The structure and function of the Haarscheibe

Atlas of Normal Microanatomy, Procedural and Processing Artifacts, Common Background Findings, and Neurotoxic Lesions in the Peripheral Nervous System of Laboratory Animals

The ability to differentiate among normal structures, procedural and processing artifacts, spontaneous background changes, and test article–related effects in the peripheral nervous system (PNS) is essential for interpreting microscopic features of ganglia and nerves evaluated in animal species commonly used in toxicity studies evaluating regulated products and chemicals. This atlas provides images of findings that may be encountered in ganglia and nerves of animal species commonly used in product discovery and development. Most atlas images are of tissues from control animals that were processed using routine methods (ie, immersion fixation in neutral-buffered 10% formalin, embedding in paraffin, sectioning at 5 µm, and staining with hematoxylin and eosin) since these preparations are traditionally applied to study materials produced during most animal toxicity studies. A few images are of tissues processed using special procedures (ie, immersion or perfusion fixation using methanol-free 4% formaldehyde, postfixation in glutaraldehyde and osmium, embedding in hard plastic resin, sectioning at 1 µm, and staining with toluidine blue), since these preparations promote better stabilization of lipids and thus optimal resolution of myelin sheaths. Together, this compilation provides a useful resource for discriminating among normal structures, procedure- and processing-related artifacts, incidental background changes, and treatment-induced findings that may be seen in PNS tissues of laboratory animals.

Neural Hedgehog signaling maintains stem cell renewal in the sensory touch dome epithelium

The touch dome is a highly patterned mechanosensory structure in the epidermis composed of specialized keratinocytes in juxtaposition with innervated Merkel cells. The touch dome epithelium is maintained by tissue-specific stem cells, but the signals that regulate the touch dome are not known. We identify touch dome stem cells that are unique among epidermal cells in their activated Hedgehog signaling and ability to maintain the touch dome as a distinct lineage compartment. Skin denervation reveals that renewal of touch dome stem cells requires a perineural microenvironment, and deleting Sonic hedgehog (Shh) in neurons or Smoothened in the epidermis demonstrates that Shh is an essential niche factor that maintains touch dome stem cells. Up-regulation of Hedgehog signaling results in neoplastic expansion of touch dome keratinocytes but no Merkel cell neoplasia. These findings demonstrate that nerve-derived Shh is a critical regulator of lineage-specific stem cells that maintain specialized sensory compartments in the epidermis.

Proliferative and non-proliferative lesions of the rat and mouse integument

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) project is a joint initiative of the societies of toxicological pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP). Its aim is to develop an internationally-accepted nomenclature for proliferative and non-proliferative lesions in laboratory rodents. A widely accepted international harmonization of nomenclature in laboratory animals will decrease confusion among regulatory and scientific research organizations in different countries and will provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists. The purpose of this publication is to provide a standardized nomenclature for classifying microscopical lesions observed in the integument of laboratory rats and mice. Example colour images are provided for most lesions. The standardized nomenclature presented in this document and additional colour images are also available electronically at http://www.goreni.org. The nomenclature presented herein is based on histopathology databases from government, academia, and industrial laboratories throughout the world, and covers lesions that develop spontaneously as well as those induced by exposure to various test materials. (DOI: 10.1293/tox.26.27S; J Toxicol Pathol 2013; 26: 27S-57S).

The effect of denervation of Merkel cells in cats

The morphology of Merkel cells in sinus hair follicles of the upper lip (vibrissae with a circular blood sinus), and in touch domes of the glabrous skin of the nose, was investigated in cats up to 12 weeks after resection of the infraorbital nerve. Even 12 weeks after denervation there was neither an ultrastructural alteration of Merkel cells nor a reduction of their number in sinus hair follicles. Also in touch domes the Merkel cells did not disappear.

Neuroendocrine (Merkel Cell) Carcinomas of the Skin: An Ultrastructural Study of Nine Cases

Nine neuroendocrine tumors of the skin were examined by light and electron microscopy. The patients (7 males and 2 females) had an average age of 57 years. Seven tumors were located in the head and neck region. Light microscopic examination showed sheets of cells. In 4 cases, a tendency to form small groups of cells was observed. Contact with the epidermis was seen in only 2 cases. The dominant ultrastructural feature was the presence of cytoplasmic processes that contained membrane-bound granules 100-200 nm. Three tumors recurred locally, and in 7 patients, regional nodal metastases occurred. Three patients died of disseminated disease.

Central and peripheral anatomy of slowly adapting type I low-threshold mechanoreceptors innervating trunk skin of neonatal mice

Despite intensive study, our understanding of the neuronal structures responsible for transducing the broad spectrum of environmental energies that impinge upon the skin has rested on inference and conjecture. This major shortcoming motivated the development of ex vivo somatosensory system preparations in neonatal mice in the hope that their small size might allow the peripheral terminals of physiologically identified sensory neurons to be labeled intracellularly for direct study. The present report describes the first such study of the peripheral terminals of four slowly adapting type I low-threshold mechanoreceptors (SAIs) that innervated the back skin of neonatal mice. In addition, this report includes information on the central anatomy of the same SAI afferents that were identified peripherally with both physiological and anatomical means, providing an essentially complete view of the central and peripheral morphology of individual SAI afferents in situ. Our findings reveal that SAIs in neonates are strikingly adult-like in all major respects. Afferents were exquisitely sensitive to mechanical stimuli and exhibited a distinctly irregular, slowly adapting discharge to stimulation of 1-4 punctate receptive fields in the skin. Their central collaterals formed transversely oriented and largely nonoverlapping arborizations limited to regions of the dorsal horn corresponding to laminae III-V. Their peripheral arborizations were restricted entirely within miniaturized touch domes, where they gave rise to expanded disc-like endings in close apposition to putative Merkel cells in basal epidermis. These findings therefore provide the first direct confirmation of the functional morphology of this physiologically unique afferent class.

Human Merkel cells – aspects of cell biology, distribution and functions

Human Merkel cells were first described by Friedrich S. Merkel in 1875 and named "Tastzellen" (touch cells) assuming a sensory touch function within the skin. Only ultrastructural research revealed their characteristics such as dense-core granules, plasma membrane spines and dendrites as well as a loosely arranged cytoskeleton. Biochemical analysis identified the expression of very specific cytokeratins (most notably CK 20) allowing the immunohistochemical detection of Merkel cells. In humans, they occur within the basal epidermis, being concentrated in eccrine glandular ridges of glabrous skin and in Haarscheiben of hairy skin, within belt-like clusters of hair follicles, and in certain mucosal tissues. Within the human skin, the dense-core granules contain heterogeneously distributed neuropeptides, some of which might work as neurotransmitters through which Merkel cells and their associated nerves exert their classical function as slowly adapting mechanoreceptors type I. This is the case in the Haarscheiben, small sensory organs containing keratinocytes with a special program of differentiation that includes the expression of CK 17 and Ber-EP4. Other peptides may act as growth factors and thus might participate in growth, differentiation and homeostasis of cutaneous structures. It is not yet clear whether the Merkel cell carcinomas, aggressive skin carcinomas, indeed arise from Merkel cells. We summarize and discuss data on the distribution, function and heterogeneity of human Merkel cells in normal and diseased skin.

