Formation of epidermal and dermal Merkel cells during human fetal skin development

Merkel cell carcinoma of the head and neck associated with Bowen's disease

The Merkel cell carcinoma occurs primarily in the skin of the head and neck, and develops in the dermis with a trabecular growth pattern. Immunohistochemistry reveals positive staining for neuron-specific enolase, neurofilaments, cytokeratin and chromogranin A. Electron microscopically, the tumor cells contain dense-core granules, spinous cytoplasmic processes, desmosomes, zonulae adherentes and paranuclear filament aggregates besides frequent mitoses, focal necroses and lymphocyte and plasma cell infiltrates. The Merkel cell carcinoma is often co-existent with other malignancies such as squamous cell carcinoma or, as in the present study, with Bowen's disease. The definite diagnosis of the Merkel cell carcinoma can be effected only by electron microscopic examination of the tumor.

Origin of spinal cord meninges, sheaths of peripheral nerves, and cutaneous receptors including Merkel cells. An experimental and ultrastructural study with avian chimeras

The origin of cells covering the nervous system and the cutaneous receptors was studied using the quail-chick marking technique and light and electron microscopy. In the first experimental series the brachial neural tube of the quail was grafted in place of a corresponding neural tube segment of the chick embryo at HH-stages 10 to 14. In the second series the leg bud of quail embryos at HH-stages 18-20 was grafted in place of the leg bud of the chick embryos of the same stages and vice versa. It was found that all meningeal layers of the spinal cord, the perineurium and the endoneurium of peripheral nerves, as well as the capsular and inner space cells of Herbst sensory corpuscles, develop from the local mesenchymal cells. Schwann cells and cells of the inner core of sensory corpuscles are of neural crest origin. The precursors of Merkel cells migrate similarly to the Schwann cells into the limb bud where they later differentiate. This means that in addition to the Schwann cells and the melanocytes a further neural crest-derived subpopulation of cells enters the limb.

The Merkel cell and associated neoplasms in the eyelids and periocular region

Merkel cells are clear oval cells in the epidermis and outer root sheaths of hair follicles, which are probably of epithelial origin, share ultrastructural features with neuroendocrine cells, and are found in association with touch receptors. In the eyelid, they occur singly in the epidermis and external root sheaths of hairs and eyelashes, and in specialized touch spots alternating with eyelashes. Their typical electron microscopical and antigenic features include dense-core granules, intranuclear rodlets, spinous processes, and a positive reaction for specific cytokeratins, epithelial membrane antigen, neuron-specific enolase, chromogranin and synaptophysin. Merkel cell carcinoma probably develops from precursor cells which give rise to keratinocytes and Merkel cells, and nearly one out of ten Merkel cell carcinomas occur in the eyelid and periocular region. They tend to be bulging lesions near the lid margin of elderly patients, reddish in color, and erythematous with telangiectatic vessels. The diagnosis is based on the frequent presence of neurofilaments and paranuclear aggregates of intermediate filaments in addition to features typical of normal Merkel cells. The tumor often mimics lymphoma or undifferentiated carcinoma and frequently invades lymphatic vessels. One third of Merkel cell carcinomas recur, almost two thirds give rise to regional node metastases, and up to one half metastasize widely and result in death. Initial treatment should be prompt and aggressive, with wide resection and routine postoperative irradiation. Although metastatic lesions often respond to radiation therapy and cytostatic drugs, these treatments are mainly of palliative value.

Relationship between Merkel cells and nerve endings during embryogenesis in the mouse epidermis

Close relationships between Merkel cells (MC) and nerve endings (NE) exist in the adult mouse. Because MC may serve as targets for the ingrowth of NE during embryogenesis, the purpose of the present study was to analyze the relationship between MC and NE during embryogenesis. Frozen tissue from whisker pads and backs of NMRI mouse embryos (12-17 d gestational age) were studied by double-labeling indirect immunofluorescence (IIF) with a cytokeratin monoclonal antibody that recognizes MC and with a neurofilament anti-serum. Such an approach allowed the analysis of a large number of MC (up to 5000), thus yielding quantitative data. At day 12 of gestational age, no MC were observed by IIF. From day 13 to 17, the number of MC, as well as their association with NE, progressively increased. On day 13, only 57% of whisker pad MC were NE associated, whereas by day 17, 95% were NE associated. These results were confirmed by electron microscopic (EM) observations. On the back, the same chronologic relationship between MC and NE was observed, but was later in the course of embryogenesis. There was also a time- and zone-dependent increase in MC association with NE in the epidermal zones studied (isthmic, parafollicular, interfollicular). These observations 1) establish the time course of MC and NE contacts during embryogenesis in the mouse epidermis, 2) show that MC are present in the epidermis and appendages before NE reach the epithelium, and 3) support the hypothesis that MC could act as targets for the growing NE.

Presence of Merkel cells in sun-exposed and not sun-exposed skin: a quantitative study

Merkel cells (MCs), the neuroendocrine cells of the skin cannot be identified with certainty using conventional light microscopic staining methods. Using immunoperoxidase microscopy with antibodies specific for cytokeratin 18, which has been established as a marker protein of MCs, we have evaluated the numbers of MCs per mm2 skin in normal and sun-damaged upper arm skin. The sun-exposed skin contained twice as many MCs as the not sun exposed skin. Further quantification of MC density at various body sites (trunk, leg) showed a rather variable but often unexpectedly high MC density. The possible role of MC in development of actinic elastosis is discussed.

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.

