Cell-cell interactions during the formation of primordial follicles in humans

Gametogenesis is a complex and sex-specific multistep process during which the gonadal somatic niche plays an essential regulatory role. One of the most crucial steps during human female gametogenesis is the formation of primordial follicles, the functional unit of the ovary that constitutes the pool of follicles available at birth during the entire reproductive life. However, the relation between human fetal germ cells (hFGCs) and gonadal somatic cells during the formation of the primordial follicles remains largely unexplored. We have discovered that hFGCs can form multinucleated syncytia, some connected via interconnecting intercellular bridges, and that not all nuclei in hFGC-syncytia were synchronous regarding meiotic stage. As hFGCs progressed in development, pre-granulosa cells formed protrusions that seemed to progressively constrict individual hFGCs, perhaps contributing to separate them from the multinucleated syncytia. Our findings highlighted the cell-cell interaction and molecular dynamics between hFGCs and (pre)granulosa cells during the formation of primordial follicles in humans. Knowledge on how the pool of primordial follicle is formed is important to understand human infertility.

Biocompatibility of a fish scale-derived artificial cornea: Cytotoxicity, cellular adhesion and phenotype, and in vivo immunogenicity

PURPOSE

To determine whether a fish scale-derived collagen matrix (FSCM) meets the basic criteria to serve as an artificial cornea, as determined with in vitro and in vivo tests.

METHODS

Primary corneal epithelial and stromal cells were obtained from human donor corneas and used to examine the (in)direct cytotoxicity effects of the scaffold. Cytotoxicity was assessed by an MTT assay, while cellular proliferation, corneal cell phenotype and adhesion markers were assessed using an EdU-assay and immunofluorescence. For in vivo-testing, FSCMs were implanted subcutaneously in rats. Ologen(®) Collagen Matrices were used as controls. A second implant was implanted as an immunological challenge. The FSCM was implanted in a corneal pocket of seven New Zealand White rabbits, and compared to sham surgery.

RESULTS

The FSCM was used as a scaffold to grow corneal epithelial and stromal cells, and displayed no cytotoxicity to these cells. Corneal epithelial cells displayed their normal phenotypical markers (CK3/12 and E-cadherin), as well as cell-matrix adhesion molecules: integrin-α6 and β4, laminin 332, and hemi-desmosomes. Corneal stromal cells similarly expressed adhesion molecules (integrin-α6 and β1). A subcutaneous implant of the FSCM in rats did not induce inflammation or sensitization; the response was comparable to the response against the Ologen(®) Collagen Matrix. Implantation of the FSCM in a corneal stromal pocket in rabbits led to a transparent cornea, healthy epithelium, and, on histology, hardly any infiltrating immune cells.

CONCLUSION

The FSCM allows excellent cell growth, is not immunogenic and is well-tolerated in the cornea, and thus meets the basic criteria to serve as a scaffold to reconstitute the cornea.

Electron diffraction study of lipids in non-lesional stratum corneum of atopic eczema patients

Skin barrier impairment is thought to be an important factor in the pathogenesis of atopic eczema (AE). The skin barrier is located in the stratum corneum (SC), consisting of corneocytes embedded in lipids. Ceramides, cholesterol and free fatty acids are the major lipid classes and are crucial for the skin barrier function, but their role in relation to AE is indistinct. Filaggrin is an epidermal barrier protein and common mutations in the filaggrin gene strongly predispose for AE. However, there is no strong evidence that filaggrin mutations are related to the reduced skin barrier in AE. In this study, electron diffraction is used in order to study the lipid organization of control SC and non-lesional SC of AE patients in vivo. An increased presence of the hexagonal lipid organization was observed in non-lesional SC of AE patients, indicating a less dense lipid organization. These changes correlate with a reduced skin barrier function as measured with transepidermal water loss but do not correlate with the presence of filaggrin mutations. These results are indicative for the importance of the lipid organization for a proper skin barrier function.