Recent progress in studies on Merkel cell biology

Merkel cells ubiquitously distribute in the skin of vertebrates, from cyclostomes to mammals. It is well known that mammalian Merkel cells coupled with axon terminals of type I sensory nerve fibers form slowly adapting mechanoreceptors, Merkel endings, within the epidermis. However, there are still many unresolved problems in the biology of Merkel cells. We reviewed recently acquired knowledge about the histochemical nature of Merkel cell granules, the morphological heterogeneity of Merkel cells and the roles of neurotrophins and their receptors for the development and survival of the cells. We discuss the functional significance of Merkel cell granules and the heterogeneity of Merkel cell populations.

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.

Merkel cells are absent in basal cell carcinomas but frequently found in trichoblastomas. An immunohistochemical study

The possibility of a neuroendocrine differentiation in basal cell carcinomas (BCCs) has been a matter of debate for many years. In the present immunohistochemical study, applying the cytokeratins 8, 18 and 20 as the most established markers for Merkel cells (MCs), we did not find elevated numbers of MCs in any of 205 BCCs. This speaks against a neuroendocrine line of differentiation in BCCs. In contrast, we found various amounts of MCs in 15 of 36 trichoblastomas, which are the main benign differential diagnosis of BCC. In 4 trichoblastomas abundant MCs were spread over the whole epithelial tumor area. Additionally, the trichoblastomas' overlying epidermis exhibited significantly much higher numbers of MCs than the uninvolved adjacent skin or the epidermis overlying the BCCs. These findings might be an additional aid in the distinction between trichoblastomas and BCCs. Furthermore, concerning the recent discussion about the role of MC in growth and development of follicular germ, our observations are another sign of regulative influences of the MC, also in follicular germ under pathological conditions. Trichoblastomas with high numbers of MCs could be characterized as showing advanced differentiation toward the neuroendocrine component of the hair follicle, i.e., the MCs.

Cutaneous overexpression of NT-3 increases sensory and sympathetic neuron number and enhances touch dome and hair follicle innervation

Target-derived influences of nerve growth factor on neuronal survival and differentiation are well documented, though effects of other neurotrophins are less clear. To examine the influence of NT-3 neurotrophin overexpression in a target tissue of sensory and sympathetic neurons, transgenic mice were isolated that overexpress NT-3 in the epidermis. Overexpression of NT-3 led to a 42% increase in the number of dorsal root ganglia sensory neurons, a 70% increase in the number of trigeminal sensory neurons, and a 32% increase in sympathetic neurons. Elevated NT-3 also caused enlargement of touch dome mechanoreceptor units, sensory end organs innervated by slowly adapting type 1 (SA1) neurons. The enlarged touch dome units of the transgenics had an increased number of associated Merkel cells, cells at which SA1s terminate. An additional alteration of skin innervation in NT-3 transgenics was an increased density of myelinated circular endings associated with the piloneural complex. The enhancement of innervation to the skin was accompanied by a doubling in the number of sensory neurons expressing trkC. In addition, measures of nerve fibers in cross-sectional profiles of cutaneous saphenous nerves of transgenics showed a 60% increase in myelinated fibers. These results indicate that in vivo overexpression of NT-3 by the epidermis enhances the number of sensory and sympathetic neurons and the development of selected sensory endings of the skin.

Merkel cells are not the mechanosensory transducers in the touch dome of the rat

The identity of the mechanosensory transducing elements in the vertebrate touch receptors that contain Merkel cell-neurite complexes is unknown. The Merkel cells, however, have long been the favoured candidates. We have now selectively eliminated the Merkel cells from rat touch domes by first loading them with quinacrine, and then irradiating the domes with near-UV light. Mechanical stimulation of these domes revealed a range of mechanosensory function, evaluated qualitatively, that varied from non-responsive to normal. Since irradiation eliminated the quinacrine fluorescence, the status of the Merkel cells was evaluated by EM. In both responsive and unresponsive domes fixed for EM immediately following irradiation, the Merkel cells and associated nerve endings appeared to be normal. After 2 or more days, even in domes that continued to be normally responsive, there was a striking reduction in the normal complement of about 90 Merkel cells, and most of the remaining Merkel cells appeared to be degenerating. However, numerous 'isolated' (Merkel cell-free) nerve endings remained in the basal epidermis. A few of these nerve endings showed signs of damage, but in the non-responsive domes abnormal nerve endings were routinely observed. The EM studies did not exclude the possibility that a few surviving innervated Merkel cells, or even one such, had escaped detection and were responsible for a persisting mechanosensitivity. To resolve this issue a mechanical stimulating technique with a spatial resolution of 55 microns was used to map the mechanosensory profile of a single responsive dome irradiated 2.75 days earlier. This dome was then serially sectioned for EM study. Only seven Merkel cells had survived which appeared to be both viable and innervated, but almost all of the tested sites were normally responsive. When the correlation was made, seven of these sites were located 55-100 microns away from the nearest surviving Merkel cell, four were 110-165 microns away, and three were more than 165 microns away. Even when allowance is made for errors in the positioning of the stimulus, the responses at the last seven sites cannot be attributed to the presence of underlying Merkel cells. We conclude that mechanosensory transduction within touch domes is not a function of the Merkel cells, but must reside in the associated nerve endings.

Hairy skin: psychophysical channels and their physiological substrates

Experiments were conducted in which threshold-frequency characteristics were measured on the hairy skin of the forearm of human observers. Thresholds were measured with two stimulus probe areas (2.9 and 0.008 cm2) at three skin-surface temperatures (15 degrees, 30 degrees, and 40 degrees C). The results suggest that whereas glabrous skin uses four distinct channels of information, only three channels may be involved in mediating the sense of touch for hairy skin. The three channels are defined as Ph, (Pacinian, hairy skin), NPh low (non-Pacinian, hairy skin, low frequencies) and NPh mid (non-Pacinian, hairy skin, middle frequencies). In addition, it is proposed that the neural substrates for the three psychophysically characterized channels are, respectively, the Pacinian corpuscle (PC) nerve fibers, the slowly adapting type II (SAII) fibers, and the rapidly adapting (RA) fibers.

