Alterations in cutaneous immune reactivity to dinitrofluorobenzene in graying C57BL/vi.vi mice

The fur of the C57BL/vi.vi mouse is black at 6 weeks of age. By 6 months of age the animals are white and there are no identifiable pigment cells within the epidermis or hair bulbs. Human subjects with vitiligo exhibit loss of epidermal pigment cells. The loss of pigment cells in human subjects with vitiligo has been associated with loss of cutaneous immune reactivity to contact allergens. Therefore, studies were performed to determine whether loss of pigment cells in these depigmenting mice also was associated with loss of the cutaneous immune response. The number of Ia-positive (Ia +) Langerhans cells (LC)/mm2 on the back and the ear, the sites of sensitization and challenge with dinitrofluorobenzene (DNFB), was quantified before, during, and after depigmentation. We observed that there were fewer LC/mm2 on the back and the ear before and after pigment loss in the graying mice than in the normal control C57BL/6 mice. The young pigmented C57BL/vi.vi mice were capable of developing moderate contact hypersensitivity; the older depigmented mice did not sensitize to DNFB. We conclude that the depigmented mice, like human subjects with vitiligo, have a loss of contact hypersensitivity associated with a loss of pigment cells within the epidermis. In the mouse, loss of melanocytes is associated with a decrease in the population density of Ia + cells.     

Leukoderma in the nude C57BL/6 mouse

The nu gene bred into mice causes the animals to be athymic and nude, that is their hairs do not project past the follicular orifice making them appear to be hairless. The gene has been bred into C57BL/6 mice. C57BL/6 mice which are homozygous for the gene are runted and nude and develop leukoderma around 6 weeks of age. The leukoderma is caused by shedding of short stubby, barely visible black hairs, probably a normal hair cycle. This type of leukoderma is not vitiligo, a disease which causes the skin to turn white because of destruction of the melanocytes in the epidermis or hair bulbs.     

不同黑素细胞群在有色毛囊重建中作用的研究

目的观察不同种属小鼠的毛囊细胞能否嵌合重建有色毛囊, 探讨不同黑素细胞群在小鼠有色毛囊重建中的作用。方法选取C57BL/6J、BALB/C胎鼠或乳鼠皮肤, 分离出表皮细胞群、毛囊上皮细胞群和真皮细胞群, 培养、纯化从表皮细胞群获得的表皮黑素细胞。实验包含三部分, ①乳鼠C57BL/6J毛囊重建实验:分为表皮细胞+毛囊上皮细胞组、真皮细胞组;②嵌合毛囊重建实验:分为乳鼠C57BL/6J真皮细胞组、乳鼠BALB/C真皮细胞组、乳鼠BALB/C真皮+乳鼠C57BL/6J真皮细胞组、胎鼠BALB/C真皮细胞+胎鼠C57BL/6J真皮细胞组;③有色毛囊重建实验:分为乳鼠BALB/C真皮细胞+乳鼠C57BL/6J表皮细胞组、乳鼠BALB/C真皮细胞+乳鼠C57BL/6J毛囊上皮细胞组、乳鼠BALB/C真皮细胞+培养的C57BL/6J表皮黑素细胞组。采用小室移植法将不同细胞接种于裸鼠的背部, 每组4只。于移植后4周和8周, 通过大体观察、组织学及免疫荧光评估毛囊重建情况。结果移植后4周和8周, 乳鼠C57BL/6J毛囊重建实验中(本部分实验2组共8只BALB/C裸鼠, 7只存活, 1只因创面感染死亡...     

A human placental extract: in vivo and in vitro assessments of its melanocyte growth and pigment-inducing activities

