Richard S. Snell

AN ELECTRON MICROSCOPIC STUDY OF THE DENDRITIC CELLS IN THE BASAL LAYER OF GUINEA-PIG EPIDERMIS.

Summary1.The dendritic cells in the basal layer of the epidermis of the anterior abdominal wall skin of the black guinea-pig has been studied with the electron microscope. The tissue was embedded in epoxy resins and sections were stained with uranyl acetate.2.The dendritic cells could be recognized by the fact that they were lying free between adjacent keratinocytes and the basement membrane and did not possess desmosomes or hemi-desmosomes. The cytoplasm contained no tono-filaments but many mitochondria and there was a well developed Golgi complex.3.Three types of dendritic cells could be identified.Type 1.The melanocyte which had a slightly indented nucleus and contained melanin granules in different stages of maturity within its cytoplasm.Type 2.Non-pigmented cell which had a deeply indented nucleus and characteristic rod-shaped granules in the cytoplasm. The dendritic processes were well developed.Type 3.Non-pigmented cell similar to that of type 2 but not having rod-shaped granules in the cytoplasm and possessing poorly developed dendritic processes.4.The possible relationship which may exist between the different types of dendritic cells and their significance was discussed.

The fate of epidermal desmosomes in mammalian skin

Summary1.The fine structure of epidermal desmosomes and hemi-desmosomes has been studied in the different layers of the epidermis of anterior abdominal wall skin of guinea-pigs. The tissue was embedded in epoxy resins and the sections were stained with uranyl acetate.2.In the basal layer, the stratum spinosum and in the deeper layers of the stratum granulosum the desmosomes had the same characteristic structure. Each consisted of localized areas of thickening of the cell membrane of two contiguous keratinocytes. As seen in section the total width of the desmosome was about 700–850 Å units. When cut tangentially the membrane thickenings were round or oval in shape and measured about 0.25–0.7 μ in diameter. In the intercellular interval between the thickenings and running parallel to them were situated three thin electron dense laminae. Converging to be attached to the intracellular surface of each thickening were bundles of tonofilaments. None of the tonofilaments crossed the intercellular space between the thickenings.3.At the junction between the stratum granulosum and the stratum corneum the structure of the desmosomes changed. The cell membrane of the stratum corneum cell showed considerable overall thickening and the desmosomal thickening could no longer be identified. In many of the desmosomes the dense intercellular laminae were partly fused together and in some of them the process of fusion was complete. In the deeper layers of the stratum corneum the now fused laminae formed a single dense fusiform body between the apposed cell membranes. In the intermediate layers the solid bodies became enlarged and were oval in shape. In the superficial layers of the stratum corneum the interior of each body showed signs of breaking down leaving a well defined limiting membrane. In the region where exfoliation was occurring, the limiting membrane of the intercellular bodies degenerated and the vesicular structures ruptured.4.Hemi-desmosomes were found on the part of the cell membrane of the keratinocyte adjacent to the basement membrane. Each consisted of a localized area of thickening of the cell membrane and a precisely apposed thickening of the basement membrane. Within the interval and lying close to the cell membrane thickening and parallel to it was a single thin electron dense lamina.5.The dendritic cells of the epidermis were seen to be lying free between adjacent keratinocytes and the basement membrane, there being no desmosomes or hemi-desmosomes. The dendritic processes of these cells extended out between adjacent keratinocytes and insinuated themselves between adjacent desmosomes.6.The functional significance of the desmosome and hemi-desmosome was considered. The relationship between the desmosome and its dynamic environment was discussed.

An electron microscopic study of the human epidermal keratinocyte

Summary1. The epidermis of the flexor surface of the upper arm of human subjects was studied with the electron microscope. 2. The cytoplasm of the keratinocytes in the basal layer contained many tonofilaments, ribosomes and other cell organelles. The tonofilaments were arranged singly or in loose bundles and many were attached to the inner membrane of the desmosomes. Along the basal border of the cells pinocytotic vesicles could be seen at different stages of development. 3. The keratinocytes in the stratum spinosum differed from those in the basal layer in two main ways: (a) The tonofilaments were grouped together into large compact bundles known as tonofibrils and it was possible to determine a definite beading or cross banding along the length of some of the filaments. (b) The cells were assuming a flattened shape. 4. The keratinocytes in the stratum granulosum possessed large numbers of irregularly shaped keratohyaline granules. The granules were strongly osmiophilic and were always situated on a meshwork of tonofibrils. The keratohyaline granules had no internal structure. The nuclei and mitochondria showed evidence of degeneration. 5. The keratinocytes in the stratum corneum were long and flattened. The cell walls showed increased electron density and were considerably thickened. The cytoplasm was filled with closely packed fibres separated by a small amount of lucent matrix. The fibres were grouped together in bundles running in different directions within the flattened squames. The fibres had along their entire length alternating areas of high and low electron density. The keratohyalin granules had disappeared and nothing remained of the nuclei or the organelles. In the deepest cells of this region the fibres were sometimes loosely packed leaving large irregular open spaces. This area corresponded to the stratum lucidum. In the most superficial layers of the stratum corneum the fibres appeared to be breaking down so that little remained within the keratinocyte except large lucent spaces. The desmosomes showed distinct structural changes. 6. An attempt was made to correlate the structural changes in the different epidermal layers with the process of keratinization. The possible part that keratohyalin may play in the process of thickening of the cell walls was discussed. The relationship between the desmosome and its dynamic environment was considered.

The Adrenocorticotrophic Hormone and Mammalian Epidermal Melanocytes

The biological significance of the adrenocorticotrophic hormone (ACTH) of the pituitary in the production of hyperpigmentation of the skin in Addison’s disease still remains obscure. A considerable amount of circumstantial evidence has been produced to show that the pituitary hormones — α- and β-MSH and possibly ACTH are implicated. Unfortunately, due to the close similarity of the chemical structure of the pituitary peptides and the large volume of the starting material necessary, it is impossible to determine the contribution of each of these hormones to the total melanocyte stimulating activity found in the body fluids of these patients. From the work of Shepherd et al. [13], Dixon [2] and Lee and Lerner [7] there remains little doubt that pure ACTH can cause the dispersion of melanin granules in frog melanophores and bring about temporary skin darkening. The object of the present work was to determine whether large doses of pure ACTH could stimulate melanin dispersion and melanogenesis in mammalian epidermal melanocytes. It was felt that the success or failure of the hormone to influence mammalian melanocyte activity would provide evidence for the participation or non-participation of this hormone in the production of Addisonian pigmentation.