Localization of Epidermal Growth Factor Immunoreactivity in Sheep Skin During Wool Follicle Development

Gene network analysis reveals candidate genes related with the hair follicle development in sheep

Background

Merino sheep are the most famous fine wool sheep in the world. They have high wool production and excellent wool quality and have attracted worldwide attention. The fleece of the Merino sheep is composed predominantly of wool fibers grown from secondary wool follicles. Therefore, it is necessary to study the development of hair follicles to understand the mechanism of wool production. The hair follicle is a complex biological system involved in a dynamic process governed by gene regulation. The hair follicle development process is very complex and poorly understood. The purpose of our research is to identify candidate genes related to hair follicle development, provide a theoretical molecular breeding basis for the cultivation of fine wool sheep, and provide a reference for the problems of hair loss and alopecia areata that affect human beings.

Results

We analyzed mRNAs data in skin tissues of 18 Merino sheep at four embryonic days (E65, E85, E105 and E135) and two postnatal days (P7 and P30). G1 to G6 represent hair follicles developmental at six stages (i.e. E65 to P30). We identified 7879 differentially expressed genes (DEGs) and 12623 novel DEGs, revealed different expression patterns of these DEGs at six stages of hair follicle development, and demonstrated their complex interactions. DEGs with stage-specific expression were significantly enriched in epidermal differentiation and development, hair follicle development and hair follicle morphogenesis and were enriched in many pathways related to hair follicle development. The key genes (LAMA5, WNT10A, KRT25, SOSTDC1, ZDHHC21, FZD1, BMP7, LRP4, TGFβ2, TMEM79, SOX10, ITGB4, KRT14, ITGA6, and GLI2) affecting hair follicle morphogenesis were identified by network analysis.

Conclusion

This study provides a new reference for the molecular basis of hair follicle development and lays a foundation for further improving sheep hair follicle breeding. Candidate genes related to hair follicular development were found, which provided a theoretical basis for molecular breeding for the culture of fine wool sheep. These results are a valuable resource for biological investigations of fleece evolution in animals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08552-2.

Transcriptome Reveals Long Non-coding RNAs and mRNAs Involved in Primary Wool Follicle Induction in Carpet Sheep Fetal Skin

Murine primary hair follicle induction is driven by the communication between the mesenchyme and epithelium and mostly governed by signaling pathways including wingless-related integration site (WNT), ectodysplasin A receptor (EDAR), bone morphogenetic protein (BMP), and fibroblast growth factor (FGF), as observed in genetically modified mouse models. Sheep skin may serve as a valuable system for hair research owing to the co-existence of sweat glands with wool follicles in trunk skin and asynchronized wool follicle growth pattern similar to that of human head hair follicles. However, the mechanisms underlying wool follicle development remain largely unknown. To understand how long non-coding RNAs (lncRNAs) and mRNAs function in primary wool follicle induction in carpet wool sheep, we conducted high-throughput RNA sequencing and revealed globally altered lncRNAs (36 upregulated and 26 downregulated), mRNAs (228 elevated and 225 decreased), and 80 differentially expressed novel transcripts. Several key signals in WNT (WNT2B and WNT16), BMP (BMP3, BMP4, and BMP7), EDAR (EDAR and EDARADD), and FGF (FGFR2 and FGF20) pathways, and a series of lncRNAs, including XLOC_539599, XLOC_556463, XLOC_015081, XLOC_1285606, XLOC_297809, and XLOC_764219, were shown to be potentially important for primary wool follicle induction. GO and KEGG analyses of differentially expressed mRNAs and potential targets of altered lncRNAs were both significantly enriched in morphogenesis biological processes and transforming growth factor-β, Hedgehog, and PI3K-Akt signaling, as well as focal adhesion and extracellular matrix-receptor interactions. The prediction of mRNA-mRNA and lncRNA-mRNA interaction networks further revealed transcripts potentially involved in primary wool follicle induction. The expression patterns of mRNAs and lncRNAs of interest were validated by qRT-PCR. The localization of XLOC_297809 and XLOC_764219 both in placodes and dermal condensations was detected by in situ hybridization, indicating important roles of lncRNAs in primary wool follicle induction and skin development. This is the first report elucidating the gene network of lncRNAs and mRNAs associated with primary wool follicle early development in carpet wool sheep and will shed new light on selective wool sheep breeding.