BACKGROUND: The authenticity of various prototype human placental extracts with biological activity, such as that inducing vitiligo repigmentation, is under serious criticism, mainly due to a lack of demonstration at the cellular level. Considering the present worldwide scenario with regard to the occurrence and treatment of vitiligo, a thorough scientific exploration of such extracts should be undertaken. METHOD: One such prototype placental preparation was prepared, and was evaluated with regard to its melanogenic action in C57BL/6J mice in vivo and its mitogenic and melanogenic activity on B16F10 mouse melanoma cells and normal human melanocytes in vitro. The extract was applied topically to mice with age-induced prolonged telogenic phase of hair growth (grey body coat hair). Standard 3H-thymidine incorporation and spectrophotometric methods were followed to illustrate mitogenic and melanogenic effects at the cellular level. RESULTS: The resurgence of blue skin, followed by shiny black hair, at the regions of application of the extract demonstrated the reversal of the age-induced prolonged telogenic phase of hair growth to the anagenic phase after topical application of the extract on C57BL/6J mice. Further support was obtained from histology where, at the extract-treated sites, the development of new melanogenic centers and hair follicles was observed. During in vitro studies, the vehicle-free extract constituents stimulated both mitogenesis and melanogenesis of B16F10 mouse melanoma cells in a concentration-dependent manner. The cell morphology and extent of melanogenesis also showed significant changes. In addition, two known melanocyte activity-modulating peptides, endothelin-1 (ET-1) and adrenocorticotropic hormone (ACTH), were determined in the extract, chiefly in the total lipid fraction, indicating their effective cutaneous permeation. CONCLUSIONS: The extract was found to be a potent mitogen in the in vitro condition and a potent melanogen in both the in vitro and in vivo situations. This strongly suggests its therapeutic potential for the repigmentation of vitiligo patches.     

Small‐animal PET imaging analysis with [18F]FHBG in a mouse model of HSV1‐tk gene expression in melanoma

The purpose of this study was to establish a small‐animal model for molecular imaging and to acquire basic data on assessing the efficacy of candidate melanoma drugs using small‐animal PET imaging analysis with [¹⁸F]FHBG for herpes simplex virus 1‐thymidine kinase (HSV1‐tk) gene expression in a melanoma mouse model. The B16 melanoma cell line was transduced with a recombinant lentiviral vector containing the HSV1‐tk gene and inoculated into the back skin of C57BL/6J mice. [¹⁸F]FHBG PET imaging showed better contrast for HSV1‐tk(+) melanomas compared to brain, heart, gall bladder, intestine and kidney than did [¹⁸F]FDG PET imaging.     

Increased mitochondrial respiratory chain enzyme activities correlate with minor extent of liver damage in mice suffering from erythropoietic protoporphyria

Mitochondrial dysfunction might play a role in the pathogenesis of liver damage in erythropoietic protoporphyria (EPP). Changes in mitochondrial respiratory chain activities were evaluated in the Fech(m1pas)/Fech(m1pas) mouse model for EPP. Mice from different strains congenic for the same ferrochelatase germline mutation manifest variable degrees of hepatobiliary injury. Protoporphyric animals bred into the C57BL/6J background showed a higher degree of hepatomegaly and liver damage as well as higher protoporphyrin (PP) accumulation than those bred into the SJL/J and BALB/cJ backgrounds. Whereas mitochondrial respiratory chain activities remained unchanged in the liver of protoporphyric mice C57BL/6J, they were increased in protoporphyric mice from both SJL/J and BALB/cJ backgrounds, when compared to wild-type animals. Mitochondrial respiratory chain activities were increased in Hep G2 cell line after accumulation of PP following addition of aminolevulinic acid. As a direct effect of these elevated mitochondrial activities, in both hepatic cells from mutant mouse strains and Hep G2 cells, adenosine 5'-triphosphate (ATP) levels significantly increased as the intracellular PP concentration was reduced. These results indicate that PP modifies intracellular ATP requirements as well as hepatic mitochondrial respiratory chain enzymatic activities and further suggest that an increase of these activities may provide a certain degree of protection against liver damage in protoporphyric mice.     

Characterization of an in vivo model for the study of eyelash biology and trichomegaly: mouse eyelash morphology,development, growth cycle,and anagen prolongation by bimatoprost

Summary Background Hypertrichosis or alopecia of the eyelashes is associated with various diseases or may be drug induced. Although neither increase nor loss of eyelashes is life threatening, eyelash disorders can be psychologically disturbing. However, as control of eyelash growth and the underlying mechanisms of eyelash hypo‐ or hypertrichosis are largely obscure, available therapy is limited. Objectives To improve this situation, we sought to establish a pragmatic, well‐defined mouse model for the study and pharmacological investigation of eyelash follicle biology. Methods We took a morphometric approach to establish an eyelash model using female C57BL/6J mice by comparing with pelage hairs and highlighting the differences. We next applied a hypertrichosis‐triggering agent and investigated its effect using the model. Results In eyelashes, a synchronized growth cycle was observed after morphogenesis but was completed earlier than pelage hairs. Exogen was strictly regulated and occurred in every cycle in the eyelash. Otherwise, general morphological features of mouse eyelashes (shafts, follicles, morphogenesis and growth cycle) were comparable with those of pelage hairs. The eyelash growth‐stimulatory agent in humans, bimatoprost, significantly extended the duration of anagen, resulting in more and longer eyelashes, but there was no evidence of follicle neogenesis. Conclusions This study shows that mouse eyelashes offer an excellent in vivo model for the quantitative and qualitative analysis of eyelash morphology, development, growth cycle, exogen and pharmacological modulation. This model will help to elucidate the unknown molecular controls of eyelash growth, and to develop novel drugs to treat eyelash disorders.     

