Immunohistochemical demonstration of nerve-Merkel cell complex in fetal human skin

Human fetal skin derived merkel cells display distinctive characteristics in vitro and in bio-engineered skin substitutes in vivo

Human skin contains specialized neuroendocrine Merkel cells responsible for fine touch sensation. In the present study, we performed in-depth analysis of Merkel cells in human fetal back skin. We revealed that these Merkel cells expressed cytokeratin 20 (CK20), were positive for the neuroendocrine markers synaptophysin and chromogranin A, and the mechanosensitive ion channel Piezo2. Further, we demonstrated that Merkel cells were present in freshly isolated human fetal epidermal cells in vitro, and in tissue-engineered human dermo-epidermal skin substitutes 4 weeks after transplantation on immune-compromised rats. Merkel cells retained the expression of CK20, synaptophysin, chromogranin A, and Piezo2 after isolation and in culture, and in the skin substitutes after transplantation. Interestingly, we observed that in fetal skin and in skin substitutes, only Merkel cells were positive for CK8, while in culture, also non-Merkel cells showed positivity for CK8. In summary, human fetal Merkel cells showed phenotypical features confirming their cell identity. This findings are of pivotal importance for the future application of fetal tissue-engineered skin in clinics.

The acquisition of mechanoreceptive competence by human digital Merkel cells and sensory corpuscles during development: an immunohistochemical study of PIEZO2

BACKGROUND

PIEZO2 is a transmembrane protein forming part of an ion channel required for mechanotransduction. In humans, PIEZO2 is present in axon terminals of adult Meissner and Pacinian corpuscles, as well as Merkel cells in Merkel cell-neurite complexes.

METHODS

To study the acquisition of functional capability for mechanotransduction of developing type I slowly adapting low-threshold mechanoreceptors, i.e., Merkel cell-neurite complexes, a battery of immunohistochemical and immunofluorescence techniques was performed on human skin specimens covering the whole development and growth, from 11 weeks of estimated gestational age to 20 years of life. In addition, developmental expression of PIEZO2 type I (Meissner's corpuscles) and type II (Pacinian corpuscles) rapidly adapting mechanoreceptors was studied in parallel.

RESULTS

The first evidence of Merkel cells showing the typical morphology and placement was at 13 weeks of estimated gestation age, and at this time positive immunoreactivity for PIEZO2 was achieved. PIEZO2 expression in axons terminals started at 23 WEGA in Pacinian corpuscles and at 36 WEGA in the case of Meissner corpuscles. The occurrence of PIEZO2 in Merkel cells, Meissner and Pacinian corpuscles was maintained for all the time investigated. Interestingly PIEZO2 was absent in most Aβ type I slowly adapting low-threshold mechanoreceptors that innervate MC while it was regularly present in most Aβ type I and type II rapidly adapting low-threshold mechanoreceptors that supplies Meissner and Pacinian corpuscles.

CONCLUSION

The present results provide evidence that human cutaneous mechanoreceptors could perform mechanotransduction already during embryonic development.

Epigenetic regulation and signalling pathways in Merkel cell development

Merkel cells are specialized epithelial cells connected to afferent nerve endings responsible for light-touch sensations, formed at specific locations in touch-sensitive regions of the mammalian skin. Although Merkel cells are descendants of the epidermal lineage, little is known about the mechanisms responsible for the development of these unique mechanosensory cells. Recent studies have highlighted that the Polycomb group (PcG) of proteins play a significant role in spatiotemporal regulation of Merkel cell formation. In addition, several of the major signalling pathways involved in skin development have been shown to regulate Merkel cell development as well. Here, we summarize the current understandings of the role of developmental regulators in Merkel cell formation, including the interplay between the epigenetic machinery and key signalling pathways, and the lineage-specific transcription factors involved in the regulation of Merkel cell development.