Epidermal stem and progenitor cells in murine epidermis accumulate UV damage despite NER proficiency

Ultraviolet (UV) radiation induces cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs) in DNA, which through gene mutations (e.g. in P53) may lead to skin carcinogenesis. Upon chronic low-level UV exposure, certain basal cells in mouse epidermis were reported to accumulate CPDs. These observations raised questions on whether these cells were fully DNA-repair deficient, and whether they were stem or progenitor cells, as suggested by their long residence time. We found that CPD-retaining basal cells (CRBCs) in SKH-1 hairless mice were repair proficient as accumulation of (6-4)PP, which is a hallmark for complete nucleotide excision repair-deficiency in rodents, was not observed. Accumulation of 6-4PP as well as CPD did, however, occur in basal cells in the epidermis of DNA repair-deficient Xpc-/- mice. Chronic UV exposure of DDB2 transgenic mice and DDB2 knockout mice revealed that the occurrence of CRBCs was inversely correlated with DDB2-expression, indicating that a boost in DNA repair lowered CPD accumulation. Stem cells are quiescent cells and can be identified as 5-bromo-2'-deoxyuridine-label retaining cells (BrdU-LRCs). Induction of BrdU-LRCs followed by chronic UV irradiation showed that all BrdU-label retaining stem cells were also CPD-retaining cells. As most CRBCs were not BrdU-labeled we surmized that these cells must include BrdU-negative stem cells and early progenitor cells. In confirmation of the latter, we found that CRBCs occurred among MTS24+ hair follicle progenitor cells. These findings provide the first evidence that epidermal stem and progenitor cells are prone to the accumulation of UV-induced DNA-damage and can be a prominent target in skin carcinogenesis.

Keratinocytes constitutively express the CD95 ligand molecule on the plasma membrane: an in situ immunoelectron microscopy study on ultracryosections of normal human skin

BACKGROUND

Tissue homeostasis is mainly preserved by cytolytic functions. Cytolytic cells, when expressing the CD95 ligand (Fas-L) molecule on the cell membrane, are able to kill CD95 (Fas)-expressing target cells. Although cultured epidermal keratinocytes (KC) have been shown to express Fas-L, and normal skin has been shown to bear Fas-L mRNA, efforts so far to find possible constitutive Fas-L expression on the cell membrane by resting KC in normal human epidermis (i.e. in a functionally active location) have been inconclusive.

OBJECTIVES

The aim of the present study was therefore to show the constitutive expression of Fas-L on the plasma membrane of KC.

METHODS

Gold immunoelectronmicroscopy, a highly specific and sensitive immunodetection system, was performed in situ on skin sections obtained by ultracryomicrotomy, without previous embedding (i.e. in conditions strictly similar to the in vivo situation).

RESULTS

Relatively few (51.55 +/- 28.61), 10-nm colloidal gold particles were observed at the cell surface of KC in the basal layer of the epidermis and an even smaller (P < 0.005) number of gold granules was detected in the KC of the spinous layer.

CONCLUSIONS

Although scanty, the constitutive Fas-L expressed on the surface of KC can bind Fas expressed by possible occasional inflammatory cells entering the epidermis, and kill them, so preventing inflammation. Fas-L-expressing KC could moreover induce apoptosis of epidermal cells bearing viral or neoplastic antigens. Thus, the expression of Fas-L by KC may contribute to the preservation of epidermal homeostasis in vivo.

Human epidermal Langerhans cells lack functional mannose receptors and a fully developed endosomal/lysosomal compartment for loading of HLA class II molecules

Langerhans cells (LC) represent the dendritic cell (DC) lineage in the epidermis. They capture and process antigens in the skin and subsequently migrate to the draining lymph nodes to activate naive T cells. Efficient uptake and processing of protein antigens by LC would, therefore, seem a prerequisite. We have now compared the capacity of human epidermal LC, blood-derived DC and peripheral blood mononuclear cells to endocytose and present (mannosylated) antigens to antigen-specific T cells. Moreover, we have determined the expression of mannose receptors, and the composition of the intracellular endosomal/lysosomal MHC class II-positive compartment. The results indicate that LC have poor endocytic capacity and do not exploit mannose receptor-mediated endocytosis pathways. Furthermore, the composition of the class II compartment in LC is distinct from that in other antigen-presenting cells and is characterized by the presence of relatively low levels of lysosomal markers. These results underscore the unique properties of LC and indicate that LC are relatively inefficient in antigen uptake, processing and presentation. This may serve to avoid hyper-responsiveness to harmless protein antigens that are likely to be frequently encountered in the skin due to (mechanical) skin damage.

