Plasticity of epidermal stem cells: survival in various environments

Progress in studies of epidermal stem cells and their application in skin tissue engineering

The epidermis, which is the outermost layer of mammalian skin, provides an essential barrier that is essential for maintenance of life. The epidermis is a stratified epithelium, which is maintained by the proliferation of epidermal stem cells (EPSCs) at the basal layer of the epidermis. As a unique cell population characterized by self-renewal and differentiation capabilities, EPSCs ensure the maintenance of adult skin homeostasis and participate in repair of the epidermis after injury. Recently, the utilization of EPSCs for wound healing and tissue regeneration has been attracting increased attention from researchers. In addition, the advances in tissue engineering have increased the interest in applying EPSCs in tissue-engineered scaffolds to further reconstitute injured tissues. In this review, we introduce research developments related to EPSCs, including methods recently used in the culture and enrichment of EPSCs, as well as advanced tools to study EPSCs. The function and mechanism of the EPSC-dermal units in the development and homeostasis of the skin are also summarized. Finally, the potential applications of EPSCs in skin tissue engineering are discussed.

Biochemistry of epidermal stem cells

BACKGROUND

The epidermis is an important protective barrier that is essential for maintenance of life. Maintaining this barrier requires continuous cell proliferation and differentiation. Moreover, these processes must be balanced to produce a normal epidermis. The stem cells of the epidermis reside in specific locations in the basal epidermis, hair follicle and sebaceous glands and these cells are responsible for replenishment of this tissue.

SCOPE OF REVIEW

A great deal of effort has gone into identifying protein epitopes that mark stem cells, in identifying stem cell niche locations, and in understanding how stem cell populations are related. We discuss these studies as they apply to understanding normal epidermal homeostasis and skin cancer.

MAJOR CONCLUSIONS

An assortment of stem cell markers have been identified that permit assignment of stem cells to specific regions of the epidermis, and progress has been made in understanding the role of these cells in normal epidermal homeostasis and in conditions of tissue stress. A key finding is the multiple stem cell populations exist in epidermis that give rise to different structures, and that multiple stem cell types may contribute to repair in damaged epidermis.

GENERAL SIGNIFICANCE

Understanding epidermal stem cell biology is likely to lead to important therapies for treating skin diseases and cancer, and will also contribute to our understanding of stem cells in other systems. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Isolation of hematopoietic stem cells and the effect of CD38 expression during the early erythroid progenitor cell development process

The aim of this study was to investigate changes in primitive hematopoietic cells through CD38 expression, identify the stage at which erythrocyte differentiation CD38 gains activity and the effects of serum factors on this expression by establishing a hematopoietic stem cell system in the erythroid development process. Using an immunomagnetic labeling and separation technique, CD34(+) cells were selected from cord blood. The CD34(+) cells were cultured in a 2 mM L-glutamine-enriched medium containing erythropoietin (Epo), penicillin-streptomycin and stem cell factor (SCF), and were incubated in 5% CO(2) at 37°C. In erythroid development pathways following CD38 expression, primitive/progenitor human hematopoietic cells obtained from cord blood were assessed through the erythroid development process in a serum-free medium in the presence of proper SCF and Epo. At the end of the 26-day process, using staining with a Megacult-c staining kit, it was determined that progenitor cells nucleate and differentiate into erythroid cell lines of 8-10 μm. During the course of this process, we analyzed increases over time in NAD glycohydrolase activity rates using the supernatant liquid samples. Results of co-culture experiments in cell culture studies showed that the stimulating effects of CD38 expression originate from specific serum factors. CD38 expression has been shown to occur at hematopoietic cell sources as well as at a number of differentiation levels. In the proliferation process the possible induction of CD38 through specific serum factors leads us to conclude that it may be involved in proliferation with a physiological task or that it may be involved in an event, such as an apoptotic process.

