Naturally occurring ω-Hydroxyacids.

ω-Hydroxyacids are fatty acids bearing a hydroxyl group on the terminal carbon. They are found in mammals and higher plants and are often involved in providing a permeability barrier, the primary purpose of which is to reduce water loss. Some ω-hydroxyacid derivatives may be involved in waterproofing and signalling. The purpose of this review was to survey the known natural sources of ω-hydroxyacids. ω-Hydroxyacids are produced by two different P450-dependent mechanisms. The longer (30-34 carbons) ω-hydroxyacids are produced by chain extension from palmitic acid until the chain extends across the membrane in which the extension is taking place, and then the terminal carbon is hydroxylated. Shorter fatty acids can be hydroxylated directly to produce C16 and C18 ω-hydroxyacids found in plants and 20-eicosatetraenoic acid (20-HETE) by a different P450. The C16 and C18 ω-hydroxyacids are components of polymers in plants. The long-chain ω-hydroxyacids are found in epidermal sphingolipids, in giant-ring lactones from the sebum of members of the equidae, as a component of meibum and in carnauba wax and wool wax.

Spectrophotometric Evaluation of Potassium Nitrate Penetration Into the Pulp Cavity.

OBJECTIVES: The aim of this study was to evaluate the penetration level of potassium nitrate-containing desensitizers or whitening materials into the pulp cavity with regard to the concentration and viscosity of the formulation. METHODS AND MATERIALS: Fifty extracted human molar teeth were prepared and randomized into five groups of 10 specimens each. The control received a 30-minute treatment without any treatment material; the other four groups corresponded to treatment with DayWhite, a 14% hydrogen peroxide whitening material containing potassium nitrate; PreviDent 5000 Sensitive, a desensitizing toothpaste; Relief ACP, a desensitizing gel; or UltraEZ, a desensitizing gel. Potassium nitrate penetration levels were measured spectrophotometrically based on the Griess assay method. Treatment materials were measured for viscosity as a function of shear rate through the use of a cone-and-plate rheometer. RESULTS: Nitrate penetration levels were significantly different among the five groups (p<0.0001, Kruskal-Wallis test). After adjustment for multiple comparisons using an overall 0.05 level of type I error, the distribution of nitrate penetration values was found to differ significantly among all groups with the exception of DayWhite (median: 10.72 µM) and UltraEZ (median: 9.22 µM), which differed significantly from other groups but not from each other. The highest levels of nitrate penetration value were observed for PreviDent (median: 27.61 µM) followed by Relief ACP (median: 19.64 µM). The lowest penetration level was observed for the control group (median: 3.41 µM). Stable end-point viscosities of 11.43 ± 0.67 Pa/s, 1.33 ± 0.06 Pa/s, 0.85 ± 0.09 Pa/s, and 0.40 ± 0.01 Pa/s were observed for UltraEZ, ReliefACP, DayWhite, and PreviDent, respectively. CONCLUSION: Potassium nitrate included in different formulations can penetrate the enamel and dentin within 30 minutes. The level of potassium nitrate penetration is influenced by concentration and may also be partly affected by the viscosity of the material as well as other constituents of proprietary preparations.

Current understanding of skin biology pertinent to skin penetration: skin biochemistry.

The purpose of this review is to summarize some of the biochemical or chemical findings that have contributed most significantly to our current understanding of the permeability barrier of the skin. This literature survey covers the period from the 1970s up to the present. This seems appropriate since earlier progress was comprehensively covered in a 1978 review by Bob Scheuplein entitled 'Permeability of the skin: a review of major concepts' and in the earlier review by Scheuplein and Blank entitled 'Permeability of the skin'. Both of these review articles are still being cited, and the earlier one has been cited more than 800 times. Overlap with material covered in these earlier publications will be minimized. The overall significance of findings from some of the most recent years may not yet be determined. The emphasis will be placed on the determination of the composition and structures of the epidermal lipids, especially those of the stratum corneum, key enzymes in the biosynthesis of these lipids and some of the physical chemical properties of these lipids as revealed by X-ray diffraction, infrared spectroscopy and other physical methods.

Organization, barrier function and antimicrobial lipids of the oral mucosa.

