VEGETATIVE GROWTH OF ACETABULARIA ACETABULUM (CHLOROPHYTA): STRUCTURAL EVIDENCE FOR JUVENILE AND ADULT PHASES IN DEVELOPMENT1

We characterized vegetative development in two inbred cell lines of Acetabularia acetabulum (L.) Silva. Cell growth occurred at the apex and by elongation of older interwhorls throughout vegetative development. Although cell length and hairs per whorl increased regularly during development, interwhorl length, hair persistence on the stalk, and complexity of each whorl (degree of branching of whorl hairs) showed sharp discontinuities during development in both cell lines. The first (earliest) discontinuity, formation of a short interwhorl, was the sixth interwhorl made in all cells. Even though cell line Aa1055 was twice the height ofAa4010 when mature, cells in both lines were 0.8–1.0 cm tall after formation of the short interwhorl. The second discontinuity, increases in hair persistence on the stalk and complexity of each whorl of hairs, began shortly before cap initiation. We propose the following nomenclature: 1) that slower growth before formation of the short interwhorl be called “juvenile”; 2) that more rapid growth after formation of the short interwhorl be called “adult”; and 3) that adult growth be separated into “early” and “late” phases by the discontinuities in whorl hair characteristics. The proposed developmental phases (juvenile, early adult, and late adult) are temporally sequential and spatially stacked.     

Molecular genetics of the hair follicle: the state of the art

For those who are interested in the biology of skin and its derivatives, these are interesting times indeed. In a mere 5 years, the field has been revolutionized by the application of molecular genetics to human congenital skin disorders. Where dermatology first was limited to observation and empirics, there are now DNA-diagnostics, rational drug design, and perhaps even gene therapy available soon. In particular, the study of rare human syndromes involving abnormalities of hair growth and structure has yielded new insights into the regulation of cell growth and differentiation in the hair follicle. As this structure shows a cyclic pattern of differentiation, it may give new information concerning the regulation of cell differentiation in general. This review covers the recent developments in this fast-moving field. First, we will give a short introduction to (structural) hair biology. Next, we will try to fit these data into the framework of what is already known and attempt to present a unified model for hair follicle growth and differentiation.     

The adaptive response of human skin to the savanna

In the early stage of human evolution, as the hominids began to inhabit the savanna mosaic in Africa some three or four million years ago, a functional complex of skin features contributed to their effective exploitation of resources and survival in the new environment. Thermal radiation from the sun combined with internally generated heat from muscular effort posed problems of thermoregulation. As a mechanism for dissipating body heat and maintaining brain temperature, eccrine sweat glands throughout the body surface combined with reduction in body hair enhanced the evaporative cooling effects of sweating. As body hair diminished, deeply pigmented skin was selected for as a protection against harmful ultraviolet radiation. When human populations left the equatorial regions of Africa, the adaptive significance of deeply pigmented skin may have shifted in response to other factors, such as latitude, diet and cultural pratices. We view the structure and function of human skin within a comparative and evolutionary framework that focuses on the environment in which the hominids evolved.     

Distinct mechanisms underlie pattern formation in the skin and skin appendages

Patterns form with the break of homogeneity and lead to the emergence of new structure or arrangement. There are different physiological and pathological mechanisms that lead to the formation of patterns. Here, we first introduce the basics of pattern formation and their possible biological basis. We then discuss different categories of skin patterns and their potential underlying molecular mechanisms. Some patterns, such as the lines of Blaschko and Naevus, are based on cell lineage and genetic mosaicism. Other patterns, such as regionally specific skin appendages, can be set by distinct combinatorial molecular codes, which in turn may be set by morphogenetic gradients. There are also some patterns, such as the arrangement of hair follicles (hair whorls) and fingerprints, which involve genetics as well as stochastic epigenetic events based on physiochemical principles. Many appendage primordia are laid out in developmental waves. In the adult, some patterns, such as those involving cycling hair follicles, may appear as traveling waves in mice. Since skin appendages can renew themselves in regeneration, their size and shape can still change in the adult via regulation by hormones and the environment. Some lesion patterns are based on pathological changes involving the above processes and can be used as diagnostic criteria in medicine. Understanding the different mechanisms that lead to patterns in the skin will help us appreciate their full significance in morphogenesis and medical research. Much remains to be learned about complex pattern formation, if we are to bridge the gap between molecular biology and organism phenotypes.     

