Talreja PS, Kleene NK, Pickens WL, Wang TF and Kasting GB Visualization of the Lipid Barrier and Measurement of Lipid Pathlength in Human Stratum Corneum AAPS PharmSci 2001;
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article 13
(https://www.pharmsci.org/scientificjournals/pharmsci/journal/01_13.html).
Visualization of the Lipid Barrier and Measurement of Lipid Pathlength in Human Stratum Corneum
Submitted: August 31, 2000; Accepted: April 23, 2001; Published: May 15, 2001
Priya S. Talreja1, Nancy K. Kleene2, William L. Pickens3, Tsuo-Feng Wang4 and Gerald B. Kasting1
1College of Pharmacy, University of Cincinnati
2Department of Cell Biology, Neurobiology and Anatomy, University of Cincinnati
3Skin Sciences Institute, Children's Hospital Medical Center
4Department of Chemical Engineering, State University of New York at Buffalo
Correspondence to: Gerald B. Kasting Telephone: 513-558-1817 Facsimile: 513-558-0978 E-mail: gerald.kasting@uc.edu
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Keywords: Stratum Corneum Alkaline Expansion Microscopy Lipid Pathlength Tortuosity
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Abstract
Detailed models of solute transport through the stratum corneum (SC) require
an interpretation of apparent bulk diffusion coefficients in terms of
microscopic transport properties. Modern microscopy techniques provide a tool
for evaluating one key property-lipid pathway
tortuosity-in more detail than previously possible.
Microscopic lipid pathway measurements on alkali expanded human SC stained with
the lipid-soluble dyes methylene blue, Nile red, and oil red O are described.
Brightfield, differential interference contrast, fluorescence, and laser
scanning confocal optics were employed to obtain 2-dimensional (2-D) and
3-dimensional (3-D) images. The 2-D techniques clearly outlined the corneocytes.
Confocal microscopy using Nile red yielded a well-delineated 3-D structure of
expanded SC. Quantitative assessment of the 2-D images from a small number of
expanded SC samples led to an average value of 3.7 for the ratio of the shortest
lipid-continuous pathway to the width of the membrane. This was corrected for
the effect of alkaline expansion to arrive at an average value of 12.7 for the
same ratio prior to swelling.
Introduction
Since the elucidation of the biphasic structure of the stratum corneum (SC)
(ie, flattened, proteinaceous corneocytes embedded within an ordered lipid
matrix), much has been written about the implications of this structure for
solute transport. It has long been known that the SC provides the skin's primary
diffusion barrier1 . Correlations of skin permeability coefficients,
k p , versus physical properties of a wide variety of permeants have
shown that skin can be effectively modeled as a simple lipid barrier to
compounds having at least moderate water and oil solubilities2-6 . In
combination with the structural detail and evidence from electron microscopy7
and other physical characterization techniques8,9 , this observation has led
many researchers to conclude that the primary transport pathway for most
materials traversing the SC is intercellular6-8,10 . If this is true, it
follows that the arrangement of the corneocytes within the lipid matrix is a key
determinant of the skin's permeability, as it would influence the effective
pathlength for diffusion. Related analyses of the impact of corneocyte shape on
skin permeability have shown that shape and, by implication, stacking
arrangement are important for compounds whose lipid/protein permeability ratio
is greater than 10002, 11,12 .
Stacking and layering of corneocytes have been studied by 2-dimensional (2-D)
light microscopy, using the alkaline expansion technique to swell the tissues
prior to analysis13-20 . Methylene blue, an absorbent dye, has most commonly
been used as a lipid stain13, 18,19 ; however, phase contrast (15,18) and
fluorescent staining16 techniques have also been described. Alkaline
expansion clarifies the corneocytes13,14 and improves the visualization of
the lipid layers, which are otherwise too closely spaced to be resolved.