Observation of Merkel cells with scanning electron microscopy

This paper first elucidated the overall morphology of Merkel cells in the rat touch dome with scanning electron microscopy (SEM). Quinacrine-fluorescent Merkel cells in the touch dome were exposed by enzymatic treatment following application of dithiothreitol, photographed and then fixed. By referring to the photograph, the same fluorescent cells were easily identified under the SEM. Enzymatically isolated Merkel cells were also examined with SEM. Unlike quinacrine negative, ordinary epidermal cells, the Merkel cells had numerous finger-like processes, ranging from 0.1 to 0.25 micron in diameter and attaining to 2.5 microns in length.

The hairlet follicle in the skin of the wild Norway rat (Rattus norvegicus Berkenhout, 1769). Its morphology and function

The hairlet follicle of the wild Norway rat (Rattus norvegicus Berkenhout 1769) differs from the other follicles in position and in structural characteristics. As an element of the guard (tylotrich) hair follicle group the hairlet follicles are arranged at the margin of the hair disk. The hairlet follicle, which produces the finest hair fibre, is enclosed by an active unilobular sebaceous gland. The arrector pili muscle is absent, but the follicle does have a supply of lanceolate nerve endings and Ruffini endings. The axons of the lanceolate endings are enclosed sandwich-like by Schwann cells and contact the basal lamina of the outer epithelial root sheath. Ruffini endings run within the band of connective tissue fibres that girdle the follicle at the level of the sebaceous gland. The appropriate stimulus for excitation of the nerve fibres can not be achieved by bending the tiny hairlet fibre. Tissue compression, induced by bending the central guard (tylotrich) hair, which is transformed via the sebaceous gland and the slim outer epithelial root sheath, may exicite the nerves of the hairlet follicle. It can be assumed that the hairlet follicle functions as a mechanoreceptor.

Special program of differentiation expressed in keratinocytes of human haarscheiben: an analysis of individual cytokeratin polypeptides

Human haarscheiben, epidermal Merkel cell-rich sensory organs of hairy skin, were studied for the expression of various cytokeratin (CK) polypeptides and other epithelial and neuronal differentiation markers by applying immunoperoxidase and immunofluorescence microscopy to frozen sections and by two-dimensional gel electrophoresis. The basal clusters of Merkel cells were specifically detected by antibodies against CK 20. Haarscheiben keratinocytes were unique mainly by the prominent expression of CK 17 in the lower and middle layers. Further differences as compared to keratinocytes of usual epidermis included the enlargement of the basal compartment, characterized by the expression of CK 5 and the absence of the maturation-associated CKs 1/10/11, and the reduction of CK 15, which is a constituent of normal basal cells. Using CK 17 as a highly sensitive Haarscheibe marker in skin tissue sections, variabilities in the spatial relationship of the haarscheibe and the corresponding hair follicle were recorded. The results show that haarscheibe keratinocytes express a special program of differentiation that may be important for optimal stimulus perception. Immunohistochemical stainings for CK 17 will facilitate further studies on the distribution and biology of haarscheibe.

Vibrotactile sensitivity of slowly adapting type I sensory fibres associated with touch domes in cat hairy skin

1. Recordings were made from single slowly adapting type I (SAI) afferent fibres associated with touch domes in the cat hairy skin. Controlled vibratory stimuli were used first, to characterize the precision with which these SAI afferents reflect the temporal aspects of vibrotactile stimuli, and second, to determine whether earlier disparate reports of SAI responsiveness to vibration may be attributable to highly specific stimulus requirements. 2. Eighteen SAI fibres from femoral cutaneous nerve branches were examined; each was associated with one to three touch domes. SAI responses to both steps and sinusoidal vibration (1-1.5 s in duration) were affected profoundly by both probe size and position. Punctate stimulus probes (250 microns) produced much higher response levels and steeper stimulus-response relations than those elicited with large (2 mm) probes, probably on account of focal distortion created within the dome by the smaller probes. SAI sensitivity to vibration was also affected markedly by the amplitude of any pre-indentation on which the vibration was superimposed; sensitivity was much lower when the pre-indentation exceeded 100 microns, in particular with larger stimulus probes. 3. Measures of both vibration sensitivity and the precision of impulse patterning demonstrated that, if appropriate stimulus parameters are chosen, the SAI fibres can respond to 1 s trains of vibration (amplitude < or = 100 microns) in a tightly phase-locked, 1:1 manner for frequencies up to 500 Hz. At frequencies from approximately 100-500 Hz the SAI fibres displayed broad 1:1 plateaus, where their response rate remained constant over a range of amplitudes, and phased-locking was tightest. Responses remained phase-locked up to 1000 Hz, but could not follow the vibration with a 1:1 pattern above 500 Hz. 4. The results demonstrate that with appropriate stimulus parameters, touch dome-associated SAI fibres are capable of signalling vibrotactile information over a similar bandwidth of frequencies as do Pacinian sensory fibres. The variability in past reports of SAI vibration sensitivity may relate principally to differences in stimulus conditions. However, in view of the SAI capacity for responding to vibration with temporally precise, patterned activity, it appears that their reported failure to contribute to vibrotactile sensibility must be attributed to limitations imposed in the central processing of SAI signals.

Merkel cells in vitro: production of nerve growth factor and selective interactions with sensory neurons