EGFR-ras-raf signaling in epidermal stem cells: roles in hair follicle development, regeneration, tissue remodeling and epidermal cancers

The mammalian skin is the largest organ of the body and its outermost layer, the epidermis, undergoes dynamic lifetime renewal through the activity of somatic stem cell populations. The EGFR-Ras-Raf pathway has a well-described role in skin development and tumor formation. While research mainly focuses on its role in cutaneous tumor initiation and maintenance, much less is known about Ras signaling in the epidermal stem cells, which are the main targets of skin carcinogenesis. In this review, we briefly discuss the properties of the epidermal stem cells and review the role of EGFR-Ras-Raf signaling in keratinocyte stem cells during homeostatic and pathological conditions.

Beyond wavy hairs: the epidermal growth factor receptor and its ligands in skin biology and pathology

The epidermal growth factor receptor (EGFR) network, including its seven ligands and four related receptors, represents one of the most complex signaling systems in biology. In many tissues, including the skin and its appendages (notoriously the hair follicles), its correct function is necessary for proper development and tissue homeostasis, and its deregulation rapidly results in defects in cellular proliferation and differentiation. The consequences are impaired wound healing, development of psoriasis-like lesions, structural and functional defects of the hair follicles, and tumorigenesis. In addition to in vitro experiments and data from clinical studies, several genetically modified mouse models displaying alterations in the interfollicular skin and hair follicles attributable to mutations in components of the EGFR system have been reported. These animals, in many cases representing bona fide models of known human diseases, have been seminal in the study of the role of EGFR and its ligands in the skin and its appendages. In this review, we take the multiple phenotypes of these animal models as a basis to summarize and discuss the effects elicited by members of the EGFR system in diverse aspects of skin biology and pathology, including cellular proliferation and differentiation, wound healing, hair follicle morphogenesis, and tumorigenesis.

Epidermal growth factor as a biologic switch in hair growth cycle

The hair growth cycle consists of three stages known as the anagen (growing), catagen (involution), and telogen (resting) phases. This cyclical growth of hair is regulated by a diversity of growth factors. Although normal expression of both epidermal growth factor and its receptor (EGFR) in the outer root sheath is down-regulated with the completion of follicular growth, here we show that continuous expression of epidermal growth factor in hair follicles of transgenic mice arrested follicular development at the final stage of morphogenesis. Data from immunoprecipitation and immunoblotting showed that epidermal growth factor signals through EGFR/ErbB2 heterodimers in skin. Furthermore, topical application of tyrphostin AG1478 or AG825, specific inhibitors of EGFR and ErbB2, respectively, completely inhibited new hair growth in wild type mice but not in transgenic mice. When the transgenic mice were crossed with waved-2 mice, which possess a lower kinase activity of EGFR, the hair phenotype was rescued in the offspring. Taken together, these data suggest that EGFR signaling is indispensable for the initiation of hair growth. On the other hand, continuous expression of epidermal growth factor prevents entry into the catagen phase. We propose that epidermal growth factor functions as a biologic switch that is turned on and off in hair follicles at the beginning and end of the anagen phase of the hair cycle, guarding the entry to and exit from the anagen phase.

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.

Role of hormones in pilosebaceous unit development

Androgens are required for sexual hair and sebaceous gland development. However, pilosebaceous unit (PSU) growth and differentiation require the interaction of androgen with numerous other biological factors. The pattern of PSU responsiveness to androgen is determined in the embryo. Hair follicle growth involves close reciprocal epithelial-stromal interactions that recapitulate ontogeny; these interactions are necessary for optimal hair growth in culture. Peroxisome proliferator-activated receptors (PPARs) and retinoids have recently been found to specifically affect sebaceous cell growth and differentiation. Many other hormones such as GH, insulin-like growth factors, insulin, glucocorticoids, estrogen, and thyroid hormone play important roles in PSU growth and development. The biological and endocrinological basis of PSU development and the hormonal treatment of the PSU disorders hirsutism, acne vulgaris, and pattern alopecia are reviewed. Improved understanding of the multiplicity of factors involved in normal PSU growth and differentiation will be necessary to provide optimal treatment approaches for these disorders.

