Dermal Merkel Cells Research Articles

Sensory Innervation of the Female Human Umbilical Skin: Morphological Studies

Background: Sensory stimuli are conducted by several cutaneous sensory nerves and tactile corpuscles. The latter are specialized sensory organs that represent the starting point of many afferent sensory pathways. To date, our knowledge about the distribution of the sensory innervation in the umbilical skin of females is incomplete.Aim of the study: To elucidate the morphology of the cutaneous innervation of the normal female umbilical skin.Materials and methods: Biopsies of normal umbilical skin were obtained from female patients undergoing umbilical hernial repair. The specimens were processed for both immunohistological (antibodies against PGP9.5, pan-neuronal marker, and S-100 protein, marker of Schwann cells) and ultrastructural (transmission electron microscopy) examinations.Results: The authors found abundant genital end-bulb-like structures, numerous epidermal and dermal Merkel cells, Meissner and Ruffini corpuscles, intraepidermal nerve terminals, and multiple free nerve endings surrounding the ducts and acini of the sweat glands.Conclusions: The umbilical skin of females has abundant sensory innervation similar to that of the glans penis.

Nonmelanoma Skin Cancer

Nonmelanoma Skin Cancer

Overexpression of neurotrophin‐3 enhances the mechanical response properties of slowly adapting type 1 afferents and myelinated nociceptors

Constitutive overexpression of neurotrophin-3 (NT3) in murine skin results in an increased number of sensory neurons within the dorsal root ganglia, an increase of myelinated axons in cutaneous nerves, hyperinnervation of the skin, and an increased number of Merkel cells found in flank skin. Here we used a saphenous skin/nerve preparation to determine if these anatomical changes affect the functional response characteristics of cutaneous sensory neurons. Overexpression of NT3 significantly increased the responses of slowly adapting type 1 (SA1) low-threshold mechanoreceptors and Adelta high-threshold mechanoreceptors to suprathreshold mechanical stimulation. It also resulted in significantly faster conduction velocities of SA1 fibers. In contrast to earlier findings in flank skin, no differences were noted in the numbers of Merkel cells in the touch domes in hindlimb skin of NT3-overexpressing mice. In addition, the number of dermal Merkel cells, located around hair follicles on the dorsum of the foot, was reduced by 55%. The increase in mechanical sensitivity was found to correlate with significant increases in the expression of acid-sensing ion channels (ASIC) 1 and 3. Additional experiments using intracellular recordings and staining procedures confirmed that at least some cutaneous myelinated nociceptors and SA1 mechanoreceptors stained positively for both trkC and ASIC3. These results indicate that cutaneous NT3 overexpression alters the response properties of specific cutaneous sensory neurons, and that these changes may be due to the modulation of putative mechanosensitive ion channels.

Merkel cells participate in the induction and alignment of epidermal ends of arrector pili muscles of human fetal skin

The follicular proximal ends of arrector pili muscles anchor the bulge areas of human hair follicles. However, the location of the ends of these muscles towards the epidermis, especially the fine points of attachment, is not fully understood. Conventional serial sections of the scalp skin of a 12-month-old infant were examined. Most of the distal ends were situated in the upper dermis, while some seemed to be in close contact with the epidermal basal layers. In vertical sections of the scalp skin of 36-week-old fetus, double immunoenzyme staining showed a close topographic correlation between the epidermal ends of the arrector pili muscles and the epidermal and dermal Merkel cells. These findings allow speculation that fetal Merkel cells participate in the induction and alignment of arrector pili muscles.

Immunohistochemical demonstration of the expression of neurofilament proteins in Merkel cells.