A method has been developed for obtaining mixed primary cultures of dissociated epidermis enriched in Merkel cells. Merkel cells obtained from embryonic rat buccal pads were grown in serum-free medium and identified in vitro using a variety of histological and immunohistochemical markers. Quinacrine, a fluorescent amine, which has been used to identify Merkel cells in situ, labeled a morphologically distinct population of cells in vitro. Cells labeled with quinacrine had a large, phase bright nucleus with prominent nucleoli, surrounded by a phase dark perinuclear ring. Antibodies directed against neuron-specific enolase, another marker for Merkel cells in situ, and antibodies against a well-characterized neuroendocrine vesicle antigen also labeled this population of quinacrine fluorescent cells. Electron microscopic examination of our cultures indicated that cells containing characteristic features of Merkel cells including cytoplasmic dense-cored granules were present. A small but significant increase in the number of Merkel cells was observed over time in culture. Merkel cells supported the survival and outgrowth of both trigeminal ganglion sensory neurons and sympathetic neurons from the superior cervical ganglion in serum-free medium in the absence of exogenous nerve growth factor (NGF). Immunoblots probed with antibodies directed against NGF demonstrated that NGF was present in the medium taken from these cultures. NGF-like immunoreactivity colocalized to cells containing quinacrine fluorescence in situ and in vitro. Addition of antibodies directed against NGF to cocultures of Merkel cells and neurons decreased survival of sympathetic neurons by 90% and decreased survival of sensory neurons by 60%. These results suggest that Merkel cells are capable of providing trophic support for their normal complement of sensory neurons by producing NGF. Selective recognition of these targets was studied in vitro by characterizing the interactions between Merkel cells and growth cones from sensory or sympathetic neurons using both time-lapse videomicroscopy and standard morphometry of fixed cocultures. The majority of trigeminal ganglion sensory neurons (approximately 60%) extended growth cones onto clusters of Merkel cells. Neurites which contacted clusters of Merkel cells were significantly more highly branched than those growing on collagen. In contrast, the majority of sympathetic neurons (greater than 90%) failed to grow onto Merkel cells. Growth cones of sympathetic neurons often "collapsed" and retracted when contact was made with a cluster of Merkel cells. Fixation of Merkel cells with paraformaldehyde prior to coculture did not affect this difference between sensory and sympathetic neurite extension onto the Merkel cells. However, prior fixation of Merkel cells eradicated the apparent Merkel ce-induced branching of sensory neurites.(ABSTRACT TRUNCATED AT 400 WORDS)

HVEM serial-section analysis of rabbit foliate taste buds: I. Type III cells and their synapses

Serially sectioned rabbit foliate taste buds were examined with high voltage electron microscopy (HVEM) and computer-assisted, three-dimensional reconstruction. This report focuses on the ultrastructure of the type III cells and their synapses with sensory nerve fibers. Type III cells have previously been proposed to be the primary gustatory receptor cells in taste buds of rabbits and other mammals. Within rabbit foliate taste buds, type III cells constitute a well-defined, easily recognizable class and are the only taste bud cells observed to form synapses with intragemmal nerve fibers. Among 18 type III cells reconstructed from serial sections, 11 formed from 1 to 6 synapses each with nerve fibers; 7 reconstructed type III cells formed no synapses. Examples of both convergence and divergence of synaptic input from type III cells onto nerve fibers were observed. The sizes of the active zones of the synapses and numbers of vesicles associated with the presynaptic membrane specializations were highly variable. Dense-cored vesicles 80-140 nm in diameter were often found among the 40-60 nm clear vesicles clustered at presynaptic sites. At some synapses, these large dense-cored vesicles appeared to be the predominant vesicle type. This observation suggests that there may be functionally different types of synapses in taste buds, distinguished by the prevalence of either clear or dense-cored vesicles. Previous investigations have indicated that the dense-cored vesicles in type III cells may be storage sites for biogenic amines.

Localization of serotonin-like immunoreactivity in the Merkel cells of pig snout skin

The presence of serotonin in the Merkel cells of pig snout epidermis was investigated by the peroxidase-antiperoxidase immunohistochemical technique. Serotonin-like immunoreactive Merkel cells were found in groups located at the base of epidermal rete pegs and in the external root sheath of sinus hair follicles (vibrissae). Immunoreactivity was stronger on the basal side of the Merkel cells, where dense-cored granules are most numerous. Neither the nerve terminal associated with the Merkel cell nor the neighbouring epidermal cells were immunostained. These results are the first evidence of serotonin-like immunoreactivity in mammalian Merkel cells. The fact that immunoreactivity is strongest in those parts of the Merkel cells with the highest granule density suggests that in these cells serotonin is probably localized in the dense-cored granules.

Changes in nerves and neuropeptides in skin from 100 leprosy patients investigated by immunocytochemistry

The cutaneous innervation is now known to contain neuropeptides including substance P (SP) and calcitonin gene-related peptide (CGRP) in sensory nerves, and vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY), principally in autonomic nerves. Skin biopsies from 100 leprosy patients and equivalent areas from 50 non-leprosy controls were fixed in p-benzoquinone solution for immunofluorescence staining and in Bouin's fluid for classification of leprosy type. Antisera to the neural markers, neurofilaments, and protein gene product 9.5 (PGP 9.5), and to neuropeptides were used. Cutaneous nerves and nerve endings immunoreactive for neuropeptides, neurofilaments, and PGP 9.5 were seen in all non-leprous control cases. In leprosy, PGP 9.5- and neurofilament-immunoreactive nerve fibres were seen in all 14 cases of the indeterminate (early) type and in the majority (33/43) of lepromatous cases, but in a smaller proportion (15/43) of tuberculoid cases. Neuropeptide immunoreactivity was seen in only 2/14 of the indeterminate leprosy specimens and was completely absent in other types. This early disappearance may be of diagnostic significance. Thus, cutaneous sensory and autonomic dysfunctions in leprosy are well reflected by changes in nerve fibres and neuropeptides.

Axonal domains within shared touch domes in the rat: a comparison of their fate during conditions favoring collateral sprouting and following axonal regeneration

Low-threshold mechanosensory nerves in the adult rat differ both from their counterparts in lower vertebrates and from high-threshold nociceptive nerves in mammals in that they appear not to undergo collateral sprouting into adjacent denervated skin, although they will clearly regenerate into it after they are damaged. We have now studied the growth capabilities of the low-threshold nerves supplying touch domes, the visible mechanosensory structures scattered throughout the hairy skin. Touch domes in the rat are often multiply innervated. A serendipitous observation on such domes allowed us to investigate the possibility that a functional collateral sprouting of their nerves can indeed occur, but only to a spatially very restricted extent, e.g., within the confines of a partially denervated dome. We used a "prodder" with a tip diameter of 16 micron to examine the mechanosensory profile across single domes that were preselected as being supplied by only two axons, one running in each of two adjacent dorsal cutaneous nerves (DCNs). Simultaneous recordings were made of the afferent discharges evoked in these nerves when the prodder was applied at about 17 or more locations on a selected dome; the spatial resolution was better than 55 micron. We found that within such a shared dome, one axon can supply a discrete territory (its "domain"), which may or may not overlap with the corresponding domain of the other axon. In a preliminary electron microscopic study, we found no evidence for a sharing of single Merkel cells, which are the specialized sensory cells in touch domes, even in the regions of a shared dome where two domains overlapped; each innervated Merkel cell appeared to be contacted by a single nerve ending, implying that in a shared dome each axon probably supplies an exclusive subpopulation of the Merkel cells. We tested for functional collateral sprouting by eliminating one nerve to a shared dome, and at a selected time thereafter mapping the domain of the remaining axon to see whether it had enlarged. The result was the same whether the two domains initially had a region of overlap or not; no expansion of the surviving domain occurred over postoperative periods up to 4 months (an expansion of the domain by 55 micron would have been detected). Thus functional collateral sprouting had failed to occur.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of Merkel cell populations with contrasting sensitivities to neonatal deafferentation in the rat whisker pad

