EXPERIMENTS ON ELECTRICAL RESISTANCE OF THE HUMAN EPIDERMIS TAIZAN SUCHI* Institute of Physiology, University of Nagoya In deeper layers of the epidermis, there are fine intercellular clefts separating the cells from one another. All the clefts are connected together and filled up with tissue fluid so that an intercellular fluid labyrinth is formed. The fluid is produced from the blood capillaries in the corium, and it has been thought that there is some continuous movement of the fluid in the labyrinth (Melczer (1) ). The spaces become smaller as they pass towards the surface of the epidermis and entirely disappear in the superficial layer of the stratum corneum where horny squamae stick to each other closely. From the view point of water content, the epidermis can be regarded as a tissue the water content of which becomes progressively smaller from down upward and is covered with a dry epithelial layer. Rothman suggested that transepidermal water loss is closely connected with the keratinization process, and it has later been demonstrated that insensible perspiration is increased under various pathological conditions of the skin where keratinization is accelerated (Rothman (2)). According to Kuno's estimation. (unpublished), however, the total volume of the epidermal tissue where keratinization takes place cannot be larger than 80 cc. Keratinization may be a partial source for insensible perspiration, but it is too small to cover it wholly which may reach over one liter per day. Evaporation of the water in the intercellular labyrinth through the dry epithelial layer seems therefore to be the essential mechanism for insensible perspiration. The question how thick is the dry epithelial layer covering the intercellular fluid, or how deep the surface of the intercellular water is situated below the surface of the skin is therefore important to be answered for consideration of the mechanism of insensible perspiration. As to the mechanism of passage of water through the epidermis, the sweat ducts in the epidermis present another important problem. Very many investigators believe that the part of the sweat duct passing through the epidermis has no wall, but is merely a channel excavated between the epithelium cells. In contrast to this opinion, Takagi (3) has recently demonstrated with certaincy that the sweat ducts in the epidermis possess their proper walls. The free shift of fluid from the interepithelial spaces to the lumen of the sweat duct, which has once widely been alleged, seems therefore to be improbable. Yet the wall of the sweat duct is always wet and it may allow to pass water much easier than it is the case in the dry epithelial layer of the stratum corneum. Received for publication September 10, 1954. 75
76 T. SUCHI At suggestion of Prof. Kuno, the above two problems were studied by measuring electrical resistance of the epidermis. METHOD Minute electrodes were used for placing over the skin surface and thrusting into the epidermis. A silver wire sharpened by electrolysis was mounted in a glass tube ending in a capillary with a diameter varying from 10 to 20 p. The other electrode was an Ag.AgCl-electrode which was inserted into the subcutaneous tissue. The two electrodes were connected with a skin ohmmeter, specially made on the principle of balb-voltmeter, through a cell of about 2 volts and grid biases with resistances of 1 mƒ and 10 mƒ respectively. The current strength was therefore 2.0 ƒêa and 0.2 ƒêa respectively when the electrodes were connected directly. For measurement of the electrical resistances of various points of the skin surface, the minute electrode was simply placed over the skin and moved from point to point. In order to estimate the distribution of water in different layers of the epidermis, the minute electrode was mounted in an adjusting screw of microscope and its tip was driven gradually into the epidermis. At every 50 ƒê advance in micrometer the ohmmeter was read. This was made under microscopic observation of the site of the skin where the insertion took place. Since the skin was deformed as it was pressed by the tip of electrode, the reading of the micrometer could not give an accurate length of the electrode actually introduced into the epidermis. After each observation, therefore, the point of electrode at the skin surface was marked, the electrode was pulled out and the length of the inserted portion of the electrode was directly measured. Observations were made on the palm of the hand, the nail mantle, and the forearm. 70 observations all together were carried out, on three adult men. RESULTS Experiments Placing the Electrode over the Skin. Observations were made on the palm and nail mantle of the finger under the aid of a magnifying lens. On these skin areas there were always some sweat glands discharging sweat almost constantly. The electrode was placed on the skin area between the sweat pores either close to, or more apart from them, or directly on the sweat pores themselves. The electrical resistance was distinctly low (several decade ) when the electrode was placed on sweat pores and much higher (several Mƒ hundred Mƒ ) when it was between them. In the latter there were no exact proportional variations in resistance according to the distance from the sweat pores but a tendency could be noticed that the resistance was low on the points of skin adjacent to a sweat pore discharging sweat. The resistance was high on sweat pores which were not secreting sweat. The difference in resistance between the dry and wet sweat pores was more pronounced on the palm than on the nail mantle, probably because, in the former, the part of sweat duct passing through the epidermis was longer and presented a higher resistance