Swelling occurs largely in the apical-to-basal direction21 and is likely to
be confined to the corneocytes, as discussed in the Appendix. Corneocyte
arrangement is maintained during swelling13,14 , as the corneocytes are
fastened together by desmosomes, which restrict their movement22,23 . The same
result is expected for mild tissue-handling procedures that do not disrupt
desmosomes. Qualitative analysis of these earlier studies has established that
corneocytes in rodent skin stack in highly aligned columns, except for the
footpad area15-19 . However, the stacking of corneocytes in human SC appears
to be less ordered19,22 . A quantitative description of this difference and
its implications for diffusive transport through the lipids has not heretofore
been presented.
In the current study, we present alternative methods for staining and
examining alkaline-expanded SC in both 2 and 3 dimensions. The 2-D methods
include brightfield microscopy using an oil red O stain24 and differential
interference contrast. Fluorescent staining with Nile red allows the extension
to 3-dimensional (3-D) visualization.
We analyzed the 2-D micrographs obtained in this study by making quantitative
estimates of the lipid pathlength across the SC. These data provide the basis
for designing a 3-D microstructural model of human SC (T.F.W. and J.M. Nitsche,
unpublished data, 2001). Pathlengths were compared
with the values predicted from 2-D "brick-and-mortar" models of the SC2,6 to
provide a quantitative measure of corneocyte stacking in these tissue samples.
The results show that a considerable degree of disorder can exist in human SC.
Materials and Methods
Chemicals
Methylene blue was purchased from Fisher Scientific (Pittsburgh, PA). Nile
red was purchased from Sigma Chemicals (St Louis, MO). Oil red O was obtained
from Polyscientific (Gaithersburg, MD). OCT embedding media was purchased from
Electron Microscopy Sciences (Washington, PA). Sorensen-Walbum buffer (0.1 M
glycine, 0.1 M NaCl, and 0.1 M NaOH), pH 12.5, was used for alkaline expansion of SC. Water was deionized
and distilled, and all chemicals were reagent grade.
Human skin
Cryoprotected, cadaveric, split-thickness skin specimens (stored in 10%
glycerol) were obtained from Ohio Valley Tissue and Skin Center (Cincinnati, OH)
and stored at -70°C until use. Donor age group (60-75
years) and sampling sites-unspecified (back, abdomen,
or thigh) for brightfield microscopy and dorsum for fluorescence and confocal
work-were recorded.
Preparation of skin specimens for cryosectioning
A piece of the frozen split-thickness skin approximately 6 cm2 was cut and
thawed in water at room temperature. A hand-held scalpel was used to cut 4 mm
x 7 mm pieces, which were placed dermal side down on
aluminum foil-wrapped glass microscope slides. The
specimens were covered with OCT embedding medium and frozen on dry ice in an
orientation that allowed sectioning perpendicular to the epithelial surface. The
molds were placed on dry ice, and the tops of the specimens were covered with
the media to facilitate thorough freezing. The tissues were sectioned on a
cryotome (Cryostat MHR; Slee Technik, Mainz, Germany) at varying thicknesses,
ranging from 8 µm for light microscopy to 40 µm for confocal imaging. They were then placed on poly-L-lysine coated glass slides, which were stored at 4°C until examination.
Alkaline expansion and staining of stratum corneum
The tissue section slide was immersed in 0.5% (wt/vol) methylene blue in 95:5
(vol/vol) ethanol:water for 2 minutes and rinsed gently in water. Excess water
was wicked off with absorbent tissue. The SC was then expanded using
half-strength Sorensen-Walbum buffer for 10 to 20 minutes, and the preparation
was covered with a glass coverslip.
Oil red O staining
The tissue section slide was placed in 100% propylene glycol at room
temperature for 5 minutes. The slide was then placed in 0.7% (wt/vol) oil red O
in propylene glycol at 60°C for 7 minutes and
transferred to 85:15 (vol/vol) propylene glycol:water at room temperature for 3
minutes, followed by rinsing with water25 . Alkaline expansion of the SC was
performed as described above.