白癜风的实验研究进展

作者综述以动物模型作为研究治疗白癜风药物的实验材料,动物模型有：（1）SL鸡;（2）C57BL小鼠;（3）豚鼠.     

Animal model of sclerotic skin. III: Histopathological comparison of bleomycin-induced scleroderma in various mice strains

Abstract We have recently established a mouse model for scleroderma by repeated local bleomycin treatment. In this study, we compared the susceptibility to bleomycin in the development of dermal sclerosis among Balb/c, C3H/He, C57BL/6J, A/J, DBA/2, B10.BR, B10.A, and B10.D2 mouse strains. After either bleomycin or PBS treatment, skin from the injection site was histologically examined. Dermal sclerosis was induced by bleomycin treatment for 4 weeks in all of the strains examined. In particular, C3H/He, DBA/2, B10.D2 and B10.A mice developed intense dermal sclerosis characterized by deposition of homogeneous material in the dermis and thickened collagen bundles. Dermal thickness showed a more than twofold increase following bleomycin treatment, as compared with PBS treatment, except in C57BL/6J and DBA/2 mice. In A/J, C3H/He, B10.A, and B10.D2 mice, dermal thickness showed a more than 2.5-fold increase. Mast cell numbers in sclerotic skin were significantly greater than in PBS-treated skin in Balb/c and B10.A mice after 4 weeks of treatment. We also examined whether bleomycin treatment for 3 weeks could induce dermal sclerosis in C3H mice. Histological examination revealed that epidermal thickness as well as dermal sclerosis was increased in C3H mice following bleomycin treatment for 3 weeks. Increased hydroxyproline content as well as mRNA expression of α1(I) collagen, as determined by Northern blot analysis, were observed following bleomycin treatment. Taken together, we conclude that C3H/He and B10.A mouse strains are bleomycin-’susceptible’, and these strains are considered to be a suitable experimental model of bleomycin-induced scleroderma. Received: 8 June 2000 / Revised: 1 August 2000 / Accepted: 4 September 2000     

Langerhans' cells in hair follicles of the depigmenting C57Bl/Ler-vit.vit mouse. A model for human vitiligo

The C57Bl/Ler-vit.vit mouse grows a black pelage after birth. During successive hair molts, the fur loses its pigmentation. By 6 months of age, most of the fur of the animal is white. The epidermis of the ears and tail also loses its pigmentation. Histologic studies confirm that in the epidermis and hair follicles there is an absence of pigment cells identifiable by various histochemical or electron microscopic techniques. This mouse may be an excellent model in which to study the role of Langerhans' cells and the immune response in the pathogenesis of vitiligo, a study not easily done in humans. From results of prior studies, we postulated that if Langerhans' cells were involved in the destruction of melanocytes, they would be abnormal (either more or less numerous) in number during the active phase of depigmentation and normal in number after depigmentation was complete. To determine whether the Langerhans cell (Ia+/adenosine triphosphatase dendritic epidermal cell) might be involved in destruction of pigment cells, we quantified the number of Ia+ and adenosine triphosphatase dendritic cells in the hair follicles in skin from the ear, abdomen, back, and tail from male C57Bl/Ler-vit.vit mice while the fur and skin were depigmenting and after depigmentation was almost completed. We found that Langerhans' cells were normal in number during depigmentation and were most numerous after depigmentation. Previous studies indicate that Langerhans' cells in these mice are functionally defective and respond poorly to some contact allergens. From these morphologic and functional data, we conclude that Langerhans' cells probably are uninvolved in causing depigmentation in these mice. We also observed that the epithelium of hair follicles has a significantly higher (up to 1600/mm2) population density of Langerhans' cells than interfollicular skin.     

