Detection of BrdU-label retaining cells in the lacrimal gland: implications for tissue repair

[Cell technologies as a basis for the development of regenerative principles for the treatment of lacrimal gland diseases]

The lacrimal gland (LG) is a tubuloacinar exocrine gland composed of acinar, ductal, and myoepithelial cells. Three-dimensional distribution of acinar lobules, ducts, and myoepithelial cells is necessary for the effective functioning of the organ. LG is the main organ of immune surveillance of the ocular surface system. The embryogenesis of the gland is regulated by the interaction of genetic mechanisms, internal epigenetic (enzyme systems, hormones) and exogenous factors. There is no doubt that there is a clear genetic program for the implementation of the complex process of embryonic development. The mechanisms regulating LG organogenesis initiate the work of a huge number of structural oncogenes, transcription and growth factors, etc. Studying the expression and selective activity of regulatory genes during organ development, their participation in the differentiation of different cell types is a current trend at the nexus of clinical genetics, molecular biology, embryology and immunocytochemistry. Due to its relatively simple structure and accessibility, human LG is a suitable object for potential application in regenerative medicine. Development of a universal protocol for obtaining functional differentiated secretory epithelium of LG capable of expressing tissue-specific markers is an urgent task. Determining the nature and origin of stem cells and progenitor cells will allow the isolation and multiplication of these cells in culture. After obtaining a functionally active culture of LG cells, it is possible to create a model of autoimmune diseases.

Role of stem cells in regenerative treatment of dry eye disease caused by lacrimal gland dysfunction

An ageing population and increased screen use in younger people have contributed to a rise in incidence of dry eye disease (DED). Quality of life can be significantly affected by DED, with patients experiencing eye dryness, burning, pain and sensitivity to light. If left untreated, DED may progress to cause lasting damage to the delicate cell layers of the ocular surface. The aqueous-deficient form of DED is characterized by decreased tear volume. This can occur through underlying disease or damage to the lacrimal gland (LG), which results in increased inflammation at the ocular surface and decreased tear secretion. Regenerative therapy for treatment of aqueous-deficient DED would ideally restore LG function without causing adverse side effects and be feasible in terms of cost, production and practical application in the clinic. In this review, we evaluate research directed at the development of clinical procedures for regeneration of the LG using various stem cell types and their products. We also discuss work identifying potential therapeutic targets that may alter pathways to effect healing and ameliorate development of DED. Finally, we discuss shortcomings and recommend future avenues for research. These include determination of the best tissue of origin for mesenchymal cells and transference of knowledge gleaned from animal studies to clinical investigations.

Lacrimal gland regeneration: The unmet challenges and promise for dry eye therapy

DED (Dry eye disease) is a common multifactorial disease of the ocular surface and the tear film. DED has gained attention globally, with millions of people affected.. Although treatment strategies for DED have shifted towards Tear Film Oriented Therapy (TFOT), all the existing strategies fall under standard palliative care when addressed as a long-term goal. Therefore, different approaches have been explored by various groups to uncover alternative treatment strategies that can contribute to a full regeneration of the damaged lacrimal gland. For this, multiple groups have investigated the role of lacrimal gland (LG) cells in DED based on their regenerating, homing, and differentiating capabilities. In this review, we discuss in detail therapeutic mechanisms and regenerative strategies that can potentially be applied for lacrimal gland regeneration as well as their therapeutic applications. This review mainly focuses on Aqueous Deficiency Dry Eye Disease (ADDE) caused by lacrimal gland dysfunction and possible future treatment strategies. The current key findings from cell and tissue-based regenerative therapy modalities that could be utilised to achieve lacrimal gland tissue regeneration are summarized. In addition, this review summarises the available literature from in vitro to in vivo animal studies, their limitations in relation to lacrimal gland regeneration and the possible clinical applications. Finally, current issues and unmet needs of cell-based therapies in providing complete lacrimal gland tissue regeneration are discussed.