The presence of immunoreactive neurofilament proteins has previously been reported in Merkel cell carcinomas but not in normal human epidermal and dermal Merkel cells. We have studied the immunoreactivity of epidermal Merkel cells for neurofilament triplet proteins (68 KD, 70 KD, 160 KD, 200 KD), using epidermal sheets prepared from the plantar skin of human adults, which enabled us to survey large numbers of Merkel cells. Neurofilament protein 200 KD-positive cells were readily identified, while neurofilament protein 68 KD-, 70 KD- and 160 KD-positive cells were largely absent. 200 KD-positive cells in the epidermis were confirmed to represent Merkel cells by a sequential immunoenzyme labeling for the simple epithelial type cytokeratin (No. 8). 200 KD-positive cells were 5.9% of the total number of epidermal Merkel cells. Despite a heterogeneous expression of neurofilament protein subspecies between the normal and transformed Merkel cells, the presence of neurofilament proteins in epidermal Merkel cells may link them to Merkel cell carcinomas.

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.

Biological significance of dermal Merkel cells in development of cutaneous nerves in human fetal skin.

We detected epidermal Merkel cells in 12-week fetuses with monoclonal antibodies (MAb) against simple epithelium keratin and epithelial membrane antigen. In 15-week fetuses these Merkel cells began to descend into the dermis and expressed nerve growth factor receptors (NGF-R). At approximately the same time, cutaneous nerves, as detected with an MAb against neurofilaments, extended from the subcutaneous trunk and branched to form the subepidermal nerve plexus. The expression of NGF-R on dermal Merkel cells preceded their connection with immunoreactive small nerves. Initially, most of these fine nerve endings were directed towards dermal Merkel cells. In 23-week fetuses the subepidermal nerve plexus was well developed and immunoreactive dermal Merkel cells began to disappear. At all stage of fetal development the epidermal Merkel cells did not strongly express NGF-R. We postulate that dermal Merkel cells play an inductive and a promotional role in development of the cutaneous nerve plexus in the upper dermis.

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

Using Merkel cell specific antikeratin antibodies and neurofilament antibody the nerve-Merkel cell relationship was studied with a double staining method on frozen sections. Merkel cells were stained with monoclonal anti-cytokeratin CK-5 and CAM 5.2 which react against human cytokeratin polypeptide 45 kDa and 52.5 kDa, respectively. Peripheral nerves were stained with monoclonal murine antibody Neurofilament 70 and 200 kDa. Epidermal Merkel cells were recognized in 12-week fetuses in the plantar skin. In 15-week fetuses dermal Merkel cells were found. Most of the dermal Merkel cells initially lacked a close association with immunoreactive nerve fibers. In 16-week fetuses immunoreactive small nerves reached the epidermis and a few dermal Merkel cells became heavily entangled with the meshwork of nerve endings. Nerve-Merkel cell complex in the dermis was confirmed by electron microscopy. The appearance of epidermal Merkel cells preceded the attachment of immunoreactive nerve endings to the epidermis. It an analysis of 448 dermal Merkel cells the nerve-Merkel cell complex became more frequent as the age of fetus advanced. It was concluded that Merkel cells do not arrive at the epidermis with peripheral nerves. Rather, the peripheral nerves are attracted to the dermal Merkel cells which originated in the epidermis.

Intraepidermal Formation of Merkel Cells in Xenografts of Human Fetal Skin

An experimental transplantation model using human fetal skin was applied to approach the question of the embryologic origin of human Merkel cells. Palmar and plantar skin from five fetuses, between 8 and 11 weeks of estimated gestational age (EGA), was xenografted to subcutaneous beds of nude mice. After 4 or 8 weeks of growth, biopsies were taken from these xenografts and examined for the presence of Merkel cells, using immunocytochemistry with antibodies specific for simple epithelial-type cytokeratins and neuron-specific enolase (NSE) as well as using electron microscopy. Skin from the same fetuses at the time of transplantation was screened in the same way. In all fetuses, no (or very scarce) epidermal Merkel cells were detected at the transplantation time, but in all cases abundant epidermal Merkel cells of apparent human origin were found after 4 or 8 weeks in xenograft culture. Dermal nerve fibers, as recognized by neurofilament antibodies, were scarce or essentially absent in the xenografts. These results indicate that Merkel cells regularly develop in epidermis dissected and xenografted in an early fetal stage, although the dissection implies the interruption of the dermal nerves. The results strongly support the notion of the origin of Merkel cells from epidermal precursor cells. The apparent absence of dermal Merkel cells and dermal nerve fibers in the xenografts suggests that the presence of dermal sensory nerve fibers may be required for the dropping off of epidermal Merkel cells into the upper dermis, which occurs in normal fetal development.