Bleomycin‐induced fibrosis in MC1 signalling‐deficient C57BL/6J‐Mc1re/e mice further supports a modulating role for melanocortins in collagen synthesis of the skin

The melanocortin‐1 receptor (MC₁) binds α‐melanocyte‐stimulating hormone (α‐MSH) with high affinity and has a physiological role in cutaneous melanin pigmentation. Previously, we reported that human dermal fibroblasts also express functional MC₁. α‐MSH suppressed transforming growth factor‐β₁‐ and bleomycin (BLM)‐induced collagen synthesis in vitro and in vivo. Using MC₁ signalling‐deficient C57BL/6J‐Mc1r^e/e mice, we tested as to whether MC₁ has a regulatory role on dermal collagen synthesis in the BLM model of scleroderma. Notably, mice with a C57BL/6J genetic background were previously shown to be BLM‐non‐susceptible. Interestingly, treatment of C57BL/6J‐Mc1r^e/e but not of C57BL/6J‐wild‐type mice with BLM increased cutaneous collagen type I content at RNA and protein level along with development of skin fibrosis. Cutaneous levels of connective tissue growth factor and monocyte chemotactic protein‐1 were also increased in BLM‐treated C57BL/6J‐Mc1r^e/e mice. Primary dermal fibroblasts from C57BL/6J‐wt mice further expressed MC₁, suggesting that these cells are directly targeted by melanocortins to affect collagen production of the skin. Our findings support the concept that MC₁ has an endogenous regulatory function in collagen synthesis and controls the extent of fibrotic stress responses of the skin.     

Hair interior defect in AKR/J mice

Background.  All AKR/J mice have a subtle defect that involves malformation of the central portion of hair fibres that is best visualized under white and polarized light microscopy.
Aims.  This study sought to characterize the clinical and ultrastructural features of the hair interior defect (HID) phenotype and to determine the chromosomal localization of the hid mutant gene locus.
Methods.  White and polarized light microscopy combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the HID phenotype. Complementation testing and gene-linkage studies were performed to map the locus.
Results.  Using SEM, the hair-fibre structure on the surface was found to be similar to hairs obtained from normal BALB/cByJ^+/+and C57BL/6 J^+/+mice. There were also no differences in sulphur content. TEM revealed degenerative changes in the medulla similar to that seen by light microscopy. This autosomal recessive mutation is called HID (locus symbol: hid ). We mapped the hid locus to the distal end of mouse chromosome 1. No genes reported to cause skin or hair abnormalities are known to be within this interval except for the lamin B receptor (Lbr), which had been excluded previously as the cause of the hid phenotype in AKR/J mice.
Conclusion.  A potentially novel gene or known gene with a novel phenotype resides within this interval, which may shed light on human diseases with defects in the inner structure of the hair fibre.     

不同剂量刺蒺藜对小黑鼠毛囊黑皮素1受体表达的影响

目的观察不同剂量的刺蒺藜对C57BL/6 J小鼠毛囊黑皮素1受体(MC-1R)表达的影响,探讨不同剂量刺蒺藜在毛囊黑素细胞活化过程中的作用。方法选择SPF级C57BL/6 J小黑鼠为实验动物,采用免疫组化技术观察用不同剂量中药刺蒺藜的水提物灌服后小鼠皮肤中MC-1R水平的变化,以正常小黑鼠皮肤为对照。结果MC-1R的表达定位于表皮基底部黑素细胞、角质形成细胞胞浆;在各组皮片中均呈阳性表达,刺蒺藜用量越大表达越明显;MC-1R在各组皮片中的表达与正常对照组皮肤比较差异均有显著性(P<0.05)。结论不同剂量刺蒺藜都能够有效上调MC-1R的表达,MC-1R在刺蒺藜作用下的表达可能是通过扩增生物学效应的方式使黑素合成增强,从而起到调节黑素代谢的作用。     

Effects of 17-beta-estradiol and ICI 182 780 on hair growth in various strains of mice