Kartogenin mediates cartilage regeneration by stimulating the IL-6/Stat3-dependent proliferation of cartilage stem/progenitor cells

A decrease in the number of endogenous stem cells in cartilage is regarded as the cause of cartilage degeneration. Kartogenin (KGN) is known to induce chondrogenesis of cartilage stem/progenitor cells (CSPCs). Using CSPCs isolated from rat cartilage, we analysed changes in the transcriptome after treatment with KGN in vitro. An animal model of destabilization of the medial meniscus (DMM) was then used to identify the effect of intra-articular (IA) KGN injection on CSPC proliferation in vivo. Here, we demonstrated that KGN promoted the proliferation of CSPCs isolated from cartilage. The percentage of G2-M phase cells in the KGN-treated group reached over 10%, nearly twice that in the control group. Transcriptomic profiling of rat CSPCs revealed significant changes in KGN-treated samples compared to control samples. The gene expression levels of IL-6 and its coreceptor Gp130 were much higher in the KGN-treated group than in the control group. Phosphorylation of the IL-6 downstream molecule Stat3 was enhanced via KGN stimulation. The DMM animal model showed increased articular cartilage thickness after IA KGN injection. IHC staining also demonstrated upregulation of Stat3 phosphorylation and enhanced distribution of CD44⁺/CD105⁺ cells in cartilage following IA KGN injection. Thus, our data suggested that KGN promoted cartilage regeneration at least partially by stimulating IL-6/Stat3-dependent proliferation.

Comparative Analysis of Age-Related Changes in Lacrimal Glands and Meibomian Glands of a C57BL/6 Male Mouse Model

It is not known how biological changes in the lacrimal (LGs) and meibomian (MGs) glands contribute to dry eye disease (DED) in a time-dependent manner. In this study, we investigated time-sequenced changes in the inflammation, oxidative stress, and senescence of stem cells in both glands of an aging-related DED mouse model. Eight-week (8W)-, one-year (1Y)-, and two-year (2Y)-old C57BL/6 male mice were used. MG areas of the upper and lower eyelids were analyzed by transillumination meibography imaging. The number of CD45⁺, 8-OHdG⁺, Ki-67⁺, and BrdU⁺ cells was compared in both glands. Increased corneal staining and decreased tear secretion were observed in aged mice. The MG dropout area increased with aging, and the age-adjusted MG area in lower lids was negatively correlated with the National Eye Institute (NEI) score. Increased CD4⁺ interferon (IFN)-γ⁺ cells in LGs were found in both aged mice. An increase in 8-OHdG⁺ cells in both glands was evident in 2Y-old mice. Reduced Ki-67⁺ cells, but no change in CD45⁺ cells, was observed in the MGs of 1Y-old mice. Increased BrdU⁺ cells were observed in the LGs of aged mice. This suggests that age-dependent DED in C57BL/6 mice is related to inflammation of the LGs, the development of MG atrophy, and oxidative stress in both glands.

Origin and Lineage Plasticity of Endogenous Lacrimal Gland Epithelial Stem/Progenitor Cells

The lacrimal gland (LG) is an exocrine organ responsible for the secretion of aqueous tear film. Regenerative and stem cell therapies that target LG repair are coming to the fore, although our understanding of LG cell lineage hierarchy is still incomplete. We utilize the analysis of label-retaining cells (LRCs) and genetic lineage tracing to define LG cell lineage hierarchy. Our study suggests that embryonic LG contains unique long-lived multipotent stem cells that give rise to all postnatal epithelial cell types. Following birth, lineages become established and the fate of progenitor cell descendants becomes restricted. However, some cell lineages retain plasticity after maturation and can trans-differentiate into other cell types upon injury. The demonstration that the LG contains progenitor cells with different levels of plasticity has profound implications for our understanding of LG gland function in homeostasis and disease and will be helpful for developing stem cell-based therapies in the future.

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Multipotent stem cells differentiate into distal Sox10+ and proximal Sox10− lineages

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Lineage-restricted progenitor cells sustain the long-term lacrimal gland maintenance

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Label-retaining cells are localized in the intercalated ducts and excretory ducts

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Some cell lineages in the adult lacrimal gland retain plasticity

Three Dimensional Culture of Potential Epithelial Progenitor Cells in Human Lacrimal Gland

Purpose

We investigate human lacrimal gland tissue to determine the presence of progenitor cells in this adult human tissue.

Methods

Six human lacrimal gland tissues from donors were collected and stored immediately in the culture medium at 4°C until the next procedure. One part of the lacrimal gland tissue was prepared for immunofluorescence staining and the other part was prepared for primary cell culture. Immunofluorescence analysis was conducted to evaluate cultured lacrimal epithelial phenotype and progenitor cell markers for five passages. Real-time polymerase chain reaction (PCR) was performed to assess proliferation markers in the different passages. Three-dimensional culture and PCR were conducted to determine the differentiation potential of cultured human lacrimal gland cells.