Human pigmentation genetics: the difference is only skin deep

There is no doubt that visual impressions of body form and color are important in the interactions within and between human communities. Remarkably, it is the levels of just one chemically inert and stable visual pigment known as melanin that is responsible for producing all shades of humankind. Major human genes involved in its formation have been identified largely using a comparative genomics approach and through the molecular analysis of the pigmentary process that occurs within the melanocyte. Three classes of genes have been examined for their contribution to normal human color variation through the production of hypopigmented phenotypes or by genetic association with skin type and hair color. The MSH cell surface receptor and the melanosomal P-protein are the two most obvious candidate genes influencing variation in pigmentation phenotype, and may do so by regulating the levels and activities of the melanogenic enzymes tyrosinase, TRP-1 and TRP-2. BioEssays 20 :712–721, 1998.© 1998 John Wiley & Sons, Inc.     

Tyrosinase Depletion Prevents the Maturation of Melanosomes in the Mouse Hair Follicle

The mechanisms that lead to variation in human skin and hair color are not fully understood. To better understand the molecular control of skin and hair color variation, we modulated the expression of Tyrosinase (Tyr), which controls the rate-limiting step of melanogenesis, by expressing a single-copy, tetracycline-inducible shRNA against Tyr in mice. Moderate depletion of TYR was sufficient to alter the appearance of the mouse coat in black, agouti, and yellow coat color backgrounds, even though TYR depletion did not significantly inhibit accumulation of melanin within the mouse hair. Ultra-structural studies revealed that the reduction of Tyr inhibited the accumulation of terminal melanosomes, and inhibited the expression of genes that regulate melanogenesis. These results indicate that color in skin and hair is determined not only by the total amount of melanin within the hair, but also by the relative accumulation of mature melanosomes.     

Analysis of the expression pattern of involucrin in human scalp skin and hair follicles: hair cycle-associated alterations

Involucrin is a structural component of the keratinocyte cornified envelope that is expressed early in the keratinocyte differentiation process. It is a component of the initial envelope scaffolding and considered as a marker for keratinocyte terminal differentiation. The expression pattern of involucrin in human scalp skin and hair follicle cycle stages is not fully explored. This study addresses this issue and tests the hypothesis that "the expression of involucrin undergoes hair follicle cycle-dependent changes". A total of 50 normal human scalp skin biopsies were examined (healthy females, 51-62?years) using immunofluorescence staining methods and real-time PCR analysis. In each case, 50 hair follicles were analyzed (35, 10 and 5 follicles in anagen, catagen and telogen, respectively). Involucrin was prominently expressed in the human scalp skin and hair follicles, on both gene and protein levels. The protein expression showed hair follicle cycle-associated changes i.e. a very strong expression during early and mature anagen, intermediate to strong expression during catagen and prominent decline in the telogen phase. The expression value of involucrin in both anagen and catagen was statistically significantly higher than that of telogen hair follicles (p?相似文献   

Whorl Formation in Polysphondylium violaceum: Relevance to Organization by Cyclic AMP

Whorl formation in Polysphondylium is a simple and good system for the study of pattern formation. The first step of whorl formation is characterized by separation of the rear cell mass from an advancing primary mass during culmination. Using the iontophoresis method, it has been shown that after the establishment of multicellular organization cells respond chemotactically to 3',5' -cyclic adenosine monophosphate (cAMP), but not to glorin, the chemoattaractant at the aggregating stage. P. violaceum cell masses were also found to secrete actively cAMP. Therefore, morphogenetic movement of P. violaceum cells after aggregation could be controlled mainly by the cAMP signalling system. Vital staining of cells with neutral red (NR) revealed that there are anterior-like cells stained well with NR in the posterior region of a migrating P. violaceum slug, and that the staining pattern changes markedly during whorl formation. Just before separation of the whorl mass, the anterior-like cells altered their distribution, and eventually were arranged as an equatorial band on the surface of the presumptive whorl mass, which probably would turn to organizing tip cells of the secondary masses left on the primary stalk. Thus whorl formation may be caused by separation of the strongest cAMP-source into two regions; the primary (anterior) and secondary (posterior) tips.     

The role of a reaction-diffusion system in the formation of hair fibres

Epithelial cells destined to form the hair fibre begin to differentiate while still in the hair follicle bulb. The fibre cells continue to differentiate as they migrate out of the bulb and up the follicle towards the skin surface. The anatomy of the hair follicle and the different cell types observed within the follicle are briefly reviewed. A theoretical scheme for cell differentiation, capable of producing all the observed mature cell types, is presented. A major component of the scheme is a reaction-diffusion system of morphogens similar to that originally proposed by Turing (1952). The mathematical solution of the equations defining the reaction-diffusion system within the follicle bulb is discussed. The sequence of patterns in the spatial distribution of the morphogens expected in the hair follicle bulb is calculated and found to be in good agreement with the sequence of patterns of orthocortical and paracortical cells in the fibre cross section as the diameter of the fibre increases. The spatial patterns of the morphogens are also compared with the shape of the fibre cross section. It is concluded that a reaction-diffusion system may play a major role in the morphogenesis of hair fibres.     