Nile red staining
A stock solution containing 0.05% (wt/vol) Nile red in acetone was stored at
4°C, protected from light. Prior to each staining, the
stock was diluted to 2.5 µg/mL with 75:25 (vol/vol)
glycerol:water, followed by brisk vortexing26 . The tissue sections were
expanded in alkaline buffer as described above and gently rinsed with water. A
drop of the glycerol-dye solution was applied to each tissue section and
immediately covered with a coverslip.
Brightfield microscopy
Photomicrographs of expanded SC stained with methylene blue were obtained
with a 35-mm Zeiss MC63 (Oberkochen, Germany) camera mounted on a Zeiss
microscope using a 40X objective lens and Kodak (Rochester, NY) TMAX-100 black
and white print film. The oil red O image was captured by a Nikon (Tokyo, Japan)
Microphot-FXA microscope with a Spot2 digital camera (Diagnostic Instruments,
Inc, Sterling Heights, MI) using a 40X objective lens.
Fluorescence microscopy
Nile red fluorescence was captured with a Leitz (Wetzlar, Germany) microscope
equipped with a fluorescein filter cube (450- to 490-nm excitation filter,
510-nm dichroic mirror, and 515-nm long pass emission filter) and a
Leitz-Vario-Orthomat camera, using a 40X objective lens and Kodak Elite
Chrome-400 color film. This fluorescein filter cube enabled the differentiation
of polar and neutral lipids. A fluorescent image of Nile red-stained SC was obtained with a Nikon Microphot-FXA
microscope and a Spot2 digital camera, but using a 60X oil-immersion objective
lens and a slightly different fluorescein filter cube (460- to 500-nm excitation
filter, 505-nm dichroic mirror, and 510- to 560-nm emission filter). The image
was captured in the black-and-white mode to decrease the exposure time and
photobleaching of the dye. Subsequently, this Nile red image was pseudocolored
using Metamorph Imaging System software (Universal Imaging Corp, West Chester,
PA).
Nomarski Differential Interference Contrast (DIC) microscopy
A DIC image of an oil red O-stained specimen was
obtained with a Spot2 digital camera mounted on a Nikon Microphot-FXA microscope
with a 63X oil immersion objective. The DIC optical components were adjusted to
obtain bright/dark effects for an apparent 3-D representation.
Confocal microscopy
An inverted Zeiss Axiovert 100M Laser Scanning Confocal Microscope 510
(LSM510) with a 63X water immersion objective lens was used to capture the
confocal images. Nile red was excited with the 488-nm argon laser line. A set of
optical sections through the specimen (called a "Z series") was obtained by
coordinating the movement of the fine focus of the objective with image
collection to get a stack of 2-D images27 . Sections were acquired in
consecutive planes perpendicular to the apical surface. The images were 512
x 512 pixels, with a pixel size of 0.14 µm x 0.14 µm. The optical section thickness was 0.9 µm. The distance between optical sections was 0.8 µm for
the 2-D image and 0.45 µm for the 3-D image. A 3-D
view was modeled by making maximum projections of the Z series at several
different angles about the apical to basal axis (LSM510 software). Audio Video Interlaced (AVI) files were generated using Animation Shop (JASC Software, Minneapolis,
MN).
Lipid pathlength measurement and tortuosity calculation
Prints of the light micrographs were prepared and the corneocyte boundaries
identified. Paths were constructed on the print from arbitrary points on the SC
surface to the viable epidermal surface using, as a rule, the principle that the
path should follow the shortest route across the membrane that does not traverse
the interior of a corneocyte. These paths were determined using the analyst's
judgment. The pathlength was evaluated by adding segment lengths measured with a
ruler, then normalizing by the average width of the membrane in the vicinity of
the chosen path. Four to five such paths per micrograph were constructed, and
the results were averaged to give a value, t ge ,
related to membrane tortuosity in 2 dimensions. This value is equal to the
"geometrical tortuosity" of the expanded membrane, as defined in the Appendix.