Results

Human lacrimal gland tissue expressed a number of epithelial progenitor cell markers. Precursor cell markers C-Kit, K15, Nestin, and P63 were observed in lacrimal gland tissues. Lacrimal gland epithelial cells were cultured successfully and passaged to P5. The cultured lacrimal gland epithelial cells were positive for pan-cytokeratin (PCK), AQP5, Rab3D, ABCB5, C-kit, K15, Ki67, and P63. Human lacrimal gland cells could form spheroids in vitro and then grow into mini-gland-like structures. PCR results showed proliferation and differentiation capability of those cultured cells.

Conclusions

Human lacrimal gland tissues contain precursor marker-positive cells and marker expression also was detected in ex vivo cultured cells, which showed differentiation capability.

Translational Relevance

Future studies of differentiation in human lacrimal gland tissue may aid in developing stem cell-based therapies for dry eye disease.

Hyaluronan Regulates Eyelid and Meibomian Gland Morphogenesis

Purpose

The Meibomian gland (MG) produces the lipid layer of the tear film, and changes to the MG that lead to a decrease or alteration in lipid quality/content may lead to MG dysfunction, a major cause of evaporative dry eye disease with prevalence ranging from 39% to 50%. Little is known about the developmental cues that regulate MG morphogenesis and homeostasis. Our study investigates the role of hyaluronan (HA), a major extracellular matrix component, in eyelid formation and MG development and function.

Methods

Hyaluronan synthase (Has) knockout mice were used to determine the role of HA in the eyelid and MG. Eyelids were obtained during different developmental stages and MG morphology was analyzed. Tet-off H2B-GFP/K5tTA mice and 5-ethynyl-2'-deoxyurdine (EdU) incorporation were used to determine the role of HA in maintaining slow-cycling and proliferating cells within the MG, respectively. Data were confirmed using an in vitro proliferation assay, differentiation assay and spheroid cultures.

Results

Has knockout mice present precocious MG development, and adult mice present MG hyperplasia and dysmorphic MGs and eyelids, with hyperplastic growths arising from the palpebral conjunctiva. Our data show that a highly organized HA network encompasses the MG, and basal cells are embedded within this HA matrix, which supports the proliferating cells. Spheroid cultures showed that HA promotes acini formation.

Conclusions

HA plays an important role in MG and eyelid development. Our findings suggest that Has knockout mice have abnormal HA synthesis, which in turn leads to precocious and exacerbated MG morphogenesis culminating in dysmorphic eyelids and MGs.

Lacrimal Gland Repair after Short-term Obstruction of Excretory Duct in Rabbits

Aqueous tear-deficient dry eye is a multifactorial chronic disorder in which the lacrimal glands fail to produce enough tears to maintain a healthy ocular surface. The existence of lacrimal gland stem/progenitor cells was proposed in several species, yet their origin and characteristics are not very clear. Here, we investigated the presence of resident progenitor cells and their regenerative potential in a rabbit model with lacrimal gland main excretory duct ligation-induced injury. The ligation-injured lacrimal glands temporarily decreased in weight and had impaired tear secretion. Protein expression profiles and transcriptional profiles were obtained from injured tissue. Isolated lacrimal gland progenitor cells were tested and characterized by stem cell-related marker evaluation, single cell clonal assay and three-dimensional (3-D) culture. The results of our study indicate that lacrimal glands are capable of tissue repair after duct ligation-induced injury, likely involving resident stem/progenitor cells and epithelial-mesenchymal transitions. Lacrimal gland progenitor cells isolated from ligated tissue can differentiate in 3-D culture. The results provide further insights into lacrimal gland stem/progenitor cell physiology and their potential for treating severe cases of tear deficiency.

RNA-Seq and CyTOF immuno-profiling of regenerating lacrimal glands identifies a novel subset of cells expressing muscle-related proteins

