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The ultrastructure of the epidermis in psoriaris vulgaris as revealed by electron microscopy
580
J. ULTRASTRUCTURE RESEARCH 8, 580--594 (1963)
The Ultrastructure of the Epidermis in Psoriasis Vulgaris as Revealed by Electron Microscopy 6. The Transition Cells in Hyperkerotosis I. BRODYz
Department of Zoology, University of California; Department of Medicine, Division of Dermatology, Medical School, University of California, Los Angeles, California, U.S.A.; and Laboratory for Biological Ultrastructure Research, Department of Anatomy, Karolinska Institutet, Stockholm, Sweden Received February 6, 1963 A two to four cell thick, continuous layer of transition cells between the stratum intermedium and stratum corneum has been demonstrated in hyperkeratotic psoriatic epidermis. Two main types of T-cells are described: type A and type B. Type A is referred to those transition cells in which intensely stained regions appear in the tonofibrils. I n type B heterogeneous areas of an opaque material are found in. the fibrils. It has been assumed that these differences in the tonofibrillar morphology of the T-cells of types A and B indicate different intermediate stages of the fibrillar substance in its development into keratin. A further subclassification of the T-cells into A~, A s, A s, B~, and B~ is possible on the basis of the density and opacity of the cytoplasm, the presence or absence of a speckled pattern in the fibrils, and of small, opaque particles in the cytoplasm and the opacity and thickness of the dense layer at the cell surfaces. I n the cells of type A no keratin pattern is discerned, whereas in B2 occasionally a faint pattern is seen. The density of the cells is mainly due to the appearance of a nonfibrillar substance. Nuclei, mitochondria, polymorphic components, and round or oval granules are observed in various numbers in the transition cells. Different alternatives for a transformation of the stratum intermedium cells by the Way of the T-cells into the stratum corneum cells are discussed. The development of the stratum intermedium cells via the different stages of type A and B in this order of progression is proposed as the most.likely. I n the n o r m a l h u m a n epidermis from sites other t h a n the p a l m a r and p l a n t a r regions, the t r a n s i t i o n between s t r a t u m i n t e r m e d i u m a n d s t r a t u m c o r n e u m usually occurs abruptly. Single t r a n s i t i o n or T-cells are sometimes observed between these two layers (1). These cells exhibit features similar b o t h to those seen in the cells of the 1 Present address: Department of Dermatology, Karolinska Sjukhuset, Stockholm 60, Sweden.
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stratum intermedium and to those of the stratum cornemn. In the T-cells the tonofibrils show the most advanced intermediate stage in their development into keratin with indications of a keratin pattern (l). In parakeratotic psoriatic epidermis without keratohyalin, where pronounced disturbances in the differentiation process of the tonofibrils occur with failure of the development of the keratin pattern, T-cells also fail to appear (2). In hyperkeratosis, on the other hand, where the successive transformation stages of the tonofibrils in the noncornified layers are still more distinct than in the normal epidermis (4), numerous T-cells are present. Here they form a continuous layer. MATERIAL AND METHODS A detailed account of Material and Methods has been given in an earlier report (4). RESULTS In the material under investigation here the transition between the stratum intermedium and the stratum corneum occurs via a continuous layer of T-cells. This layer is two to four cells thick. The transition cells have elongate profiles with the long axes always oriented parallel to the skin surface. The cells attenuate into long processes which project between adjacent cells (Figs. 1 and 2). Their height is usually less than that of the cells of the stratum intermedium. Nuclei are sometimes seen in the T-cells (Fig. 1). A varying number of mitochondria occur. Their triple-layered membranes show negative contrast with unstained outer layers and an opaque interspace. Round or oval granules are scattered all over the cytoplasm. They usually occur singly (Figs. 1 and 2). A large number of polymorphic components of unknown nature are observed in the cells. They show negative contrast (Figs. 2 and 6). They are similar in ultrastructure to those observed in the horny layer of parakeratotic psoriatic epidermis without keratohyalin (3). The T-cells may show fairly even cell surfaces with few microvilli (Fig. 2) or there may appear numerous microvilli (Fig. 1). Between the frequent desmosomes the intercellular space appears mainly electron optically "empty." Sometimes the space contains a homogeneous substance of fairly low opacity. This substance does not fill out the space completely but is separated from the cell surface by a narrow, less opaque space. More frequently, however, when an intercellular substance occurs, it is seen only as a thin membrane along the cell surface. This substance occasionally parts the intercellular space into compartments (Figs. 1-5). On the basis of the ultrastructure of the tonofibrils, two main types of transition cells can be distinguished: types A and B. Differences of the cells with regard to density and opacity of the cytoplasm, the appearance of a speckled pattern in the
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tonofibrils, the presence or absence of small, opaque particles in the cytoplasm, a n d the opacity a n d thickness of the dense layer at the cell surfaces permit a further subclassification of the cells into A1, A~, A3, B1, a n d B2. The different t r a n s i t i o n cells are n o t seen at a c o n s t a n t level i n the sections, b u t appear very irregularly m r e l a t i o n to each other a n d to the cells of the s t r a t u m i n t e r m e d i u m a n d s t r a t u m c o r n e u m .
