Lattice and rodlet nuclear inclusions in Merkel cells in rabbit epidermis

Ultrastructural characterization of normal Merkel cells in the dog

BACKGROUND

Involvement of Merkel cells (MKs) in different cutaneous diseases as well as in the growth, differentiation and homeostasis of the skin has been previously documented.

HYPOTHESIS/OBJECTIVES

The aim was to assess the ultrastructural features of MKs in canine skin, including morphometrics, highlighting their similarities with and differences from those described for other mammals.

ANIMALS

Hard palate, nasal planum, lower lip and whisker pad samples were taken from two healthy young dogs destined for academic purposes.

METHODS

Ultrathin sections of samples fixed in osmium tetroxide and embedded in Epon 812 resin were stained with uranyl acetate and lead citrate and examined using a JEOL JEM 2010 transmission electron microscope.

RESULTS

Ultrastructural characteristics included the following: (i) arrangement in clusters in the basal layer of the epidermis, oral mucosa and external follicular root sheath; (ii) inconstant link with nerve terminal; (iii) oval (10.27 ± 1.64 μm major axis) cell shape with large lobulated nuclei (5.98 ± 1.16 μm major axis); (iv) spine-like and thick cytoplasmic processes interdigitating with surrounding keratinocytes; (v) presence of desmosomes in the cell body or at the base of spine-like processes attaching to neighbouring keratinocytes; and (vi) cytoplasm containing loosely arranged intermediate filaments (10.04 ± 1.17 nm) and numerous dense-core granules (100.1 ± 17.12 nm) arranged in the basal portion of the cytoplasm.

CONCLUSIONS AND CLINICAL IMPORTANCE

This study provides the first complete description of the ultrastructural characteristics of MKs in the dog, enhancing our knowledge of the skin structure in this species and providing a basis for future physiological and pathological studies of the role of these cells in normal and damaged canine tissues.

Merkel cell carcinoma. Diagnosis and treatment

BACKGROUND

Merkel cell carcinoma is an uncommon malignancy of the skin that often portends a poor prognosis. Since its first description by Toker in 1972, a plethora of case reports and articles regarding the etiopathogenesis and treatment have been published spanning multiple medical and surgical disciplines. Much confusion still exists regarding the diagnosis and treatment of this ominous tumor.

OBJECT

Through extensive review of the medical, surgical, and pathological literature, to collate the observations of multiple investigators and summarize these findings.

METHODS

Articles from journals of multiple subspecialties were carefully reviewed with particular emphasis placed on epidemiology, prognosis, histology, immunohistochemistry, electron microscopy, tumor origin, treatment, and work-up of Merkel cell carcinoma.

RESULTS

Merkel cell carcinoma is an aggressive malignant neoplasm. Local recurrence develops in 26-44% of patients despite therapy. Up to three-fourths of patients eventually develop regional nodal metastases with distant metastases occurring in one-third of all patients. Reported overall 5-year survival rates range from 30% to 64%.

CONCLUSION

Treatment recommendations unfortunately are based more on anecdotal than scientific data because of the rarity of the tumor and its recognized high risk. Most authors recommend wide local excision of the primary lesion and regional lymph node resection if lymph nodes are palpable followed by x-irradiation of both the postsurgical bed and lymph node basin. The role of elective lymph node resection in the absence of clinically positive nodes remains controversial.

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.

An ultrastructural study of intranuclear rodlets in a malignant extracranial neuroepithelial neoplasm

This report describes for the first time the presence of intranuclear rodlets in a malignant extracranial neuroepithelial neoplasm. This malignant neoplasm appeared poorly differentiated by light microscopy and presented initially as a diagnostic problem. Electron microscopy revealed neoplastic cells joined by macula adherens-type junctions and containing numerous interdigitating cell processes diagnostic of a neuroepithelial neoplasm. A quantitative analysis of 500 cells revealed the presence of intranuclear rodlets in 5% of the neoplastic cells. The rodlets were composed of individual filaments that measured 7-9 nm in width. An extensive review of the literature revealed the high frequency of reported rodlets in normal neurons, normal paraneurons, neoplasms of the nervous system, and paraneuromas. The significance of this interesting structure to the physiology of the cell is discussed.

Small-cell neuroepithelial tumor of skin: a Merkel-cell neoplasm?