ELECTRICAL RESISTANCE OF EPIDERMIS 77 when it was empty. The above circumstances can be seen from figure 1 in which three examples of experiments are presented. FIG. 1. Electrical resistances measured over the skin surface. A: Cathodal electrode was placed on nail mantle. B: Anodal electrode on nail mantle. C: Cathodal electrode on palm. on sweat pores discharging sweat. on sweat pores producing no sweat. over the skin. on epidermal surface. in water droplets put The average resistance per unit of area was lower in the palm (for example 54 M12) than on the nail mantle (225 Mf2). This was probably so because, on the former, there were sweat glands secreting constantly in a greater density. A minute water droplet was placed over the skin where no sweat pore was present, and the electrode was placed in the water droplet. A high resistance similar to that on the ordinary skin surface was always found. This indicates that there is no change in resistance of the epidermis when its surface is dry or wet. Experiments inserting the Electrode into the Epidermis. The electrode was placed over the skin between sweat pores and it was slowly inserted into the epidermis, being driven by means of a microscrew. The resistance was read at every 50 Đ advance. This was made when the skin surface was dry and when it was thoroughly wet with water. In the experiments with the dry skin surface, the electrical resistance was the highest when the electrode was placed on the skin and it fell as it was inserted into the epidermis. The fall in resistance was however not gradual but characteristic of different layers, and it could roughly divided into four stages : (1) The initial drop of resistance, usually slight, taking place in the outermost layer. The thickness of this layer is from 50 to 70 Đ in the palm and very much thinner in the nail mantle and the forearm. (2) In a layer of varying thickness immediately below the above, the resistance is still high and kept in a steady state. (3) In the next thin layer, the resistance falls very slowly. (4) At a further insertion, the resistance suddenly falls to the level of the own resistance of the electrode so that no further change in resistance occurs while the electrode is driven further. The depths from the surface at which the
78 T. SUCHI changes of (2), (3) and (4) appear vary considerably in different regions of the skin as will be dealt with later. In the experiments with wet skin surface, the electrical resistance was relatively low when the electrode was placed on the skin. It rose gradually, though not considerably, when the electrode was inserted into the outermost layer of the epidermis where a high resistance was found in the above experiments with the dry skin. Changes in resistance on further introduction of the electrode were similar to those described above. In order to see if the above changes can be regarded as simple changes in resistance, or they are participated by some active potentials in the epidermis, observations were performed with reversed direction of current. The results were almost the same in the two directions (fig. 2). To make sure, the potential was tested with the same electrodes connected with a galvanometer instead of the ohmmeter. When the electrode was inserted into the epidermis and reached about the layer where the sudden drop in resistance was usually found, a swing of the galvanometer was found. There was a current of from 0.2 to 3.4 mv (0.9 mv on an average) flowing from the minute electrode to the other. In the experiments above described, small variations in resistance were often seen just before the sudden drop of resistance took place (fig. 2). They were probably due to active potentials produced from deeper layers of the epidermis. Figure 2 shows examples of the experiments made on the palm and nail mantle. FIG. 2. Changes in resistance on gradual insertion of electrode into epidermis. The four stage changes in resistance above described can be seen in the palm, the nail mantle as well as in the forearm. The depth from the surface of the skin at which the characteristic changes in resistance occur varies considerably in these three regions (see figure 2). Among the characteristic changes in resistance, the sudden drop seems to be the important one as it indicates the boundary between the dry and wet layers of the epidermis. The dept