Ultrastructure and neuron-specific enolase (NSE) immunohistochemistry of Merkel cells in normal toad epidermis, and following ablation of the pars distalis of the pituitary gland

The present study was performed in order to re-examine the possible Merkel cell dependency upon the anterior pituitary by a combined ultrastructural and NSE immunohistochemical analysis of Merkel cells in normal toads, and following pars distalis ablation. Ultrastructurally, toad Merkel cells appeared similar to those in previous reports. They were found in normal as well as in operated toads, but with lower frequency in the latter group. By NSE immunohistochemistry, Merkel cells were seen in normal toads only. Even in individual, operated toads, in which Merkel cells were found with relative high frequency by electron microscopy, no NSE could be demonstrated immunohistochemically. It is discussed whether the amount of NSE present depends upon the physiological state of the Merkel cell and in some cells occurs in so low an amount that the NSE antigen cannot be detected by the immunocytochemical method applied. If it is so, the failure to demonstrate Merkel cells in the operated toads by means of the specific marker NSE may be interpreted as an inhibition of NSE expression following pars distalis ablation. This interpretation combined with the lower number of Merkel cells found ultrastructurally in operated toads support a previous indication of an influence of the anterior pituitary--directly or indirectly--upon toad Merkel cell function.

Characterization of subcolumnar reserve cells and other epithelia of human uterine cervix. Demonstration of diverse cytokeratin polypeptides in reserve cells

We have analyzed the expression of cytokeratin polypeptides in subcolumnar reserve cells of the human uterine endocervical mucosa and the other epithelial cells using immunoperoxidase and immunofluorescence microscopy as well as by applying two-dimensional gel electrophoresis to microdissected cytoskeletal preparations. Endocervical columnar cells were uniformly positive for antibodies directed against the simple epithelium-type cytokeratins nos. 7, 8, 18, and 19, while a variable proportion of these cells was stained by an antibody against cytokeratin no. 4. Reserve cells were not only positive for cytokeratins nos. 8 (weakly and variably) and 19 but were also decorated by antibody KA 1, which reacts with cytokeratins present in stratified squamous epithelia. This last antibody selectively decorated reserve cells even when they were flat and inconspicuous. Antibody KA 1 uniformly stained the ectocervical squamous epithelium, the basal cells of which were also decorated by antibodies directed against cytokeratins nos. 8 (weakly and variably) and 19. Ectocervical suprabasal cells were positive, to a variable extent, for antibodies against cytokeratins nos. 4, 10/11, and 13. Gel electrophoresis revealed the presence of squamous-type cytokeratins nos. 5 and 17 in reserve cell-rich, but not in reserve cell-free, endocervical mucosa. We also analyzed the distribution pattern of these cells, as revealed by antibody KA 1, in the endocervical mucosa of 26 uteri. In all the specimens examined reserve cells were present, but their numbers exhibited considerable variation. In some cases these cells were confined to small islets localized deep within the cervical canal and lacked any continuity with the squamous epithelium. The expression of cytokeratins nos. 5 and 17 in reserve cells indicates that these cells have undergone a low level of squamous differentiation. The additional expression of cytokeratins nos. 8 and 19 in these cells points to a relationship with simple epithelial cells. The present data would seem to favor the view that reserve cells originate in situ from the columnar epithelium; however, this would imply an acquisition of new differentiation properties.

Distribution of a special subset of keratinocytes characterized by the expression of cytokeratin 9 in adult and fetal human epidermis of various body sites

Biochemical analyses have previously shown that palmar and plantar epidermis, unlike the epidermis of other body sites, contain cytokeratin 9 (Mr 64,000), an unusually large acidic (type I) cytokeratin. Guinea-pig antibodies that specifically and selectively react with bovine and human cytokeratin 9 were used for the immunocytochemical identification of cytokeratin 9 in adult and fetal human epidermis from various body sites. In the epidermis of palms and soles, antibodies against cytokeratin 9 stained a high proportion of the keratinocytes in suprabasal locations. These suprabasal cytokeratin-9-positive keratinocytes were often arranged in vertical columns and concentrated around intraepidermal sweat-gland ducts, but they sometimes also formed extended continuous sheets. In contrast, another type-I component, cytokeratin(s) 10/11, was uniformly distributed among suprabasal keratinocytes. By double-labeling immunofluorescence microscopy using a monoclonal antibody against cytokeratin(s) 10/11, we found that cytokeratin 9 usually appears in cells located one or two layers above the cells in which cytokeratin(s) 10/11 is detected, indicating that most keratinocytes expressing cytokeratin 9 also express cytokeratin(s) 10/11. At other body sites, cytokeratin 9 was only detected in sparsely distributed keratinocytes localized in upper epidermal layers; these cells were scattered or formed small clusters, and often exhibited a conspicuous association with the epidermal portion of eccrine sweat-gland ducts. During human fetal development, cytokeratin 9 was first detected at week 15 of gestation in some suprabasal cells of the foot-sole epidermis and, occasionally, in basal cells. At later fetal stages, most of the cytokeratin-9-positive cells appeared in clusters that were mainly concentrated in glandular ridges and interridges. Our results show that two major types of terminally differentiating keratinocytes can be distinguished in human epidermis, i.e. those that do and those that do not express cytokeratin 9. This special program of keratinocyte differentiation identified by the presence of cytokeratin 9 appears to be related to the morphogenesis of palm and sole epidermis, where this protein is expressed early in fetal life. Possible biological functions of this subset of cytokeratin-9-positive keratinocytes are discussed.