As one moves from the skin across the vermilion region of the lip and into the oral cavity, the oral mucosa is encountered. The oral mucosa consists of connective tissue known as the lamina propria covered by a stratified squamous epithelium. In the regions of the hard palate and gingiva, the epithelium is keratinized like the epidermis. In the buccal region, the floor of the mouth and the underside of the tongue, the epithelium is non-keratinized. The epithelium on the dorsum of the tongue is a specialized epithelium, but can be approximated as a mosaic of keratinized and non-keratinized epithelia. The non-keratinized epithelial regions do not produce a stratum corneum. Nuclei with intact DNA are retained in the superficial cells. In all regions, the outer portions of the epithelium provide a protective permeability barrier, which varies regionally. Antimicrobial lipids at the surfaces of the oral mucosa are an integral part of innate immunity.

The relationship of hydrogen peroxide exposure protocol to bleaching efficacy.

The purpose of this study was to compare two in-office bleaching methods with respect to tooth color change and level of hydrogen peroxide penetration into the pulp cavity and to evaluate relationships between penetration level and color change. Eighty extracted canines were exposed to two different bleaching regimens (conventional vs sealed bleaching technique). After exposure to 38% hydrogen peroxide gel for one hour, hydrogen peroxide amount was estimated spectrophotometrically. Color change was measured per Commission Internationale de l'Eclairage methodology. Linear regression was used to evaluate factors affecting color change, including bleaching technique. The conventional and sealed bleaching groups showed no difference for any color change parameters (ΔL, Δa, Δb, ΔE); however, there was significantly greater hydrogen peroxide penetration in the conventional bleaching group (p<0.05). Linear modeling of the change in lightness (ΔL) showed that the increase in lightness tended to be greater for teeth with lower initial L* values (r=-0.32, p<0.05). After adjustment for initial L*, there was no evidence that ΔL differed with hydrogen peroxide penetration levels (p>0.05) or bleaching technique (mean group difference in ΔL=0.36; p>0.05).

Sphingoid bases are taken up by Escherichia coli and Staphylococcus aureus and induce ultrastructural damage.

Sphingoid bases found in the outer layers of the skin exhibit antimicrobial activity against gram-positive and gram-negative bacteria. We investigated the uptake of several sphingoid bases by Escherichia coli and Staphylococcus aureus, and assessed subsequent ultrastructural damage. E. coli and S. aureus were incubated with D-sphingosine, dihydrosphingosine, or phytosphingosine at ten times their MIC for 0.5 and 4 h, respectively, to kill 50% of viable bacteria. Treated bacterial cells were immediately prepared for SEM, TEM, and analyzed for lipid content by QTLC. E. coli and S. aureus treated with sphingoid bases were distorted and their surfaces were concave and rugate. Significant differences were observed in the visual surface area relative to controls for both E. coli and S. aureus when treated with dihydrosphingosine and sphingosine (p < 0.0001) but not phytosphingosine. While sphingoid base-treated S. aureus exhibited disruption and loss of cell wall and membrane, E. coli cytoplasmic membranes appeared intact and the outer envelope uncompromised. Both E. coli and S. aureus cells contained unique internal inclusion bodies, likely associated with cell death. QTLC demonstrated extensive uptake of sphingoid bases by the bacteria. Hence, sphingoid bases induce both extracellular and intracellular damage and cause intracellular inclusions that may reflect lipid uptake.

The emerging role of peptides and lipids as antimicrobial epidermal barriers and modulators of local inflammation.

Skin is complex and comprised of distinct layers, each layer with unique architecture and immunologic functions. Cells within these layers produce differing amounts of antimicrobial peptides and lipids (sphingoid bases and sebaceous fatty acids) that limit colonization of commensal and opportunistic microorganisms. Furthermore, antimicrobial peptides and lipids have distinct, concentration-dependent ancillary innate and adaptive immune functions. At 0.1-2.0 µM, antimicrobial peptides induce cell migration and adaptive immune responses to coadministered antigens. At 2.0-6.0 µM, they induce cell proliferation and enhance wound healing. At 6.0-12.0 µM, they can regulate chemokine and cytokine production and at their highest concentrations of 15.0-30.0 µM, antimicrobial peptides can be cytotoxic. At 1-100 nM, lipids enhance cell migration induced by chemokines, suppress apoptosis, and optimize T cell cytotoxicity, and at 0.3-1.0 µM they inhibit cell migration and attenuate chemokine and pro-inflammatory cytokine responses. Recently, many antimicrobial peptides and lipids at 0.1-2.0 µM have been found to attenuate the production of chemokines and pro-inflammatory cytokines to microbial antigens. Together, both the antimicrobial and the anti-inflammatory activities of these peptides and lipids may serve to create a strong, overlapping immunologic barrier that not only controls the concentrations of cutaneous commensal flora but also the extent to which they induce a localized inflammatory response.

