Cultivation of murine hair follicles as organoids in a collagen matrix

Hair-Shaft Growth in Gelfoam® Histoculture of Skin and Isolated Hair Follicles

Human scalp skin with abundant hair follicles in various stages of the hair growth cycle was histocultured for up to 40 days on Gelfoam^® at the air/liquid interface. The anagen hair follicles within the histoculture scalp skin produced growing hair shafts. Hair follicles could continue their cycle in histoculture; for example, apparent spontaneous catagen induction was observed both histologically and by the actual regression of the hair follicle. In addition, vellus follicles were shown to be viable at day 40 after initiation of culture. Follicle keratinocytes continued to incorporate [³H]thymidine for up to several weeks after shaft elongation had ceased. Intensive hair growth was observed in the pieces of shaved mouse skin histocultured on Gelfoam^®. Isolated human and mouse hair follicles also produced growing hair shafts. By day 63 in histoculture of mouse hair follicles, the number of hair follicle-associated pluripotent (HAP) stem cells increased significantly and the follicles were intact. Gelfoam^® histoculture of skin demonstrated that the hair follicle cells are the most sensitive to doxorubicin which prevented hair growth, thereby mimicking chemotherapy-induced alopecia in Gelfoam^® histoculture.

Isolation and Fluorescence-Activated Cell Sorting of Mouse Keratinocytes Expressing β-Galactosidase

During the past decade, the rapid development of new transgenic and knock-in mouse models has propelled epidermal stem-cell research into "fast-forward mode". It has become possible to identify and visualize defined cell populations during normal tissue maintenance, and to follow their progeny during the processes of homeostasis, wound repair, and tumorigenesis. Moreover, these cells can be isolated using specific labels, and characterized in detail using an array of molecular and cell biology approaches. The bacterial enzyme, β-galactosidase (β-gal), the product of the LacZ gene, is one of the most commonly used in vivo cell labels in genetically-engineered mice. The protocol described in this chapter provides a guideline for the isolation of viable murine epidermal cells expressing β-gal, which can then be subjected to further characterization in vivo or in vitro.

Bioengineering the hair follicle

The hair follicle develops from the primitive embryonic epidermis as a result of complex epithelial-mesenchymal interactions. The full follicle, consisting of epithelial cylinders under control of a proximal lying mesenchymal papilla, grows in cycles giving rise to a new hair shaft during each cycle. The ability to cycle endows the follicle with regenerative properties. The evolution of hair follicle engineering began with the recognition in the early 1960's that hair follicles could be transplanted clinically into a foreign site and still grow a shaft typical of the donor site. Since that time, it has been found that the follicular papilla has hair follicle inducing properties and that the hair follicle houses within it epithelial stem cells that can respond to hair inductive signals. These findings have laid the foundation for isolating hair-forming cells, for expanding the cells in culture, and for forming new follicles in vivo.

Skin tissue engineering--in vivo and in vitro applications

Significant progress has been made over the years in the development of in vitro-engineered substitutes that mimic human skin, either to be used as grafts for the replacement of lost skin or for the establishment of human-based in vitro skin models. This review summarizes these advances in in vivo and in vitro applications of tissue-engineered skin. We further highlight novel efforts in the design of complex disease-in-a-dish models for studies ranging from disease etiology to drug development and screening.

Skin tissue engineering--in vivo and in vitro applications

Significant progress has been made over the years in the development of in vitro-engineered substitutes that mimic human skin, either to be used as grafts for the replacement of lost skin or for the establishment of human-based in vitro skin models. This review summarizes these advances in in vivo and in vitro applications of tissue-engineered skin. We further highlight novel efforts in the design of complex disease-in-a-dish models for studies ranging from disease etiology to drug development and screening.

Tissue-engineered skin preserving the potential of epithelial cells to differentiate into hair after grafting

The aim of this study was to evaluate whether tissue-engineered skin produced in vitro was able to sustain growth of hair follicles in vitro and after grafting. Different tissues were designed. Dissociated newborn mouse keratinocytes or newborn mouse hair buds (HBs) were added onto dermal constructs consisting of a tissue-engineered cell-derived matrix elaborated from either newborn mouse or adult human fibroblasts cultured with ascorbic acid. After 7-21 days of maturation at the air-liquid interface, no hair was noticed in vitro. Epidermal differentiation was observed in all tissue-engineered skin. However, human fibroblast-derived tissue-engineered dermis (hD) promoted a thicker epidermis than mouse fibroblast-derived tissue-engineered dermis (mD). In association with mD, HBs developed epithelial cyst-like inclusions presenting outer root sheath-like attributes. In contrast, epidermoid cyst-like inclusions lined by a stratified squamous epithelium were present in tissues composed of HBs and hD. After grafting, pilo-sebaceous units formed and hair grew in skin elaborated from HBs cultured 10-26 days submerged in culture medium in association with mD. However, the number of normal hair follicles decreased with longer culture time. This hair-forming capacity after grafting was not observed in tissues composed of hD overlaid with HBs. These results demonstrate that epithelial stem cells can be kept in vitro in a permissive tissue-engineered dermal environment without losing their potential to induce hair growth after grafting.

The mesenchymal component of hair follicle neogenesis: background, methods and molecular characterization

Hair follicle morphogenesis and regeneration occur by an extensive and collaborative crosstalk between epithelial and mesenchymal skin components. A series of pioneering studies, which revealed an indispensable role of follicular dermal papilla and dermal sheath cells in this crosstalk, has led workers in the field to study in detail the anatomical distribution, functional properties, and molecular signature of the trichogenic dermal cells. The purpose of this paper was to provide a practical summary of the development and recent advances in the study of trichogenic dermal cells. Following a short review of the relevant literature, the methods for isolating and culturing these cells are summarized. Next, the bioassays, both in vivo and in vitro, that enable the evaluation of trichogenic properties of tested dermal cells are described in detail. A list of trichogenic molecular markers identified by those assays is also provided. Finally, this methods review is completed by defining some of the major questions needing resolution.

