Skin adapts to protect against mechanical forces

The inherent adaptivity of nipple-areolar complex skin in response to repetitive high mechanical loads may explain why women are particularly vulnerable to nipple pain and damage in the first week of breastfeeding, as their nipple and areola skin begins to adapt.

It also explains why women may report that their nipples visibly change over the course of lactation, from the lesser elasticity of the nipple-areolar complex in primiparous women to greater elasticity as breastfeeding progresses.

How does skin adapt to mechanical forces in the short term?

Skin deforms elastically in response to force or mechanical load, which occurs constantly in daily life, to protect against mechanical injury. Most human skin can be stretched to several times its initial size and yet return to its original genetically determined size and shape.58

The weakest component of skin is the keratinocyte cell. If there is a low level of mechanical stress, structures within the keratinocytes absorb the load.
At first, mechanical stretching causes elongation of the skin in the direction of the acting forces, so that it thins. Keratinocytes become more elongated and change their orientation to align along the direction of the load, dissipating the load.153
The desmosomes protect the keratinocytes by locking after a certain point of epithelial stretching, known as the’ yield point’. At the yield point, actomyosin in the desmosomes contracts, resulting in catch bonds, in an attempt to avoid epithelial fracture (Figure 1).57

How does skin adapt to mechanical forces in the long term?

Skin has properties of plasticity which allow for progressive and permanent adaptation to mechanical forces applied over time.

The skin develops a thickened epidermal layer as keratinocytes elongate, divide, and proliferate in the direction of the force to alleviate stress within each cell, and stem cells migrate in, resulting in production of expanded skin tissue. More collagen is laid down in the dermal layer.
When the surface area of skin increases, mechanical load is dissipated, strain applied at any one point decreases, and the risk of epithelial fracture is mitigated. Increased dermal collagen also protects against fracture.58, 134, 153

Selected references

Citation numbers in this article can be used to locate the references in the following publication:

Douglas PS. Re-thinking lactation-related nipple pain and damage. Women's Health. 2022;18:17455057221087865.

Tokuyama E, Nagai Y, Takahashi K. Mechanical stretch on human skin equivalents increases the epidermal thickness and develops the basement membrane. Plos One. 2015;10(11):e0141989.