In this study, we used the quinacrine fluorescence technique to investigate the embryonic and early postnatal development of two distinct populations of Merkel cells in the rat whisker pad and the consequences of neonatal deafferentation on their subsequent development. Annular clusters of Merkel cells first appear in the epidermis near the caudal margin of the mystacial region between embryonic days E14 and E15 at dome sites located on horizontal ridges where the primordial vibrissal follicles develop. The development of these cells progresses in a caudorostral sequence across the whisker pad as does the development of the vibrissal follicles. Each cluster eventually forms a conical ridge or collar of about 130 Merkel cells that surrounds the vibrissal hair shaft as it penetrates the overlying pad epidermis. In the vibrissae, which develop as downgrowths from the horizontal ridges at the dome sites, Merkel cells first appear (caudally) between E16 and E17 and form a cylindrical cuff within the outer root sheath; cells are added progressively until about the end of the first postnatal week when a plateau level of about 750-800 cells is reached. Following unilateral transection of the infraorbital nerve at 24-36 hr after birth, these vibrissal Merkel cells continued to develop along a time course that was indistinguishable from normal, at least over the first 2 weeks of postnatal life. In contrast, all or most of the Merkel cells that normally develop within collars or annular clusters in the pad epidermis (around both the vibrissal and intervibrissal or pelage hairs) either disappeared within a few days or failed to develop. Other light and electron microscopic procedures supported the main findings and confirmed that the denervation was successful. Thus, the vibrissal Merkel cells, like those in the glabrous hindpaw, behaved as a distinct class which develops postnatally and is maintained (at least over a 2-week period) without the presence of sensory nerves. Since both the mystacial vibrissae and glabrous hindpaw have specialized cortical representations, a possible relationship between these findings and the organization of the somatosensory cortex during development is discussed.

Identification of Merkel cells in oral epithelium using antikeratin and antineuroendocrine monoclonal antibodies

Merkel cells are a rare cell type located at or near the basal lamina of oral epithelia and epidermis. They are associated with nerves, and may function as touch receptors. Merkel cells are difficult to identify by routine light microscopy, but have been identified by electron microscopy and specific antibodies. We demonstrate here that Merkel cells can be identified by immunohistochemistry, using a monoclonal antibody (LK2H11, Lloyd and Wilson, 1983) to neuroendocrine granules, and a monoclonal antibody (35 beta H11, Gown and Vogel, 1982) to a 54-kD keratin generally located in simple epithelia but not in stratified epithelia. Human oral tissue was fixed in Carnoy's fixative, and adjacent serial sections were stained with the two antibodies. Individual positively-stained cells were generally located at the bases of rete ridges. Cells in the same position in adjacent sections stained with both antibodies. Essentially all cells (94%) staining for neuroendocrine granules also gave a positive reaction for the 54-kD keratin, evidence that a single Merkel cell contains neuroendocrine granules and the 54-kD keratin. Staining of epithelial sheets of human oral tissues revealed a linear distribution of Merkel cells along the bases of rete ridges. We concluded that antibodies to simple epithelial keratins and neuroendocrine granules may be excellent immunohistochemical markers for Merkel cells, thus facilitating future studies to examine changes in Merkel cells and their distribution in pathologic conditions. The observation that Merkel cells can be stained with an antikeratin antibody strongly supports the epithelial origin of these cells.

Sensory neurons supplying touch domes near the body midlines project bilaterally in the thoracic spinal cord of rats

Sensory neurons that project bilaterally to laminae III-V of the dorsal horns in the thoracic spinal cord of rats were described in a previous anatomical study. The electrophysiological experiments reported here show that these neurons innervate skin near the dorsal and ventral midlines. Three series of experiments were undertaken: the regions of skin supplied by subdivisions of thoracic nerves were mapped by recording from the subdivisions while mechanically stimulating the skin; subdivisions containing the axons of contralaterally projecting sensory neurons were identified by antidromic activation from the contralateral dorsal horn; the receptive fields of individual sensory neurons and dorsal horn neurons were characterized by single-unit recording methods. Contralaterally projecting sensory neurons had axons in subdivisions of the thoracic nerves that supply skin adjacent to the body midlines. Their receptive fields were located near the dorsal or ventral midline but did not extend onto the contralateral side of the body. Most of the neurons were slowly adapting "type I" units supplying touch domes. Dorsal horn neurons receiving input from contralateral sensory neurons had receptive fields that overlapped the dorsal or ventral midline. This bilateral representation of skin near the midlines may be important for the fusion of the sensory maps of the right and left sides of the body.

Neuron-specific enolase and serotonin in the Merkel cells of conger-eel (Conger conger) epidermis. An immunohistochemical study

Immunocytochemical techniques were used to investigate the distribution and co-localization of neuron-specific enolase (NSE) and serotonin (5-HT) in the skin of the conger eel, Conger conger. NSE and 5-HT immunoreactivity were found in Merkel cells; these cells were also identified at the electron-microscope level by the presence of characteristic granules and their association with an intraepithelial nerve ending. For the first time, it was demonstrated that Merkel-cell granules of vertebrate skin exhibit an immunoreaction with 5-HT. The production of amines may indicate that the Merkel cells of C. conger have both secretory capabilities and transduction functions. However, immunocytochemical investigation of the synaptic zones at the electron microscope level will be necessary to confirm this hypothesis. The present histochemical results suggest that NSE and 5-HT may be marker substances for Merkel cells, and that immunocytochemistry is a useful tool for the light-microscopic localization of these cells.

Cat carotid bodies reinnervated by normal or foreign nerves

The carotid body of the cat was reinnervated with either its native nerve, the carotid sinus nerve (CSN, re-anastomosis), or a foreign nerve, the lingual branch of the IXth cranial nerve (LN, cross-anastomosis). In both types of preparations, regenerating axons from the LN or CSN readily penetrated carotid body parenchymal tissue, as demonstrated by axoplasmic transport of radiolabeled material from the petrosal (sensory) ganglion. Electron microscopy revealed nearly normal fiber invasion into lobules of glomus (type I) and sustentacular (type II) cells following reinnervation by either the foreign or native nerve. However, while the regenerated CSN fibers formed a normal complement of specialized axon terminals in contact with type I cells, the incidence of such terminals in LN reinnervated carotid bodies was reduced by over 90% (2-19 months survival time). This low incidence of specialized LN endings was correlated with reductions in the magnitude of the chemosensory discharge elicited in these preparations by asphyxia, NaCN or acetylcholine. These data suggest that chemosensitivity depends upon intimate association between glomus cells and afferent nerve endings; and that the ability to form such contacts may reside in particular axons whose incidence is higher in the CSN than in the LN.