Genetic interplays between Msx2 and Foxn1 are required for Notch1 expression and hair shaft differentiation

Hair shafts are produced from stem cells located in the bulge. Our knowledge of the genetic pathways regulating cell fate acquisition in the immediate descendents of these stem cells, and fate maintenance in their committed progeny, is still incomplete. One pathway involved in fate maintenance within the hair matrix is the Notch pathway. Here we use compound genetic mutants to demonstrate that two transcription factors, Msx2 and Foxn1, are both required to maintain Notch1 expression in the hair follicle matrix. In their absence, Notch1 is markedly reduced in hair matrix; as a consequence, medulla and inner root sheath (IRS) differentiation is impaired. Our studies also suggest that Foxn1 is a direct activator of the Notch1 promoter activity through one or more putative Foxn1 consensus binding sites located within the 4.7 kb of mouse Notch1 promoter. Since recombinant human BMP4 can induce Foxn1 expression in Msx2-deficient hair follicles, and that their effect on cortical keratin expression appears synergistic, we suggest that these two genes function in parallel pathways downstream of BMP signaling and upstream of Notch1. Independent from their role in Notch activation, Msx2 and Foxn1 also contribute to the expression of several cortical and cuticle keratins. The impact of these additional defects is the complete loss of all visible external hairs, not seen in Notch1 mutants. Our results position Msx2 and Foxn1 upstream of Notch1 within the hair matrix and demonstrate that together these factors play a pivotal role in IRS, cortex and medulla differentiation.

In vitro expansion of immature melanoblasts and their ability to repopulate melanocyte stem cells in the hair follicle

Elucidation of the molecular mechanisms underlying stem cell regulation is of great importance both for basic biology and for clinical applications. Melanocyte stem cells (MSCs) are an excellent model in which to study the molecular basis of stem cell regulation, as the genetic alterations involved in the maintenance of the stem cells are readily identifiable by a premature hair graying phenotype. Research on MSCs has been hampered by the lack of a reliable system to assay their function. Here, by co-culturing highly purified melanoblasts (MBs) with XB2 keratinocytes, we describe an efficient culture method that allows the expansion of immature MBs in vitro. These MBs are also capable of undergoing terminal differentiation into mature melanocytes (MCs) when differentiation is induced. Furthermore, by performing a hair-follicle reconstitution assay in which expanded MBs in a mixture of epidermal and dermal cells were grafted to reconstitute a hair follicle, we demonstrate that the expanded MBs retain their capacity to become incorporated into newly developed hair follicles and repopulate the MC stem cell population there. Thus, by integrating genetic manipulations in cultured MBs in vitro, this method provides a powerful tool with which to study the molecular basis of stem cell regulation.

Isolation of a mesenchymal cell population from murine dermis that contains progenitors of multiple cell lineages

The skin contains two known subpopulations of stem cells/epidermal progenitors: a basal keratinocyte population found in the interfollicular epithelium and cells residing in the bulge region of the hair follicle. The major role of the interfollicular basal keratinocyte population may be epidermal renewal, whereas the bulge population may only be activated and recruited to form a cutaneous epithelium in case of trauma. Using 3-dimensional cultures of murine skin under stress conditions in which only reserve epithelial cells would be expected to survive and expand, we demonstrate that a mesenchymal population resident in neonatal murine dermis has the unique potential to develop an epidermis in vitro. In monolayer culture, this dermal subpopulation has long-term survival capabilities in restricted serum and an inducible capacity to evolve into multiple cell lineages, both epithelial and mesenchymal, depending on culture conditions. When grafted subcutaneously, this dermal subpopulation gave rise to fusiform structures, reminiscent of disorganized muscle, that stained positive for smooth muscle actin and desmin; on typical epidermal grafts, abundant melanocytes appeared throughout the dermis that were not associated with hair follicles. The multipotential cells can be repeatedly isolated from neonatal murine dermis by a sequence of differential centrifugation and selective culture conditions. These results suggest that progenitors capable of epidermal differentiation exist in the mesenchymal compartment of an abundant tissue source and may have a function in mesenchymal-epithelial transition upon insult. Moreover, these cells could be available in sufficient quantities for lineage determination or tissue engineering applications.

Smad7-induced beta-catenin degradation alters epidermal appendage development

To assess whether Smad signaling affects skin development, we generated transgenic mice in which a Smad antagonist, Smad7, was induced in keratinocytes, including epidermal stem cells. Smad7 transgene induction perturbed hair follicle morphogenesis and differentiation, but accelerated sebaceous gland morphogenesis. Further analysis revealed that independent of its role in anti-Smad signaling, Smad7 bound beta-catenin and induced beta-catenin degradation by recruiting an E3 ligase, Smurf2, to the Smad7/beta-catenin complex. Consequently, Wnt/beta-catenin signaling was suppressed in Smad7 transgenic hair follicles. Coexpression of the Smurf2 and Smad7 transgenes exacerbated Smad7-induced abnormalities in hair follicles and sebaceous glands. Conversely, when endogenous Smad7 was knocked down, keratinocytes exhibited increased beta-catenin protein and enhanced Wnt signaling. Our data reveal a mechanism for Smad7 in antagonizing Wnt/beta-catenin signaling, thereby shifting the skin differentiation program from forming hair follicles to sebaceous glands.

The hair follicle as an estrogen target and source

For many decades, androgens have dominated endocrine research in hair growth control. Androgen metabolism and the androgen receptor currently are the key targets for systemic, pharmacological hair growth control in clinical medicine. However, it has long been known that estrogens also profoundly alter hair follicle growth and cycling by binding to locally expressed high-affinity estrogen receptors (ERs). Besides altering the transcription of genes with estrogen-responsive elements, 17beta-estradiol (E2) also modifies androgen metabolism within distinct subunits of the pilosebaceous unit (i.e., hair follicle and sebaceous gland). The latter displays prominent aromatase activity, the key enzyme for androgen conversion to E2, and is both an estrogen source and target. Here, we chart the recent renaissance of estrogen research in hair research; explain why the hair follicle offers an ideal, clinically relevant test system for studying the role of sex steroids, their receptors, and interactions in neuroectodermal-mesodermal interaction systems in general; and illustrate how it can be exploited to identify novel functions and signaling cross talks of ER-mediated signaling. Emphasizing the long-underestimated complexity and species-, gender-, and site-dependence of E2-induced biological effects on the hair follicle, we explore targets for pharmacological intervention in clinically relevant hair cycle manipulation, ranging from androgenetic alopecia and hirsutism via telogen effluvium to chemotherapy-induced alopecia. While defining major open questions, unsolved clinical challenges, and particularly promising research avenues in this area, we argue that the time has come to pay estrogen-mediated signaling the full attention it deserves in future endocrinological therapy of common hair growth disorders.

