Three-dimensional analyses of touch domes in the hairy skin of the cat paw reveal morphological substrates for complex sensory processing.

The three-dimensional morphology of the innervation of touch domes in the hairy skin folds of cat forepaws was investigated by the confocal laser scanning microscopic analyses of sections stained immunocytochemically with primary antibodies for protein gene product 9.5, neurofilament 200 and cytokeratin 20 in combination with transmission electron microscopic observations. One square centimeter of interdigital skin can contain as many as 68 touch domes. Each touch dome can have up to 150 Merkel cells and all are innervated by a single large-caliber afferent myelinated nerve fiber at the level of the palisade endings around the guard hair. It gives rise to multiple long, myelinated branches. Each final myelinated branch gives rise to several short and fine unmyelinated branches, supplying approximately 15 Merkel cell-axon complexes. Each Merkel cell is typically contacted by multiple small discoid endings instead of by a large single one. Discoid endings on separate Merkel cells were usually the distal ends of the unmyelinated branches, although, some were en-passant swellings of the branches. Only a few Merkel cell-axon complexes at the marginal zone of each territory could also be supplied by adjacent final myelinated branches. Each Merkel cell is surrounded by protrusions of keratinocytes that are penetrated by several collagen bundles of the dermis. This intricate pattern of innervation may explain the unique irregular discharges of action potentials typical for this type of mechanoreceptor.

Apoptosis of Merkel cells in neurotrophin-3 null mice.

Postnatal mice lacking neurotrophin-3 (NT3) are deficient in Merkel cells of touch domes and whisker follicles. We examined the mechanism of Merkel cell loss by immunocytochemistry and electron microscopy. Merkel cell of whisker follicles of NT3 null newborns exhibited decreased immunoreactivity for cytokeratin 8 and contained apoptotic bodies that were positive for cleaved caspase-3, a marker of active apoptosis. By electron microscopy, the Merkel cells displayed aggregation of chromatin along the nuclear membrane, with the marginated chromatin forming caps at the periphery of the nucleus. Ribosomes aggregated in the cytoplasm, while dense core granules characteristic of Merkel cells were still discernible. Finally, the Merkel cells and their nuclei fragmented into apoptotic bodies. None of the apoptotic Merkel cells were contacted by nerve fibers, and their desmosomal contacts with surrounding keratinocytes disappeared. After postnatal day 6 apoptotic Merkel cells were no longer observed, and the number of surviving Merkel cells was severely reduced. They were flat and contained few osmiophilic granules. We conclude that perinatal apoptosis is responsible for the loss of Merkel cells lacking innervation in NT3 null mice.

FOXN1 is critical for onycholemmal terminal differentiation in nude (Foxn1) mice.

Nude mice have a mutation in the transcription factor Foxn1(nu), resulting in downregulation of hair keratins. Although hair follicles develop normally, the hair fibers become structurally weak, curl, and break off at the surface. Nails in nude mice are deformed, based on alterations of the onychocyte differentiation process. Elemental microanalysis of the nail plate reveals marked decreases in sulfur concentrations in the nude mouse nail plates. Immunohistochemistry shows a lack of keratin 1 expression in terminally differentiating keratinocytes of the nail matrix. Instead, the typical differentiation process of the matrix is altered toward an epidermis-like differentiation pattern, comprising the production of filaggrin-containing keratohyalin granules in cells resembling those of the stratum granulosum, which are never observed in normally haired mice. The nail plate has diffuse basophilic stippling. It is thinner than normal, weak, and in most Foxn1(nu)/Foxn1(nu) mice breaks where it separates from the hyponychium. These studies indicate that the Foxn1(nu) mutated gene has effects beyond downregulating keratin expression, including changes in filaggrin expression, and is critical for normal onycholemmal differentiation. The nails of nude mice provide new insights into the molecular controls of onychocyte differentiation, and they offer a useful model to investigate the pathogenesis of nail hypergranulosis, a common feature in human nail diseases.

Friedrich Sigmund Merkel and his "Merkel cell", morphology, development, and physiology: review and new results.

