After the completion of skin development, angiogenesis, i.e., the growth of new capillaries from pre-existing blood vessels, is held to occur in the skin only under pathologic conditions. It has long been noted, however, that hair follicle cycling is associated with prominent changes in skin perfusion, that the epithelial hair bulbs of anagen follicles display angiogenic properties, and that the follicular dermal papilla can produce angiogenic factors. Despite these suggestive observations, no formal proof is as yet available for the concept that angiogenesis is a physiologic event that occurs all over the mature mammalian integument whenever hair follicles switch from resting (telogen) to active growth (anagen). This study uses quantitative histomorphometry and double-immunohistologic detection techniques for the demarcation of proliferating endothelial cells, to show that synchronized hair follicle cycling in adolescent C57BL/6 mice is associated with substantial angiogenesis, and that inhibiting angiogenesis in vivo by the intraperitoneal application of a fumagillin derivative retards experimentally induced anagen development in these mice. Thus, angiogenesis is a physiologic event in normal postnatal murine skin, apparently is dictated by the hair follicle, and appears to be required for normal anagen development. Anagen-associated angiogenesis offers an attractive model for identifying the physiologic controls of cutaneous angiogenesis, and an interesting system for screening the effects of potential antiangiogenic drugs in vivo.

Numerous spontaneous and experimentally induced mouse mutations develop a hair phenotype, which is often associated with more or less discrete abnormalities in hair follicle development. In order to recognize these, it is critically important to be able to determine and to classify accurately the major stages of normal murine hair follicle morphogenesis. As an aid, we propose a pragmatic and comprehensive guide, modified after previous suggestions by Hardy, and provide a list of easily recognizable classification criteria, illustrated by representative micrographs. Basic and more advanced criteria are distinguished, the former being applicable to all mouse strains and requiring only simple histologic stains (hematoxylin and eosin, Giemsa, periodic acid Schiff, alkaline phosphatase activity), the latter serving as auxiliary criteria, which require a pigmented mouse strain (like C57BL/6J) or immunohistochemistry (interleukin-1 receptor type I, transforming growth factor-beta receptor type II). In addition, we present simplified, computer-generated schematic drawings for the standardized recording and reporting of gene and antigen expression patterns during hair follicle development. This classification aid serves as a basic introduction into the field of hair follicle morphogenesis, aims at standardizing the presentation of related hair research data, and should become a useful tool when screening new mouse mutants for discrete abnormalities of hair follicle morphogenesis (compared with the respective wild type) in a highly reproducible, easily applicable, and quantifiable manner.

We have previously shown that the ubiquitously expressed lysosomal cysteine protease, cathepsin L (CTSL), is essential for skin and hair follicle homeostasis. Here we examine the effect of CTSL deficiency on hair follicle development and cycling in ctsl ؊/؊ mice by light and electron microscopy, Ki67/ terminal dUTP nick-end labeling, and trichohyalin immunofluorescence. Hair follicle morphogenesis in ctsl ؊/؊ mice was associated with several abnormalities. Defective terminal differentiation of keratinocytes occurred during the formation of the hair canal, resulting in disruption of hair shaft outgrowth. Both proliferation and apoptosis levels in keratinocytes and melanocytes were higher in ctsl ؊/؊ than in ctsl ؉/؉ hair follicles. The development of the hair follicle pigmentary unit was disrupted by vacuolation of differentiating melanocytes. Hair cycling was also abnormal in ctsl ؊/؊ mice. Final stages of hair follicle morphogenesis and the induction of hair follicle cycling were retarded. Thereafter, these follicles exhibited a truncated resting phase (telogen) and a premature entry into the first growth phase. Further abnormalities of telogen development included the defective anchoring of club hairs in the skin, which resulted in their abnormal shedding. Melanocyte vacuolation was again apparent during the hair cycleassociated reconstruction of the hair pigmentary unit. A hallmark of these ctsl ؊/؊ mice was the severe disruption in the exiting of hair shafts to the skin surface. This was mostly because of a failure of the inner root sheath (keratinocyte layer next to the hair shaft) to fully desquamate. These changes resulted in a massive dilation of the hair canal and the abnormal routing of sebaceous gland products to the skin surface. In summary, this study suggests novel roles for cathepsin proteases in skin, hair, and pigment biology. Principal target tissues that may contain protein substrate(s) for this cysteine protease include the developing hair cone, inner root sheath, anchoring apparatus of the telogen club, and organelles of lysosomal origin (eg, melanosomes). The hair follicle (HF) is a unique neuroectodermal-mesodermal interactive organoid that results in the elaboration of at least 15 distinct interacting cell subpopulations, organized into five or six concentric cylinders.