The temporal and spatial expression of the novel Ca++-binding proteins, Scarf and Scarf2, during development and epidermal differentiation

During the process of epidermal differentiation, intracellular and extracellular calcium (Ca++) concentrations induce an array of signaling pathways . Keratinocytes follow a complex Ca++-dependent program of differentiation moving from the basal proliferative layer, through the spinous and granular differentiated layers to ultimately culminate in the formation of the cornified layer of the epidermis. Members of the Ca++-binding proteins play a central role in the transduction of Ca++ signals. Utilizing a suppressive subtractive hybridization screen comparing basal and differentiated keratinocytes, we identified the novel Ca++-binding protein genes, Scarf (skin Calmodulin-related factor) and Scarf2, which have homology to calmodulin (CaM). In this study, we present a comprehensive analysis of the expression pattern for Scarf and Scarf2 transcripts and proteins in the developing mouse. To examine Scarf2 expression during embryogenesis, we performed in situ hybridization, and detected expression in the hair follicle, skin and nasal epithelium. These results showed substantial overlap with the previously reported Scarf gene expression [Hwang, M., Morasso, M.I., 2003. The novel murine Ca2+-binding protein, Scarf, is differentially expressed during epidermal differentiation. J. Biol. Chem. 278, 47827-47833]. Comparing the expression patterns of Scarf and Scarf2 proteins in neonatal and adult mouse skin with several structural epidermal proteins, i.e. keratin 14 (K14), keratin 1 (K1), loricrin (LOR) and filaggrin (FIL) showed that their expression overlaps K1, an early marker of keratinocyte differentiation. Interestingly, Scarf and Scarf2 were also detected in the tongue and oral epithelia, rib bone undergoing ossification and in the medullar region of thymus.

Controls of hair follicle cycling

Nearly 50 years ago, Chase published a review of hair cycling in which he detailed hair growth in the mouse and integrated hair biology with the biology of his day. In this review we have used Chase as our model and tried to put the adult hair follicle growth cycle in perspective. We have tried to sketch the adult hair follicle cycle, as we know it today and what needs to be known. Above all, we hope that this work will serve as an introduction to basic biologists who are looking for a defined biological system that illustrates many of the challenges of modern biology: cell differentiation, epithelial-mesenchymal interactions, stem cell biology, pattern formation, apoptosis, cell and organ growth cycles, and pigmentation. The most important theme in studying the cycling hair follicle is that the follicle is a regenerating system. By traversing the phases of the cycle (growth, regression, resting, shedding, then growth again), the follicle demonstrates the unusual ability to completely regenerate itself. The basis for this regeneration rests in the unique follicular epithelial and mesenchymal components and their interactions. Recently, some of the molecular signals making up these interactions have been defined. They involve gene families also found in other regenerating systems such as fibroblast growth factor, transforming growth factor-beta, Wnt pathway, Sonic hedgehog, neurotrophins, and homeobox. For the immediate future, our challenge is to define the molecular basis for hair follicle growth control, to regenerate a mature hair follicle in vitro from defined populations, and to offer real solutions to our patients' problems.

Regulation of a hair follicle keratin intermediate filament gene promoter

During hair growth, cortical cells emerging from the proliferative follicle bulb rapidly undergo a differentiation program and synthesise large amounts of hair keratin proteins. To identify some of the controls that specify expression of hair genes we have defined the minimal promoter of the wool keratin intermediate filament gene K2.10. The region of this gene spanning nucleotides −350 to +53 was sufficient to direct expression of the lacZ gene to the follicle cortex of transgenic mice but deletion of nucleotides −350 to −150 led to a complete loss of promoter activity. When a four base substitution mutation was introduced into the minimal functional promoter at the binding site for lymphoid enhancer factor 1 (LEF-1), promoter activity in transgenic mice was decreased but specificity was not affected. To investigate the interaction of trans-acting factors within the minimal K2.10 promoter we performed DNase I footprinting analyses and electrophoretic mobility shift assays. In addition to LEF-1, Sp1, AP2-like and NF1-like proteins bound to the promoter. The Sp1 and AP2-like proteins bound sequences flanking the LEF-1 binding site whereas the NF1-like proteins bound closer to the transcription start site. We conclude that the LEF-1 binding site is an enhancer element of the K2.10 promoter in the hair follicle cortex and that factors other than LEF-1 regulate promoter tissue- and differentiation-specificity.

Proliferation and differentiation of organoid hair follicle cells co-cultured with fat cells in collagen gel matrix culture

Using rat skin, we studied the influence of fat cells on the proliferation and differentiation of organoid hair follicle cells in a three-dimensional collagen gel matrix culture system. We cultured organoid hair follicles embedded in collagen gel under each of the following three conditions: cell-free collagen gel for control experiments (condition 1); co-culture with fat cells in close apposition (condition 2); and co-culture with fat cells in spatial separation (condition 3). Outgrowths of epithelial cells from the organoid hair follicles associated with perifollicular proliferation of fibroblasts were observed under conditions 1 and 3. Under condition 2, proliferation of both organoid hair follicle cells and fibroblasts was inhibited, but differentiation of the hair follicle cells appeared to be accelerated. Fat cells are considered to have an inhibitory effect on the proliferation of perifollicular fibroblasts, which might have resulted in the inhibition of hair follicle cell proliferation and also in the better maintenance of normal follicular structure and integrity, allowing for hair-type differentiation to proceed. A direct accelerating effect of fat cells on hair follicle differentiation may also have been responsible. In a physiological state (co-culture with keratinocytes on the collagen gel), similar results were observed under conditions 1 and 2. The different findings under conditions 2 and 3 may be due to either of two possibilities: either the concentration gradient of the soluble factors released from fat cells, acting on either the hair follicle cells or the perifollicular fibroblasts as an inhibitor of proliferation, caused the difference in the results, or direct contact between the organoid hair follicle cells and fat cells may have influenced the accelerating effect of fat cells on the differentiation of hair follicle cells.

Expression of two Ig family adhesion molecules in the murine hair cycle: DCC in the bulge epithelia and NCAM in the follicular papilla

The hair cycle involves remodeling of cells and of cell groups into a complex follicular structure. During skin appendage development, adhesion molecules such as neural cell adhesion molecule (NCAM) and deleted in colon carcinoma (DC) participate in the formation of cell groups. NCAM has been found to be expressed in the mesenchyme during mouse hair follicle induction. DCC expression has been observed in the epithelial cells of the developing feather. We postulate that these two molecules may also define cell groups in the cycling hair follicle. Here we report their spatio-temporal expression patterns during the depilation-induced murine hair cycle. NCAM expression was also examined in positive and negative hair-inductive follicular papilla cell lines. Throughout the hair cycle, DCC expression was confined to the basal keratinocytes of the epidermis and the epithelial portion of the hair follicle. During mid-anagen, two types of deleted in colon carcinoma staining were observed. One was a cell surface pattern seen in the epithelial cells in the bulge region where the follicular stem cells reside. The other was a diffuse cytoplasmic staining pattern in the transient hair follicle epithelia located below the bulge region. Prominent NCAM staining was observed in the follicular papilla throughout the hair cycle and was accompanied by weak staining of the matrix epithelia. NCAM expression correlated with hair induction by a follicular papilla cell line. The results suggest that DCC and NCAM define the permanent cell groups of the hair follicle and that NCAM is important for hair induction.