Embryonic development of the mouse mutant pupoid foetus (pf/pf)

The pupoid foetus mutation in the mouse is a recessive lethal mutation causing death of homozygous (pf/pf) embryos immediately after birth. From 11.3 days gestation onwards, these embryos are characterised externally by the development of a tail twist, followed by apparent stunting of the limbs and tail (when compared with the development of these structures in normal embryos), lack of digits, distortion of facial features, and possession of a smooth, mottled skin. Embryos ranging in age from 11.3 days gestation to full term have been examined using light microscopy and scanning and transmission electron microscopy. The skeletal structure and internal organs of the embryo are normal, but abnormalities occur in the external epidermis, the dermis, and the peripheral sensory nerves. Development of the palate and the eyes are affected by the behaviour of these tissues. The epidermis undergoes hypertrophy and fails to differentiate, and, on the basis of morphological criteria and theoretical considerations, it is suggested that the pf gene is activated in the epidermis during the keratinization pathway, preventing differentiation and altering the cell surface characteristics of the cells. Other abnormalities are explained in terms of interactions with the epidermis. This mutant is compared with other similar mutants.

Sympathetically induced changes in the responses of slowly adapting type I receptors in cat skin

The effects of sympathetic efferent activity on slowly adapting Type I receptors in the hairy skin of cats were studied by recording from single afferent units in the saphenous nerve. Stimulation of the sympathetic trunk at 10 Hz had predominantly excitatory effects, which were seen in some units as the development of a background discharge in the absence of overt mechanical stimulation, or in most other units as a reduction in the threshold for mechanical activation. These effects generally persisted throughout the 3-min period of sympathetic stimulation (SS). The percentage of afferent units that began to discharge during SS was significantly greater in female cats than in males (53% vs. 19%). An increase in the force exerted by the skin on the stimulus probe was also observed during SS. Several tests were conducted to assess possible neurochemical and mechanical mechanisms of action. Administration of the alpha-adrenergic blocking agent phentolamine produced a marked reduction in the sympathetic effects. However, histochemical analysis of sections from the touch domes showed no catecholamine fluorescence near the sensory fibers. Cessation of local blood flow just prior to SS, produced by occlusion of the descending aorta, had no apparent effect on the sympathetically induced changes in afferent activity. It was concluded that sympathetic activity has an excitatory action on most Type I afferents in the cat. Because this sympathetic action is neither replicated nor altered by aortic occlusion, it appears not to be mediated by changes in blood flow. It also appears not to be mediated by direct neurotransmitter action on the sensory receptor, because no catecholamine fluorescence was observed, yet the action was blocked by an alpha-adrenergic blocker. It is likely, therefore, that this sympathetic action is mediated by some unidentified mechanical response within the skin.

Identification of Merkel cells in human skin by specific cytokeratin antibodies: changes of cell density and distribution in fetal and adult plantar epidermis

Merkel cells are special neurosecretory cells which, in adult human skin, are usually very scarce. By immunofluorescence microscopy using antibodies to human cytokeratin polypeptide no. 18, we localized distinct non-keratinocyte cells in the glandular ridges of human fetal and adult plantar epidermis. Using electron and immunofluorescence microscopy, these cells were identified as Merkel cells containing typical neurosecretory granules as well as bundles of intermediate-sized filaments and desmosomes. Two-dimensional gel electrophoresis of the cytoskeletal fractions of microdissected epidermal preparations highly enriched in Merkel cells indicated the presence of cytokeratin polypeptides nos. 8, 18 and 19 which are typical of diverse simple epithelia of the human body. Double immunofluorescence microscopy showed that these human Merkel cells contain neither neurofilaments nor vimentin filaments. In human fetuses of 18-24 weeks of age, conspicuously high concentrations of Merkel cells, reaching a density of approximately 1,700 Merkel cells/mm2 skin, were found in the glandular ridges of plantar skin. The concentration decreased considerably at newborn and adult stages. Thin cell processes (up to 20 microns long) were observed in many fetal epidermal Merkel cells. In addition, we detected isolated Merkel cells deeper in the dermis (i.e. at distances of, at most, 100 microns from the epidermis) in fetal and newborn plantar skin. Our results show that Merkel cells are true epithelial cells which, however, differ profoundly from epidermal keratinocytes in their cytokeratin expression. The findings are discussed in relation to the much disputed question of the origin of Merkel cells. The present data speak against the immigration of Merkel cells from the neural crest, but rather suggest that they originate from epithelial cells of the skin, although most probably not from differentiated keratinocytes.

Reinnervation of the rat touch dome restores the Merkel cell population reduced after denervation

By using the fluorescent dye quinacrine as a marker for the Merkel cells in the rat touch dome, we previously showed that a sustained denervation of the dome causes a rapid and persistent loss of about 60% of its Merkel cells [Nurse, Macintyre and Diamond (1984) Neuroscience 11, 521-533]. We now show that if the sensory nerves to the skin are crushed (or cut) in 2-week old pups and allowed to regenerate, the Merkel cell population within touch domes shows a biphasic response; there is an initial loss of Merkel cells associated with the early phase of denervation, followed by an increase, associated with the phase of reinnervation. Physiological tests revealed that many (though not all) domes within initially deafferented skin had become functionally reinnervated and had their Merkel cell numbers either wholly or partially restored some 40-100 days post operatively. In one case an adult reinnervated dome, that appeared normal physiologically and by its complement of quinacrine fluorescent (Merkel) cells, also had normal histological features in toluidine blue sections and normally innervated Merkel cells in the electron microscope. These results, based on the use of quinacrine to visualize the Merkel cell population in the touch dome, suggest that sensory nerves may induce the differentiation of new Merkel cells in domes where these cells have become reduced after denervation.

Effects of vincristine sulfate on touch dome function in the rat

The responses of touch domes in hairy skin of the rat to mechanical stimulation were examined after single doses of vincristine sulfate. Within 24 h of drug administration, the mean thresholds of domes to brief mechanical pulses had increased threefold, from 5.2 +/- 2.0 to 14.2 +/- 8.8 microns. This elevated threshold was maintained for 2 weeks but by the 3rd week the domes had recovered normal excitability. Measurements of response latency suggested that the increase in receptor thresholds occurred without impulse propagation being impaired in the axons.