Merkel Cells: A Collective Review of Current Concepts

Merkel cells (MCs) constitute a very unique population of postmitotic cells scattered along the dermo-epidermal junction. These cells that have synaptic contacts with somatosensory afferents are regarded to have a pivotal role in sensory discernment. Several concerns exist till date as to their origin, multiplication, and relevance in skin biology. The article, a collective review of literature extracted from PubMed search and dermatology books, provides novel insights into the physiology of MCs and their recent advances.

Merkel cells and touch domes: more than mechanosensory functions?

The touch dome (TD) is an innervated structure in the epidermis of mammalian skin. Composed of specialized keratinocytes and neuroendocrine Merkel cells, the TD has distinct molecular characteristics compared to the surrounding epidermal keratinocytes. Much of the research on Merkel cell function has focused on their role in mechanosensation, specifically light touch. Recently, more has been discovered about Merkel cell molecular characteristics and their cells of origin. Here we review Merkel cell and TD biology, and discuss potential functions beyond mechanosensation.

Sox2 modulates the function of two distinct cell lineages in mouse skin

In postnatal skin the transcription factor Sox2 is expressed in the dermal papilla (DP) of guard/awl/auchene hair follicles and by mechanosensory Merkel cells in the touch domes of guard hairs. To investigate the consequences of Sox2 ablation in skin we deleted Sox2 in DP cells via Blimp1Cre and in Merkel cells via K14Cre. Loss of Sox2 from the DP did not inhibit hair follicle morphogenesis or establishment of the dermis and hypodermis. However, Sox2 expression in the DP was necessary for postnatal maintenance of awl/auchene hair follicles. Deletion of Sox2 via K14Cre resulted in a decreased number of Merkel cells but had no effect on other epithelial compartments or on the dermis. The reduced number of Merkel cells did not affect the number or patterning of guard hairs, nerve density or the interaction of nerve cells with the touch domes. We conclude that Sox2 is a marker of two distinct lineages in the skin and regulates the number of differentiated cells in the case of the Merkel cell lineage and hair follicle type in the case of the DP.

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Sox2 is a marker of two distinct lineages in the skin.

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Sox2 is required for postnatal maintenance of awl/auchene hair follicles.

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Loss of Sox2 results in a reduction in Merkel cells.

Epidermal progenitors give rise to Merkel cells during embryonic development and adult homeostasis

Lineage-tracing experiments show that the origin of specialized mechanosensory Merkel cells in the skin is epidermal progenitors, not the neural crest.

Merkel cells (MCs) are located in the touch-sensitive area of the epidermis and mediate mechanotransduction in the skin. Whether MCs originate from embryonic epidermal or neural crest progenitors has been a matter of intense controversy since their discovery >130 yr ago. In addition, how MCs are maintained during adulthood is currently unknown. In this study, using lineage-tracing experiments, we show that MCs arise through the differentiation of epidermal progenitors during embryonic development. In adults, MCs undergo slow turnover and are replaced by cells originating from epidermal stem cells, not through the proliferation of differentiated MCs. Conditional deletion of the Atoh1/Math1 transcription factor in epidermal progenitors results in the absence of MCs in all body locations, including the whisker region. Our study demonstrates that MCs arise from the epidermis by an Atoh1-dependent mechanism and opens new avenues for study of MC functions in sensory perception, neuroendocrine signaling, and MC carcinoma.

An organotypic culture system of Merkel cells using isolated epidermal sheets

BACKGROUND

Merkel cells (MCs) exist in the epidermal basal layer, in contact with keratinocytes. This direct contact seems critical for maintaining MCs in vitro.

OBJECTIVES

To estimate the effects of nerve cells on the maintenance of MCs within epidermal sheets in a new organotypic culture system of MCs.

METHODS

We developed a new organotypic culture system of MCs, using MC-containing epidermal sheets embedded in collagen gel. To estimate the effects of nerve cells on the maintenance of MCs within the epidermal sheets, we cocultured nerve cells and MC-containing epidermal sheets. In these culture assemblies, cellular behaviour was analysed by histochemistry, immunohistochemistry, electron microscopy and enzyme-linked immunosorbent assay.

