on the tissues inside your nipple, resulting in deep tissue bruising.
-
Breast inflammation (such as mastitis or engorgement)
-
If there isn't enough milk removed from the inside of a milk gland, high volumes of milk press back on the lactocytes which line it on the inside, stretching and thinning them. This growing pressure also stretches the tight junctions between the lactocytes, placing mechanical stress or strain upon them. If the backpressure from the milk is high enough, the milk gland might break apart.
-
The pressure or tension inside the breast increases
-
If there is increased blood flow in the capillaries
-
More fluid than usual leaking out of the capillaries
-
Bruising (which is breaking of the capillaries so they leak some blood) of the deep tissues.
-
-
The rising pressure of this tension acts upon the milk ducts, which are easily compressed, and may occlude them. This worsens the cycle of rising milk volume and backpressure inside the alveolar gland, which triggers more blood flow into the area and more fluid than usual leaking out, in a cycle of inflammation.
Externally applied pressure
-
Mechanical pressure might be experienced inside the breast from externally applied forces.
-
Mechanical forces such as a kick by the baby or massage may result in deep breast tissue bruising, resulting in raised tension or pressure inside your breast, which in turn might compress the milk ducts, resulting in inflammation.
-
Prolonged application of mechanical pressures upon the breast result in prolonged ductal compression, which results in backpressure of milk, triggering inflammation. This prolonged pressure could come from an ill-fitting garment or bra or sleeping with your arm pressed against your breast or from using a breast shell or silverette, to give just a few examples.
-
-
Fit and hold problems are a form of externally applied mechanical pressure which results in nipple and breast tissue drag, as discussed above.
Recommended resources (PBL Advanced)
The NDC mechanobiological model explains downregulation of breast milk production
The NDC mechanobiological model explains clinically inflamed lactating breast stroma
Selected references
Basree M, Shinde N, Koivisto C. Abrupt involution induces inflammation, estrogenic signaling, and hyperplasia linking lack of breastfeeding with increased risk of breast cancer. Breast Cancer Research. 2019;21(80):https://doi.org/10.1186/s31058-019-1163-7
Douglas P. Re-thinking benign inflammation of the lactating breast: a mechanobiological model. Women's Health. 2022;18:17455065221075907.
Douglas PS. Re-thinking benign inflammation of the lactating breast: classification, prevention, and management. Women's Health. 2022;18:17455057221091349.
Douglas PS. Re-thinking lactation-related nipple pain and damage. Women's Health. 2022;18:17455057221087865.
Fetherstone C. Mastitis in lactating women: physiology or pathology? Breastfeeding Review. 2001;9:5-12.
Ingman WV, Glynn DJ, Hutchinson MR. Inflammatory mediators in mastitis and lactation insufficiency. Journal of Mammary Gland Biology and Neoplasia. 2014;19:161-7.
Jindal S, Narasimhan J, Vorges VF, Schedin P. Characterization of weaning-induced breast involution in women: implications for young women's breast cancer. Breast Cancer. 2020;6(55):https://doi.org/10.1038/s41523-020-00196-3.
Kim T-J. Mechanobiology: a new frontier in biology. Biology. 2021;10(570):https://doi.org/10.3390/biology10070570.
Kobayashi K, Han L, Lu S-N, Ninomiya K, Isobe N, Nishimura T. Effects of hydrostatic ompression on milk production-related signaling pathways in mouse mammary epithelial cells. Experimental Cell Research. 2023;432:113762.
Noam Zuela-Sopilniak, Lammerding J. Can’t handle the stress? Mechanobiology and disease. Trends in Molecular Medicine. 2022;28(9):710-725.
Stewart TA, Hughes K, Stevenson AJ, Marino N, Ju AL, Morehead M, et al. Mammary mechanobiology - investigating roles for mechanically activated ion channels in lactation and involution. Journal of Cell Science. 2021;134:doi:10.124/jcs.248849.
Weaver SR, Hernandez LL. Autocrine-paracrine regulation of the mammary gland. Journal of Dairy Science. 2016;99:842-53.
Zaragoza R, Garcia-Trevijano ER, Lluch A, Ribas G, Vina JR. Involvement of different networks in the mammary gland involution after the pregnancy/lactation cycle: implications in breast cancer. International Union of Biochemistry and Molecular Biology. 2015;67(4):227-38.