Mannose receptor-mediated uptake of antigens strongly enhances HLA class II-restricted antigen presentation by cultured dendritic cells

Dendritic cells (DC) efficiently take up antigens by macropinocytosis and mannose receptor-mediated endocytosis. Here we show that endocytosis of mannose receptor-antigen complexes takes place via small coated vesicles, while non-mannosylated antigens were mainly present in larger vesicles. Shortly after internalization the mannose receptor and its ligand appeared in the larger vesicles. Within 10 min, the mannosylated and non-mannosylated antigens co-localized with typical markers for major histocompatibility complex class II-enriched compartments and lysosomes. In contrast, the mannose receptor appeared not to reach these compartments, suggesting that it releases its ligand in an earlier endosomal structure. Moreover, we demonstrate that mannosylation of protein antigen and peptides resulted in a 200-10,000-fold enhanced potency to stimulate HLA class II-restricted peptide-specific T cell clones compared to non-mannosylated peptides. Our results indicate that mannosylation of antigen leads to selective targeting and subsequent superior presentation by DC which may be applicable in vaccine design.

Mannose receptor mediated uptake of antigens strongly enhances HLA-class II restricted antigen presentation by cultured dendritic cells

Dendritic cells (DCs) use macropinocytosis and mannose receptor mediated endocytosis for the uptake of exogenous antigens. Here we show that the endocytosis of the mannose receptor and mannosylated antigen is distinct from that of a non-mannosylated antigen. Shortly after internalization, however, both mannosylated and non-mannosylated antigen are found in an MIIC like compartment. The mannose receptor itself does not reach this compartment, and probably releases its ligand in an earlier endosomal structure. Finally, we found that mannosylation of peptides strongly enhanced their potency to stimulate HLA class II-restricted peptide-specific T cell clones. Our results indicate that mannosylation of antigen leads to selective targeting and subsequent superior presentation by DCs which may be useful for vaccine design.

The use of vibratome sections for the ruthenium tetroxide protocol: a key for optimal visualization of epidermal lipid bilayers of the entire human stratum corneum in transmission electron microscopy

In this paper we describe a preparative procedure which allows maximal visualization of all lipid bilayers in the human stratum corneum (SC). We used 50-microns-thick vibratome sections of paraformaldehyde/glutaraldehyde-fixed human skin. The sections were post-fixed in 1% osmium tetroxide and 0.5% ruthenium tetroxide. The vibratome sections were dehydrated only in 70% ethanol in order to prevent dissolution of the lipids. Lipid bilayers, including the alternating electron-dense and electron-lucent lamellae, were visible between all cell layers of the SC. In addition, this preparative procedure also appeared to be excellent for the ultrastructural preservation of lamellar bodies.

Impaired desquamation in the in vitro reconstructed human epidermis

A fully differentiated epidermis generated in vitro by culturing normal human keratinocytes at the air-liquid interface shares many similarities with native tissue. However, in contrast to native epidermis, its stratum corneum consists of a larger number of compactly packed corneocyte layers, indicating that the processes involved in corneocyte desquamation are disturbed. Although the tri-lamellar appearance of desmosomes in viable cell layers of both types of epidermis is similar, abnormalities in the transformation of desmosomes into the corneosomal plug in the stratum corneum of reconstructed epidermis have been observed. In the native epidermis, desmosomes are transformed into corneosomes at the stratum granulosum/stratum corneum interface. This process is retarded in vitro, since desmosomal structures with preserved lamellar appearance are present in the lower parts of the stratum corneum. Moreover, the corneosome frequency in reconstructed epidermis is significantly higher than in native human skin. A comparison of reconstructed epidermis with hyperproliferative epidermis, such as UV-irradiated epidermis, psoriatic epidermis and recently healed burn-wounds treated with cultured epidermal autografts, has revealed that only the structure of the stratum corneum derived from psoriatic skin is similar to that of reconstructed epidermis. The stratum corneum organization in the UV-treated epidermis and in recently healed burn-wound is, however, close to that seen in native skin.

Distribution of HLA class II molecules in epidermal Langerhans cells in situ

We performed immunoelectron microscopic studies to investigate the expression of HLA class II molecules in Langerhans cells (LC). In the epidermis, LC expressed class II molecules on the plasma membrane of the dendrites, resulting in a class II positive reticulo-epithelial network, but not on the surface of the cell body. In contrast, isolated LC as well as activated LC in situ displayed an even and strong expression of class II molecules on their entire cell surface. Therefore, isolation and/or activation results in redistribution and up-regulation of class II molecules on the cell surface. In addition, double-labeling experiments were carried out with monoclonal antibodies to class II antigens and to LAMP-1, CD63 and alpha-glucosidase, specific markers for organelles of the endosomal/lysosomal system. The results show expression of class II molecules on intracellular, electron-dense vesicular structures, and co-localization of class II molecules and the markers for late endosomes and early lysosomes in human LC in situ. Expression of these markers was not found on Birbeck granules, an LC-specific organelle.