The purpose of the present studies was to use CyTOF and RNA-Seq technologies to identify cells and genes involved in lacrimal gland repair that could be targeted to treat diseases of lacrimal gland dysfunction. Lacrimal glands of female BALB/c mice were experimentally injured by intra-glandular injection of interleukin 1 alpha (IL-1α). The lacrimal glands were harvested at various time points following injury (1 to 14 days) and used to either prepare single cell suspensions for CyTOF immuno-phenotyping analyses or to extract RNA for gene expression studies using RNA-Seq. CyTOF immuno-phenotyping identified monocytes and neutrophils as the major infiltrating populations 1 and 2 days post injury. Clustering of significantly differentially expressed genes identified 13 distinct molecular signatures: 3 associated with immune/inflammatory processes included genes up-regulated at days 1–2 and 3 associated with reparative processes with genes up-regulated primarily between days 4 and 5. Finally, clustering identified 65 genes which were specifically up-regulated 2 days post injury which was enriched for muscle specific genes. The expression of select muscle-related proteins was confirmed by immunohistochemistry which identified a subset of cells expressing these proteins. Double staining experiments showed that these cells are distinct from the myoepithelial cells. We conclude that experimentally induced injury to the lacrimal gland leads to massive infiltration by neutrophils and monocytes which resolved after 3 days. RNAseq and immunohistochemistry identified a group of cells, other than myoepithelial cells, that express muscle-related proteins that could play an important role in lacrimal gland repair.

Defining epithelial cell dynamics and lineage relationships in the developing lacrimal gland

The tear-producing lacrimal gland is a tubular organ that protects and lubricates the ocular surface. The lacrimal gland possesses many features that make it an excellent model in which to investigate tubulogenesis, but the cell types and lineage relationships that drive lacrimal gland formation are unclear. Using single-cell sequencing and other molecular tools, we reveal novel cell identities and epithelial lineage dynamics that underlie lacrimal gland development. We show that the lacrimal gland from its earliest developmental stages is composed of multiple subpopulations of immune, epithelial and mesenchymal cell lineages. The epithelial lineage exhibits the most substantial cellular changes, transitioning through a series of unique transcriptional states to become terminally differentiated acinar, ductal and myoepithelial cells. Furthermore, lineage tracing in postnatal and adult glands provides the first direct evidence of unipotent KRT5⁺ epithelial cells in the lacrimal gland. Finally, we show conservation of developmental markers between the developing mouse and human lacrimal gland, supporting the use of mice to understand human development. Together, our data reveal crucial features of lacrimal gland development that have broad implications for understanding epithelial organogenesis.

Lacrimal Gland Repair Using Progenitor Cells

In humans, the lacrimal gland (LG) is the primary contributor to the aqueous layer of the tear film. Production of tears in insufficient quantity or of inadequate quality may lead to aqueous‐deficiency dry eye (ADDE). Currently there is no cure for ADDE. The development of strategies to reliably isolate LG stem/progenitor cells from the LG tissue brings great promise for the design of cell replacement therapies for patients with ADDE. We analyzed the therapeutic potential of epithelial progenitor cells (EPCPs) isolated from adult wild‐type mouse LGs by transplanting them into the LGs of TSP‐1^−/− mice, which represent a novel mouse model for ADDE. TSP‐1^−/− mice are normal at birth but progressively develop a chronic form of ocular surface disease, characterized by deterioration, inflammation, and secretory dysfunction of the lacrimal gland. Our study shows that, among c‐kit‐positive epithelial cell adhesion molecule (EpCAM⁺) populations sorted from mouse LGs, the c‐kit⁺dim/EpCAM⁺/Sca1⁻/CD34⁻/CD45⁻ cells have the hallmarks of an epithelial cell progenitor population. Isolated EPCPs express pluripotency factors and markers of the epithelial cell lineage Runx1 and EpCAM, and they form acini and ducts when grown in reaggregated three‐dimensional cultures. Moreover, when transplanted into injured or “diseased” LGs, they engraft into acinar and ductal compartments. EPCP‐injected TSP‐1^−/− LGs showed reduction of cell infiltration, differentiation of the donor EPCPs within secretory acini, and substantial improvement in LG structural integrity and function. This study provides the first evidence for the effective use of adult EPCP cell transplantation to rescue LG dysfunction in a model system. Stem Cells Translational Medicine2017;6:88–98

Renewal of the Holocrine Meibomian Glands by Label-Retaining, Unipotent Epithelial Progenitors

The meibomian and sebaceous glands secrete lipids to prevent desiccation of the ocular surface and skin, respectively. Precisely how these holocrine tissues regenerate is not well understood. To address this, we characterized keratin 5(+) (K5) label-retaining cells (LRCs) and the lineage tracing of keratin 14 (K14) progenitors in mouse meibomian glands. Using the tet-off H2B-GFP/K5tTA mouse, H2B-GFP fluorescence dilutes 2-fold with every division in K5(+) cell nuclei after doxycycline administration. In 3D reconstructions generated over a >28-day doxycycline chase, we observed LRCs at the acinus entrance where K6(+) ductal epithelium terminates. For lineage tracing, K14CreER(T2)-Confetti mice were injected intraperitoneally with tamoxifen and euthanized at 23 and 59 weeks later. Meibomian gland acini in these mice were either monochromatic or dual-colored, whereas the duct exhibited multiple colors. In conclusion, LRCs are likely to direct meibomian gland turnover and may exist as two distinct unipotent progenitors that renew ductal and acinar tissue separately.