Cellular features of differentiation in the nail

Biochemical analysis indicated that the human nail plate contains two distinct types of keratin (skin-type and hair-type keratins), and several population of keratinocytes are thought to be associated with development of the nail. To elucidate the nature of the differentiation occurring in nail development, we examined the patterns of molecular markers relevant to the course of differentiation in the skin and hair in the nail matrix as well as in cultured nail cells. The nail matrix was characterized by the mutually exclusive localization of skin-type and hair-type markers, while in the apical matrix the localization of two groups of keratins partially overlapped. Double-label immunofluorescence showed the existence of unusual cells coexpressing both keratins, thereby indicating that the nail matrix consists of skin-type and hair-type and additionally intermediate-type differentiating cells. The cultured cells taken from the ventral matrix which develop under hair-type differentiation in vivo were found to express skin-type along with hair-type keratins, suggesting alteration of the pathway of differentiation in vitro. The cellular diversification as seen in in vitro cultured cells provides further insight into nail differentiation which is related to the multiple patterns of keratin expression that generate in the nail matrix.

Hair cycle stage of the mouse vibrissa follicle determines subsequent fiber growth and follicle behavior in vitro

The establishment of culture models representative of all aspects of in vivo hair follicle behavior is an important goal for theoretic and analytic studies. Rodent vibrissa follicles have regular, predictable, and relatively short growth cycles. In this investigation, we took advantage of these properties; we classified mouse vibrissa follicles according to different phases in the hair cycle and then compared fiber growth and morphologic changes in culture. Follicles isolated in the early phase of the growth cycle produced fine growing fibers with an average growth that exceeded 3 mm over 15 d. Even when hair growth had slowed or halted subsequently, histology showed that these follicles retained an anagen-like morphology. By contrast, follicles isolated toward the end of the growing cycle produced thicker fibers for much shorter periods, after which growth ceased and the fibers lifted up from the base of the follicle. Internally, these specimens resembled their telogen counterparts in situ. Follicles isolated in mid-growth demonstrated intermediate fiber growth characteristics. In organ culture, mouse vibrissa follicles therefore closely reflect their in vivo origin in growth characteristics and cycle timing. These data provide new opportunities for studying hair growth cycle mechanisms in vitro, but present a caveat for quantitative studies because there may be a greater growth cycle-related variation than has previously been assumed.

Hair follicle growth controls

Research in hair biology has embarked in the pursuit for molecules that control hair growth. Many molecules already have been associated with the controls of hair patterning, hair maturation, and hair cycling and differentiation. Knowing how these molecules work gives us the tools for understanding and treating patients with hair disorders.

Whole hair follicle culture

In this article the authors have reviewed the historical background behind the organ culture of whole hair follicles. The methods developed by the authors and others for the isolation and whole organ maintenance of hair follicles from both human and other species are described. How whole organ models have been used to further understanding of the biology of the hair follicle and how they may be used in the future are discussed.

In vivo cytokine and receptor gene expression during the rat hair growth cycle. Analysis by semi-quantitative RT-PCR

A number of cytokines have previously been localised within the developing and adult hair follicle, however, the role they play in producing a mature hair follicle remains unknown. In an attempt to identify dermal papilla specific cytokines and thus those that may have an important controlling role, cytokine gene expression profiles, obtained by reverse transcriptase-polymerase chain reaction (RT-PCR), were compared between whole anagen rat hair follicles, passage 2 dermal papillae (a cell type with hair inductive capacity), and footpad fibroblasts (a non-hair inducing cell type). Based on this qualitative data, we were unable to identify a dermal papilla specific gene. The analysis of the pattern and timing of cytokine gene expression during the hair cycle is likely to be more informative. A semi-quantitative RT-PCR technique was therefore developed for studying trends in the level of in vivo expression of the following cytokines and their receptors from early anagen to early catagen in the rat hair growth cycle: insulin-like growth factor I, transforming growth factor beta 1, tumour necrosis factor, and basic fibroblast growth factor. These genes were found to be differentially expressed and this was correlated with their possible functions in controlling the hair growth cycle, providing valuable insights into the role of cytokines in regulating the hair growth process.

Immortalized rat whisker dermal papilla cells cooperate with mouse immature hair follicle buds to activate type IV procollagenases in collagen matrix coculture: correlation with ability to promote hair follicle development in nude mouse grafts

An in vivo nude mouse graft model and an in vitro collagen matrix culture system were used to study interactions of immature hair follicle buds from newborn mice with clonally derived AdE1A-12S-immortalized rat whisker dermal papilla cell lines. Of the 19 available dermal papilla cell lines, four consistently supported good hair follicle development and hair growth in grafts. Seven cell lines were clearly negative in this assay, and the remaining eight cell lines yielded poor to moderate hair growth. As a correlate to in vivo extracellular matrix remodeling accompanying hair follicle development, type IV collagenase activity in the medium from cocultures of dermal papilla cells and hair follicle buds was analyzed by gelatin zymography. Hair follicle buds cultured alone secrete primarily the 92-kDa type IV procollagenase. Cocultivation of hair follicle buds with eight of the dermal papilla cell lines resulted in activation of this proenzyme and activation of the 72-kDa and 92-kDa type IV procollagenases produced by the dermal papilla cells. Seven of these eight dermal papilla cell lines support hair growth in the graft system. In the absence of dermal papilla cells, several growth factors induced activation of the 92-kDa procollagenase secreted by hair follicle buds cultured in serum-free medium: epidermal growth factor, transforming growth factor alpha, acidic fibroblast growth factor, and keratinocyte growth factor. The current working hypothesis is that a) hair follicle epithelial cells interact with dermal papilla cells in coculture by mutual induction of growth factors and cytokines that stimulate the release and activation of matrix remodeling proteases; and b) the ability of dermal papilla cells to interact with hair follicle epithelial cells in this way may be crucial for controlled dermal matrix remodelling during HF development.

Regrowth of grafted human scalp hair after removal of the bulb

BACKGROUND

The bulbar region of the hair follicle contains the dermal papilla, hair germinative epithelial component, and active melanocytes. Thus, it has been assumed that the bulbar region plays a central role in hair growth, differentiation, and pigmentation.