Penetration pattern of rhodamine dyes into enamel and dentin: confocal laser microscopy observation.

Enamel and dentin are susceptible to extrinsic and intrinsic stains. The purposes of this study were to determine the penetration pattern of Rhodamine B and dextran-conjugated Rhodamine B into the enamel and dentin as observed by confocal laser microscopy and to relate it to the penetration pattern of hydrogen peroxide commonly used as an active ingredient in tooth-whitening agents and high-molecular-weight staining molecules. Eighteen recently extracted human maxillary anterior teeth were used. Teeth were cleaned and painted with nail varnish except for the crown area above the cemento-enamel junction (CEJ). The painted teeth were then immersed in Rhodamine B and dextran-conjugated Rhodamine B (70 000 MW) for 4, 7, 10 and 15 days. Teeth were sliced to 3 mm thickness in transverse plane and mounted on a glass slide just prior to observation with confocal laser microscopy. Rhodamine B and dextran-conjugated Rhodamine B readily penetrated into the enamel and dentin when exposed for 4 and 7 days, respectively. Rhodamine B penetrated along the interprismatic spaces of the enamel into the dentin. The penetration was accentuated in sections with existing crack lines in the enamel. Rhodamine B was readily absorbed into the dentinal tubules at the dentino-enamel junction and continued to penetrate through the dentin via the dentinal tubules into the pre-dentin. Within the limitations of this study, it is concluded that Rhodamine B and dextran-conjugated Rhodamine B when applied to the external surface of the tooth readily penetrate into the enamel and dentin via the interprismatic spaces in the enamel and dentinal tubules in the dentin, suggesting that stain molecules and bleaching agents possibly exhibit similar penetration pathways.

Comparative analysis of skin surface lipids of the labia majora, inner thigh, and forearm.

The purpose of this study was to develop a procedure for the collection of skin surface corneocyte lipids from the semioccluded and intimate regions of the labia majora and inner thigh of women, to evaluate the polar and nonpolar composition, and to compare the distribution of the lipid classes relative to a collection of lipids from the forearm. The solvent system of ethanol-cyclohexane was well tolerated across all sites. While the yield of polar lipids was similar across all 3 sites, there were only marginal differences in the relative abundance of ceramides, a class of lipids closely associated with skin barrier activity. The yield of neutral lipids was significantly less for the labia majora and was associated with a reduced yield of wax esters, triglycerides and free fatty acids, likely associated with reduced sebaceous gland activity. Factors that may contribute to an inferior skin barrier activity for the labia majora are discussed and suggest a possible deficiency of ω-6 fatty acid linked to the sphingosine base of ceramide EOS.

Essential fatty acids and dietary stress.

There are two families of essential fatty acids that must be obtained from the diet: the omega-6 fatty acids consisting of linoleic and arachidonic acids and the omega-3 fatty acids consisting of linolenic, eicosapentaenoic and docosahexaenoic acids (Prog Chem Fats Other Lipids 1968:9;275-348; Recent Pat Cardiovasc Drug Discov 2007; 2:13-21; Mini Rev Med Chem 2008; 8:107-115). Vegetables and vegetable oils are sources of linoleic and linolenic acids, and the higher omega-3s are obtained from fish. The estimated ratio of omega-6:omega-3 fatty acids in the typical Western diet is about 20:1, whereas, several lines of evidence indicate that a ration of 1:1 would be optimal. Both series of fatty acids can be oxidatively metabolized to a range of products. The oxidative metabolites of arachidonic acid are all proinflammatory and/or prothrombotic, while the corresponding omega-3 metabolites are anti-inflammatory and/or antithrombotic. The imbalanced consumption of the two families of essential fatty acids contributes to a range of diseases. Greater awareness of this problem is leading to increased use of dietary supplements and new products intended to decrease omega-6 consumption while increasing omega-3 intake.

Human synthetic sebum formulation and stability under conditions of use and storage.

