38 JOURNAL OF THE SOCIETY poor mucous membrane this .factor is obviated or that, as in acanthotic processes, there is a looser type of tissue so that the processes are better seen or better developed. If one examines a skin showing a fairly strong dopa reaction or one that is fairly well pigmented, one is struck by the clarity and the number of dendritic cells, seen in arete peg cut on a bias. It is possible that in many cases dendrites are given off in a plane which runs parallel to the body surface, so that in the usual section which cuts this plane at right angles the dendrites are poorly visualized. Above all, the state of pigment activity as well as the method employed for its demon- stration are of prime importance in the demonstration of dendritic cells. Many workers (Bloch [7]) have investigated the • problem of the nature, origin, anatomy, distribu- tion and biologic significance of the dendritic cell in the human skin, since the original demonstration of them in syphilitic leukoderma and in the normal hair matrix by Riehl (24) (1884). The first thorough investigation of their distribution in the normal white skin was made by Adachi (2'5) (1903). Recently Becker (26), in a detailed and thor- ough piece of work, undertook a systematic investigation of the pig- ment (melanin) of the upper mucous membranes and the skin, with special consideration of the den- dritic cells. His investigations showed that dendritic cells were much more frequent in the normal skin and mucous membranes than OF COSMETIC CHEMISTS has been supposed that they were found in all regions examined-- nipple, abdomen, axilla, toe, pre- puce, mouth, pharynx and esopha- gous--and that they varied in size, shape, nature of branching and numerical relation to nondendritic cells in different regions of the skin and mucous membranes and even in the same location in different per- sons. The dendritic cell in its appearance and variable form may appear very different from the ordi- nary basal cell. However, all tran- sitions between the most bizarre shapes with long, gnarled branches to basal cells with short stubby proc- esses can be seen (transition forms). The cell body is tisually larger than that of a basal cell and may be globular, irregularly oval, club- shaped or polyhedral. The proto- ß plasm is spongy, lacks fibrillation and contains pigment in its cell body as well as in the processes. The processes may be fine and whip- like, or thicker and irregularly formed, extending in the inter- cellular .spaces ahnost up to the stratum corneum (27). They may run along just below the basal cells, extending under three or four cells, and often curving upward at the end to go up between the basal cells again. From such a long process branches may be given off at right angles, also extending upward. The branches of a dendritic process often end in a Y-form or show at their terminations a button-like swelling. This knoblike or button- like swelling at the end of a process can be seen especially well in dopa

THE PIGMENT MELANIN OF THE SKIN AND HAIR 39 sections. It is perhaps a cross-sec- tion of a dendrite which is running at right angles to the section. According to Masson (28), each dendrite ends in the supranuclear region of a malpighian cell. The dendrites never dip below the epi- dermiscuds line. Sections in which they seem to do so can sometimes be seen,' but serial sections will show that these are artefacts. The proc- esses may spring from any point on the surface of the cells and may be three or more in number and show repeated branching. Often the longest branches spring from the poles of the cell at its longest di- ameter. Since the dendrites are not all in the same plane and often change their direction, it is unusual to get all the processes in the same section or to be able to follow the dendrites to their final termination. The dendrites often seem to com- municate with one another, espe- cially in the dopa sections and some- times form a swelling at their point of junction. The nucleus is large, round and/or oval, contains several nucleoli and is often eccentric. Mitotic figures have not been seen in dendritic cells (Bloch (7), Mieschre (29) and Becker (26), except by Masson (28)). The cell lies between the basal cells, usually at the basal cell level, but sometimes it is seen at a slightly higher level, or almost half of the cell body .may be below the basal line in the cutis. The whole cell, however, has never been seen below the epidermis. The dendrites are ordinarily made visible by their impregnation with melanin, although Miescher has seen cells with dendrites in mela- nomas, when they did not contain any pigment (29). It is a curious fact t.hat the cytoplasm of the den- dritic processes does not take the ordinary stains. This is true of the dendritic cytoplasm in lower animals also (Biedermann (130), Ballowitz (al)). In sections treated with silver nitrate the dendrites are still better seen because their pigment content is brought more to the fore by the reduction of the •ilver (form- ation of silver melanin) which makes visible the fine pale pig- ment granules and perhaps propig- ments. By far the clearest and truest pic- ture is obtained with Bloch's dopa method which demonstrates the pigment-forming oxydase. With the dopa reaction the dendrites are seen either through the content of granular dopa melanin or, better still and as is usually the ease, by the diffuse protoplasmic reaction. This is the only method that clearly demonstrates the cytoplasm, i.e., protoplasm-containing oxidase. In the native as well as in the silver sections the dendrites are demon- strated through their content of pigment granules, which give a pic- ture that is often only outlined by scattered dots, so that it is difficult to trace component parts of the cells (132). With the dopa reaction, how- ever, especially in the diffuse re- action, one obtains a more solid and complete picture. Further- more, in a strong dopa reaction