ELECTRICAL RESISTANCE OF EPIDERMIS 79 from the surface of the skin to the point of the sudden drop of resistance indicates the thickness of the dry layer of epidermis covering the intercellular fluid labyrinth. The thickness of this layer varied considerably in the palm the nail mantle and forearm. But in each region, individual variations were not considerable so that the mean thickness could be easily obtained. These average figures are given in the following table. Mean Values of Epidermal Layers (y) DISCUSSION The human body is covered with a thin epithelial layer, the dryness of which is revealed by a high electrical resistance. This layer is pierced by a great number of sweat ducts with considerably lower electrical resistance, especially when sweat is secreted. In the experiments of thrusting the electrode into the epidermis, it was found that, when the skin surface was wet, the electrical resistance was relatively low so long as the electrode was placed over the skin and that it rose gradually as the electrode was introduced into the superficial layer of the epidermis. The resistance was low on the skin probably because the electric current could flow from the electrode along the water over the skin and sweat ducts into the corium. This pathway was interrupted when the electrode was entered into the dry epithelial layer and the resistance was therefore increased. From the view point of water distribution, the human epidermis may be divided into three layers: (1) The dry outermost layer showing usually little change in electrical resistance; (2) a very thin layer containing a small amount of water, showing minor changes in resistance: and (3) the deepest layer containing abundant water, where the electric resistance falls suddenly. The important question is at which layer in the epidermis the sudden fall in electrical resistance takes place as it may roughly represent the boundary between the dry and wet layers. As no histological observation was made in conjunction with these experiments the above question cannot be answered with certaincy. On the basis of the current knowledge concerning the thicknesses of the epidermal layers of various regions of the body surface, we may roughly say that this boundary layer may lie just below the deepest layer of the stratum corneum, or the stratum lucidum in the case of the palm. The fluid in the intercellular labyrinth is undoubtedly produced from the blood capillaries in the corium. It can be assumed that when the skin blood vessels dilate the production of the intercellular fluid increases, resulting in an increase in its volume and pressure. It may be surmised that the surface of
80 T. SUCHI the intercellular fluid is raised when its production is augmented so that the dry epithelial layer is reduced in thickness. Determinations of changes in resistance from layer to layer were repeated on the same part of the skin while the hand and forearm were heated by light bath or exposed to cold. Contrary to our expectation, changes in the level of the layer at which the resistance fell suddenly could be noticed in no case. It seems therefore probable that the distribution of water in the epidermis is conditioned by the constitional arrangement of the intercellular clefts which seems to be always filled up with fluid and that the space of the clefts cannot be enlarged outward even when the hydrostatic pressure in it is raised. From the above account the mechanism of insensible perspiration may be considered as follows : The fluid contained in the interstitial spaces in the deeper layers of epidermis serves as the chief source of water. A part of it passes through the sweat ducts in fluid form and evaporated from their orifices. But the total surface of the orifices being very small, the amount of water eliminated by this way cannot be large. The essential part of insensible perspiration seems to consist of water vapour which passes through the dry layer in vapour form from the surface of the interstitial' fluid. The vapour has to pass through a dry layer of almost constant thickness. This thickness may vary according to regions of the skin. SUMMARY The electrical resistance of the human epidermis was measured with a minute electrode with the purpose to study the water distribution in it. When the electrode was placed over the skin, the resistance was much smaller on sweat pores especially when they discharge sweat. When the electrode was gradually inserted into the epidermis, no marked change in resistance was found in the outermost layer, followed by a thin layer where slight fall in resistance began to occur. On further introduction, the resistance fell suddenly. The layer where the resistance fell suddenly was situated at a depth of about 350 p in the palm, 170 p in the nail mantle and 50 p in the forearm, reckoned from the skin surface. The expenses of this work were defrayed by a grant from the Ministry of Education. REFERENCES 1. MELCZER, N. Derm. Z. 47: 255, 1926. 2. ROTHMAN, S. Physiol. and Biochem. of the Skin, 234. Chicago: Univ. Chicago Press, 1954. 3. TAKAGI, S. Jap. J. Physiol. 3: 65, 1952.