Challenges and Strategies for Regenerating the Lacrimal Gland

The lacrimal gland produces the aqueous component of tears, including electrolytes, peptides, and glycoproteins necessary to maintain homeostasis and optical properties of the ocular surface. Stem cells that contribute to the homeostasis of the lacrimal gland are under extensive study. It is still unclear whether such stem cells are of mesenchymal or epithelial origin. It is also possible that a unique epithelial stem cell undergoes epithelial-mesenchymal transition and contributes to the mesenchyme. Developmental studies in mice have shown that a network of growth factors contributes to epithelial-mesenchymal interaction during morphogenesis of the lacrimal gland. Recently, the developmental process was successfully recapitulated in vitro, providing a valuable tool for study of lacrimal gland development and possibly opening doors to regenerative therapy. While further studies are required to identify and appreciate the potential of lacrimal gland stem cells, advances in stem cell biology in general should become a catalyst towards developing regenerative therapy of the lacrimal gland.

Role of Matrix Metalloproteinases 2 and 9 in Lacrimal Gland Disease in Animal Models of Sjögren's Syndrome

PURPOSE

Chronic inflammation of the lacrimal gland results in changes in the composition of the extracellular matrix (ECM), which is believed to compromise tissue repair. We hypothesized that increased production/activity of matrix metalloproteinases (MMPs), especially MMP-2 and -9, in inflamed lacrimal glands modifies the ECM environment, therefore disrupting tissue repair.

METHODS

The lacrimal glands from female MRL/lpr and male NOD mice along with their respective control strains were harvested and divided into three pieces and processed for histology, immunohistochemistry, zymography, Western blotting, and RNA analyses. In another study, MRL/lpr mice were treated for 5 weeks with a selective MMP2/9 inhibitor peptide or a control peptide. At the end of treatment, the lacrimal glands were excised and the tissue was processed as described above.

RESULTS

There was a 2.5- and 2.7-fold increase in MMP2 gene expression levels in MRL/lpr and NOD mice, respectively. Matrix metalloproteinase 2 and 9 enzymatic activities and protein expression levels were significantly upregulated in the lacrimal glands of MRL/lpr and NOD mice compared to controls. Treatment with the MMP2/9 inhibitor resulted in decreased activity of MMP-2 and -9 both in vitro and in vivo. Importantly, MMP2/9 inhibitor treatment of MRL/lpr mice improved aqueous tear production and resulted in reduced number and size of lymphocytic foci in diseased lacrimal glands.

CONCLUSIONS

We conclude that MMP2/9 expression and activity are elevated in lacrimal glands of two murine models of Sjögren's syndrome, suggesting that manipulation of MMP2/9 activity might be a potential therapeutic target in chronically inflamed lacrimal glands.

Myoepithelial Cells: Their Origin and Function in Lacrimal Gland Morphogenesis, Homeostasis, and Repair

Lacrimal gland (LG) is an exocrine tubuloacinar gland that secretes the aqueous layer of the tear film. LG epithelium is composed of ductal, acinar, and myoepithelial cells (MECs) bordering the basal lamina and separating the epithelial layer from the extracellular matrix. Mature MECs have contractile ability and morphologically resemble smooth muscle cells; however, they exhibit features typical for epithelial cells, such as the presence of specific cytokeratin filaments. Increasing evidence supports the assertion that myoepithelial cells (MECs) play key roles in the lacrimal gland development, homeostasis, and stabilizing the normal structure and polarity of LG secretory acini. MECs take part in the formation of extracellular matrix gland and participate in signal exchange between epithelium and stroma. MECs have a high level of plasticity and are able to differentiate into several cell lineages. Here, we provide a review on some of the MEC characteristics and their role in LG morphogenesis, maintenance, and repair.

