When discussing fun facts about the body, it is not uncommon to mention that the human epidermis is thickest on the soles of the feet (and thinnest on the eyelid) – but where does this evidence come from and just how true is this? A recent publication has attempted to pull together the many papers which all attempt to address this issue (1).
The researchers decided to collate and analyse as much data as possible as a systematic review from papers to answer a few questions:
1. To collate multiple studies to establish the epidermal thickness at various body sites.
2. Look at different skin thickness measurement techniques to see if they gave different results.
3. Examine if age, gender, phototype or ethnic origin had any bearing on these results.
As part of the study, they undertook a literature review of all published papers which met the eligibility criteria. By searching Embase® and MEDLINE® from 1946 to 2020. Papers were independently screened and selected or rejected on methodological grounds. They also had to define which measures to use. If you think about plantar and palmar skin for example the epidermis runs from the outermost stratum corneum down to the dermo-epidermal junction (DEJ) but the DEJ can be very undulating (see figure 1 below) consequently an average measure needs to be calculated.
Figure 1 : Undulations in the dermo-epidermal junction give rise to variability to epidermal thickness, therefore an average value was required.
The main methods used to measure skin thickness include light microscopy, high frequency ultrasound, optical coherence tomography, multiphoton laser tomography and laser scanning microscopy.
Results
Following screening and selection 142 papers were included in the review (qualitatively) with 133 of these being able to undergo a more detailed quantitative analysis. Figure 2 highlights the average epidermal thickness for various regions on the foot (expressed in millimetres).
Figure 2 : Mean Epidermal Thicknesses at different points on the foot
Other average epidermal thicknesses were as follows:
Lower leg: 0.097mm
Thigh: 0.071mm
Gluteal area: 0.084mm
Palm: 0.172mm
Finger (volar): 0.337mm
The research also demonstrated that the skin thickness for all methods of measurements were very similar with the exception of high frequency ultrasound which tended to over-estimate the skin thickness. As previously reported, epidermal thickness was shown to decrease with age. Gender differences were minor with women showing slightly thinner skin on the forehead, abdomen, thorax and cheek areas.
Examination of ethnicity demonstrated that Caucasian skin was virtually always thinner compared to Asian and African skin types, but this difference was not statistically significant.
Consequently, the epidermis is confirmed to be thickest on the soles but by probably not as thick as frequently quoted - its just over half a millimeter thick. As for the eyelid, alas, other parts of the body have a thinner epidermis - the skin was found to be thinnest on the tip of the penis (0.031mm), the pubic area (0.045mm) and then the eyelid (0.048mm).
This work helps to establish normal values from which further studies can examine the effects of various diseases may have on skin thickness and structure such as diabetic foot disease for example.
Reference
1. Lintzeri DA, Karimian N, Blume-Peytavi U, Kottner J. Epidermal thickness in healthy humans: a systematic review and meta-analysis. J Eur Acad Dermatol Venereol. 2022;36(8):1191-200.