Morphogenesis of Haarscheiben in rats

Merkel cell carcinoma of the skin: the structure and origin of normal Merkel cells

A series of 15 Merkel cell tumours of skin is reported. They occur dominantly on the head and neck and on the extremities of elderly women, frequently presenting as a reddish nodule. Three cases were associated with squamous carcinoma at the same site, an association deserving further study. There are two main patterns: the commoner one takes the form of a trabecular carcinoma in the dermis mimicking metastatic carcinoma, including oat-cell carcinoma and neuroblastoma: a dissociated-cell form mimicks malignant lymphoma. The triad of vesicular nuclei with very small nucleoli, abundant mitotic activity and apoptosis is so characteristic as to be virtually pathognomonic in conjunction with structural features. Argyrophilia is common, but Bouin fixation is necessary to demonstrate it regularly. Small round secretory granules (89 +/- 18 nm) with narrow haloes, and an abundance of intermediate size filaments are among the ultrastructural hallmarks. There is a close similarity between better differentiated tumour cells and normal Merkel cells. The neural crest origin of MC is in doubt both on the basis of studies of the development and regeneration of MC and from the study of Merkel cell tumours.

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.

Development of rat Merkel cells

The ultrastructure of Merkel cells and cutaneous nerves was examined in fetal, newborn, and 7-day-old rats. The earliest observation of cells having some, but not all, of the features of mature Merkel cells was at 16 days gestation in snout skin. These early presumptive Merkel cells resembled the neighboring keratocytes, except that they contained dense-cored vesicles scattered in the cytoplasm. Presumptive Merkel cells were seen only in the epidermis, although a careful search was made of the dermis. Developing neurons were not observed to penetrate the epidermal basal lamina when presumptive Merkel cells were first seen. The earliest observation of identifiable nerve axons in the snout epidermis was at 17-171/2 days gestation. Study of the presumptive Merkel cells through successively older gestational stages showed that the cells became innervated and progressively developed the characteristics of adult Merkel cells. We suggest that Merkel cells arise from keratocyte-like precursors in rat epidermis, at a time when skin nerves may still be several micrometers away.

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.

Branched cells in the epidermis: an overview

Current knowledge concerning the nature, lineage, and function of the Langerhans cell, Merkel cell, and, to a lesser extent, the melanocyte, are reviewed under headings that emphasize the confederate constitution of the epidermis as a compound tissue composed of a variety of cellular elements; the role of the lymphocyte as a component of normal epidermis is also considered. It appears that the function of the Langerhans cell has finally been established, i.e., it serves as a front-line element in immune reactions of the skin. Developmentally, it is of mesenchymal origin. The Merkel cell still presents a number of problems centering around questions of its lineage, the nature of its characteristic granules, and the "synaptic" relationship between it and the associated neurite. The melanocyte continues to hold the attention of investigators, mainly from the point of view of the chemistry of melanin and the rational treatment of pigmentary disorders based upon findings derived from fundamental research into all aspects of its biology.

Development and differentiation of dermal cells in man

Development and differentiation of the single free cells of mesenchyme and dermis of human embryos and fetuses from week 6 to term is described. From week 6 to week 14, three cell types are present: stellate general mesenchymal cells with long processes, phagocytic macrophages of probable yolk-sac origin, and a granule-secretory type of cell, which could be either a melanoblast or a mast stem cell. From week 14 to week 21, fibroblasts are numerous and active, and perineurial cells, pericytes, melanoblasts, mast cells, and Merkel cells can be individually identified. There is also present another cell type, possible of bone marrow origin, that may be ancestral to the Langerhans cell and that may be carried over into postanal dermis as the "histiocyte" or fixed dermal macrophage. From week 24 to term there is little change apart from the development of fat cells in the deeper dermis. Neither lymphocyte nor plasma cell was observed at any stage of development. These observations were used in the specification and identification of cells of fully developed postnatal dermis.

The Haarscheibe

The Haarscheibe is a specialized spot of epidermis containing many Merkel cell-neurite complexes. It is a highly sensitive, slowly adapting, modality-specific touch receptor occurring in all mammals. Its exact role in signaling sensation in human beings is undetermined, and the function of Merkel cells, with their distinctive cytoplasmic granules, in transducing mechanical force into neural action potentials remains to be determined.

Neural-epithelial interactions in sensory receptors

The specific types of relationships that exist between sensory nerve fibers and the associated epithelial cells have been reviewed in a variety of sensory systems. In many sensory systems, perhaps most, specificity of terminal epithelial element provides a degree of specificity to the cytologic pattern of the terminal receptor. We can see examples of such interaction, not only in the case of intraepithelial nerve fibers where nerves tend to be associated with Merkel cells, but also in the case of corpuscular receptors, both of these being examples of mechanosensory systems. Equivalent types of specificity are present within the special senses, where a specific receptor cell can be described. These findings strongly suggest that specificity in the peripheral nervous system is the rule rather than the exception, a thesis proposed over a hundred years ago by F. Merkel.