OBJECTIVE

To assess the regenerative capacity of human hair.

METHODS

Individual anagen hair follicles were isolated from the occipital scalp and grafted onto the leg after removal of the bulb.

RESULTS

The grafts of follicles from which the bulb and complete papilla have been excised regenerated new papillae and grew new pigmented hairs.

CONCLUSION

The middle portion of the outer root sheath and dermal sheath may also contain epithelial, mesenchymal, and melanocyte reservoirs.

Targeted expression of SV40 T antigen in the hair follicle of transgenic mice produces an aberrant hair phenotype

Directed expression of SV40 large T antigen (TAg) in transgenic mice can induce tissue-specific tumorigenesis and useful cell lines exhibiting differentiated characteristics can be established from resultant tumor cells. In an attempt to produce an immortalised mouse hair follicle cortical cell line for the study of hair keratin gene control, SV40 TAg expression was targeted to the hair follicles of transgenic mice using a sheep hair gene promoter. Expression of SV40 TAg in the follicle cortex disrupted normal fiber ultrastructure, producing a marked phenotypic effect. Affected hairs were wavy or severely kinked (depending on the severity of the phenotype) producing an appearance ranging from a ruffled coat to a stubble covering the back of the mouse. The transgenic hairs appeared to be weakened at the base of the fibers, leading to premature hair-loss and a thinner pelage, or regions of temporary nudity. No follicle tumors or neoplasia were apparent and immortalisation of cortical cells could not be established in culture. In situ hybridisation studies in the hair follicle using histone H3 as a cell proliferation marker suggested that cell proliferation had ceased prior to commencement of K2.10-TAg expression and was not re-established in the differentiating cortical cells. Hence, TAg was unable to induce cell immortalisation at that stage of cortical cell differentiation. However, transgenic mice developed various other abnormalities including vertebral abnormalities and bladder, liver and intestinal tumors, which resulted in reduced life expectancy.

Cytokine gene expression in intact anagen rat hair follicles

Substantial cellular proliferative activity is necessary to produce a mature hair follicle. Therefore, it is likely that cytokines and their receptors play an important controlling role. To provide an understanding of the mechanisms involved during hair growth, we investigated the expression of cytokines in rat anagen hair follicles. A new technique was developed that allowed the rapid isolation of large numbers of intact, viable, anagen, rat pelage hair follicles. Total RNA was isolated from these follicles using an acid-phenol-chloroform extraction and analyzed for cytokine expression. Using the conventional technique of Northern blotting, it was only possible to detect transcripts for transforming growth factor beta (TGF beta) and insulin-like growth factor I (IGFI). Polymerase chain reaction amplification of reverse-transcribed mRNA detected cDNA fragments for TGF beta, IGF I, IGF II, nerve growth factor beta (NGF beta), and interleukin-1 alpha (IL-1 alpha). The amplified products were confirmed by digestion with restriction endonucleases. The proteins themselves for TGF beta and IGF I have been shown to be present within the anagen hair follicle using immunogold antibody labeling. This study has provided the first reported cytokine expression profile of rat anagen hair follicles. It is likely that the analysis of the pattern and timing of expression of these cytokines in the follicle will provide valuable insights into hair growth regulation.

Hair follicle reconstruction in vitro

The in vitro creation of a follicular structure capable of hair growth from cultured adult cells has long been an aim of researchers in hair biology. Basal outer root sheath cells (ORS) attached to the hair follicle glassy membrane (GM) were isolated and cultured, where they revealed greater replicative potential and longevity than ORS cells from plucked fibres. Hair follicles were reconstructed in vitro using the collagenous shells of rat vibrissa follicles as natural containers for 4 hair follicle cell types. Basal ORS cells were initially seeded onto the residual vibrissa GM, and cultured dermal papilla, dermal sheath and germinative epidermal cells were then added. Histology revealed that after 2 or 3 weeks in combined culture, cell interactions and tissue morphogenesis had resulted in the formation of irregular but recognizable hair fibres, produced from unusual bulb structures. To our knowledge this represents the first example of adult cell populations instigating the de novo creation of hair fibres in a culture dish. While the usefulness of the current methodology relates immediately to hair growth research, the generation of hair follicles and fibres in vitro establishes the enormous potential of this type of interactive work for practical purposes.

Molecules of the cycling hair follicle--a tabulated review

In this review we tabulated molecules which have been experimentally identified to be associated with, or play a role in, hair follicle growth. While compiling these data we were impressed by the fact that this field is only now beginning to be developed in terms of molecular analysis. Ironically, hair was used in some of the earliest molecular approaches to biologic structure (e.g. Astbury and Street, 1931), but the field did not develop from there. From our review we have come to the following conclusions. (1) As indicated by the growing number of reports dealing with follicle-associated molecules in the past 3 years, the field of hair biology has entered a new molecular era. (2) In many reported hair biology studies not enough emphasis has been placed on the fact that the follicle is a dynamic structure. All too often a study is limited to follicles of one particular phase of the cycle or one phase of development. Students in the field have to be more sensitive to the remarkable changes that this deceptively simple structure can undergo during its cycle. (3) Although we have not been able to find any molecules unique to the follicle, some of the structural molecules come close to an ideal tool. It is our impression that even more specific molecule tags will be found. Whether this requires a subtraction library approach or gene mapping of specific mutants is not yet clear. It would appear that the large, diverse family of intermediate filament-associated proteins will prove to be an excellent source of unique follicle-labeling molecules. (4) There is an acute need for molecules which distinguish the phases of the cycle, e.g. telogen from early anagen. Telogen is by far the most difficult phase to identify morphologically since the earliest phase of anagen and the latest phase of catagen may appear structurally like telogen. That these phases are functionally distinguishable must imply a molecular difference. As the number of recognized hair follicle-associated molecules and their interactions increase, it will be essential to assemble libraries of highly specific RNA and antibody probes for localization and mapping studies. We recognize that this review, as written, is imperfect. It is particularly deficient in making any effort towards identifying unifying principles of structure and function. We look forward to returning to this subject within 3 years.(ABSTRACT TRUNCATED AT 400 WORDS)

Human hair growth in vitro: a model for the study of hair follicle biology

The factors that regulate hair follicle growth are still poorly understood. In vitro models may be useful in elucidating some aspects of hair follicle biology. We have developed an in vitro human hair growth model that enables us to maintain isolated human hair follicles for up to 10 days, during which time they continue to grow at an in vivo rate producing a keratinised hair fibre. We have shown that epidermal growth factor (EGF) in our system mimics the in vivo depilatory action of EGF in sheep, and suggest that this occurs as a result of EGF stimulating outer root sheath (ORS) cell proliferation which results in the disruption of normal mechanisms of cell-cell interaction in the hair follicle. We identify transforming growth factor-beta (TGF-beta) as a possible negative regulator of hair follicle growth and show that physiological levels of insulin-like growth factor-I (IGF-I) can support the same rates of hair follicle growth as supraphysiological levels of insulin. Furthermore, in the absence of insulin hair follicles show premature entry into a catagen-like state. This is prevented by physiological levels of IGF-I. Finally we demonstrate that the hair follicle is an aerobic glycolytic, glutaminolytic tissue and discuss the possible implications of this metabolism.