Subsequent reanalysis of several of these paths using the Metamorph software led
to very similar pathlength estimates.
The value of t ge determined in this manner
reflects the length of the lipid pathway in the expanded SC specimen, relative
to the membrane thickness. Because the SC swells primarily in the transverse
dimension21 , the corresponding value for the membrane prior to expansion,
t g , must be larger than t ge . Values of t g were
calculated as follows.: First, an estimate of the SC thickness for each sample
prior to expansion was made by counting the number of corneocyte layers,
N , and multiplying the result by 0.875 µm. This
procedure makes use of commonly accepted values for corneocyte (0.8 µm) and lipid lamellae (0.075 µm)
thickness in air-dried skin6,12 . The transverse expansion factor, E t ,
was calculated as ratio of the measured width of the SC for the alkali-expanded
sample to that calculated from 0.875 µm x N. The lateral expansion factor, E l , was
taken to be 1.11, in accordance with measurements of lateral swelling of human
epidermis in water21 . Values of t g were then
calculated using Equation 1, which is derived in the Appendix.
....................(1)
Pathlength calculation via this procedure is insensitive to small angular
variations in sample alignment. Only rotations along the axis of rotation
parallel to the plane of sectioning and to the plane of the corneocytes have an
effect on t ge . Using Equation A-7 listed in the
Appendix, it is possible to show that a misalignment of θ degrees along this axis lowers the measured value of
t ge by a factor slightly less than cos θ. The same deviation increases the calculated value
of E t by a factor of 1/cos θ. Since t g is related to the product of E t and t ge (Equation 1), the alignment error cancels to first
order in the calculation of t g . Numerical
estimates of this effect made using Equations 1 and A-7 show that a 10° misalignment yields an error in t g of less than 1%, while a 20°
misalignment yields an error of only 2%. These effects are much smaller than the
other sources of uncertainty in this analysis, ie, the small number of samples
analyzed, subjective judgment as to the shortest path, and local variations in
SC thickness.
Results
Figure 1 shows corneocyte stacking in cross sections of human epidermis
following alkaline expansion using a variety of histochemical stains and
methods. Brightfield optics with methylene blue (Figure 1A ) and oil red O
(Figure 1B) clearly outline the lipid boundaries of the corneocytes, while
fluorescence and DIC methods reveal additional features. Nile red fluoresces
yellow-gold in the presence of neutral lipids and red in the presence of polar
lipids26,28,29 . The red to yellow shift in fluorescence wavelength seen in
Figure 1C illustrates the transition from polar lipids in the granular and
spinous layers to neutral intercellular lipids in the SC (brightly stained
boundaries). Nile red fluorescence in Figure 1D was captured with a different
fluorescein filter cube, then pseudocolored to appear green. The oil red O-stained DIC photomicrograph in Figure 1E presents an
apparent 3-D image due to the shadowing effect caused by variations in
refractive index within the specimen. This image shows the corneocyte boundaries
in sharp contrast to their interior and seems to be an attractive method to use
in combination with digital image analysis techniques.
The images in Figure 1 show varying degrees of order for stacking of
corneocytes within the SC. Figure 1A shows the highest degree of alignment;
however, the arrangement is less ordered than has been reported for rodent
epithelia17-19 and some human specimens19,22 .
The stacking arrangement appears to be random in the other images (
Figure 1B-E . It would be of interest
to test for correlations between these arrangements-and
the resulting differences in effective lipid pathlengths-with regional differences in skin permeability. If the
diffusion pathways are truly intercellular and if they are significantly
different from site to site, then one would anticipate a direct relationship
between the precision of the stacking arrangement and skin permeability. This
difference in stacking arrangement could be a factor in the higher permeability
of rodent skin as compared to human skin.
Figure 2 shows an example of a lipid pathlength determination performed on
the Nile red-stained image in Figure 1C . Algorithms for
determining such shortest paths have been known since the time of Euler (30);
however, the paths in Figure 2 reflect human judgment only.