Cultivation of epithelia from the secretory coil of the ovine apocrine gland: evidence of secretory cell function and ductal morphogenesis in vitro

The secretory coil of the ovine apocrine gland is composed predominantly of two cell types, secretory cells lining the lumen and myoepithelial cells adjacent to the basement membrane. The glands synthesize a number of hormones and growth factors, but analysis of the functions of these molecules may be hampered by the mixing of apocrine and sebaceous secretions in the pilary canal. The purpose of this study was to isolate the glands and devise simple culture procedures to facilitate investigations of secretory cell function. The most successful approach involved microdissection of the secretory coils individually from skin biopsies and culture in Dulbecco's modified Eagle's medium. After 1-2 wk in medium, cell outgrowths were seen from explants. These consisted predominantly of populations of epithelial cells, many containing granules. Smaller granules were usually concentrated around the cell nuclei and accumulated lipophilic dyes. Large granules were unreactive. Western analysis showed that cells in culture synthesized nerve growth factor-like peptides, a feature consistent with one of the functions of the gland in vivo. When isolated secretor, coils were explanted to culture dishes coated with matrigel, highly compact, multilayered masses of cells grew out. Subsequently, tubular structures formed. The observations suggest that some differentiated functions of gland cells were retained in vitro and that the procedures described provide a system for the study, of apocrine secretions in isolation from those of other skin glands.

Immunohistochemical localization of epidermal growth factors in mouse olfactory epithelium

Our object was to clarify changes in epidermal growth factor (EGF) occurring in the olfactory epithelium with aging. We investigated the changes in EGF in the murine olfactory epithelium with aging by an immunohistochemical method. We examined six mice at each of the following stages: embryonal (embryonic days 14 and 16), neonatal (postnatal days 1 and 7), adult (postnatal weeks 5 and 12), and aged (postnatal years 1.5-2). The olfactory epithelium of each mouse was stained with anti-EGF polyclonal IgG using an immunohistochemical method. EGF were observed in the whole layer of the olfactory epithelium at all stages. The mesenchyme under the epithelium was stained in the embryonal stage, and the circumference of the olfactory gland cells was also stained, although weakly, in the adult stage. We believe that EGF contribute to cell proliferation, growth and turnover of the olfactory epithelium. However, the decrease in EGF was less than expected in the aged mice.

Expression of midkine in normal and burn sites of rat skin

Expression of midkine (MK), a retinoic acid-inducible heparin binding growth factor, was examined immunohistochemically in normal and burn sites of rat skin. In the normal skin, MK was localized in the epidermis and dermal appendages such as hair follicles and sebaceous glands. Mast cells in the subdermal connective tissue also accumulated MK. After burn injury, MK-positive cells began to infiltrate into subdermal connective tissue, and the number of MK-positive cells in the region increased to a maximum at postburn day two and then decreased gradually. Western blotting analysis of both normal and postburn skin revealed a 30 kDa band reactive with anti-MK antibody; this band was concluded to be a dimer of MK. These findings were discussed from the viewpoint of the possible role of MK in wound healing.

Hair follicle has a novel anagen-specific protein, mKAP13

To identify the anagen hair follicle-specific proteins, we screened the cDNA library prepared from the murine skins of anagen phase by the differential hybridization technique. Fifty-four cDNA clones expressed specifically in anagen phase were isolated, and most were found to correspond to known proteins in the hair follicles. Alternatively, we isolated a cDNA clone encoding a novel protein that possessed an entire open reading frame of 501 base pairs. This protein with a molecular weight of 17.9 kDa has no specific motifs nor significant homology to proteins already reported, although it contains some direct repeats that are often observed in intermediate filament-associated proteins and has a similar amino acid composition as a member of them. Northern blot analysis demonstrates that the transcript of this protein is skin specific, and that it is present in mid- and late anagen but not in catagen, telogen, and early anagen phases. The transcript appears to be expressed specifically in the keratogenous zone of the cortical cells of hair follicles, as exhibited by in situ hybridization. Furthermore, immunohistochemical study confirms that the protein is distributed in the cytoplasms of the keratinizing cortical cells and is undetected in the completely keratinized ones. These results suggest that this protein can be identified as a new member of intermediate filament-associated proteins and is related to the keratinization of the cortical cell layer in mouse hair. Therefore, we have termed this novel protein mKAP13 according to the unified nomenclature.