RESULTS

This culture, even in the absence of neurotrophin (NT)-3 and nerve growth factor (NGF) (which are crucial for MC biology), retained cytokeratin (CK)-20-positive and neuroendocrine granule-containing MCs within the sheets for over 2 weeks. Coculture of MCs with PC-12 nerve cells significantly increased the number of MCs within the epidermal sheets, and the keratinocytes had almost identical expression levels of CK1, CK10, CK14 and the progenitor marker p63 to those produced by keratinocytes in vivo. Uptake of the growth marker bromodeoxyuridine by MCs and levels of NT-3 and NGF in the culture supernatants were undetectable in this system, regardless of the presence or absence of PC-12.

CONCLUSIONS

The data suggest, first, that direct contact between MCs and keratinocytes may be critical for retaining MCs in vitro; second, that nerve cell-affected maintenance of keratinocyte differentiation, but not NT-3 and NGF, may contribute to MC maintenance; and third, that MCs are not able to grow, at least in our system. Our method would be useful for studying MC biology.

Current considerations about Merkel cells

Since the discovery of Merkel cells by Friedrich S. Merkel in 1875, knowledge of their structure has increased with the progression of new technologies such as electron and laser microscopy, and immunohistochemical techniques. For most vertebrates, Merkel cells are located in the basal layer of the epidermis and characterized by dense-core granules that contain a variety of neuropeptides, plasma membrane spines and cytoskeletal filaments consisting of cytokeratins and desmosomes. The presence of the two latter structures would suggest that Merkel cells originate from the epidermis rather than from the neural crest, even though such a hypothesis is not unanimously accepted. The function of the Merkel cell is also very controversial. For a long time, it has been accepted that Merkel cells with associated nerve terminals act as mechanoreceptors although the transduction mechanism has not yet been elucidated. Merkel cells that do not make contact with nerve terminals have an endocrine function. The present review aims to shed new and comparative light on this field with an attempt to investigate the stimuli that Merkel cells are able to perceive.

Merkel Cell-Nerve Cell Interaction Undergoes Formation of a Synapse-like Structure in a Primary Culture

Merkel cells have been assumed to guide nerve fibers to the skin. However, there has been little in vitro evidence that supports this hypothesis, because there is no suitable established culture system of Merkel cells. Here we show that Merkel cells isolated from rat footpad skin were successfully cultured in a monolayer with keratinocytes. Keratinocytes did not affect any structural changes in Merkel cells. When nerve cells (NG108-15 or PC12) were added to the culture system, both nerve fibers and cytoplasmic processes of Merkel cells outgrew and cooperatively organized synapse-like structures at their contact points. Nerve cells promoted Merkel cell survival, compared with keratinocytes only. Merkel cell proliferation was not detected in all conditions, even with nerve growth factor, neurotrophin-3, interleukin-6 and tumor necrosis factor-alpha. The data suggest, firstly, that Merkel cells may guide nerve fibers to the skin by interacting with nerve cells; and, secondly, that nerve cells, but not keratinocytes, may produce some survival factors other than the cytokines above for Merkel cells, although Merkel cells may be a terminally differentiated cell type. Our method could open a way to study Merkel cell biology.

Characterization of the peri-implant epithelium in hamster palatine mucosa: Behavior of Merkel cells and nerve endings