The safety of photochemical tissue bonding for treating damaged corneal epithelium using limbal stem cells pre-cultured on human amniotic membrane

We previously demonstrated the feasibility of treating limbal stem cell deficiency (LSCD) with limbal stem cells (LSCs) pre-cultured on human amniotic membrane (HAM), using a suture-free technique called photochemical tissue bonding (PTB). However, important issues regarding the safety and the influence of PTB on LSCs have not been elucidated. In this study, LSCs, isolated from rabbit eyes and identified by cell markers, were labeled with BrdU prior to cultivation on de-epithelialized HAM to fabricate grafts. Rabbit LSCD models were created and randomly divided into groups for transplantation of fabricated grafts using sutures or PTB (n=10). Possible phototoxicity of PTB to LSCs was analyzed in vitro and in vivo. Restoration of corneal epithelium was evaluated at 28 days after grafting. Our results showed that phototoxicity did not occur in the LSCs cultured on HAM after PTB in vitro. Transplantation of grafts with PTB restored the damaged cornea epithelium effectively and no significant influences on LSC characteristics were found in both sutured and PTB groups. BrdU positive cells were tracked at 28 days post grafting suggesting that the restored epithelium was derived from the in vitro fabricated HAM/LSC graft. These data suggest that PTB is a safe and potential strategy for securing LSC/HAM grafts that produces with better outcomes than sutured attachment.

Label-retaining cells in the adult murine salivary glands possess characteristics of adult progenitor cells

Radiotherapy is the primary treatment for patients with head and neck cancer, which account for roughly 500,000 annual cases worldwide. Dysfunction of the salivary glands and associated conditions like xerostomia and dysphagia are often developed by these patients, greatly diminishing their life quality. Current preventative and palliative care fail to deliver an improvement in the quality of life, thus accentuating the need for regenerative therapies. In this study, a model of label retaining cells (LRCs) in murine salivary glands was developed, in which LRCs demonstrated proliferative potential and possessed markers of putative salivary progenitors. Mice were labeled with 5-Ethynyl-2′-deoxyuridine (EdU) at postnatal day 10 and chased for 8 weeks. Tissue sections from salivary glands obtained at the end of chase demonstrated co-localization between LRCs and the salivary progenitor markers keratin 5 and keratin 14, as well as kit mRNA, indicating that LRCs encompass a heterogeneous population of salivary progenitors. Proliferative potential of LRCs was demonstrated by a sphere assay, in which LRCs were found in primary and secondary spheres and they co-localized with the proliferation marker Ki67 throughout sphere formation. Surprisingly, LRCs were shown to be radio-resistant and evade apoptosis following radiation treatment. The clinical significance of these findings lie in the potential of this model to study the mechanisms that prevent salivary progenitors from maintaining homeostasis upon exposure to radiation, which will in turn facilitate the development of regenerative therapies for salivary gland dysfunction.

Distribution of BrdU label-retaining cells in eccrine sweat glands and comparison of the percentage of BrdU-positive cells in eccrine sweat glands and in epidermis in rats

Bromodeoxyuridine (BrdU) has commonly been used for detecting of label-retaining cells (LRCs). To determine if there are LRCs and the distributions of LRCs in eccrine sweat glands, 20 newborn SD rats within 24 h after birth were injected intraperitoneally with 50 mg/kg/time BrdU four consecutive times at 2-h intervals, or twice daily at 2-h intervals for four consecutive days. Six weeks after the last BrdU injection, rats were sacrificed, and the hind footpads were harvested, fixed and embedded in paraffin. Five-micrometer thickness tissue sections were cut and the expression of BrdU was detected immunohistochemically. The results showed that BrdU(+) cells were scatteredly distributed in coiled secretory part and coiled duct, as well as the straight duct, but not the intraepidermal duct of eccrine sweat glands. In secretory part, besides secretory cells, myoepithelial cells showed label retaining. The percentage of BrdU(+) cells in eccrine sweat gland of rat footpads had no significant difference between the two injection methods of BrdU (50 mg/kg/time BrdU four consecutive times at 2-h intervals vs. 50 mg/kg/time BrdU twice daily at 2-h intervals for four consecutive days) (P > 0.05). The percentage of BrdU(+) cells in eccrine sweat glands (4.2 ± 1.3 %) was significantly higher than that in stratum basale of epidermis (0.5 ± 0.1 ‰) (P < 0.05). In conclusion, there were LRCs in eccrine sweat glands of rat footpads, and these LRCs might play important roles in the homeostasis of skin and its appendages.