Ciliogenesis in human bronchial epithelial cells cultured at the air-liquid interface

We present a study on modification of culture conditions in serially cultured human bronchial epithelial cells (HBEC), necessary to achieve bronchial epithelial cells similar to the native epithelium. Cells were obtained from bronchial biopsies and serially cultured using a previously described method (In Vitro Cell. Dev. Biol. 1993; 29A:379-387). At the air-liquid interface, the second and the subsequent passages of HBEC cultures were grown 7 to 31 days, in medium containing fetal calf serum, using de-epidermized dermis or collagen discs as substratum. Scanning and transmission electron microscopy revealed ciliogenesis after 7 days and maturation of the cilia up to 31 days, irrespective of whether de-epidermized dermis or collagen membrane was used. The transmission electron microscopy of the developing cilia showed fibrogranular masses, procentrioles, basal bodies, and in the mature cilia a normal ultrastructure of the axoneme, the nine doublets, the central pair, radial spokes, and dynein arms in the ciliary shaft. In contrast, the submerged cultures showed no signs of ciliogenesis in the same time course. Results of experiments, in which cell seeding density, the substrate used, and the manner of nutrient supplementation were modulated, revealed that the air-exposure of the cultured HBEC is a necessary requirement for the ciliogenesis. The development pathway of ciliated cells in air-exposed HBEC cultures was similar to the differentiation and maturation pattern in human fetal tracheal cells. The in vitro model of human bronchial epithelial cells derived from biopsies obtained by fiberoptic bronchoscopy offers an attractive model for future studies on the function of human bronchial epithelial cells under normal and pathologic conditions.

Long-term ultraviolet B-induced impairment of Langerhans cell function: an immunoelectron microscopic study

The influence of low-dose, long-term ultraviolet B (UVB) light exposure on HLA class II-positive human epidermal Langerhans cells (LC) was studied using a sensitive immunoelectron microscopic technique for the ultrastructural assessment of HLA class II expression on LC and for quantification of these cells in situ. Six healthy Caucasian volunteers participated in the experiments and received thrice weekly UVB treatments for 4 weeks. The initial dose ranged from 30 to 50 mJ/cm2 and the total dose from 600 to 3500 mJ/cm2, depending on skin type. Suction blisters and biopsies were obtained before the start of the UVB protocol and 48 h after the last UVB irradiation, and processed for the mixed epidermal cell-lymphocyte reaction (MECLR) and electronmicroscopy, respectively. The MECLR was used as a measure of the immune response. The distribution of HLA class II molecules on LC was studied by incubating ultrathin cryosections of human skin tissue with an anti-HLA class II MoAb that was conjugated to 10 nm colloidal gold. Furthermore, the number of LC was assessed ultrastructurally, when they could be recognized by their unique cytoplasmic organelle, the Birbeck granule (BG). The UVB protocol that was employed caused a marked suppression of the MECLR responses. This UVB-induced reduction of the immune response was not paralleled by changes in HLA class II expression on LC, nor in the number of epidermal LC. These findings are further support for our hypothesis that UVB-induced immune suppression in the skin is not due to a depletion of local LC.

Ultrastructural studies bearing on the mechanism of UVB-impaired induction of contact hypersensitivity to DNCB in man

In both murine and human experimental systems, acute, low dose exposure of skin to ultraviolet B light (UVB) impairs the induction of allergic contact dermatitis (ACD) by haptens such as dinitrochlorobenzene (DNCB) in a significant proportion of individuals. By light microscopy, epidermal Langerhans cells (LC) have been reported to be depleted by UVB exposure as well as by epicutaneous hapten application, implying that LC may be the locus of action of the effects of both UVB and DNCB. However, light microscopy can not readily distinguish cell density changes secondary to LC necrosis from changes resulting from down-modulation of expression of LC surface molecules. Using a highly sensitive immunogold electron microscopic approach, we have evaluated the differential effects of UVB and/or DNCB on human epidermal LC. The results reveal that DNCB alone caused significant up-regulation of cell surface HLA class II expression on a very small number of LC, the major fraction of LC expressing normal levels of HLA class II. Furthermore, DNCB alone caused a modest reduction in the density of LC at the treated sites without evidence of cell necrosis. Treatment with UVB alone or UVB exposure followed by DNCB resulted in a reduction in the density of LC, with widespread evidence of LC necrosis. However, the few remaining intact LC were all intensely HLA class II-positive after UVB exposure followed by DNCB, whereas treatment with UVB alone did not result in changes in LC HLA class II expression. The findings that after DNCB painting only a small proportion of the LC were strongly HLA class II-positive, but after UVB exposure followed by DNCB all intact LC displayed significant up-regulation of cell surface HLA class II expression, imply that UVB exposure inhibits the migration of epidermal LC. This is consistent with the view that DNCB fails to induce ACD when hapten is painted on UVB-exposed skin because insufficient LC are available to initiate T cell activation in the draining lymph node.