Cryopreserved lip mucosa tissue derived keratinocytes can fabricate tissue engineered palatal mucosa equivalent

Clinical application of tissue engineered palatal mucosa is hampered by unavailability of suitable oral keratinocytes as seeding cells. The aim of this study is to fabricate a tissue engineered palatal mucosa equivalent from the oral keratinocytes which cultured from cryopreserved lip mucosa tissues. Abundant lip mucosa tissues during cheilorrhaphy were firstly cryopreserved in liquid nitrogen for four to six months, and then recovered to culture oral keratinocytes for the fabrication of oral mucosa equivalent. In the control groups, oral keratinocytes cultured from fresh lip mucosa, fresh palate mucosa, and cryopreserved palate mucosa were used to fabricate oral mucosa equivalents. Attachment rate of the oral keratinocytes derived from cryopreserved lip mucosa was lower than that of the keratinocytes from fresh lip mucosa samples, however, the cell cycle distribution of oral keratinocytes cultured from all four groups of mucosa samples were similar. Histologically, the fabricated mucosa equivalents from these four groups had four- to six epithelial layers, the basal cells were cubic and the outmost cells were flatten with narrow nuclei which paralleled to the surface of the dermal matrix. Additionally, Ki-67 positive stained cells were mainly located in the basal layer of the epithelium of these equivalents. These characteristics disclosed that the oral mucosa equivalent cultured from the cryopreserved lip mucosa tissue was not different with the equivalents from other groups and similar to the native palate mucosa tissue. It suggested that the cryopreserved lip mucosa tissues could be used for the construction of palatal mucosal equivalent for clinical application.

Limiting dilution analysis of murine epidermal stem cells using an in vivo regeneration assay

Epidermal stem cells are of major importance for tissue homeostasis, wound repair, tumor initiation, and gene therapy. Here we describe an in vivo regeneration assay to test for the ability of keratinocyte progenitors to maintain an epidermis over the long-term in vivo. Limiting dilution analysis of epidermal repopulating units in this in vivo regeneration assay at sequential time points allows the frequency of short-term (transit amplifying cell) and long-term (stem cell) repopulating cells to be quantified.

Transit-amplifying cell frequency and cell cycle kinetics are altered in aged epidermis

Aged epidermis is less proliferative than young, as exemplified by slower wound healing. However, it is not known whether quantitative and/or qualitative alterations in the stem and/or transit-amplifying (TA) compartments are responsible for the decreased proliferation. Earlier studies found a normal or decreased frequency of putative epidermal stem cells (EpiSCs) with aging. We show, using long-term repopulation in vivo and colony formation in vitro, that, although no significant difference was detected in EpiSC frequency with aging, TA cell frequency is increased. Moreover, aged TA cells persist longer, whereas their younger counterparts have already differentiated. Underlying the alteration in TA cell kinetics in the aged is an increase in the proportion of cycling keratinocytes, as well as an increase in cell cycle duration. In summary, although no significant difference in EpiSC frequency was found, TA cell frequency was increased (as measured by in vivo repopulation, growth fraction, and colony formation). Furthermore, the proliferative capacity (cellular output) of individual aged EpiSCs and TA cells was decreased compared to that of young cells. Although longer cell cycle duration contributes to the decreased proliferative output from aged progenitors, the greater number of TA cells may be a compensatory mechanism tending to offset this deficit.

Epidermal stem cells have the potential to assist in healing damaged tissues

Homeostasis of continuously renewing tissues, such as the epidermis, is maintained by somatic undifferentiated, self-renewing stem cells, which are thought to persist throughout life. Through a series of labeling experiments, we previously showed that stem cells from mouse skin did not divide often, but they did divide at a steady rate in vivo. Using our recently redefined sorting method, we isolated epidermal stem and transit amplifying (TA) cells from mouse skin. When injected into a developing blastocyst or into damaged tissues, the stem cells, but not the TA cells, could participate in the formation of new tissues. We hypothesize that all tissues contain reserved undifferentiated stem cells that are primed to react if needed. These reserve stem cells could restore the tissue in which they reside or they could be called upon to help restore another tissue that was severely damage.