Branching in Polysphondylium whorls: Two-dimensional patterning in a three-dimensional system

During morphogenesis in the slime mold Polysphondylium pallidum, spherical masses of cells called whorls pinch off from the slug at regular intervals. Soon afterward, branches form at equidistant positions around the whorl equator. We have quantified the relationship between the number of cells in a whorl, and the number of branches that form. We find that the number of branches produced is proportional to the surface area of the whorl, suggesting that the patterning process is confined to the whorl surface. This observation is consistent with theoretical arguments that mechanisms for pattern formation would likely operate in one or two dimensions, not three.     

The Distribution of Polycomb-Group Proteins During Cell Division and Development in Drosophila Embryos: Impact on Models for Silencing

The type I keratin 17 (K17) shows a peculiar localization in human epithelial appendages including hair follicles, which undergo a growth cycle throughout adult life. Additionally K17 is induced, along with K6 and K16, early after acute injury to human skin. To gain further insights into its potential function(s), we cloned the mouse K17 gene and investigated its expression during skin development. Synthesis of K17 protein first occurs in a subset of epithelial cells within the single-layered, undifferentiated ectoderm of embryonic day 10.5 mouse fetuses. In the ensuing 48 h, K17-expressing cells give rise to placodes, the precursors of ectoderm-derived appendages (hair, glands, and tooth), and to periderm. During early development, there is a spatial correspondence in the distribution of K17 and that of lymphoid-enhancer factor (lef-1), a DNA-bending protein involved in inductive epithelial–mesenchymal interactions. We demonstrate that ectopic lef-1 expression induces K17 protein in the skin of adult transgenic mice. The pattern of K17 gene expression during development has direct implications for the morphogenesis of skin epithelia, and points to the existence of a molecular relationship between development and wound repair.     

Analysis of the expression pattern of the carrier protein transthyretin and its receptor megalin in the human scalp skin and hair follicles: hair cycle-associated changes

Transthyretin is a serum and cerebrospinal fluid protein synthesized early in development by the liver, choroid plexus and several other tissues. It is a carrier protein for the antioxidant vitamins, retinol, and thyroid hormones. Transthyretin helps internalize thyroxine and retinol-binding protein into cells by binding to megalin, which is a multi-ligand receptor expressed on the luminal surface of various epithelia. We investigated the expression of transthyretin and its receptor megalin in the human skin; however, their expression pattern in the hair follicle is still to be elucidated. This study addresses this issue and tests the hypothesis that “the expression of transthyretin and megalin undergoes hair follicle cycle-dependent changes.” A total of 50 normal human scalp skin biopsies were examined (healthy females, 53–62 years) using immunofluorescence staining methods and real-time PCR. In each case, 50 hair follicles were analyzed (35, 10, and 5 follicles in anagen, catagen, and telogen, respectively). Transthyretin and megalin were prominently expressed in the human scalp skin and hair follicles, on both gene and protein levels. The concentrations of transthyretin and megalin were 0.12 and 0.03 Ul/ml, respectively, as indicated by PCR. The expression showed hair follicle cycle-associated changes i.e., strong expression during early and mature anagen, very weak expression during catagen and moderate expression during telogen. The expression values of these proteins in the anagen were statistically significantly higher than those of either catagen or telogen hair follicles (P ≤ 0.001). This study provides the first morphologic indication that transthyretin and megalin are variably expressed in the human scalp skin and hair follicles. It also reports variations in the expression of these proteins during hair follicle cycling. The clinical ramifications of these findings are open for further investigations.     

Intragenic deletion in the Desmoglein 4 gene underlies the skin phenotype in the Iffa Credo "hairless" rat

The Iffa Credo (IC) "hairless" rat is an autosomal recessive hypotrichotic animal model actively used in pharmacological and dermatological studies. Although the molecular basis of the IC rat phenotype was never defined, the designation "hr/hr" (hairless) has been used for this rat mutation. Despite the observation that IC rats share many phenotypic similarities with Charles River (CR) 'hairless rats', crossbreeding between CR and IC rats indicated that these mutations are not allelic, and moreover, genetic analysis of both CR and IC hairless mutant rats showed no mutations in the hr gene. Here, we present a detailed analysis of the skin phenotype in the IC rat. While the initial stages of hair follicle (HF) morphogenesis reveal no significant abnormalities, the subsequent processes of inner root sheath and hair shaft formation are severely disturbed due to impaired proliferation in the hair matrix and abnormal differentiation in the precortex zone. This results in significant reduction of hair bulb volume, and the formation of dysmorphic "blebbed" hair shafts lacking medullar structure and resembling "lanceolate" hairs. Based on the presence of lance-head hairs typical of rodent lanceolate mutants, we performed molecular analysis of the desmoglein 4 gene and found a large intragenic deletion encompassing nine exons of the gene. This finding, together with specific morphological features of skin and hairs, confirms that the IC rat is allelic with the lanceolate hair (lah) mutations in mice and rats. Our results elucidate the genetic and morphological basis of the IC rat mutation, thus providing a new model to study molecular mechanisms of hair growth control.     