The purpose of this study was to investigate the relationship between Merkel cells and nerve elements during tissue regeneration after receiving dental implants. Golden hamsters were divided into 3 groups and titanium alloy implants were fixed in their left-side maxilla through the third palatine rug. Animals were sacrificed at 1, 2, 3, 4, 5, 6, and 7 days after the implantation and tissues were characterized at the immunohistochemical and morphological levels. CK 20 and PGP 9.5 antibodies which react with Merkel cells and nerve fibers were used. Immunohistochemically, no CK 20-positive Merkel cells were seen in the peri-implant epithelium throughout the 7 days. However, starting at day 4, PGP 9.5-positive nerve fibers appeared in the connective tissue, and by day 7, nerve fibers had invaded the more superficial layer of the peri-implant epithelium compared to the mucosa removal control group. At the electron microscopic level, the intercellular spaces of the regenerating epithelium in the mucosa removal control group were small. In contrast, intercellular spaces of the peri-implant epithelium tended to be wide and regenerating nerve fibers invaded those intercellular spaces. In both the mucosa removal control group and the implantation group, the basal lamina and connective tissues regenerated completely. However, clear Merkel cells containing neurosecretory granules were not observed. Taken together, our results indicate that Merkel cells in the hamster palatine mucosa do not regenerate in the peri-implant epithelium. However, regenerative nerve fibers seem to play essential roles as part of the defense and sensory systems around the peri-implant epithelium to compensate for the weakened defense mechanism.

Neurotrophin-3 signaling in mammalian Merkel cell development

Merkel cells are sensory cells of neural crest origin. Because little is known about the mechanisms that direct their differentiation, we have investigated the potential role of a candidate regulatory factor, neurotrophin-3 (NT-3). At embryonic day 16.5 (E 16.5), neither NT-3 nor its primary receptors, TrkC and p75NTR are expressed by Merkel cells in the murine whisker. At the time of birth, however, Merkel cells are immunoreactive for NT-3, TrkC and p75NTR. In TrkC null and NT-3 null mice, Merkel cells differentiate initially, but undergo apoptosis perinatally. These results show that NT-3 signaling is not required for the differentiation of Merkel cells, but that it is essential for their postnatal survival.

Postnatal loss of Merkel cells, but not of slowly adapting mechanoreceptors in mice lacking the neurotrophin receptor p75

Merkel cells are specialized epidermal cells which are abundantly found in touch-sensitive areas and which are innervated by slowly adapting mechanosensitive afferent fibres with large myelinated (Abeta) axons. The role of Merkel cells in mechanosensation, their developmental regulation and their influence on sensory neuron function are, however, incompletely understood. Here, we used mice lacking the neurotrophin receptor p75 which is expressed on Merkel cells to investigate their postnatal development and that of their innervating sensory neurons. Using morphological studies we now show that Merkel cells develop normally in both hairy and glabrous skin in these animals until 2 weeks old, but are progressively lost thereafter and have almost completely disappeared 2 months after birth. Using standard extracellular electrophysiological recording techniques we find that despite the profound loss of Merkel cells there is no corresponding reduction in the number of myelinated slowly adapting afferent fibres. Moreover, the mean mechanical threshold of these neurons and their average stimulus response function to suprathreshold mechanical stimuli does not change during the time period when more than 99% of Merkel cells are lost. We conclude that Merkel cells require p75 during the late postnatal development. However, neither the survival nor the mechanical sensitivity of slowly adapting mechanoreceptive Abeta-fibres depends on the presence of Merkel cells.

The existence of Merkel cells in the lingual connective tissue of the Surinam caiman, Caiman crocodilus crocodilus (order Crocodilia)

The tongue of the Surinam caiman (a reptilian species) was studied by light microscopy including immunohistochemistry for protein gene product 9.5 (PGP 9.5), and transmission electron microscopy. The connective tissue immediately under taste buds housed a cluster of cells immunoreactive for PGP 9.5. These cells synapsed on nerves, and their cytoplasm contained characteristic granules of 90 nm in the mean diameter, glycogen particles, and bundles of intermediate filaments. In light of these ultrastructural features, they were identified as Merkel cells. The Merkel cells were also surrounded by Schwann cells. These findings indicate that the present Merkel cell-neurite-Schwann cell complex is comparable to the avian Merkel corpuscle. On the basis of the granule localization in the cytoplasm, the caiman Merkel cell was presumed to be involved in not only mechanoreception but also endocrine or paracrine functions.