Table 1 shows the
results of applying this method of analysis to the remainder of the images in
Figure 1 . The calculated values of
t ge ranged
from 3.0 to 4.4 (mean, 3.7). The values were corrected for the effect of
alkaline expansion according to Equation 1 to arrive at a 2-D value for the
unperturbed membrane, t g . Results of this
calculation are shown in
Table 2 . The mean value of t g was 12.7, more than 3 times the value of t ge . Relating this value to the mean pathlength and
effective diffusivity of the full 3-D structure is nontrivial and beyond the
scope of the present manuscript. It is, however, one of our long-range
objectives. As Johnson et al6 point out, in order to do this one must account
also for the excluded volume of the corneocytes in addition to pathlength. The
latter leads to a much greater reduction in permeability than suggested by
t g alone.
It is worth noting that the alkaline expansion technique produces swelling of
the SC comparable to that obtained with full hydration. Thus, the expanded SC
value, t ge , of 3.7, calculated in our study may
be applicable for in vivo exposure under occlusion or other hydrating
conditions.
It is interesting to compare the results in Table 2 with those obtained from
brick-and-mortar models of the SC. Michaels et al2 obtained an expression for
the lipid pathlength in a 2-D model consisting of a fully offset array of
corneocytes embedded in a lipid matrix-the "brick mason's" model of corneocyte arrangement. We show in the Appendix, using current
best estimates for corneocyte and lipid layer dimensions, that this model leads
to an estimated value for t g of 22.5. Johnson et
al6 adopted nearly the opposite arrangement, with slightly overlapped
corneocyte stacks traceable to micrographs of mouse ear epithelia17,18 . Their
model leads to a value for t g of 5.8 (see
Appendix. The average value for t g of 12.7
determined in this report indicates that the human SC samples we examined had
lipid pathlengths intermediate between that calculated for fully offset
corneocytes and that calculated for highly aligned columns.
shows images of expanded SC obtained by laser scanning confocal
microscopy. The 2-D optical sections stained with Nile red in Figure 3A are
devoid of the "out of focus" flare that is seen with nonconfocal microscopes
(eg, Figure 1D ). To obtain a model of the 3-D arrangement of the SC, maximum
projections were made through a stack of optical sections at several different
angles about the apical to basal axis (Figure 3B ). Although the apical-to-basal
dimension of the tissue in Figure 3B has been greatly increased by the alkaline
expansion, desmosomal linkages between the cells prevent their sliding over one
another to any great extent. Thus, one may be able to directly calculate lipid
pathlengths in 3 dimensions by tracing fluorescent boundaries using a suitable
image analysis algorithm. Alternatively, appropriate analysis of the 2-D
projections (Figures 1 and 3A may allow one to reconstruct the 3-D pathlengths. Analysis of these images is an ongoing effort in our laboratories.
Conclusion
Fluorescence, brightfield, and DIC microscopy, in combination with alkaline
expansion and a geometrical model to account for the effect of swelling, can be
used to estimate intercellular lipid pathlengths across human SC. Confocal
optics show potential for extending this work from 2 to 3 dimensions.
Application of these techniques to a small number of human SC samples led to
calculated average lipid pathlengths, relative to the SC width, of about 3.7
after expansion and 12.7 in unexpanded membranes.
Acknowledgements
Financial support was provided by the Procter & Gamble Company's
International Program for Animal Alternatives and by the National Science
Foundation GOALI program. We thank Drs Raymond Boissy and Ravi Kothari for
helpful discussions.
Appendix
Geometrical tortuosity estimates for 2-D brick-and-mortar models of the stratum corneum - Consider the arrangement of corneocytes embedded in a lipid matrix shown in Figure 4 . This model, originally presented by Johnson et al6 , is entirely analogous to the earlier model by
Michaels et al2 . The only difference is the variable offset ratio, w = d l s/d . Johnson and coworkers chose a columnar
arrangement of corneocytes corresponding to w =
8 based on references presenting micrographs of mouse ear epithelium17,18 .