Correlation of proteolytic activities of organ cultured intact mouse skin with defined hair cycle stages

The cyclic growth activity of the hair follicle is characterized by substantial remodelling of the extracellular matrix, yet, little is known about the proteolytic activities regulating this process. In murine skin, hair cycling is highly synchronized and is associated with dramatic remodeling of all skin compartments. We therefore have assessed, in this pilot study, proteolytic activities of murine skin from various stages of the depilation-induced hair cycle. We show that the defined proteolytic activities displayed by organ cultured intact mouse skin differ between hair cycle stages. Skin with all follicles in telogen or mid anagen displayed only minimal lysis of collagen type I gels, while early anagen skin had significant collagenase activity. Skin cultured on gelatin gels at the air-liquid interphase ('histoculture') completely lysed the gel within 5 days when all follicles were in early anagen, while this was not observed with mid and very late-anagen skin. Zymography of conditioned medium from these cultures revealed the secretion of activated interstitial collagenase and of gelatinases of 72 and 92 kDa, with the maximum of interstitial collagenase activity secreted by anagen IV skin. Addition of TPA or TNF-alpha to the culture medium stimulated secreted collagenase type I activity. The C 57 BL-6 mouse offers an attractive model for dissecting and manipulating hair cycle-associated proteolysis in a physiologically relevant system.

Variation of differentiation in nail and bovine hoof cells

Human nail plate contains two distinct types of keratins, skin-type and hair-type keratins. To elucidate that nature of the differentiation pathway of nail, we examined the expression of these keratins in human nail as well as in cultured cells taken from bovine hoof matrix. In this study we succeeded in showing that human nail matrix is characterized by the segregated localization of skin- and hair-type keratins except for the apical matrix in which both types of keratins are co-expressed. These results allow us to infer that some of the nail cells possibly divert the pattern of keratin expression during differentiation in vivo. Cultured cells taken from the ventral matrix of bovine hoof, which undergo the pathway of hair-type differentiation in vivo, expressed skin-type keratins together with hair-type keratins, thereby indicating that these cells develop into another pathway of differentiation (skin-type differentiation) from hair-type differentiation developed in vivo. These results provide us with a further insight into nail differentiation under which nail cells develop into multiple patterns of differentiation in vivo and in vitro.

Hair fibre production by human hair follicles in whole-organ culture

We have used both pulse-chase radiolabelling and morphometric techniques to investigate the ability of whole-organ cultures of human hair follicles to synthesize hair fibre. Anagen hair follicles were obtained from human facial skin by microdissection. Follicles were pulse-labelled with 3H-leucine, and radioactivity in alkali-soluble protein (ASP) and alkali-insoluble hair fibre protein (HFP) fractions was determined by differential extraction. There was a slow decline in ASP radioactivity over 7 days of culture, with a half-life of 4 days. In contrast, leucine incorporation into HFP increased linearly from day 1 to day 5, following an initial delay of 17 h. Morphometric analysis revealed that hair follicles and hair fibres grew at the same rate (0.25 mm/day) for 4 days in culture, after which the rate of hair follicle growth declined progressively, whereas fibre growth continued at the initial rate for a further 3 days. Concurrent with the onset of the decline in follicle growth, the base of the hair fibre began to descend towards the follicle base, until hair fibre occupied almost the full length of the hair follicle at day 15. The decline in follicle growth was preceded by a decline in the rate of follicular 3H-thymidine incorporation, with a delay of approximately 1 day. Overall, these data demonstrate that human hair follicles in whole-organ culture are capable of the production of hair fibre, and suggest that cessation of hair growth in vitro results from depletion of the pool of differentiated follicular keratinocytes, secondary to a loss of matrix cell proliferative activity.

Growth of wool follicles in culture

A procedure for the culture of isolated wool follicles from Merino sheep is described. Follicles were microdissected from midside skin samples of 2-yr-old wethers and transferred, individually, to 24-well tissue culture plates. When maintained in supplemented Williams' E medium containing 5 to 10% fetal bovine serum (FBS), insulin, hydrocortisone, and a trace element mixture, fibre growth rates of 40 to 80 microns/day were observed. Follicles maintained their morphologic integrity for up to 7 days, incorporated [methyl-3H]thymidine into DNA and [35S]methionine into intermediate-filament keratins of the growing fiber. Insulin and hydrocortisone stimulated fiber growth at concentrations of 10 micrograms/ml and 50 ng/ml, respectively, but higher doses were inhibitory. The growth of fibers in response to hydrocortisone and the changes in follicle morphology was similar to those induced in skin after systemic administration of cortisol in vivo. A positive interaction between hydrocortisone and trace elements for follicle survival and hydrocortisone, insulin, and FBS for fiber growth was also found. The successful culture of Merino sheep follicles provides a model with which to study the direct influence of endocrine, nutritional and local factors on wool keratin synthesis independently of systemic shifts in the animals' metabolism.

Human hair follicle germinative epidermal cell culture

Isolated human hair follicle germinative epidermal cells were observed in vitro for the first time. When cultured alone, this small, round, novel cell type did not grow, divide, take on an outer root sheath-type appearance, or display any obvious signs of epidermal differentiation. We have previously described comparable cells from rat vibrissa follicles. However, in combination with human hair follicle dermal papilla populations, the germinative epidermal cells were stimulated into proliferative and complex interactive behaviors. This included the formation of composite organotypic structures containing not only impressively intact basement membrane, but also the hair-specific form, glassy membrane.