Identification of Merkel cells by an antibody to villin

Merkel cells represent a population of epithelial cells in the skin and oral mucosa. Although Merkel cells are reliably distinguishable from other epithelial cells at the ultrastructural level, these cells are usually not discernible by standard light microscopy and need special techniques for their identification. Villin is an actin-crosslinking protein that is associated with the actin filament cores of brush border microvilli. In this study we show that an antibody against villin is an excellent marker of Merkel cells and their microvilli even at the light microscopic level. The surrounding keratinocytes and subepithelial connective tissue cells do not show any significant affinity for the antibody against villin. Confocal laser micrographs reconstructed from serial images 0.5 microm thick of Merkel cells that were immunostained with villin clearly reveal the three-dimensional morphology of Merkel cells and their microvilli. The presence of villin in Merkel cell microvilli lends support to the idea that these cells might have a mechanoreceptor function.

An immunocytochemical study of the carbonic anhydrase I isoenzyme in human oral Merkel cells

Merkel cells in human buccal mucosa and hard palate possess the carbonic anhydrase I isoenzyme (CAI). CAI colocalized immunocytochemically with a range of Merkel cell cytokeratins, namely CK 7, 8, 18, 19 and 20. No other cells in the oral epithelium were immunoreactive for the CAI antibody. The presence of the enzyme may be related to the function of sensory receptors that produce a sustained response to a maintained mechanical stimulus.

Merkel cells do not require trophic maintenance from the nerves in adult human skin

A 34-year-old Japanese man with hereditary sensory neuropathy was examined to evaluate the distribution, density and inter-relationship between Merkel cells and peripheral nerves in the skin. An epidermal sheet of affected plantar skin showed numerous CAM 5.2-reactive Merkel cells, whereas PGP 9.5-reactive peripheral nerves were completely absent in the epidermis and dermis. These findings strongly suggest that Merkel cells do not require trophic maintenance from nerves in adult human skin.

Cytokeratin 20 is a general marker of cutaneous Merkel cells while certain neuronal proteins are absent

Merkel cells are difficult to identify in tissue sections. Previous studies have used cytokeratins (CK) 8, 18, and 19 as histologic markers of Merkel cells. However, these CKs are also expressed in some outer root sheath keratinocytes and some early fetal epidermal cells and thus are not truly specific of Merkel cells in general. Using selective antibodies against a newly described CK, number 20--originally found in intestinal epithelium and Merkel cell carcinomas--in comparison to a key protein of neuroendocrine cells, chromogranin A, we established CK 20 as a specific Merkel cell marker in skin of humans, pigs, and mice. CK 20 seems to be an even more general and sensitive Merkel cell marker as compared to CgA. In double-labeling experiments with stratified-squamous epithelial CK (numbers 5 and 13-17) and simple epithelial CK (numbers 8, 18, and 20) antibodies evaluated by confocal laser scanning microscopy, no cell expressing CKs of both types (i.e., no cell of so-called "transitional" character between Merkel cells and keratinocytes) was identified in human skin. In addition, various neuronal markers present in Merkel cell carcinomas including neurofilaments, peripherin, nerve growth factor receptor, and neuronal cell adhesion molecule appear to be absent in normal Merkel cells. Thus, Merkel cells exhibit a distinct and unique marker profile, with CK 20 being of particularly high value in various species.

The appearance, density, and distribution of Merkel cells in human embryonic and fetal skin: their relation to sweat gland and hair follicle development