Interactions between epidermal growth factor and the Tabby mutation in skin

Mutations of the X-linked genes Tabby (Ta) in mice and EDA in humans result in developmental and functional abnormalities, primarily in the skin and hair follicles. Although both genes are believed to encode membrane-associated proteins, it has been suggested that, in the mouse, the mutation is linked to a deficiency of epidermal growth factor (EGF). This study investigated relationships between the skin abnormalities of Ta mice and the EGF signal pathway. The distribution of endogenous EGF in tissues of Ta/Y and +/Y animals was examined and, because of its reported morphogenetic actions and ability to overcome receptor signalling defects in vivo, the effects of exogenous EGF on the hair follicle population were determined. EGF levels were similar in a number of tissues of Ta/Y and +/Y mice, but amounts in Ta/Y submaxillary glands were reduced, probably due to a smaller gland size. Exogenous EGF inhibited hair follicle development and decreased follicle density in both genotypes. It was concluded from comparisons of the distributions of EGF and its effects in skin with those in mice bearing mutations in the EGF signal pathway that the normal phenotype results from interactions between EGF and the Ta peptide in skin.

Skin2--an in vitro human skin model: the correlation between in vivo and in vitro testing of surfactants

The availability of an in vitro test system to replace animal testing of potential irritants is becoming more and more urgent especially in Europe as a consequence of the European Community Cosmetics Directive. To evaluate the ability of Advanced Tissue Sciences' (ATS) ZK1301 skin model to predict the skin irritation potential of surfactants, we performed a pilot validation study utilizing four different laboratories. The in vitro protocol was designed as a quantitative pre-screen for the clinical patch studies. Sixteen substances, representing various surfactant categories and ranges of irritation potential, were tested. The 3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay was used to quantitate viability in vitro. We documented the viability of tissues exposed to unknown substances for specific periods. The in vitro results were calculated as percent distilled water controls (DWC). The time required to reduce the viability of each tissue to 50% of the distilled water controls (T50) was compared to mean erythema and edema scores from the clinical studies by Pearson's correlation. The individual laboratories demonstrated coefficients of 0.72. The results indicated that the 30 min percent untreated control values best predicted the 24 h clinical patch scores. No statistically significant interlab variability was found. Only one false negative was seen when non/mild and moderate/severe irritant categories were assigned according to the in vitro scores. These results demonstrate that the skin2 in vitro test system may serve as a good screening method prior to clinical patch studies.

The regulatory biology of the human pilosebaceous unit

The last few years have witnessed an acceleration in our understanding of the regulation of the human pilosebaceous unit. Recombination and histochemical experiments are beginning to elucidate the role of homeotic genes, transcription factors, growth factors and adhesion molecules in pilosebaceous embryology. Histochemical studies, experiments in gene-modified animals, and in vitro studies on growing human hairs, have identified a number of growth factors that are central to normal hair growth. Thus epidermal growth factor and transforming growth factor-alpha appear to be involved in the triggering of both anagen and catagen. Insulin-like growth factor-I appears to sustain normal anagen growth, transforming growth factor-beta will inhibit anagen growth, while interleukin-1-alpha and tumour necrosis factor-alpha will induce matrix cell death. These complex growth factor effects are beginning to be moulded into an integrated model of pilosebaceous regulation. The role of steroid hormones in modulating these growth factor effects is also beginning to be understood.

The role of keratin proteins and their genes in the growth, structure and properties of hair

The importance of wool in the textile industry has inspired extensive research into its structure since the 1960s. Over the past several years, however, the hair follicle has increased in significance as a system for studying developmental events and the process of terminal differentiation. The present chapter seeks to integrate the expanding literature and present a broad picture of what we know of the structure and formation of hair at the cellular and molecular level. We describe in detail the hair keratin proteins and their genes, their structure, function and regulation in the hair follicle, and also the major proteins and genes of the inner and outer root sheaths. We discuss hair follicle development with an emphasis on the factors involved and describe some hair genetic diseases and transgenic and gene knockout models because, in some cases, they stimulate natural mutations that are advancing our understanding of cellular interactions in the formation of hair.

Growth factors and cytokines in hair follicle development and cycling: recent insights from animal models and the potentials for clinical therapy

Hair growth disorders, particularly those that lead to hair loss (alopecia), are common and frequently cause significant mental anguish in affected individuals. The mechanisms underlying the majority of these disorders are unknown. However, insights into the specific molecular mechanisms of hair follicle development and cycling have recently been made using animal models, particularly mice that over- or underexpress a specific gene for a growth factor or cytokine. Other animal models have demonstrated that certain growth factors and cytokines can prevent much of the alopecia caused by cancer chemotherapeutic agents. These animal models have confirmed the importance of growth factors and cytokines in hair follicle development and cycling, and have formed the foundation for potential clinical therapy of hair growth disorders, particularly alopecia. Nevertheless, important questions concerning their efficacy, safety and delivery will need to be answered before successful clinical therapy of any hair growth disorder becomes a reality.