The human skin surface and hair are generally coated with a thin film of liquid phase sebaceous lipids. This surface lipid film contributes to the cosmetic properties of the skin. Synthetic sebum has been used for studies on properties of skin and hair. However, there has been no standardized formulation of synthetic sebum and many of the synthetic sebum formulations that have been used do not closely resemble actual sebum. In this study, a formulation for a standardized and inexpensive synthetic sebum is proposed, and the chemical stability of this lipid mixture is demonstrated under conditions of use and storage. The proposed synthetic sebum consists of 17% fatty acid, 44.7% triglyceride, 25% wax monoester (jojoba oil) and 12.4% squalene. This lipid mixture takes up approximately 6% of its weight in water when equilibrated in an atmosphere saturated with water vapour. It is stable on exposure to the atmosphere at 32 degrees C for at least 48 h, and it is also stable on storage at 4 or -20 degrees C, either dry or in chloroform : methanol solution for at least 6 months. This synthetic sebum could be useful in studies on cosmetic properties of the skin surface or hair, on penetration of chemicals into the skin or in development of standardized tests of laundry detergent performance.

Characterization of rabbit ear skin as a skin model for in vitro transdermal permeation experiments: histology, lipid composition and permeability.

AIM: The aim of this work was to characterize rabbit ear skin in view of its use in transdermal permeation experiments. METHOD: The characterization included histological analysis of the tissue, qualitative and quantitative analysis of stratum corneum (SC) lipids, differential scanning calorimetry and permeation experiments (caffeine, nicotinamide, progesterone). As a reference, pig ear skin was used. RESULTS: The results obtained show that rabbit ear skin has a similar SC thickness compared to pig skin although the viable epidermis has a different structure. The lipid composition of rabbit SC was similar to pig SC but was characterized by a lower content of ceramides and a higher content of cholesterol esters and triglycerides. In terms of permeability, rabbit ear skin was 4-7 times less permeable to hydrophilic compounds, probably because of the higher lipophilicity of its SC. The permeability to progesterone was comparable between isolated pig epidermis and rabbit ear skin. CONCLUSION: Overall, the results obtained in this work support the usefulness of rabbit ear skin as barrier for skin penetration studies, for both lipophilic and hydrophilic permeants.

New arrangement of proteins and lipids in the stratum corneum cornified envelope.

A new arrangement of proteins and lipids of stratum corneum (SC) cornified envelope (CE) is proposed. The chemical analysis of CE revealed the presence of free fatty acids (FFA), ceramides (Cer), and important percentages of glutamic acid/glutamine (Glx) and serine (Ser) residues. The molecular structure of these components suggests the existence of covalent links not only between Cer and Glx but also between FFA and Ser. The protein distribution of extracellular surface of CE, i.e., the proteins that could be involved in the bonds with lipids, was studied using post- and pre-embedding immunolabeling electron microscopy. Some loricrin (protein rich in Ser) was detected in the outermost part of the CE protein layer. The external arrangement of some domains of this protein may give rise to form linkages with FFA, yielding further insight into the CE arrangement in which Cer-Glx bonds and FFA-Ser bonds would be involved. Although the importance of fatty acids in the cohesion and barrier function of SC has been widely demonstrated, their role could be associated not only to the presence of these lipids in the intercellular lamellae but also in the CE, in the same way that Cer.

A new covalently bound ceramide from human stratum corneum -omega-hydroxyacylphytosphingosine.

The outermost layer of the skin, the stratum corneum, consists of non-viable keratin-filled cells, or corneocytes, embedded in a matrix of lipids. The boundary of the cells consists of cross-linked proteins with covalently bound lipids on the outer surface. The spaces between cells are filled with a mixture of ceramides, cholesterol and fatty acids. The stratum corneum provides a protective barrier against water loss through the skin and limits the penetration of potentially harmful substances from the environment. Among the covalently bound lipids on the corneocyte surface are omega-hydroxyacylsphingosine and omega-hydroxyacyl-6-hydroxysphingosine. The previously suspected presence of omega-hydroxyacylphytosphingosine is confirmed in this report through its specific isolation and characterization based on chromatographic behaviour and proton magnetic resonance spectral data.

The human epidermis models EpiSkin, SkinEthic and EpiDerm: an evaluation of morphology and their suitability for testing phototoxicity, irritancy, corrosivity, and substance transport.

The commercially available reconstructed human epidermis models EpiSkin, SkinEthic and EpiDerm demonstrate reasonable similarities to the native human tissue in terms of morphology, lipid composition and biochemical markers. These models have been identified as useful tools for the testing of phototoxicity, corrosivity and irritancy, and test protocols have been developed for such applications. For acceptance of these tests by the authorities, prevalidation or validation studies are currently in progress. Furthermore, first results also indicate their suitability for transport experiments of drugs and other xenobiotics across skin. Still, however, the barrier function of these reconstructed human epidermis models appears to be much less developed compared to native skin. Further adaptation of the models to the human epidermis, especially concerning the barrier function, therefore remains an important challenge in this area of research.