17-beta-Estradiol (10 nmol per 200 microl acetone) applied topically twice weekly to the clipped dorsal surface of C57BL/6 or C3H female mouse skin prevented hair growth, as previously described in the CD-1 mouse strain. Twice weekly topical application of the estrogen receptor antagonist, ICI 182 780 (10nmol per 200microl acetone), induced the telogenanagen transition and produced early pigmentation appearance in skin and hair growth in C57BL/6 and C3H female mice. Whereas twice weekly topical application of 10nmol 17-beta-estradiol blocked hair growth, the intraperitoneal administration of this dose twice weekly did not block hair growth, suggesting a direct cutaneous effect of 17-beta-estradiol. We also evaluated the effect of 17-alpha-estradiol, 17-beta-estradiol, and ICI 182 780 on hair growth in male mice. As observed in female mice, 17-beta-estradiol was a potent inhibitor of hair growth and ICI 182 780 stimulated hair growth; however, unlike the results previously observed in female mice, 17-alpha-estradiol was a potent inhibitor of hair growth in male mice. These results demonstrate that (i) the route of administration of 17-beta-estradiol is critical for its ability to block hair growth; (ii) C57BL/6 and C3H mice, two commonly employed mouse strains for hair growth studies, responded to 17-beta-estradiol and ICI 182 780 in a manner similar to that described in CD-1 mice; and (iii) the hair follicles of male and female mice respond similarly to 17-beta-estradiol and ICI 182 780, but display striking sex differences in the response to 17-alpha-estradiol on hair growth.     

Comparative regenerative biology of spiny (Acomys cahirinus) and laboratory (Mus musculus) mouse skin

Wound‐induced hair follicle neogenesis (WIHN) has been demonstrated in laboratory mice (Mus musculus) after large (>1.5 × 1.5 cm²) full‐thickness wounds. WIHN occurs more robustly in African spiny mice (Acomys cahirinus), which undergo autotomy to escape predation. Yet, the non‐WIHN regenerative ability of the spiny mouse skin has not been explored. To understand the regenerative ability of the spiny mouse, we characterized skin features such as hair types, hair cycling, and the response to small and large wounds. We found that spiny mouse skin contains a large portion of adipose tissue. The spiny mouse hair bulge is larger and shows high expression of stem cell markers, K15 and CD34. All hair types cycle synchronously. To our surprise, the hair cycle is longer and less frequent than in laboratory mice. Newborn hair follicles in anagen are more mature than C57Bl/6 and demonstrate molecular features similar to C57Bl/6 adult hairs. The second hair cycling wave begins at week 4 and lasts for 5 weeks, then telogen lasts for 30 weeks. The third wave has a 6‐week anagen, and even longer telogen. After plucking, spiny mouse hairs regenerate in about 5 days, similar to that of C57Bl/6. After large full‐thickness excisional wounding, there is more de novo hair formation than C57Bl/6. Also, all hair types are present and pigmented, in contrast to the unpigmented zigzag hairs in C57Bl/6 WIHN. These findings shed new light on the regenerative biology of WIHN and may help us understand the control of skin repair vs regeneration.     

Predominant formation of heavily pigmented dermal melanocytomas resembling 'animal-type' melanomas in hepatocyte growth factor (C57BL/6 x C3H)F1 mice following neonatal UV irradiation

BACKGROUND: Transgenic mice expressing hepatocyte growth factor (HGF) develop cutaneous melanocytic tumors following neonatal UV exposure. Here, we examined the histologic spectrum of UV-induced melanocytic tumors in HGF mice on a pigmented (C57BL/6 x C3H/HeN)F(1) background. METHODS: Neonatally irradiated (4000 J/m(2)) mice were monitored for 43 weeks, and 31/34 (91%) animals developed a total of 163 melanocytic tumors. RESULTS: Of 54 primary tumors analyzed, most (49/54, 91%) demonstrated exclusively dermal collections of epithelioid cells with voluminous densely pigmented cytoplasm. Seven of these also demonstrated a population of spindled cells with mitoses. Several (3/54, 6%) tumors exhibited a junctional component with melanocytes present in the epidermis. Staining with PEP8 confirmed the presence of interfollicular melanocytes at the dermal-epidermal junction in neonatal skin. CONCLUSIONS: In contrast to HGF animals on an albino (FVB) background, HGF animals on the pigmented (C57BL/6 x C3H/HeN)F(1) background do not develop classic radial growth phase melanoma but rather predominantly develop dermal melanocytomas resembling the 'animal-type' melanoma occasionally seen in humans. These results demonstrate the influence of genetic background on histologic pattern of UV-induced melanomas in mice.     

Dermal dendritic cells,but not Langerhans cells,are critical in murine single epicutaneous sensitization

A murine repeated protein‐patch model has been established to study epicutaneous sensitization in atopic dermatitis. This model has shown a predominant Th2 and a weak Th1 response in both BALB/c and C57BL/6 mice. However, Th responses induced in the repeated model are not consistent with the generally accepted theory that BALB/c and C57BL/6 mice are Th2 and Th1 prone and are representatives of human atopy and non‐atopy, respectively. In this study, a single protein‐patch model was established, which showed in addition to the Th2 response, a remarkable Th1 response in C57BL/6 mice, but not in BALB/c mice. Moreover, using muLangerin‐DTR mice, we demonstrated that dermal dendritic cells, but not Langerhans cells, are critical in single epicutaneous sensitization in both strains of mice.     