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.

Growth factors in hair organ development and the hair growth cycle

Growth factors are polypeptides that regulate growth and differentiation of many cell types. Different growth factor families including the epidermal growth factor (EGF)-related ligands, fibroblast growth factors (FGF), transforming growth factor-beta (TGF-beta), insulin-like growth factor (IGF), hepatocyte growth factor/scatter factor (HGF/SF), and platelet-derived growth factor (PDGF) have been shown to be crucial for the regulation of the hair cycle and hair growth. Growth factors and their receptors have been localized to the skin and hair follicles. Their biological activities on cells comprising the hair follicle have been tested in vitro and increasingly in transgenic mice. Herein we review selected important aspects of growth factors with regard to the hair organ, its development, and the hair growth cycle.

Regulation of human dermal papilla cell production of insulin-like growth factor binding protein-3 by retinoic acid, glucocorticoids, and insulin-like growth factor-1

Insulin-like growth factor-1 (IGF1) has been reported to stimulate hair elongation and to facilitate maintenance of the hair follicle in anagen phase. However, little is known about IGF1 signaling in the hair follicle. In this study we investigate the effects of IGF1, glucocorticoids, and retinoids on dermal papilla (DP) cell production of insulin-like growth factor binding proteins (IGFBPs). IGFBPs comprise a family of IGF binding proteins that are produced and released by most cell types. They bind to IGFs to either enhance or inhibit IGF activity. In the present report we identify IGFBP-3 as being produced and released by cultured human dermal papilla (DP) cells. IGFBP-3 levels are increased fivefold by retinoic acid, eightfold by dexamethasone, and tenfold by IGF1. DP cells are known to produce IGF1, and so the observed stimulation of DP cell IGFBP-3 production by IGF1 is consistent with the idea that DP cells possess the IGF transmembrane receptor kinase and are autoregulated by IGFs. The level of another IGFBP, tentatively identified as IGFBP-2, is, in contrast, not regulated by these agents. IGFBP-3 has been shown to inhibit the activity of IGFs in a variety of systems. Our results are consistent with a model in which retinoids and glucocorticoids inhibit IGF action on DP cells and surrounding matrix cells by stimulating increased DP cell production of IGFBP-3. The IGFBP-3, in turn, forms a complex with free IGF1 to reduce the concentration of IGF1 available to stimulate hair elongation and maintenance of anagen phase.

Effects of epidermal growth factor and transforming growth factor alpha on the function of wool follicles in culture

The development of a procedure to culture wool follicles from Merino sheep in serum-free conditions has enabled us to investigate the actions of epidermal growth factor (EGF) and transforming growth factor alpha (TGF alpha) on follicle function, including fibre growth. Follicles grown in the absence of growth factors maintained their anagen morphology for 6 days as determined by light microscopy. During this time they incorporated [3H]thymidine into the DNA of the bulb matrix and outer root sheath (ORS) cells and produced fibre keratins as detected by immunohistochemistry. In the presence of EGF and TGF alpha, fibre production ceased after 4 days, as it does following the administration of EGF in vivo. Cessation of fibre growth was not accompanied by regression of the follicle bulb which occurs in vivo. Follicle length growth did not differ significantly from controls and cells in the bulb continued to proliferate. Usually, the structure of the dermal papillae resembled that in control follicles, which was also in marked contrast to changes reported in vivo. In EGF- and TGF alpha-treated follicles, [3H]thymidine continued to be incorporated into DNA of the ORS and bulb after fibre growth ceased. Although wool keratin synthesis ceased, cytokeratins of the epidermis and ORS continued to be produced in the bulb as detected by immunochemistry. These bulb cells were also positive for the periodic acid-Schiff (PAS) reaction indicating the presence of glycogen, a normal component of ORS cells. The observations that cell proliferation continued in the bulb, that glycogen was present and that soft keratins were expressed in these cells suggest that the bulb cell population was induced to differentiate into an ORS phenotype by EGF and TGF alpha.