In vivo regulation of murine hair growth: insights from grafting defined cell populations onto nude mice

The nude mouse graft model for testing the hair-forming ability of selected cell populations has considerable potential for providing insights into factors that are important for hair follicle development and proper hair formation. We have developed a minimal component system consisting of immature hair follicle buds from newborn pigmented C57BL/6 mice and adenovirus E1A-immortalized rat vibrissa dermal papilla cells. Hair follicle buds contribute to formation of hairless skin when grafted alone or with Swiss 3T3 cells, but produce densely haired skin when grafted with a fresh dermal cell preparation. The fresh dermal cell preparation represents the single cell fraction after hair follicles have been removed from a collagenase digest of newborn mouse dermis. It provides dermal papilla cells, fibroblasts, and possibly other important growth factor-producing cell types. Rat vibrissa dermal papilla cells supported dense hair growth at early passage in culture but progressively lost this potential during repeated passage in culture. Of 19 E1A-immortalized, clonally derived rat vibrissa dermal papilla cell lines, the four most positive clones supported hair growth to the extent of approximately 200 to 300 hairs per 1-2 cm2 graft area. The remaining clones were moderately positive (five clones), weakly positive (three clones), or negative (seven clones). Swiss 3T3 cells prevented contraction of the graft area but did not appear to affect the number of hairs in the graft site produced by dermal papilla cells plus hair follicle buds alone. The relatively low hair density (estimated 1-5% of normal) resulting from grafts of hair follicle buds with the most positive of the immortalized dermal papilla cell clones compared to fresh dermal cells suggests that optimal reconstitution of hair growth requires some function of dermal papilla cells partially lost during the immortalization process and possibly the contribution of other cell types present in the fresh dermal cell preparation, which is not supplied by the Swiss 3T3 cells. The current graft system, comprising hair follicle buds and immortalized dermal papilla cell clones, provides an assay for positive or negative influences on hair growth exerted by added selected cell types, growth factors, or other substances. Characterization of the phenotype of the dermal papilla cell lines, which differ in their ability to support hair growth when grafted with hair follicle buds, may provide insight into specific dermal papilla cell properties important for their function in this system.

Culture of hair matrix and follicular keratinocytes

In recent years, a variety of in vitro models for the cultivation of hair follicles and their constituents have been developed. Outer root sheath (ORS) keratinocytes (KC) have been mainly studied in explant cultures, planted on bovine eye lens capsules, collagen substrata, 3T3 cell feeder layers, or dermal equivalents, yielding outgrowth of a multilayered stratified epithelium with some biochemical and ultrastructural characteristics of keratinocytic differentiation. More recently, ORS KC cultures have also been initiated from single cell suspensions, and organotypic cultures have been obtained by recombination with dermal cells, inducing a higher degree of epidermal differentiation. Presumptive human hair matrix cells have been isolated from plucked anagen hair follicles and have been successfully propagated on 3T3 cell or normal human fibroblast feeder layers, giving rise to multilayered stratified KC cultures. In contrast, only preliminary data exist concerning the cultivation of bulge cells that have been suggested to represent follicular stem cells. In conclusion we dispose of several in vitro models today to cultivate ORS KC and hair matrix cells that have increased our knowledge on the regulation of the human hair cycle by soluble factors and dermal-epidermal interactions. Further comparative studies on ORS KC, bulge cells and matrix cells have to be carried out to confirm the distinct character of these hair KC subsets.

Regulation of hair follicle development: an in vitro model for hair follicle invasion of dermis and associated connective tissue remodeling

During embryonic development presumptive hair follicle cells of epithelial and mesenchymal origin are determined in defined body locations. This is followed by rapid proliferation of epithelial cells and associated penetration into the dermis in response to as yet undetermined signals. A collagen matrix culture system, which maintains the three-dimensional relationships of hair follicle cells to each other, was developed to study the regulation of the enlargement of immature hair follicles and the accompanying remodeling of the dermis. In studies with a heterogeneous dermis-derived preparation of murine hair follicles, ranging in size from the earliest down-growing budding cell mass to hair-forming follicles, we had previously shown that cell proliferation was stimulated by cholera toxin and epidermal growth factor, but only the epidermal growth factor-stimulated proliferation was accompanied by digestion of the collagen matrix due to release of collagenolytic enzymes. Further studies revealed that transforming growth factor-alpha also stimulated hair follicle cell proliferation and collagenase release. However, although transforming growth factor-beta inhibited the transforming growth factor-alpha-stimulated proliferation, it enhanced the release and activation of collagenases and other gelatin-degrading enzymes detectable by gelatin zymography. Stimulation of collagenolytic activity depended on the three-dimensional hair follicle structure and did not occur in monolayer cultures of hair follicle cells. Comparison of hair follicle buds with more developed dermis-derived hair follicles, plated at the same cell density (based on DNA content), suggested that a greater fraction of cells in the bud-stage follicle responded to the growth factors by release of collagenases. Possibly only the cells in the advancing portion of growing hair follicles that are closest to the dermal papilla cell cluster produce the collagenases in response to growth factors. To examine the participation of dermal papilla cells in collagenase release and activation, several immortalized rat whisker dermal papilla cell lines were co-cultured with mouse hair follicle buds. Co-culture resulted in a marked enlargement of follicles as well as activation of the 92-kDa type IV collagenase, produced by hair follicle buds, that correlated with ability of the dermal papilla cells to stimulate hair formation in grafts of hair follicle buds on nude mice. Dermal papilla cells cultured alone produced the 72-kDa type IV collagenase, which was also activated during co-culture with hair follicle buds. Thus, two activities, both relevant for hair follicle development, namely, cell proliferation and release and activation of collagenases, have been stimulated in immature hair follicle buds by either growth-factor supplementation or interaction with dermal papilla cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhanced in vitro hair growth at the air-liquid interface: minoxidil preserves the root sheath in cultured whisker follicles

Inasmuch as hair follicles are difficult to maintain in culture, the study of hair biology using cultured hair follicles has met with only limited success. In our attempts to solve the problem of follicle degeneration, we cultured follicles at the air-surface interface on a modified collagen matrix (Gelfoam). In follicles cultured at the air-surface or submerged, we examined follicular morphology, hair shaft growth, sulfotransferase levels, cysteine incorporation, an expression of a tissue inhibitor of metalloproteinase (TIMP), and ultra-high sulfur keratin (UHSK). Follicles cultured at the air-liquid interface produced a 2.7-fold increase in hair growth and maintained an anagen-like morphology. Substrates such as nylon mesh seeded with fibroblasts, Full Thickness Skin, or 5-microns polycarbonate filter also supported hair growth, whereas Gelfilm, GF-A glass filter, filter paper, or 1-micron polycarbonate filter did not. The UHSK expression was significantly higher in the air-liquid interface cultures compared to the submerged culture. Several potassium channel openers, including minoxidil, a minoxidil analog, and the pinacidil analog (P-1075), all stimulated significant cysteine incorporation in follicles. Minoxidil and its analog specifically preserved the follicular root sheath, in contrast to P-1075 which did not, indicating a difference in the two drug types. The preservation of the root sheath was measured by increased TIMP expression and sulfotransferase activity and indicates that the root sheath is a target tissue for minoxidil. Our results show that follicles cultured at the air-liquid interface maintain a better morphology and produced greater hair growth than follicles cultured on tissue culture plastic.