Merkel nerve endings are mechanoreceptors in the mammalian skin. They consist of large, pale cells with lobulated nuclei forming synapse-like contacts with enlarged terminal endings of myelinated nerve fibers. They were first described by F.S. Merkel in 1875. They are found in the skin and in those parts of the mucosa derived from the ectoderm. In mammals (apart from man), the largest accumulation of Merkel nerve endings is found in whiskers. In all vertebrates, Merkel nerve endings are located in the basal layer of the epidermis, apart from birds, where they are located in the dermis. Cytoskeletal filaments consisting of cytokeratins and osmiophilic granules containing a variety of neuropeptides are found in Merkel cells. In anseriform birds, groups of cells resembling Merkel cells, with discoid nerve terminals between cells, form Grandry corpuscles. There has been controversy over the origin of Merkel cells. Results from chick/quail chimeras show that, in birds, Merkel cells are a subpopulation of cells derived from the neural crest, which thus excludes their development from the epidermis. Most recently, also in mammals, conclusive evidence for a neural crest origin of Merkel cells has been obtained. Merkel cells and nerve terminals form mechanoreceptors. Calcium ions enter Merkel cells in response to mechanical stimuli, a process which triggers the release of calcium from intracellular stores resulting in exocytosis of neurotransmitter or neuromodulator. Recent results suggest that there may be glutamatergic transmission between Merkel cell and nerve terminal, which appears to be essential for the characteristic slowly adapting response of these receptors during maintained mechanical stimuli. Thus, we are convinced that Merkel cells with associated nerve terminals function as mechanoreceptor cells. Cells in the skin with a similar appearance as Merkel cells, but without contact to nerve terminals, are probably part of a diffuse neuroendocrine system and do not function as mechanoreceptors. Probably these cells, rather than those acting as mechanoreceptors, are the origin of a highly malignant skin cancer called Merkel cell carcinoma.

Histological evaluation of age-related changes in human respiratory mucosa of the middle turbinate.

Though the lining of the healthy human nose is subject of numerous investigations, age-related changes have attracted little attention. Furthermore, varying sampling sites complicate the evaluation of the available information. Reference is needed for evaluation of abnormal conditions, e.g. in persons with sleep apnea. Biopsy samples from the middle turbinate of 40 healthy humans aged 5 to 75 years were examined. Tissue samples were fixed immediately in glutaraldehyde and embedded in Epon for semi-thin sectioning and blind histological evaluation. In all age groups, patchy arrangement of different epithelial types were found. Ciliated epithelium decrease with age. Stratified and atrophic epithelial types show striking changes. Thin atrophic epithelium with conspicuously thickened basement membranes is only seen above the age of 40 years and is frequently found in older subjects. Stratified epithelial types initially increase with age, but decrease above 50 years. In the middle aged group (26 to 50 years) we found a remarkably high amount of squamous metaplasia, resulting in overall thicker epithelium becoming thinner in older subjects. In contrast, basement membranes are thinnest in the middle aged group. In conclusion, evaluating the histology of nasal respiratory epithelium must take pronounced age-related physiological changes into account.

Histologic evaluation of nasal epithelium of the middle turbinate in untreated OSAS patients and during nCPAP therapy.

OSAS-patients complain about nasal disorders. Irritation of the nasal mucosa often leads to termination of nCPAP treatment. The aim of this study is to evaluate whether symptoms are related to histologic changes of the nasal mucosa of the head of the middle turbinate in OSAS-patients. Semi-thin sections of epon-embedded middle turbinate biopsy samples from 35 male patients (age 51-75 yr) with OSAS were compared with those of 10 healthy men (age 51-75 yr). In untreated OSAS-patients atrophic epithelium is common whereas ciliated epithelial types are rare. After short-time nCPAP therapy (mean 581h) patches of ciliated epithelium and squamous metaplasia reappear. Short-term nCPAP leads to a partially restoration of the mucosal architecture. During long-time nCPAP therapy (mean 6.737h) squamous metaplasia with conspicuous intraepithelial connective tissue papillae predominates whereas pseudostratified ciliated epithelium is missing. Dense round cell infiltrates in the lamina propria are frequently found. Rhinitic symptoms in OSAS-patients are correlated with marked histological changes of the respiratory epithelium of the head of the middle turbinate. Histological changes in untreated OSAS differ from those of patients during nCPAP-treatment.

Neural crest origin of mammalian Merkel cells.

Here, we provide evidence for the neural crest origin of mammalian Merkel cells. Together with nerve terminals, Merkel cells form slowly adapting cutaneous mechanoreceptors that transduce steady indentation in hairy and glabrous skin. We have determined the ontogenetic origin of Merkel cells in Wnt1-cre/R26R compound transgenic mice, in which neural crest cells are marked indelibly. Merkel cells in whiskers and interfollicular locations express the transgene, beta-galactosidase, identifying them as neural crest descendants. We thus conclude that murine Merkel cells originate from the neural crest.