Characterization and distribution of epidermal growth factor receptors in the skin and wool follicles of the sheep fetus during development

We have determined the binding affinity and capacity and relative distribution of epidermal growth factor (EGF) receptors in the skin of the Merino sheep fetus before and during the development of the wool follicle population. Autoradiography of tissue sections incubated with [125I]EGF revealed that label was confined predominantly to the epidermis and dermoepidermal junction before follicle formation, at 30 and 55 d of gestation. During follicle initiation (Days 60 to 65), receptor activity was distributed over the epidermis, including the epidermal aggregations of primordia at the dermoepidermal junction. However, receptor concentrations, as revealed by grain counts of autoradiographs, were reduced in these regions when compared with 55-d skin. The receptor distribution over the epidermis and its derivatives did not alter during subsequent follicle development, although the intensity of labeling increased as the follicles matured. Specific receptor binding was not observed above background levels in the dermis and dermal papillae during all stages of follicle development. At follicle maturation, EGF receptors were widely distributed over the cells of the epidermis and the epidermal derivatives of the cutaneous appendages but were particularly localized in the sebaceous glands and outer root sheath (see also Wynn et al. 1989). EGF immunoreactive material has also been found at these sites (du Cros et al. 1992), suggesting an autocrine role for EGF in the regulation of cell function. It is likely that the differentiation-promoting activities of EGF may predominate over those of growth, because the receptor-bearing cells were not members of rapidly proliferating populations.

Growth factor expression in skin during wool follicle development

A variety of growth factors are likely to be involved in initiation and morphogenesis of wool follicles. To enable direct comparisons of the expression of different growth factors, reverse transcriptase-polymerase chain reactions (RT-PCR) were developed for ovine and murine TGF alpha, TGF beta 1, TGF beta 2, TGF beta 3, IGF1, IGF2, and FGF-2, which could all be carried out on a single cDNA sample. These RT-PCR were used with 16 sheep RNA samples from different foetal stages, neonatal sheep and mouse skin. The mRNAs for these growth factors were detected throughout gestation in sheep skin, except for TGF beta 1 mRNA which was not expressed in 51-day-old skin, but was expressed in 54-day and older samples. Since the first microscopically visible changes of follicle initiation occur around 62 days gestation, these results suggest that TGF beta 1 expression may be a signal for follicle initiation.

Sheep vibrissa dermal papillae induce hair follicle formation in heterotypic skin equivalents

Cultured skin equivalents were constructed by combining keratinocytes, outer root sheath cells or isolated epidermis, in vitro, with a matrix composed of collagen and cultured fibroblasts. When equivalents were grafted on to host animals, the epidermis thickened considerably, and tongues of cells penetrated the dermis, giving the dermal/epidermal junction a deeply sculptured profile. No cutaneous appendages were found in these grafts. We explored the possibility of inducing hair follicles by incorporating ovine hair follicle dermal papillae into constructs composed of an isolated epidermal sheet and a contracted dermal equivalent. In vitro, no morphogenetic changes associated with follicle formation were observed in the recombinants, but when grafted on to nude mice, follicle-like structures were identified. The follicles were large, and had developed adjacent to the epidermis, indicating that the matrix environment of the induced follicles may not have been compatible with the downgrowth of the epidermal plugs normally observed during follicle formation in living skin. Nevertheless, in histological sections, the induced structures displayed many of the morphological characteristics of follicles in vivo, including the production of keratinized hairs. These results indicate that skin equivalents provide a useful model for the study of the chemical and structural features of matrices that facilitate hair follicle development.

Follicular origin of epidermal papillomas in v-Ha-ras transgenic TG.AC mouse skin

A follicular origin for some skin tumors has been hypothesized in both humans and animal models. Because of its rapid and sensitive response to tumor promoter treatment, a v-Ha-ras transgenic (TG.AC) mouse line was used to determine the origins of epidermal papillomas. Using histological studies and transgene expression as a marker for papilloma development, we determined that pedunculated papillomas arose from focal hyperplasias of the permanent portion of the follicular epithelium in phorbol 12-myristate 13-acetate-treated TG.AC mouse skin. Damage to the hair follicle by depilation was also sufficient to induce papillomas that were histologically indistinguishable from those produced by chemical exposure. Identification of the cellular origins of papillomas in this transgenic mouse model will allow for an analysis of the role of the hair follicle and hair cycle-associated signaling in tumor development.

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)

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.