Effects of cytokines, anti-cancer agents and cocarcinogen on DNA synthesis in hair bulb cells

We analysed the effects of cytokines, anti-cancer agents and cocarcinogen on DNA synthesis in human hair germinative cells cultured in serum-free media. Epidermal growth factor and gamma interferon were found to inhibit DNA synthesis slightly, while strong inhibition was demonstrated by doxorubicin, cytosine arabinoside and tetradecanoyl-phorbolacetate. Basic fibroblast growth factor had very little influence on DNA synthesis. This organ culture model in serum-free media is a useful method by which to examine the effects of various cytokines and drugs on DNA synthesis in hair germinative cells and/or to study the pathogenesis of various alopecia diseases.

Reconstitution of hair follicle development in vivo: determination of follicle formation, hair growth, and hair quality by dermal cells

Combinations of cultured and uncultured epidermal and dermal cell preparations from newborn and perinatal mice were grafted onto the backs of athymic nude mouse hosts to elucidate the cellular requirements for skin appendage formation. All epidermal populations studied, including a total epidermal keratinocyte preparation from trypsin-split skin, developing hair follicle buds isolated from epidermis, and preformed hair follicles isolated from dermis, make haired skin when grafted with fresh dermal cells. Only pre-formed hair follicles produce haired skin on grafts without an additional dermal component. Hair follicle buds grafted alone or with cultured dermal cells will reconstitute skin but without appendage formation. Thus, cells or factors present in fresh, but not cultured, dermal cells are essential for supporting hair growth from budding follicles, whereas more developed (pre-formed) follicles appear to contain all the necessary components for hair formation. Dissociation of isolated hair follicles by trypsin/ethylenediaminetetraacetic acid prior to grafting is permissive for hair growth, suggesting that follicle cells can be re-induced or reassociate in vivo. Dermal papilla cells, microdissected from rat vibrissal follicles and cultured for up to 14 passages, stimulate hair growth from follicle buds and influence the quality of hair growth from pre-formed hair follicles. Thus, dermal papilla cells maintain inductive capacity in culture and contribute to the reconstituted skin. This reconstitution model should be useful for identifying cell populations within the hair follicle compartment necessary for hair growth and for examining the effects of specific gene products on hair follicle growth and development in vivo.

Morphological analysis of in vitro human hair growth

The histological and ultrastructural aspect of normal human hair follicles maintained ex vivo for 12 days was evaluated. Anagen hair follicles, dissected free of contaminating connective tissue, were maintained for up to 12 days in a serum-free medium. Macroscopic observations revealed continued viability for 12 days, at which time some follicles involuted in a manner morphologically similar to catagen. Increased growth of maintained follicles was measured from the abrupt ending of the connective tissue sheath (CTS), as no increase in this component was observed from initiation of culture. In general follicles maintained up to 8 days exhibited little divergence from normal in vivo morphologies including the persistence of functional hair bulb melanocytes--a marker of anagen. After this time melanin granules were present in dermal papilla cells, as occurs during impending involution in vivo. Heterotypic cell contact occurred in the middle to upper follicle between outer root sheath (ORS) keratinocytes and disorganized CTS. Herniation of some ORS cells away from the follicle and the occurrence of loose desmosomal junctions between ORS keratinocytes reflected loss of normal follicular cell interactions in upper follicles maintained after 8 days. Continued follicle growth correlated with the presence of mitotic matrix keratinocytes even at 12 days. After 12 days in culture most follicles involuted displaying apoptotic-like keratinocytes and hair bulb melanocytes and the presence of highly keratinized hair 'club' structures. While most follicles exhibited this orderly sequence of events, a few follicles involuted after 24 h with synchronous degeneration of all cells. Two follicles exhibited upregulated cortical cell differentiation at the level of the dermal papilla (DP).(ABSTRACT TRUNCATED AT 250 WORDS)

Organ culture of human hair follicles in serum-free medium

Human hair follicles were cultured in serum-free media at 31 degrees C in an atmosphere containing 95% O2 and 5% CO2. Results showed that the length of the cultured hair increased time dependently for 96 h. Histological findings revealed that the hair germinative cells maintained their normal morphology throughout the 96 h culture period. DNA synthesis in the hair bulb also increased time dependently for 96 h. Autoradiographs of 3H-thymidine-labelled follicles indicated that they were localized in the germinative cells below Auber's critical line. The effects of minoxidil sulphate on DNA synthesis in this culture system were concentration dependent. Minoxidil sulphate at concentrations of 10(-10), 10(-9) and 10(-8) M significantly increased DNA synthesis compared with DNA synthesis in the control medium. Autoradiographs of the follicles cultured in 10(-10) M minoxidil sulphate showed that 3H-thymidine localized primarily in the germinative cells below Auber's critical line. These results suggest that this organ culture system may be useful for studying DNA synthesis by hair germinative cells in serum-free media.

Rat hair follicle growth in vitro

Pelage hair follicles were isolated by gentle microdissection from 8-12-day-old rats, and maintained in supplemented Williams E medium. Length measurements made on freshly isolated hair follicles, and at 24-h intervals, showed a significant increase in hair follicle length over 48 h, after which time no further significant increase in length was observed. Photomicrographs of maintained follicles showed that this increase in hair follicle length could be attributed to the production of a keratinized hair shaft. Histology and [methyl-3H] thymidine autoradiography of freshly isolated hair follicles showed the dermal papilla to be elongated, with thymidine uptake located predominantly in the matrix cells of the hair follicle bulb adjacent to the dermal papilla. This pattern remained unaltered for the first 48 h of maintenance, but after 72 h the dermal papilla had rounded into a tight ball of cells, with very little thymidine uptake occurring in the adjacent matrix cells. On maintenance, fetal calf serum (FCS), epidermal growth factor (EGF) and 12-o-tetradecanoyl phorbol 13-acetate (TPA) all significantly stimulated [methyl-3H] thymidine and [U-14C] leucine uptake, but inhibited hair follicle elongation. Insulin-like growth factor-1 (IGF-1) had no significant effect on rates of hair follicle elongation and [methyl-3H] thymidine uptake, but significantly stimulated rates of [U-14C] leucine uptake. Transforming growth factor-beta 1 (TGF-beta 1) significantly inhibited both the rate of [methyl-3H] thymidine uptake and hair follicle elongation.