The density and distribution of Merkel cells in human embryonic and fetal skin were studied using an immunolabeling technique on epidermal and dermal sheets obtained by ethylenediamine tetraacetic acid separation. Merkel cells were identified by the known cytokeratin markers CK20 and CK18. Merkel cells showed CK20 immunoreactivity as early as 56 d estimated gestational age (EGA) in the palmar epidermis (133.11 +/- 44.27 cells/mm2). The density increased rapidly, reaching a maximum of more than 1400 cells/mm2 at 80-90 d EGA. At this stage, the cells became distributed along the primary epidermal ridges. In the palmar epidermis of fetuses older than 100 d EGA, the distribution of Merkel cells showed the same pattern, but the density then decreased gradually. Merkel cells were not observed in ductal and glandular portions of eccrine sweat glands. In the epidermal sheets of hairy skin, a few cells were first seen in the fetus at 75 d EGA. At 100 d EGA, only a few Merkel cells were observed, mostly in the hair pegs and bulbous hair pegs. In the older fetus, ring-like arrangements and aggregates of Merkel cells were prominent in the infundibulum and bulge of hair follicles, respectively. Merkel cells were both globular and dendritic in shape. The ratio of dendritic to globular cells increased gradually until the period of highest Merkel cell density in both the glabrous and hairy skin. All Merkel cells located in the dermis were globular in shape. In accord with the results obtained, we postulate that Merkel cells may have some functional role in the formation and proliferation of eccrine sweat glands and hair follicle anlagen in developing skin.

Distribution and chemical phenotypes of neuroendocrine cells in the human anal canal

The presence, morphology and distribution of anal neuroendocrine cells were investigated with a panel of antisera and antibodies for neural markers, biogenic amines, and neuropeptides by the sensitive streptavidin-biotin-peroxidase immunocytochemistry, and coexistence patterns of neurochemically characterized neuroendocrine cells were examined by double immunofluorescence cytochemistry. In the colorectal zone, endocrine-like cells were immunoreactive for chromogranin A (CGA), serotonin (5-HT), pancreastatin (PST), peptide tyrosine tyrosine (PYY), glucagon-like peptide-1 (GLP-1), and somatostatin (SOM). Coexistence patterns of endocrine-like cell phenotypes with CGA and GLP-1 were heterogeneous. In the anal transitional zone (ATZ), endocrine-like cells were immunoreactive for CGA, 5-HT and PST. Furthermore, six new phenotypes of endocrine-like cells were characterized by their immunoreactivity for PYY, GLP-1, protein gene product 9.5 (PGP), calcitonin gene-related peptide (CGRP), neurotensin (NT), and SOM. All endocrine-like cell types in the ATZ were immunoreactive for CGA. In the squamous zone and perianal skin, CGA-immunopositive Merkel cells were also immunoreactive for CGRP, PST, NT and PGP. Neuroendocrine cells in the anal canal exhibit epithelial zone-related diversities in their neurochemical phenotypes and coexistence patterns, which may indicate specific regulatory functions. In the epithelium of the ATZ, which is regarded as metaplastic, endocrine-like cells expressed phenotypes characteristic of the neuroendocrine cells of the colorectal zone and the squamous zones, indicating a possible metaplastic origin of these cells.

Merkel cells of the terminal hair follicle of the adult human scalp

Human scalp skins were treated with 20 mM ethylenediaminetetraacetic acid and terminal hair follicles were extracted with the epidermis. Some terminal hair follicles were morphologically preserved well and provided opportunity to examine three-dimensional distribution of CAM5.2 (K8, 52.5 kD) reactive Merkel cells. In anagen terminal hair of the scalp numerous immunoreactive Merkel cells were distributed in the presumptive bulge area. Distinct swelling as in the bulge of human vellus hair was usually absent; however, in rare instances anagen terminal hair demonstrated unilateral prominent swelling with dense aggregation of Merkel cells. In telogen hair the bulge becomes indistinguishable from the regressed end of the club hair follicle but Merkel cells continued to be abundant. We found morphologic variation of the bulge such as formation of knoblike swellings and villous projections. Interestingly, Merkel cells were also located in these structures. Palisading stockade-like nerve endings were observed surrounding the follicular epithelium at the sebaceous gland level. Merkel cells were sparse in this follicular segment. Variable number of Merkel cells were also scattered in the infundibulum of terminal hair in no association with peripheral nerves.