Touch dome properties as a function of age

A study was made of touch domes in the lumbar skin of rats of different ages. The mean density of the domes in 11- to 29-day-old rats ("pups") was 43.1 +/- 16.6/cm2 and was approximately four times higher than that in adult animals (11.6 +/- 2.8/cm2); no loss of domes appeared to have taken place in the oldest animals examined (21 months). By repeating measurements of density in the territory of a single T13 dorsal cutaneous nerve, it was shown that density decreased during body growth because no additional domes were formed to compensate for enlargement of the skin. The mechanical thresholds of the domes were lowest in the youngest rats and highest in the oldest ones (mean values, 3.21 +/- 1.96 and 9.19 +/- 5.88 microns, respectively). The sizes of the domes increased until adulthood, the mean diameters being 170 +/- 8 and 314 +/- 9 microns, respectively in pups and mature animals. The possible implications of the findings for spatial discrimination by the rat central nervous system have been considered.

A fluorescent microscopic study of the development of rat touch domes and their Merkel cells

The ability of the fluorescent dye quinacrine to label epidermal Merkel cells was used to study the development of touch domes (Haarscheiben) in rat skin. In embryonic and early postnatal pups, sites of touch dome primordia were reliably located within strips of separated trunk epidermis by the occurrence of discrete clusters of fluorescent cells scattered across the basal cell layer. As in excised adult domes, most of these fluorescent (Merkel) cells lay caudal to the emerging tylotrich hair follicle and the cell cluster formed an annular or crescentric disc. Though all the touch domes that comprise the adult population appear to contain labelled Merkel cells by postnatal day 4, the number of these cells per dome continues to increase some 3-4 fold after birth to reach the average adult number (ca 90 cells) on the dorsal trunk around the fifth week. At about this time the adult size (ca 250 micron) of the dorsal Haarscheiben is also reached, as estimated by the length of the long axis of the fluorescent disc. The rostro-caudal orientation of this long axis varied from dome to dome, usually in the range 0-45 degrees clockwise or counterclockwise, and a functional correlate of this is suggested. The density of labelled dome primordia is higher on the dorsal than on the ventral trunk from as early as embryonic day E20-21 and remains higher although the density of domes decreases steadily across the skin during postnatal growth of the animal. This study utilizes a novel fluorescent labelling technique that is applicable to microscopic studies on the development of Merkel cells within their epithelial locations, in this instance the rat touch domes. Although the full population of domes is established early in development, the Merkel cells within a dome continue to increase in number for several weeks after physiological function is known to appear in this structure.

A quantitative study of the time course of the reduction in Merkel cell number within denervated rat touch domes

By using the fluorescent dye quinacrine as a marker for the Merkel cells in rat touch domes we have shown that denervation results in a progressive reduction in the number of these cells to a level that remains relatively constant at about 40% of that present at the time of denervation. The time-course of quinacrine fluorescent cell changes after denervation could be described by assuming that (i) there are two populations of quinacrine fluorescent cells, one labile and the other stable, and (ii) the labile population is the one most sensitive to denervation and disappears exponentially with a half-time of ca 10 days. It appeared that this time-course of decay of the labile quinacrine fluorescent cells was the same whether the denervation was performed during the period of postnatal development studied (at 7 and 14 days), when normally Merkel cells are rapidly added to the dome, or later (at 35 and 60 days) when the adult population is virtually established. Correlative electron microscopic studies confirmed that quinacrine fluorescent cell counts reflect fairly accurately the Merkel cell population in denervated domes. These quantitative findings based on the use of quinacrine to visualize the entire Merkel cell population of touch domes show that the normal development and maintenance of these cells are trophically dependent on sensory nerves, although a subpopulation may persist even in long-term denervated domes. In addition, the similarity of the first order rate constant for the decay of quinacrine fluorescent cells after denervation and for the normal generation of quinacrine fluorescent cells suggests that the labile Merkel cell population is one that turns over continuously in the normally innervated touch dome.

Denervation reduces extractable Met-enkephalin-like immunoreactivity in guinea-pig lips

Skin extracts of the upper lip of 18 guinea pigs were made and subjected to a Met-enkephalin radioimmunoassay 3-4 weeks, 8 weeks, and 12 weeks after transection of one infraorbital nerve. There was a significant reduction in the concentration of extractable Met-enkephalin-like immunoreactivity in the denervated half of the lip as compared to the innervated side. The reduction increased with time. These results show that the enkephalin content of the guinea pig upper lip depends on an intact innervation by the infraorbital nerve.

Effects of chronic denervation in type I cutaneous mechanoreceptors (Haarscheiben)

Cutaneous type I receptor sites (Haarscheiben or tactile domes) were examined at intervals of 4 to 275 days after chronic denervation of the skin. The number of domes decreased with denervation time, and only about one-third of the domes originally present were still visible at 275 days. Most but not all of the Merkel cells from these domes were absent by 48 days, and the epithelium was significantly thinner than in nondenervated domes. Only a few of the examined domes appeared to be completely devoid of Merkel cells. It is concluded that after nerve transection, domes degenerate but do not always disappear entirely. The remnants may thus act as target sites which either attract regenerating type I nerve fibers or facilitate the formation of new dome structures after nerve regeneration.

The morphology and distribution of Merkel cells in primate gingival mucosa

The morphology and distribution of Merkel cells in primate gingival mucosa have been studied by correlated light and electron microscopic techniques. The gingival mucosa is composed of a stratified squamous epithelium with a dense underlying connective tissue stroma. The epithelium inter-digitates with the underlying connective tissue forming long interconnected rete ridges. Merkel cells and their associated axons are abundant in gingival mucosa where they are located, either individually or in clusters, at the base of epithelial rete ridges. These cells have an identical morphology to Merkel cells described by others in the hard palate, hairy skin, glabrous skin and eyelid. While individual Merkel cells are found throughout the gingival mucosa, Merkel cell clusters are most numerous in the mandibular lingual gingival mucosa. When correlating this finding with data from other investigators, it appears that Merkel cell clusters are located preferentially in the masticatory mucosa in intimate contact with the tongue and thus may function as an important source of somatosensory feedback providing valuable information regarding the position of the tongue in the oral cavity.

Granular basal cell tumors in the skin of three dogs: a distinct histopathologic entity

Granular cell variants of cutaneous basal cell tumors from three dogs are described. These tumors resemble a granular cell variant of basal cell tumor (carcinoma) in man. The tumors are characterized by classical basal cells as well as granular cells with numerous cytoplasmic granules. These granules are cytoplasmic vacuoles 0.3 micrometer to 1.5 micrometer that seem to be secondary lysosomes; they usually contain many small (42 nm), membrane-bound vesicles. No natural, non-neoplastic counterpart of these granular cells is known to exist in the skin, and the significance of the production and accumulation of the small vesicles is not known. The cytoplasmic granules, and their contents, resemble the granules of granular cell "myoblastoma" although granular basal cells are of epithelial origin.