Short-term and long-term UVB-induced immunosuppression in human skin exhibit different ultrastructural features

Short-term ultraviolet B light (UVB)-induced immune suppression was established by the ability of UVB to inhibit induction of allergic contact dermatitis (ACD) to dinitrochlorobenzene (DNCB). The long-term effect was assessed by a dramatic reduction of the mixed-epidermal cell lymphocyte reaction (MECLR) responses. Immuno electron microscopic (IEM) investigation of HLA class II localization on epidermal Langerhans cells (LC) (pivotal for the immune response of the skin) showed no difference in HLA class II expression after UVB-exposure compared to control skin, both after short-term and long-term UVB-exposure. Ultrastructural examination of the epidermis revealed a normal appearance of the epidermal cells after long-term UVB-exposure. However, after short-term UVB-exposure the epidermis exhibited many abnormalities, ranging from the stratum corneum to the basal membrane. So although both short- and long-term UVB exposures were able to cause immune suppression in the skin, those processes were not accompanied with identical morphological epidermal features.

Ontogenesis of the basement membrane zone after grafting cultured human epithelium: a morphologic and immunoelectron microscopic study

Sheets of cultured epithelial cells have been successfully used as autografts for the permanent coverage of patients with full-thickness burns and as allografts to stimulate the healing of chronic skin ulcers. The basement membrane zone (BMZ), composed of lamina lucida, lamina densa, and anchoring fibrils, plays a pivotal role in the firm adherence of the epidermis to the dermis. The present study describes the ultrastructural development during various stages of wound healing after resurfacing different wound areas by cultured epithelial grafts. For this purpose, biopsies were obtained from five patients 5 d to 4.5 years after resurfacing full-thickness burns with cultured autografts, and from five patients at various stages after treatment of excised tattoos and chronic skin ulcers with cultured allografts. One biopsy was taken from a spontaneously healed burn wound 30 years post-injury. Ultrathin sections were prepared for transmission and immunoelectron microscopy, using monoclonal antibodies against type IV and VII collagen. Findings were compared to controls of age- and site-matched normal skin. Eleven days after grafting, the first BMZ features had regenerated, including lamina lucida, a discontinuous lamina densa, hemidesmosomes, and sparse anchoring fibrils. The process of de novo synthesis of BMZ components had begun, and within 4 to 5 weeks complete reformation of BMZ was observed, including normal distribution of anchoring fibrils. Immunolabeling of type VII collagen was first observed upon the lower part of lamina densa at day 11 and steadily increased, reaching normal values 5 weeks after grafting. In contrast, gold deposition of type IV collagen upon lamina densa was strongly increased at day 19 compared to normal. This high expression reduced a little at 5 weeks, but remained high up to 30 years after injury. Long-term burn scars exhibited pseudopodia-like extensions of all basal cells, abundant anchoring fibrils, and an increased amount of arching anchoring fibrils. These features might compensate for the lack of proper rete ridges.

HLA class II expression on human epidermal Langerhans cells in situ: upregulation during the elicitation of allergic contact dermatitis

An immunoelectron-microscopic technique was applied to investigate the localization of molecules that are involved in the elicitation of allergic contact dermatitis in human epidermal cells in situ. Langerhans cells in the epidermis of lesions showed a strongly increased cell surface expression of HLA class II molecules as compared with normal skin. In addition, a high number of intracellularly located HLA class II molecules were present in Langerhans cells of lesional epidermis, suggesting increased biosynthesis of these molecules during the elicitation process. In contrast, no differences in the expression of CD1a by Langerhans cells was observed between normal and lesional skin. Frequently, the Langerhans cells were found in close apposition to mononuclear cells, which also exhibited a strong cell surface HLA class II expression. The number of Birbeck granules that are characteristic intracellular Langerhans cells organelles was increased in lesional Langerhans cells as compared with normal-skin Langerhans cells, which may correlate with the activated state of lesional Langerhans cells. These Birbeck granules were always HLA class II or CD1a negative. The increased synthesis and expression of HLA class II molecules on the cell surface of Langerhans cells suggests a direct role for these HLA class II molecules in the elicitation process of allergic contact dermatitis.