Analysis of hair lipids and tensile properties as a function of distance from scalp.

Cuticle cells form the outer covering surrounding and protecting the cortex. The cuticle cells are thin, flat and overlap, and intercellular lipid lamellae are found in the gaps between the cell boundaries. The lipid lamellae are also found within the cortex in the cell boundaries between the long fribrous corticle cells. In addition, the outer surfaces of the cuticle cells are covered by a monolayer of covalently bound fatty acids, a major component of which is 18-methyleicosanoic acid. The fatty acids are thought to be attached through thio-ester linkages. Together these lipids are thought to be major determinants of the physical properties of the hair. The present study tested the hypothesis that both free and covalently bound lipids are progressively lost during normal environmental exposures. This progressive loss within the cuticle layers may, in part, lead to an increased susceptibility of the protein and lipid lamellae in the cortex to degradation. This degradation, in turn, would contribute to a progressive decrease in the tensile properties of the hair. Research grade hair was cut into five segments from the root to the distal end. Lipids from each segment were extracted and analyzed by thin-layer chromatography in conjunction with photodensitometry. The major free polar lipid classes in the hair included ceramides, glucosylceramides and cholesterol sulfate. The concentrations of all of the free polar lipids as well as the covalently bound fatty acids decreased in going from the root to the distal end of the hair. In addition, there was a significant reduction in tensile properties of the hair from the root to distal end. In conclusion, the progressive loss of endogenous free and covalently bound lipids from hair, which are probably related to normal weathering of the hair and grooming practices, may help contribute to a marked decrease in tensile properties to the hair.

Comparison of differentiation markers between normal and two squamous cell carcinoma cell lines in culture.

This study examines differences between cultures of normal human oral epithelial cells and two squamous cell carcinoma cell lines (SCC15 and SCC25) in the expression of structural proteins, adhesion molecules, plasma membrane lipid composition, and intercellular junctions. Based on immunocytochemistry, most normal cell cultures appeared to express more E-cadherin, integrin beta-1, cytokeratin (CK) 14, CK19, and involucrin than SCC cultures. By Western blot analysis, normal cultures expressing high levels of E-cadherin also expressed high levels of involucrin and low levels of CK19. Both SCC cultures demonstrated lower expression of E-cadherin and involucrin, whereas only SCC15 cells showed high levels of CK19. Expression of beta-catenin, an E-cadherin associated protein with potential oncogene function, did not vary among normal and SCC cells. Proportions of saturated fatty acids quantified by thin layer chromatography were higher in the normal cell cultures, than in both SCC cell lines. No morphological differences were evident by transmission electron microscopy (TEM) between normal and SCC cell-cell intercellular junctions. Although no quantitation was attempted, observation suggested that normal cells form more intercellular junctions (TEM observation) and larger intercellular bridges (SEM observation) compared to both SCC cell lines. Of the factors examined, main variations between cultures of normal oral epithelium and the two SCC cell lines examined include the expression of structural and adhesion proteins, lipid composition, and intercellular junctions. The extent of the differences varies according to the stage of terminal differentiation demonstrated by the normal cell cultures.

Molecular models of the intercellular lipid lamellae from epidermal stratum corneum.

The purpose of the present study was to test the hypothesis that the 13 nm trilamellar repeat units within the intercellular spaces of epidermal stratum corneum are composed of lamellae with alternating 5-3-5 nm dimensions as presented in previous models [J. Invest. Dermatol. 92 (1989) 251, P.W. Wertz, Integral lipids in hair and stratum corneum, in: P. Jolles, H. Zahn, H. Hocker (Eds.), Hair: Biology And Structure, Birkhauser Verlag, Basel, 1996, pp. 227-238, Acta Derm.-Venereol., Suppl. 208 (2000) 23]. Electron density profiles were measured from transmission electron micrographs of porcine stratum corneum prepared using ruthenium tetroxide [J. Invest. Dermatol. 92 (1989) 251]. Center-to-center distances of adjacent electron-dense bands as well as adjacent lucent bands were measured. Dense band center-to-center measurements were consistent with a 5-3-5 nm arrangement. However, lucent band center-to-center measurements revealed uniform lamellar thickness. It is suggested that linoleate chains in the central lamella reduce more ruthenium than the predominantly saturated chains in the outer lamellae and that this additional reduced ruthenium accumulates under the polar head group regions. A similar phenomenon involving the sphingosine moieties of the covalently bound omega-hydroxyceramide molecules accounts for the three-band pattern seen between the ends of adjacent corneocytes. It is concluded that the component lamellae of the several types of 13 nm trilamellar units of the stratum corneum are all of equal thickness.