The development of the Merkel cells in the tentacles of Xenopus laevis larvae.

The Merkel cells in the larval tentacles of Xenopus laevis were examined by TEM. Different forms of Merkel cells were found, depending on the age of the larvae or the location in the tentacles. These forms have the appearance of intermediate states between Merkel cells and superficial epidermal cells; thus an epidermal origin for the Merkel cells seems more likely than an immigration from the neural crest. The forms differ in (1) their location in the epidermis, (2) their shape, (3) the number and extension of their desmosomes, (4) the content and distribution of dense-core granules, and (5) the outgrowth of their finger-like processes. Also the relation to a nerve ending is different. By marking Merkel cells with quinacrine, fluorescence spots were observed between the superficial and basal epidermal cells or, in the very tip, within the superficial epidermal cells. These latter spots represent immature Merkel cells, as confirmed by TEM. This indicates a development of Merkel cells from superficial epidermal cells and migration towards the basal layer. Dermal Merkel cells were never observed.

Formation of Epidermal and Dermal Merkel Cells During Human Fetal Skin Development

The origin of Merkel cells is still a matter of debate, specifically the question of whether they are derived from epithelial cells of the epidermis or from immigrated neural crest cells. As an argument for the latter hypothesis the occurrence of dermal, nerve-associated Merkel cells in human fetal skin has often been mentioned. Therefore, we analyzed the distribution of Merkel cells in epidermis and dermis of plantar skin of human embryos and fetuses, ranging in gestational age between 7 and 17 weeks. Merkel cells were identified by immunocytochemistry on frozen sections using antibodies against simple epithelium-type cytokeratins and by electron microscopy. In the 17-week-old fetus, 17% of the total cutaneous (epidermal and dermal) Merkel cells were located in the upper dermal compartment, whereas in the 14-week-old fetus only 3.9% of the Merkel cells were dermal, including some cells that seemed to be in the process of traversing the dermal-epidermal junction. Thirteen-week-old fetuses showed even fewer dermal Merkel cells. Twelve-week-old fetuses exhibited 660 epidermal Merkel cells per 100 mm total section length, but none in the upper or deep dermis. In 7- to 9-week embryos, no Merkel cells were recognized. However, at this stage, but not in later stages, the basal cells of the plantar epidermis expressed certain simple epithelium-type cytokeratin polypeptides. These results speak against an invasion of Merkel cells or putative neural crest-derived precursor cells into the epidermis via a dermal passage. They suggest that in plantar skin Merkel cells arise, between weeks 8-12, from precursor stages of epithelial cells of the early fetal epidermis which still express simple epithelium-type cytokeratins. The results further suggest that in subsequent stages of skin development some epidermal Merkel cells detach from the epithelium and migrate into the upper dermis where some of them may associate with small nerves.

Human dermal neuroendocrine cells (so-called dermal merkel cells): A further observation

Human dermal neuroendocrine cells (so-called dermal merkel cells): A further observation

The neuroanatomical basis for the protopathic sensibility of the human glans penis