Fine structural and morphometric studies of the Merkel cell during fetal and postnatal development

The morphological and morphometric changes of the Merkel cells during fetal and postnatal development were studied in the glabrous digital pads of rats. In 20-day-old fetus rats, the Merkel cells we observed were present in the lower spinous and basal layers, and not associated with axon terminals. The Merkel cell granules were few and sparse. The Merkel cell had clumps of fibrils and formed many desmosomes with surrounding keratinocytes. In postnatal rats, innervation was followed by an increase in the number of Merkel cell granules, and their specific accumulation. The fibrils of the Merkel cells were not prominent. It was found by t-test that the numerical density of the Merkel cell granules significantly increased from the fetal stage to 4-day-old postnatal rats. These results suggest that the Merkel cells are present in the epidermis without nerve contact in 20-day-old fetus rats, and that innervation is necessary for the increase of cellular activity in Merkel cells.

Regenerating axons reclaim sensory targets from collateral nerve sprouts

There is a critical period for the sprouting of intact low-threshold mechanosensory cutaneous nerves in rats; functional invasion of adjacent denervated skin does not occur in animals older than about 20 days of age, and it is largely confined to denervated skin within the "domain" of the parent dermatome. These nerves can regenerate readily in the adult, however, and such regenerating nerves do not respect domain borders; moreover, they functionally displace endings of intact nerves that earlier had sprouted into denervated skin.

Hormone-induced enlargement of receptive fields in trigeminal mechanoreceptive neurons. I. Time course, hormone, sex and modality specificity

The phenomenon of hormone induced enlargement of receptive field area in mechanoreceptive neurons was investigated with respect to time course, hormone, sex and modality specificity. In acute recording experiments, threshold receptive field boundaries of single trigeminal ganglion neurons innervating the facial region of rats were determined for several different treatment groups. Among ovariectomized females injected with estradiol benzoate (EB) for 2, 5 or 10 days, receptive field areas were significantly increased after only 2 days. Enlargement of field areas appeared to be maximal after 5 days of EB. Lordosis behavior was observed after 10 days of EB but not after 2 or 5 days, suggesting that receptive field enlargement occurs independently of hormone induced lordosis behavior. In normal, intact females, receptive field area varied significantly with the estrous cycle (large during estrus, small during diestrus) indicating that this phenomenon is naturally occurring, not dependent upon the method of estrogen administration. Progesterone given for 10 days had no effect on receptive field area. Treatment with 2 days of EB followed by a single progesterone injection produced reliable lordosis but did not result in any further increase in receptive field area beyond the increase produced by 2 days of EB alone. Thus the synergism between estrogen and progesterone that is recognized for induction of lordosis is absent with respect to receptive field enlargement. The small increase in field area following 10 days of testosterone treatment could be accounted for by the fact that such animals, had blood serum estradiol levels comparable to animals given 2 or 5 days of EB, possibly the result of a testosterone to estrogen conversion. It was concluded that hormone induced receptive field enlargement appears specific for estrogen. Castrated male rats treated with 10 days of EB show the same increase in receptive field area as females, indicating that the phenomenon is not specific to females. Analysis of receptive field area as a function of receptive field type and adaptation rate showed that EB induced field area enlargement was specific to rapidly adapting mechanoreceptive neurons.

Ultrastructure of Merkel cells in the hard palate of the squirrel monkey (Saimiri sciureus)

The distribution and ultrastructure of Merkel cells in the hard palate was investigated in the squirrel monkey (Saimiri sciureus) after fixation by vascular perfusion. Merkel cells were clustered at the base of the epithelial rete pegs of the hard palate. They were characterized by concentrations of dense-cored granules and closely associated intraepithelial nerve endings. Numerous spiny processes extending from the Merkel cell were intercalated with adjacent keratinocytes, probably serving to detect and amplify movement of adjacent cells. These spiny processes contained a rigid core of parallel microfilaments which were interrelated with cytoplasmic filament bundles located beneath the cell membrane. Transitional cells containing dense-cored granules and well developed tonofibrils appear to be related to both Merkel cells and keratinocytes. The findings of this study suggest that Merkel cells are highly adapted for detection of movement in adjacent keratinocytes, as well as movement of the epithelium with respect to the underlying connective tissue.

The specialized junctions between Merkel cell and neurite: an electron microscopic study

Longitudinal serial sections of one half of the entire sinus hair of a mouse were examined by the electron microscope. Three neurites entering the outer root sheath from the perifollicular blood sinus were encountered. These were separate nerve trunks from those connected with perifollicular tactile nerve endings and exclusively innervated intrafollicular Merkel cells. Two types of specialized junctions were observed at the contact regions between Merkel cell plasma membrane and neurite plasma membrane: (i) desmosome-like structures in which small clear vesicles and/or the large cored vesicles of neurite and thicker membrane (post-synaptic?) of apposed Merkel cell were found ant (ii) synapse-like structures in which Merkel cell granules were concentrated near the plasma membrane and the membrane of the apposed neurite was usually thicker (post-synaptic?). In some of the synapse-like junctions the limiting membrane of Merkel cell granules fused with the Merkel cell plasma membrane and its content seemed to be discharged into the intercellular space. This suggested actual exocytotic secretion of Merkel cell granules. Juxtaposition of 2 types of junctions, i.e. (i) and (ii) above, was also found. This suggested the possibility that the reciprocal synapse would be present between Merkel cells and afferent neurites.

The Merkel cell--a member of the APUD cell system. Fluorescence and electron microscopic contribution to the neurotransmitter function of the Merkel cell granules

Merkel cells of sinus hair follicles of nude mice were investigated by fluorescence and electron microscopy following pretreatment with amine precursors (L-Dopa, L-5-Hydroxytryptophan) and monoaminoxidase-inhibitors (Marsilid, Harmaline). Neither in control animals nor in pretreated animals any evidence for an involvement of Merkel cells in monoamine metabolism could be found. Therefore, the hypothesis that Merkel cells might share the most constant cytochemical characteristics of the cells of the APUD series, i.e., to take up amine precursors, should definitely be left. Following Harmaline treatment, however, Merkel cells were found fluorescent; cytophotometric analysis of the fluorescence emission spectra of formaldehyde-treated tissue ascertained that this fluorescence was due merely to a specific Harmaline fluorescence. The significance of Harmaline uptake in Merkel cells, most probably in the Merkel cell granules, is discussed.