Effects of EGF on the morphology and patterns of DNA synthesis in isolated human hair follicles

We have previously reported that human hair grows at a normal rate in vitro for up to 10 d. We have also reported that, on gross observation, epidermal growth factor appears to induce a catagen-like effect on cultured hair follicles, but we have not characterized the details of this. We now report that when isolated human hair follicles are maintained in the presence of epidermal growth factor, the rate of hair follicle elongation is significantly stimulated but hair fiber production is inhibited. Light microscopy showed that epidermal growth factor stimulated a thickening and vacuolation of the cells of the lower outer root sheath of the hair follicle and that the matrix cells of the hair follicle underwent an upward migration resulting in the formation of a 'club hair'-like structure that remained connected to the dermal papilla by a thin strand of epithelial cells. [Methyl-3H] thymidine autoradiography was carried out to investigate the patterns of DNA synthesis and showed that epidermal growth factor inhibited DNA synthesis in the hair follicle matrix cells but dramatically stimulated DNA synthesis in the outer root sheath. We conclude from these studies that epidermal growth factor may be inducing an artificial 'catagen-like' effect by stimulating outer root sheath proliferation, which uncouples the normal patterns of proliferation and migration that occur in the anagen hair follicle and that result in an anagen-to-catagen-like transition. Moreover, these results also suggest that, under certain conditions, outer root sheath cells in the hair follicle may be capable of downward migration.

Distribution of acidic and basic fibroblast growth factors in ovine skin during follicle morphogenesis

Acidic and basic fibroblast growth factors (aFGF and bFGF) have been localized by immunochemistry in ovine skin during wool follicle morphogenesis. At 40 days of gestation, prior to the appearance of follicle primordia, bFGF immunoreactivity was detected in the intermediate and periderm layers of the epidermis and at the dermal-epidermal junction. Antibodies to aFGF did not bind to skin at this age. During early follicle formation, at 76 days of gestation, both FGFs were found in the epidermis and associated with the follicle primordia. Antibodies to aFGF, in particular, bound to the basal cells of the epidermis and the follicle cell aggregations. With the development of epidermal plugs, bFGF was confined to the intermediate layers of the epidermis and the dermal-epidermal junction, whereas aFGF staining was associated with the cells of the epidermis and the plugs. At 90 days, when many different stages of follicle development were in evidence, immunoreactivity for both FGFs was associated with the cells of the elongating epidermal column, particularly those adjacent to the dermal-epidermal junction. During follicle maturation, bFGF was found in the suprabasal layer of the epidermis, in the outer root sheath of the follicle and in the basement membrane zone surrounding the bulb matrix. Conversely, strong staining for aFGF was observed in the epidermis and pilary canal contiguous with the epidermis, and in cells of the upper bulb matrix of the follicle in the region of the keratogenous zone. Western blotting of extracts of mature follicles that had been isolated from the skin showed the presence of a major aFGF immunoreactive band with an apparent molecular mass of 27 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

Fibroblast growth factor and epidermal growth factor in hair development

Hair follicles arise in developing skin as a result of a complex of interactions that are likely to be mediated by diffusible, cell- and matrix-bound factors. Growth factors such as fibroblast growth factor (FGF) and epidermal growth factor (EGF) have been implicated in the control of epidermal and mesenchymal cell function, and it is likely that they also affect proliferation and differentiation of the cells of the cutaneous appendages during development. Immunolocalization of basic FGF adjacent to areas of proliferation in developing and in mature follicles suggests that this factor may regulate the mitotic activity of epithelially-derived cells; acidic FGF, on the other hand, appears in the differentiating cells of the follicle bulb and may therefore participate in the formation of structural components of the follicle or of the fiber. EGF has been identified as a potent modulator of cellular growth and is also present during follicle differentiation. These factors may act through autocrine and paracrine mechanisms because their receptors are also found on epidermally derived and mesenchymal structures in the skin. We have studied the effects of these growth factors on hair follicle development in the newborn mouse. Daily injections for 1 week after birth resulted in significant changes in the morphogenesis of the hair follicle population. Histologic examination of skin of FGF-treated mice suggested that the growth factor had affected hair follicle initiation and development, which resulted in a significant delay in the first and subsequent hair cycles when compared to control animals. Because aFGF and bFGF are not readily diffusible, these effects remained confined to the area of treatment. In contrast, EGF affected the whole body coat of the treated animals, induced hyperkeratinization of the skin, and caused a significant delay in hair follicle development.