The human glans penis is covered by stratified squamous epithelium and a dense layer of connective tissue equivalent to the dermis of typical skin. Rete ridges of the epidermis are irregular and vary in height depending on location, age, and presence or absence of a foreskin. The papillary layer of the dermis blends into and is continuous with the dense connective tissue forming the tunica albuginea of the corpus spongiosum of the glans penis. The most numerous nerve terminals are free nerve endings (FNEs) present in almost every dermal papilla, as well as scattered throughout the deeper dermis. FNEs are characterized by an incomplete Schwann cell investment, and contain irregularly scattered neurofilaments and neurotubules, clusters of mitochondria, vesicles of variable size and various inclusions. The ratio of FNEs to corpuscular receptors is approximately 10:1 and a similar ratio of small to large axons is seen in dermal nerves. Genital end bulbs are present throughout the glans, but are most numerous in the corona and near the frenulum. The unique corpuscular receptor of the glans penis consists of axon terminals that at an ultrastructural level resemble a tangled skein of FNEs. Simple, Pacinian and Ruffini corpuscles were occasionally identified predominantly in the corona glandis. Epidermal Merkel nerve endings and other types of mechanoreceptors typically found in primate glabrous skin (lip or digit) are not present. Rarely, dermal Merkel cells have been identified associated with genital end bulbs. The abundance of FNEs in isolated as well as corpuscular form can be correlated with the embryogenesis and known neurophysiologic and psychophysical parameters of sensory function of the glans penis. Finally, the divergence in reported values for the threshold of tactile and painful stimuli when applied to glabrous skin of fingertip and glans penis can be considered as an example of dissociated sensibility. The anatomical basis for this dissociation is the abundance of FNEs and absence of Merkel terminals and typical Meissner corpuscles in the covering of the glans, and the converse in glabrous skin of the digit.

Dermal Merkel cells in the nevus of Ota and leopard syndrome

We observed dermal Merkel cells around vellus hair follicles in one patient with nevus of Ota and one with leopard syndrome. These Merkel cells were in contact with Schwann cells and nerve endings in the dermis. The question of whether or not Merkel cells exist normally in adult dermis remains unanswered. However, their presence in these abnormal conditions suggests that normally they do occur in the dermis but go undetected because their numbers are so few or they are in a form not readily identifiable by currently available methods.

The sensorineural apparatus of the human eyelid.

The present study describes a complex array of sensory nerve terminals in the human eyelid. In many respects this pattern of innervation resembles that previously described in the rhesus monkey, but in other respects it is unique to man. The most prominent nerve terminals are a complex array of lanceolate and circular Ruffini and free nerve endings that envelop the eyelashes. In addition, Merkel cells have not been conclusively identified to date in other nonhuman primate nonsinus hairs. The external root sheath collar contains Merkel cells, and dermal Merkel cells have also been identified close to the collar. The anterior cutaneous surface of the eyelid contains small vellus hairs with typical lanceolate, Ruffini, and free nerve endings resembling those of primate facial skin. Scattered Meissner and scant simple corpuscles as well as scattered free nerve endings (FNEs) can be identified on the occlusal surface of the eyelid. Intraepithelial as well as dermal FNEs were most easily identified in this region in areas lacking other corpuscular receptors. Corpuscular receptors are especially common at the occlusal/conjunctival angle. The inner or conjunctival surface of the eyelid appears to be a glandular epithelium, whereas in the rhesus monkey it is stratified squamous epithelium. This epithelium needs additional study. In summary, the present study confirms the unique sensory neural status of the human eyelid and verifies the presence of Ruffini nerve terminals by light and electron microscopy and of free nerve ending terminals at least by light microscopy, as well as a unique pattern of innervation of the human eyelash.

Merkel Tactile Cells of the Human Embryo Nail

The Merkel tactile cell of the human skin has mainly been described in association with the Haarscheibe (hair disc) in the part of the body where lanugo hairs are present. In the present survey a number of Merkel cells were found in the nail matrix of 15-18 weeks old human embryos (figs. 1,2). Dorsal matrix contained more cells than the ventral half. It was also found in the perivascular spaces of the finger of the same embryos (fig. 3). In both locations nerve endings rich in mitochondria and synaptic vesicles were in direct contact with the Merkel cell (figs. 1-3). In the nail matrix the Merkel cell was most commonly found within or just above the basal layer. Less frequently, they were present in the upper dermis. Such dermal Merkel cells were completely covered with basal lamina suggesting that they were projecting downward from the basal layer. Occasionally, two cells with centrioles were seen side by side suggesting that one of them was produced from the other by mitosis (fig. 1).