In mice, rats, and humans, loss of function of Foxn1, a member of the winged helix/forkhead family of transcription factors, leads to macroscopic nudity and an inborn dysgenesis of the thymus. Nude (Foxn1 nu /Foxn1 nu ) mice develop largely normal hair follicles and produce hair shafts. However, presumably because of a lack of certain hair keratins, the hair shafts that are generated twist and coil in the hair follicle infundibulum, which becomes dilated. Since hair shafts fail to penetrate the epidermis, macroscopic nudity results and generates thegrossly misleading -impression that nude mice are hairless. Here, we provide an overview of what is known on the role of Foxn1 in mammalian skin biology, its expression patterns in the hair follicle, its influence on hair follicle function, and onychocyte differentiation. We focus on the mechanisms and signaling pathways by which Foxn1 modulates keratinocyte differentiation in the hair follicle and nail apparatus and summarize the current knowledge on the molecular and functional consequences of a loss of function of the Foxn1 protein in skin. Foxn1 target genes, gene regulation of Foxn, and pharmacological manipulation of the nude phenotype (e.g. by cyclosporine A, KGF, and vitamin D3) are discussed, and important open questions as well as promising research strategies in Foxn1 biology are defined. Taken together, this review aims at delineating why enhanced research efforts in this comparatively neglected field of investigative dermatology promise important new insights into the controls of epithelial differentiation in mammalian skin.

Although 17beta-estradiol (E2) is recognized as a potent hair growth modulator, our knowledge of estrogen function, signaling, and target genes in hair biology is still very limited. Between the two recognized estrogen receptors (ERs), ER alpha and ER beta, only ER alpha had been detected in murine skin. Here we show that ER alpha, ER beta, and ER beta ins are all expressed throughout the murine hair cycle, both at the protein and RNA level, but show distinct expression patterns. We confirm that topical E2 arrests murine pelage hair follicles in telogen and demonstrate that E2 is a potent inducer of premature catagen development. The ER antagonist ICI 182.780 does not induce anagen prematurely but accelerates anagen development and wave spreading in female mice. ER beta knockout mice display accelerated catagen development along with an increase in the number of apoptotic hair follicle keratinocytes. This suggests that, contrary to previous concepts, ER beta does indeed play a significant role in murine hair growth control: whereas the catagen-promoting properties of E2 are mediated via ER alpha, ER beta mainly may function as a silencer of ER alpha action in hair biology. These findings illustrate the complexity of hair growth modulation by estrogens and suggest that one key to more effective hair growth manipulation with ER ligands lies in the use of selective ER alpha or -beta antagonists/agonists. Our study also underscores that the hair cycling response to estrogens offers an ideal model for studying the controls and dynamics of wave propagation in biological systems.