T-cell o f type A 1 This type of t r a n s i t i o n cell seems to differ f r o m the cells in the s t r a t u m i n t e r m e d i u m only with respect to the appearance of a dense layer at the surface of the cells. This layer has the same thickness ( a b o u t 80 A) as, b u t shows o n the whole a m u c h higher opacity than, the p l a s m a m e m b r a n e s of the s t r a t u m i n t e r m e d i u m cells (Figs. 1 a n d 2). The triple-layered p a t t e r n characterizing the p l a s m a m e m b r a n e s in the n o n cornified epidermis (4) is n o t discerned here. The cytoplasm exhibits the same appearance of looseness as the subjacent cells of the s t r a t u m i n t e r m e d i u m . The intensely stained parts of the tonofibrils seem to have the same opacity as the corresponding regions in the upper cells of the s t r a t u m i n t e r m e d i u m (Figs. 1 a n d 2). I n the entire fibrillar substance a speckled p a t t e r n is distinctly observed. I n Figs. 1 a n d 2, showing this type of T-cell, the tonofibrils are fairly few in n u m b e r a n d aggregate into larger b u n d l e s only to a m i n o r extent. H o w ever, the n u m b e r of tonofibrils as well as the aggregation of the fibrils vary in the cells of the different sections. Between a n d close to the tonofibrils particles a b o u t 150 A in diameter are seen (Fig. 2). They show the same opacity a n d ultrastructure as those occurring i n the cells of the n o n c o r n i f i e d epidermis (4). The particles seem to be less n u m e r o u s t h a n in the cells of the s t r a t u m i n t e r m e d i u m .
FI6. 1. Survey of upper cells in the stratum intermedium (/), T-cells of type At (/:11) and Bt (B1) and of the basal (b) and intermediate (i) layers of the stratum corneum. In the stratum intermedium cell on the right the tonofibrils are aggregated into larger bundles than in the cell on the left. Similarly as in the stratum intermedium cells the cytoplasm of A1 displays an appearance of looseness. The keratohyalin in At seems to have the same opacity as that in the stratum intermedium cells. The only difference in the electron micrograph between the At cell and the cells in the subjacent layer appears to be the appearance of a highly opaque, dense layer at the cell surface of At. The plasma membranes of the stratum intermedium cells are very faintly seen in this magnification. The cytoplasm of B1 is more dense than that of At owing to the existence of a homogeneous, nonfibrillar substance between the fibrils and in the perinuclear zone. This substance is less opaque than the fibrils. The cytoplasm of the cells in the basal layer of the stratum corneum is dense and of a fairly even opacity. In the intermediate layer cells with a lesser density usually occur but cells of the same density as in the basal layer are also seen. Nuclei (N) are observed in the B1 cell and in one of the basal horny cells. The nuclear membrane in the cornified cell has a negative contrast, x, oval or round granules. Ultraviolet irradiation on the whole block for 40 hours. × 14,000.
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T-cell of type A2 The dense layer at the cell surface of the transition cells of type A2 (Fig. 3) is similar in opacity to that of A1 but is about 100 A in thickness. The cytoplasm of the cells as a whole is denser and more opaque than that of type A1 and of the stratum intermedium cells. The difference in density and opacity may, in the main, be due to the distinct appearance of a nonfibrillar substance between the fibrils. This substance varies conspicuously in amount in the cells. It has a lower opacity than that of the fibrils and appears homogeneous. The tonofibrils in type Az (Fig. 3) usually conform in ultrastructure with those in t y p e A 1. However, cells of the Az type are also seen (Fig. 4), where the speckled pattern of the tonofibrils is less apparent than in A~. The intensely stained regions of the tonofibrils appear somewhat less opaque than the corresponding areas in the subjacent stratum intermedium cells [see Fig. 1, ref. (4)]. In the transition cells the tonofibrils usually seem to be more extensively packed than those in the stratum intermedium cells, but still great variations are seen in this respect. Particles about 150 Jk in diameter seem to occur in reduced number in the T-cells of type A2 as compared with those in the stratum intermedium cells. It is difficult to tell whether they are less numerous than in A1. The particles are usually arranged in groups of different sizes. They are mainly found between and close to the tonofibrils (Fig. 3). Only a few are seen in the perinuclear zone.