Studies on the tibiotarsal chaetotaxy of Collembola

Abstract. A system of identifying and homologizing the tibiotarsal setae of Collembola by means of letters and numerals is described. These setae are arranged in a series of whorls, each whorl consisting of up to seven setae. The whorls are identified by letters (A, B, etc.), whorl A being the most distal. The setae are numbered from the medio-dorsal surface and over the anterior face, that is, clockwise for left legs but anticlockwise for right legs. The setal pattern appears to be constatn within some genera, and may be of phylogenetic significance.     

The human cysteine protease cathepsin V can compensate for murine cathepsin L in mouse epidermis and hair follicles

Mice lacking the ubiquitously expressed lysosomal cysteine protease cathepsin L, show a complex skin phenotype consisting of periodic hair loss and epidermal hyperplasia with hyperproliferation of basal epidermal keratinocytes, acanthosis and hyperkeratosis. The recently identified human cathepsin L-like enzyme cathepsin V, which is also termed cathepsin L2, is specifically expressed in cornea, testis, thymus, and epidermis. To date, in mice no cathepsin V orthologue with this typical expression pattern has been identified. Since cathepsin V has about 75% protein sequence identity to murine cathepsin L, we hypothesized that transgenic, keratinocyte-specific expression of cathepsin V in cathepsin L knockout mice might rescue the skin and hair phenotype. Thus, we generated a transgenic mouse line expressing cathepsin V under the control of the human keratin 14 promoter, which mimics the genuine cathepsin V expression pattern in human skin, by directing it to basal epidermal keratinocytes and the outer root sheath of hair follicles. Subsequently, transgenic mice were crossed with congenic cathepsin L knockout animals. The resulting mice show normalization of epidermal proliferation and normal epidermal thickness as well as rescue of the hair phenotype. These findings provide evidence for keratinocyte-specific pivotal functions of cathepsin L-like proteolytic activities in maintenance of epidermis and hair follicles and suggest, that cathepsin V may perform similar functions in human skin.     

De novo formation and ultra-structural characterization of a fiber-producing human hair follicle equivalent in vitro

Across many tissues and organs, the ability to create an organoid, the smallest functional unit of an organ, in vitro is the key both to tissue engineering and preclinical testing regimes. The hair follicle is an organoid that has been much studied based on its ability to grow quickly and to regenerate after trauma. But hair follicle formation in vitro has been elusive. Replacing hair lost due to pattern baldness or more severe alopecia, including that induced by chemotherapy, remains a significant unmet medical need.By carefully analyzing and recapitulating the growth conditions of hair follicle formation, we recreated human hair follicles in tissue culture that were capable of producing hair. Our microfollicles contained all relevant cell types and their structure and orientation resembled in some ways excised hair follicle specimens from human skin. This finding offers a new window onto hair follicle development. Having a robust culture system for hair follicles is an important step towards improved hair regeneration as well as to an understanding of how marketed drugs or drug candidates, including cancer chemotherapy, will affect this important organ.     

Essential roles of BMPR-IA signaling in differentiation and growth of hair follicles and in skin tumorigenesis

Hair differentiation and growth are controlled by complex reciprocal signaling between epithelial and mesenchymal cells. To better understand the requirement and molecular mechanism of BMP signaling in hair follicle development, we performed genetic analyses of bone morphogenetic protein receptor 1A (BMPR-IA) function during hair follicle development by using a conditional knockout approach. The conditional mutation of Bmpr1a in ventral limb ectoderm and its derivatives (epidermis and hair follicles) resulted in a lack of hair outgrowth from the affected skin regions. Mutant hair follicles exhibited abnormal morphology and lacked hair formation and pigment deposition during anagen. The timing of the hair cycle and the proliferation of hair matrix cells were also affected in the mutant follicles. We demonstrate that signaling via epithelial BMPR-IA is required for differentiation of both hair shaft and inner root sheath from hair matrix precursor cells in anagen hair follicles but is dispensable for embryonic hair follicle induction. Surprisingly, aberrant de novo hair follicle morphogenesis together with hair matrix cell hyperplasia was observed in the absence of BMPR-IA signaling within the affected skin of adult mutants. They developed hair follicle tumors from 3 months of age, indicating that inactivation of epidermal BMPR-IA signaling can lead to hair tumor formation. Taken together, our data provide genetic evidence that BMPR-IA signaling plays critical and multiple roles in controlling cell fate decisions or maintenance, proliferation, and differentiation during hair morphogenesis and growth, and implicate Bmpr1a as a tumor suppressor in skin tumorigenesis.