Two patients with small-cell undifferentiated neuroepithelial tumors of skin are reported. The histologic and ultrastructural features of this neoplasm are presented. Characteristic membrane-bound, dense-core neurosecretory granules were seen in the cytoplasm of the tumor cells in both cases. The literature is reviewed in regard to the clinical behavior of these lesions and their possible cell of origin. Dermatologists should be aware of these tumors because they may require electron microscopy for accurate diagnosis. The tumors should be treated aggressively to minimize the chance of local recurrence and nodal or visceral metastases.

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.

A cutaneous APUDoma or Merkel cell tumor? A morphologically recognizable tumor with a biological and histological malignant aspect in contrast with its clinical behavior

Clinical, microscopic, ultrastructural, and histochemical characteristics of a primary cutaneous tumor, identified as an APUDoma possibly arising from Merkel cells, are presented. The tumor can be diagnosed by means of routine histology. In some cases the argyrophylic staining technique can be helpful. The dermatological and histological aspects of this tumor suggest a highly malignant undifferentiated process. The biological behavior of this tumor, however seems to be of a low-grade malignancy. It is therefore important to recognize it from undifferentiated, highly malignant, mostly metastatic processes in the skin. There is also a summary of the clinical history and microscopic findings of identical tumors in 4 other patients.

Fine structural analysis of the synaptic junction of Merkel cell-axon-complexes

The ultrastructure of synaptic contact areas in Merkel cell-axon-complexes from sinus hair follicles and touch domes of various mammals (nude mice, rats, cats, rabbits, opossums and monkeys) was investigated by electron microscopy of ultrathin sections from perfusion fixed tissue. Synapses between Merkel cells and axons were a common feature in all analyzed species. Special staining with digallic acid and goniometric tilting facilitated the resolution of the membranous and paramembranous synaptic elements. The synaptic contact revealed the typical characteristics of a chemical synapse, except for presynaptic clear vesicles: a postsynaptic membrane thickening and dense projections at the presynaptic membrane (i.e., the Merkel cell membrane). The cleft material was resolved as a fuzzy coating of the outer leaflets of the synaptic membranes with occasional bridges across the synaptic cleft. The presence of a synapse in the Merkel cell-axon-complexes emphasizes the receptor function of the Merkel cell besides other possible functions of this cell.

Intranuclear rodlets in undifferentiated carcinomas of salivary glands in strain A mice in a study involving a tobacco specific nitrosamine, N-nitrosonornicotine

Undifferentiated carcinomas of the salivary glands were found in 2 of 44 (4.5%) strain A mice injected intraperitoneally with a tobacco specific nitrosamine, N-nitrosonornicotine (NNN). The presence of intranuclear rodlets (INR) in the salivary carcinomas provided the first demonstration of such structures in a non-neuronal tumor in mice. Two types of rodlets were exhibited; one was composed of fibrillar filaments arranged in bundles, and the other was much thicker, branching in form. These INR appeared to be closely associated with nuclear chromatin or nucleoli.

The Merkel cell in oral human mucosa

A study of the mucosal Merkel cells from 10 patients showed that the cell contained an intranuclear rodlet, clear cell qualities, fine desmosomes, characteristic membrane-bound Merkel granules, and cellular "horns" and had a close relationship to terminal axons. The Merkel cell is an intraepithelial cell with features of neurons and of amine-storage cells.

The Merkel cell system and a comparison between it and the neurosecretory or APUD cell system

The Merkel cell, a neural crest migrant to the skin, possesses a characteristic intranuclear rodlet, cytoplasmic membrane-bound granules, and horn projections and is usually associated with nerve terminations. It is also associated with all types of known organizations of sensory nerve endings in the skin and possesses characteristic enzyme reactions in relation to the ending. The Merkel cell is found in the skin and the oral mucous membranes, and similar cells are observed in the taste buds and pulmonary tissue. A comparison between the Merkel cell and the APUD cell system of polypeptide hormone and amine-producing cells reveals many structural and chemical similarities.

Intranuclear tonofilaments in verruca vulgaris

Electron microscopic examination of a verruca vulgaris has revealed the intranuclear inclusion of tonofibrils. Although many intracellular inclusions may be related directly to the virus, nonviral inclusions may also be present.