Michaels et al considered the fully offset arrangement, corresponding to w » 1. Both groups
restricted the analysis to the case in which s = g , equivalent to
a constant distance between corneocytes.
We wish to construct an expression for the length, h lip , of the
shortest path through the lipid matrix relative to the membrane width, h.
The ratio t g = hlip /h will heretofore be
defined as the "geometrical tortuosity" of the lipid pathway. It should not be
confused with the effective tortuosity, t *,
defined by Johnson et al6 as the ratio by which the transmembrane flux is
reduced by impermeable corneocyte impediments, nor with the mean pathlength for
diffusion, t = at *,
where a is the area fraction of lipid in a
corneocyte layer. The reason for defining t g in
this manner is that this quantity can be directly estimated from SC micrographs,
regardless of the manner in which the corneocytes are stacked. Comparison of the
measured values of t g with those calculated from
precisely ordered structures such as shown in Figure 4 gives an estimate of the
average corneocyte offset in the membrane.
Referring again to Figure 4 , we construct expressions for h and
h lip as follows:
....................(A-1)
....................(A-2)
Here N is the number of corneocyte layers and N-1 is the number of lipid layers. The value of t g is given by:
....................(A-3)
Note that the corneocytes are fully aligned when d s = 0, in which case
t g = 1.
We define, as did Michaels et al2 , the corneocyte aspect ratio as a = d/t and the lipid/corneocyte thickness ratio as a = g/t . For simplicity
we consider the case g = s . Since a = (d s + d l )/t = (1 + w)d s /t , Equation A-3 can be rewritten as
....................(A-4)
For the fully aligned case, w = 8, and Equation A-4 (like Equation A-3) yields t g = 1.
For the fully offset case, w = (a- b)/(a+b). This may be seen by noting that the values of
d s and d l for this case are ½ (d +s ) and ½(d -s ), respectively. In this limit, with the further approximation N /(N -1) = 1, Equation A-4 yields
, which is equal to the value estimated by Michaels et al2 for this structure.
Johnson et al6 chose the values N = 15, d = 40 µm, t = 0.8 µm, s =
g = 0.075 µm, and w = 8 for their geometrical model of the SC. Using these
values and Equation A-4, one obtains a = 50,
ß = 0.094, and t g = 5.8. Using the same values of N , a , and ß , but choosing
w = 0.996 to represent the fully offset case,
one obtains t g = 22.5. Partially aligned arrays
of corneocytes sharing these dimensions would thus be expected to yield values
for t g within the range of 5.8 to 22.5.
Correction for alkaline expansion - When the SC
swells in water, the transverse (apical-to-basal) dimension of the tissue
increases dramatically and the lateral dimension increases slightly21 . Based
on water sorption31,32 and lateral expansion21 measurements, the
transverse expansion factor for fully hydrated SC relative to its dehydrated
state is E t = 4-6 and the lateral expansion factor is E l = 1.11. Lipid lamella spacing does not increase as a result
of hydration33 ; hence, the swelling appears to be confined to the corneocytes.
Less is known quantitatively about SC swelling in alkali. Qualitatively, it
appears to be similar to swelling in water14 . Our estimates for the
transverse expansion factor range from E t = 3.3-6.1 (
Table 2 . We assume,
for the present analysis, that the lateral expansion factor is E l = 1.11,
as found for swelling in water. The impact of these factors on SC tortuosity can
then be found by an analysis similar to that leading to Equation A-4.
Pathlengths in expanded SC are calculated as follows:
....................(A-5)
....................(A-6)
The expanded tortuosity factor is equal to the ratio of these values, as in Equation A-3:
....................(A-7)
Rearrangement of Equation A-7 and substitution of the previously defined value for h from Equation A-1 yields
....................(A-8)
Comparison of Equations A-8 and A-3 reveals that
....................(A-9)
Rearrangement of Equation A-9 yields Equation 1 in the text.
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