Merkel cells in human fetal eccrine glands

The presence of human Merkel cells in the eccrine ridges and eccrine germs was studied, using antibodies to simple epithelial keratins, in separated epidermal sheets with attached eccrine ducts. The localization of Merkel cells could be analysed three-dimensionally in the wet, whole-mount of the stained sheets. In the plantar skin of a 12-week-old human fetus, immunoreactive (ir-) Merkel cells were randomly located in the flattened epidermis. In the plantar skin of a 15-week-old human fetus, there was early development of eccrine germs, and Merkel cells were concentrated in eccrine gland ridges. In the plantar skin of a 20-week-old human fetus, eccrine germs were well formed and ir-Merkel cells were located within the developing eccrine ridges and ducts. In the plantar skin of adults, the eccrine concentration of Merkel cells was markedly reduced. Concentration of Merkel cells on the eccrine structures was also observed in the scalp skin of human fetuses. This tendency continued into adult life, although there was a marked reduction in the total number of Merkel cells. These findings suggest that epidermal Merkel cells move down into the eccrine ducts as eccrine germs extend into the mesenchyme. Alternatively, they may develop de novo from the keratinocytes of the eccrine duct. In view of the expression of nerve growth factor receptor in fetal Merkel cells, it is postulated that these eccrine gland Merkel cells play a role in the formation of the periglandular nerve plexus.

A high concentration of Merkel cells in the bulge prior to the attachment of the arrector pili muscle and the formation of the perifollicular nerve plexus in human fetal skin

The distribution of Merkel cells in human fetal hair follicles was studied using whole mounts of separated epidermis with attached hair follicles. The technique had the advantage of enabling the elucidation of the spatial relationships of Merkel cells with other cells in the skin. In a 16-week-old fetus the hair anlagen had formed one or two epithelial swellings of variable size. In a 17-week-old fetus sebaceous glands and the bulge of the hair follicle were recognizable and immunoreactive. Merkel cells were present in the bulge and surrounding the acrotrichium (intraepidermal follicular canal). In a 20-week-old fetus the sebaceous gland and bulge were well formed and immunoreactive Merkel cells were concentrated in the bulge and infundibulum. In vertical sections of a 20-week-old fetus immunoreactive Merkel cells were also situated in the vicinity of the bulge. Arrector pili muscles were first observable in a 24-week-old fetus being weakly stained with anti-desmin antibody. In a 24-week-old fetus, nerves were also stained within the arrector pili muscles with S-100 protein antibody. In the presumptive arrector pili muscle immunoreactivity for S-100 protein developed before or at the same time as immunoreactivity for desmin. Merkel cells or their products in the bulge may serve as attractants for the growing arrector pili muscle which contain peripheral nerves. Following our report that dermal Merkel cells influence the formation of the dermal nerve plexus, perifollicular Merkel cells near the bulge may also play an inductive and growth-stimulative role for the perifollicular nerve plexus.

Cytokeratin Polypeptide Profile of Merkel Cells in Human Fetal and Adult Skin: Difference of Expression of Cytokeratins in Epidermal and Dermal Merkel Cells

The origin of Merkel cells is uncertain, although current evidence by immunohistochemical keratin marker studies favors an epidermal derivation. We studied the expression of keratin species in Merkel cells of human fetus and adult using 19 anti-keratin antibodies. Epidermal and dermal Merkel cells contained not only simple epithelium-type but also some stratified epithelium-type keratins. Interestingly, expression of some keratins was different between epidermal and dermal Merkel cells, for example, AN3 (50, 58 kD) and CKB1 (50 kD) recognized epidermal Merkel cells, but not dermal Merkel cells. These results suggest that surrounding keratinocytes influence the expression of cytokeratins in Merkel cells or that dermal Merkel cells undergo a modification from keratin-producing epidermal Merkel cells to a more neural cell type by the association with nerve endings in the upper dermis. On the other hand, certain cytokeratin polypeptides recognizable with Ks19.1 (40 kD), CK5 (45 kD), and CAM5.2 (52.5 kD) were expressed in both epidermal and dermal Merkel cells. The expression of simple epithelium-type keratins in Merkel cells remained even after the epidermal basal cells gradually lost their expression.