T-cell of type A3 A particularly striking feature in the electron micrographs of the T-cell of type A 3 (Fig. 2) is the lower opacity of the dense layer at the cell surface as compared with that of the transition cells of types A 1 and A2. However, it does not appear to show as low opacity as the dense layer of a stratum corneum cell usually does. The dense layer is about 100 A thick.
FIG. 2. Survey of a stratum intermedium cell (I), T-ceUs of type A~ (A1), A3 (As), and B2 (B~) and of cells in the basal (b) and intermediate (i) layers of the stratum corneum. The tonofibrils in A1 are few in number. The keratohyalin has the same opacity as that in the stratum intermedium cell. Small, opaque particles are fairly numerous. They may appear close to the keratohyalin regions and are similar to them in opacity. A highly opaque, dense layer at the cell surf~tce of A~ is discernible. The plasma membrane of the stratum intermedium cell is only faintly seen; it shows a triple-layered pattern (arrow). The cells attenuate into long processes that project between the adjacent cells. The Tcell of type As deviates conspicuously in ultrastructure from that of A1 The cytoplasm is dense and consists here mainly of closely packed fibrils. The keratohyalin as well as the dense layer at the cell surface of Aa are less opaque than the corresponding components in Ax. The basal layer of the stratum corneum is here two cells thick. The karyoplasm of the nucleus (N) has a higher opacity than the cytoplasm of the horny cell. The nuclear membrane has a negative contrast. Close to the nucleus a mitochondrion (m) is seen, Polymorphic components, are scattered throughout the cytoplasm, x, round granules of low opacity; IS, intercellular space; de, desmosome. Ultraviolet irradiation on the whole block for 45 hours, x 42,000.
....'iili~
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The cytoplasm shows a high opacity and density with a varying amount of a nonfibrillar substance. In Fig. 2 the fibrils seem to form the main part of the cytoplasm. Here they mainly aggregate into large bundles. The intensely stained regions of the fibrils show a lesser opacity than in type A 1. The fibrillar substance does not show any speckled pattern or keratin pattern. Particles do not appear in the cytoplasm in this type of transition cell.
T-cell of type B1 The cytoplasm of these cells has a high density and opacity (Figs. 1 and 4.6). The fibrils do not show a speckled pattern or intensely stained regions, but heterogeneous areas of an opaque material. These areas display as a whole a lower opacity than the intensely stained regions of the fibrils in the T-cells of type A (Figs. 1 and 4). They may occupy small parts in the center or at the periphery of the fibrils or they may make up the main parts of the fibrillar substance (Figs. 4-6). In the cells a varying amount of a nonfibrillar substance occurs. This has a lower opacity than the fibrils (Figs. 1 and 4-6). Small, opaque particles are not seen in the cytoplasm. The dense layer at the cell surface has a fairly high opacity and a thickness of about 100/~ (Figs. 4-6).
T-cell of type B2 A type of transition cell is discernible in which the dense layer at the cell surface shows about the same opacity as that of the cornified cells. The layer is about 100
FIG. 3. T-cell of type A2. The fibrils (t) occur in a large number, they anastomose and aggregate into bundles of different widths. The fibrils exhibit a distinct speckled pattern. They are mainly longitudinally sectioned but transversally cut fibrils are also seen (--~). The keratohyalin regions of the tonofibrils appear very irregularly. Between the tonofibrils is observed a homogeneous, nonfibrillar substance (+-->) of less opacity than the fibrils. In the space between the fibrils groups of small, opaque particles (p) are also present. The dense layer at the cell surface is opaque. In the intercellular space (IS) a less opaque substance appears, separated from the dense layer by a narrow space. In the upper left corner of the picture a part of a T-cell of type B~ (B2) is seen, showing small, heterogeneous areas of an opaque material (q~--~). The dense layer at the surface of this cell has a low opacity as compared to that of the A2 cell. de, desmosome, x 65,000. FIG. 4. Parts of T-cells of type B1 (B1) and As (A2). In B1 at the bottom of the picture the tonofibrils (t) display heterogeneous areas of an opaque material (---~),whereas in As the intensely stained regions of the tonofibrils on the whole have a high opacity (+--~). A speckled pattern is not observed in the tonofibrillar substance in either T-cell. Between the fibrils in As and B1 a less opaque, nonfibrillar material exists (+~-~). In As groups of small, opaque particles (p) are seen. The dense layers at the cell surfaces of the T-cells are highly opaque. IS, intercellular space containing a small amount of a substance. The intercellular disc of the desmosome (de) indicates a layered structure, x 97,000. Fla. 5. Part of a transition cell of type B1 (B1). The opaque fibrils (t) display heterogeneous areas of a highly opaque material (---~). The areas have different shapes and sizes. The tonofibrils in the Tcells do not show any speckled pattern. A large part of the cytoplasm is formed by a less opaque, nonfibrillar substance (+-+). Whereas the plasma membrane of the stratum intermedium cell (/)is only faintly seen (-H--~), the dense layers at the cell surfaces of the B1 and A2 (As)cells show a high opacity. Groups of small, opaque particles (p). de, desmosome, x 52,000.