Nude mice have a mutation in the transcription factor Foxn1(nu), resulting in downregulation of hair keratins. Although hair follicles develop normally, the hair fibers become structurally weak, curl, and break off at the surface. Nails in nude mice are deformed, based on alterations of the onychocyte differentiation process. Elemental microanalysis of the nail plate reveals marked decreases in sulfur concentrations in the nude mouse nail plates. Immunohistochemistry shows a lack of keratin 1 expression in terminally differentiating keratinocytes of the nail matrix. Instead, the typical differentiation process of the matrix is altered toward an epidermis-like differentiation pattern, comprising the production of filaggrin-containing keratohyalin granules in cells resembling those of the stratum granulosum, which are never observed in normally haired mice. The nail plate has diffuse basophilic stippling. It is thinner than normal, weak, and in most Foxn1(nu)/Foxn1(nu) mice breaks where it separates from the hyponychium. These studies indicate that the Foxn1(nu) mutated gene has effects beyond downregulating keratin expression, including changes in filaggrin expression, and is critical for normal onycholemmal differentiation. The nails of nude mice provide new insights into the molecular controls of onychocyte differentiation, and they offer a useful model to investigate the pathogenesis of nail hypergranulosis, a common feature in human nail diseases.

The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP), and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying lesions observed in the soft tissues including skeletal muscle as well as the mesothelium of rats and mice. The standardized nomenclature of lesions presented in this document is also available electronically on the Internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous developmental and aging lesions as well as those induced by exposure to test materials. A widely accepted and utilized international harmonization of nomenclature for lesions in soft tissues, skeletal muscle and mesothelium in laboratory animals will decrease confusion among regulatory and scientific research organizations in different countries and provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists. (DOI: 10.1293/tox.26.1S; J Toxicol Pathol 2013; 26: 1S–26S)

Hair follicle morphogenesis and cycling were examined in transgenic mice that overexpress the bone morphogenetic protein (BMP) inhibitor Noggin under the control of the neuron-specific enolase promoter. The Noggin transgene was misexpressed in the proximal portion of the hair follicle, primarily the matrix cells, apart from the usual expression in neurons. Transgene expression appeared only after induction of both the primary (tylotrich) and secondary (nontylotrich) pelage hair follicles had already occurred, thus allowing examination of the role of BMP signaling in follicles that had been induced normally in the presence of BMPs. The overexpression of Noggin in these animals resulted in a dramatic loss of hair postnatally. There was an apparently normal, but shortened period of postnatal hair follicle morphogenesis, followed by premature initiation of hair follicle cycling via entry into the first catagen transformation. This resulted in a complete loss of hair shafts from the nontylotrich hair follicles in these mice while the tylotrich hair follicles were normal. The onset of anagen of the first postnatal hair follicle cycle was also accelerated in the transgenic mice. Our results show that BMP signaling is specifically required for proper proliferation and differentiation during late morphogenesis of nontylotrich hair follicles and that inhibition of this signaling pathway may be one of the triggers for the onset of catagen when the follicles are in anagen and the onset of anagen when the follicles are in telogen. Ectopic sebocyte differentiation was another hallmark of the phenotype of these transgenic mice suggesting that BMP signaling may be an important determinant of lineage selection by com- Hair follicle development involves a highly coordinated series of bidirectional epithelial-mesenchymal interactions. Because hair follicle morphogenesis is significantly affected by the sensory and autonomic innervation as well as the pigment-producing melanocytes, this miniorgan has been viewed as an epithelial-mesenchymalneuroectodermal unit.1,2 Hair follicle induction occurs with the appearance of a thickening of the embryonic ectoderm called the ectodermal placode as a result of an initial mesodermal signal. The placode signals condensation of the underlying mesoderm to form the future dermal papilla.3,4 A second signal from the condensed mesoderm then induces proliferation of the overlying ectodermal placode allowing it to grow downwards eventually surrounding the mesenchymal condensation. The latter forms the dermal papilla, the permanent mesenchymal component of the hair follicle. The basic structure of the mature hair follicle then forms by systematic differentiation of the proliferating keratinocytes, producing the outer root sheath (ORS), which is contiguous with the basal layer of the epidermis, three concentric cylinders constituting the inner root sheath (IRS) and the medulla, cortex, and cuticle of the hair shaft at the center. The sebaceous gland soon develops as an appendage to the upper part of the...