Caveolin expression and localization in human keratinocytes suggest a role in lamellar granule biogenesis.

Lamellar granules are sphingolipid-enriched organelles, probably intimately related to the tubulo-vesicular elements of the trans-Golgi network, that deliver the precursors of stratum corneum barrier lipids to the extracellular compartment. Caveolins are cholesterol-binding scaffolding proteins that facilitate the assembly of cholesterol- and sphingolipid-enriched membrane domains known as caveolae. Similarities in the composition of lamellar granules and caveolae suggest that caveolins could be involved in lamellar granule assembly, trafficking, and/or function. In order to explore this relationship, we have examined the expression of caveolins in epidermis, keratinocyte cultures, and an isolated lamellar granule fraction using immunolabeling, immunoblotting, and northern blotting. Several antibodies show immunolocalization of caveolin-1 in the basal layer of human epidermis, with a decline in the suprabasal layers and a reemergence of expression at the stratum granulosum/stratum corneum junction. Two of three caveolin-2 antibodies show little basal staining, but strong signal throughout the rest of the epidermis, whereas a third shows a pattern like caveolin-1. An antibody against caveolin-3 shows a strong signal at the stratum granulosum/stratum corneum interface. Caveolins partially colocalize with glucocerebrosidase, an enzyme known to be critical for remodeling of extruded lamellar granule contents, with AE17, a previously described lamellar-granule-associated antibody, and with glucosylceramides, a major lipid component of lamellar granules. Caveolin-1 protein is present in undifferentiated low-calcium-grown keratinocyte cultures, decreases upon induction of differentiation, and then rises to levels above those seen in undifferentiated cultures, consistent with the immunofluorescence findings. Caveolin-1 mRNA expression parallels that of the protein. Caveolin-2 mRNA and protein expression were unchanged over the course of culture differentiation. Keratinocyte caveolin-1 mRNA expression is not induced by an increase in medium calcium level and is markedly reduced by phorbol-ester-mediated protein kinase C induction. Caveolin-1 is enriched in an isolated lamellar granule fraction that is also enriched, as we have previously described, in lysosomal acid lipase and glucocerebrosidase, and localizes to structures consistent with lamellar granules on immunoelectron microscopy. The differentiation-dependent expression of caveolin-1, the colocalization of caveolins with putative lamellar-granule-associated antigens, their enrichment in isolated lamellar granules, and their presence in lamellar-granule-like structures on immunoelectron microscopy, along with their known structural role in the assembly of glycosphingolipid- and cholesterol-enriched domains in other cell types, suggest that caveolins may play a role in lamellar granule assembly, trafficking, and/or function.

Deciduous canine and permanent lateral incisor differential root resorption.

When a permanent maxillary canine erupts apical to the permanent lateral incisor and the deciduous canine, resorption typically takes place only on the deciduous canine root. An understanding of this differential resorption could provide insight into the reasons for excessive iatrogenic root resorption during orthodontic tooth movement. The purpose of the present study was to examine the response of roots of permanent lateral incisors and deciduous canines to simulated resorption, and to acid and enzyme attack, reflecting the physiologic environment of an erupting permanent canine. Groups of maxillary permanent lateral incisor and deciduous canine roots were exposed to 5 combinations of Ten Cate demineralizing solution, Ten Cate demineralizing solution with EDTA, and a Type I collagenase solution. Sections of the roots were examined under a polarized light microscope. Analysis of variation of the resulting root lesions demonstrated that the lesion depths for deciduous canines were greater than those for permanent lateral incisors when averaged across 4 of the conditions (F(1,24) = 7.49, P =.0115). On average, deciduous canine roots demonstrated lesions 10% deeper than did permanent lateral incisor roots. We concluded that when deciduous canine and permanent lateral incisor roots are subjected to acid and enzyme attack, reflecting the physiologic environment of an erupting permanent canine, significantly deeper demineralized lesions are seen in the deciduous roots compared with the permanent roots. This finding may partially explain the differential root resorption during permanent tooth eruption.