IL-1 signalling is dispensable for protective immunity in Leishmania-resistant mice

Leishmaniasis is a parasitic disease affecting ～12 million people. Control of infection (e.g. in C57BL/6 mice) results from IL-12-dependent production of IFNγ by Th1/Tc1 cells. In contrast, BALB/c mice succumb to infection because of preferential Th2-type cytokine induction. Infected dendritic cells (DC) represent important sources of IL-12. Genetically determined differences in DC IL-1α/β production contribute to disease outcome. Whereas the course of disease was not dramatically altered in IL-1RI(-/-) mice, local administration of IL-1α to infected C57BL/6 mice improved disease outcome. To definitively elucidate the involvement of IL-1 in immunity against leishmaniasis, we now utilized IL-1α/β-double-deficient C57BL/6 mice. C57BL/6 mice are believed to be a good surrogate model for human, self limited cutaneous leishmaniasis (CL). Leishmania major-infected IL-1α/β(-/-) mice were resistant to experimental CL comparable to controls. In addition, DC-based vaccination against leishmaniasis in C57BL/6 mice was independent of IL-1. Thus, in Leishmania-resistant C57BL/6 mice, IL-1 signalling is dispensable for protection.     

Dual mode reflectance and fluorescence confocal laser scanning microscopy for in vivo imaging melanoma progression in murine skin

A system was designed and developed for simultaneous fluorescence and reflectance contrast in vivo confocal imaging of murine skin using 488 nm (fluorescence mode) and 830 nm (reflectance mode) laser light sources. B16 melanoma cells and B16-enhanced green fluorescent protein (EGFP) cells were inoculated intradermally into transgenic C57BL/6-TgN (ACTbEGFP) 10sb and non-transgenic C57BL/6 mice, respectively. The inoculation sites were imaged sequentially over a 20 d period. The in vivo confocal images were correlated with ex vivo conventional microscopy. The combined modality system provided single-cell resolution and adequate image registration. In fluorescence mode, B16 melanoma cells appeared as dark objects in the bright background of the GFP expressing murine cells of the C57BL/6 transgenic mouse, and the B16-EGFP melanoma cells had a bright signal within a dark background in C57BL/6 mice. In the C57BL/6 transgenic mouse, a population of fluorescent dendritic cells was observed in the vicinity of the tumor cells. The reflectance images provide a useful reference for those areas in the dermal tissues lacking a fluorescent signal. Combined reflectance/fluorescence in vivo confocal laser scanning microscopy holds significant promise for studies of tumor progression in murine skin.     

Independent inheritance of genes regulating two subpopulations of mouse clonogenic keratinocyte stem cells

Mouse keratinocyte stem cells originate from the bulge of hair follicle, and, according to definition, possess a clonogenic activity in vitro. We have investigated seven inbred (C57BL/6, C3H, DBA/2, BALB/c, FVB) and outbred (SENCAR, CD-1) mouse strains and found that three genetically distinct subsets of mouse strains differ significantly in the frequency of clonogenic activity in vitro. The analysis of keratinocyte colonies in two reciprocal backcross [C57BL/6 x (BALB/c x C57BL/6); BALB/c x (BALB/c x C57BL/6)] and intercross [(BALB/c x C57BL/ 6)F2] of BALB/c and C57BL/6 mice allowed us to identify two subpopulations of clonogenic keratinocytes able to produce small (less than 2 mm2) and large (more than 2 mm2) colonies. We conducted linkage analysis and found that small colonies associated with mouse chromosomes 1, 6, 7, 8, and 9; but large colonies--with the chromosome 4. We defined locus on the chromosome 9 that associated with small colonies as keratinocyte stem cell locus 1 (Ksc1), and locus on the mouse chromosome 4 associated with large colonies-keratinocyte stem cell locus 2 (Ksc2). Ksc1 and loci on chromosomes 6 and 7 are close if not equal to loci associated with sensitivity to skin carcinogenesis. We conclude that two subpopulations of stem cells able to produce small and large colonies regulated by different genes and genes regulating small colonies might be responsible for sensitivity to skin carcinogenesis.