38--' 631827 J . Ultrastructure Research
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thick. These cells are sometimes found in several layers. They contain a variable amount of fibrillar and nonfibrillar substance. There are few, small heterogeneous areas o~ the fibrils as described in type B1. The fibrils occasionally display a faint keratin pattern (6). DISCUSSION
Classification of the T-cells In this investigation of the psoriatic epidermis the transition cells form a continuous layer. They exhibit differences in ultrastructure, on the basis of which two main types of transition cells have been described. The classification of the T-cells into the main types A and B has been made without any reference to the mutual relationship between these types of cells with respect to stage of development in the keratinization process. This investigation does not permit a clear interpretation in this respect. The type A transition cells show tonofibrillar substance with intensely stained regions. These regions are referred to as keratohyalin for practical reasons until a more suitable name can be given on the basis of their biochemistry. The T-cells of type B are characterized by tonofibrils showing heterogeneous areas of an opaque material. The keratohyalin in type A~ seems to show the same opacity as that in the cells of the stratum intermedium, whereas that in A 3 is less opaque. The keratohyalin in A2 seems to be somewhat less opaque than that in the subjacent stratum intermedium cells. It is not possible to tell whether the keratohyalin in A2 differs in opacity from that in A3, as these cells have not yet been observed in the same sections. The tonofibrils in A1 also agree in ultrastructure with those in the subjacent cells of the stratum intermedium with regard to the distinct speckled pattern of the fibrils. This pattern is mostly clearly observed also in the T-cells of type A2. In A 3 it is not seen at all. The tonofibrils of the transition cells of type A thus show differences which could be interpreted as representing successive changes. Differences can also be seen in respect to the occurrence of a nonfibrillar substance, the decrease in number of the small, opaque particles in the cytoplasm, and the opacity and thickness of the dense layer at the cell surfaces. These observations further support the hypothesis earlier suggested on the basis of ultrastructural studies of the normal human epidermis that the transition cells represent the most advanced intermediate stage in the development of the stratum corneum cells (1).
FIG. 6. Parts of T-cells of type B1 (B1) and B2 (B~) and a cell in the basal layer of the stratum corneum (b). The heterogeneous areas (--~) of the fibrils (t) of B1 appear here very distinctly. They vary as to shapes and sizes. Between the fibrils a nonfibrillar substance appears (+--~). Ultraviolet irradiation on the whole block for 45 hours, x 69,000.
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The heterogeneous areas of the tonofibrillar substance in type Bx differ in opacity in its whole from the keratohyalin regions in type A. However, with regard to their shape and size and the fact that they appear only in the fibrils, these areas are similar to the keratohyalin regions of the tonofibrils in type A. it therefore seems justifiable to assume that the keratohyalin regions of the T-cells of type A and the heterogeneous areas of type B only represent different intermediate stages of the tonofibrillar substance in its development into keratin.
The development of the stratum intermedium cells into horny cells The question now arises: by what route do the stratum intermedium cells develop into horny cells via T-cells? At the present time it is not possible to tell whether the cells of type A are transformed into type B or vice versa and furthermore, whether the stratum intermedium cells can be directly transformed into the T-cells both of type A and B. Three alternatives are thus open for discussion: (a) transformation independently by way of types A and B; (b) transformation via the different stages of types of A into the types of B; (c) transformation via the types of B through the various types of A, i.e., the reverse of alternative (b). In alternative (a) the two types of transition cells would represent different lines of development in the keratinization process. However, any morphologic evidence on the basis of the tonofibrillar picture of the cells of the stratum intermedium suggesting a direct transformation of the stratum intermedium cells into the T-cells of type B has not yet been obtained. In normal h u m a n epidermis it was observed that the speckled pattern distinctly seen in the fibrils of the cells of the stratum spinosum became less apparent in the cells of the stratum intermedium. In the T-cells the speckled pattern was no longer seen (1). The demonstration of a distinct speckled pattern in the cells of the stratum intermedium (4) and in the transition cells of types A1 and A2 may indicate a slowing down of the keratinization process in the hyperkeratotic psoriatic epidermis. The absence of this pattern in the T-cells of type Aa, B1, and B2 favors the assumption that these cells represent the more advanced intermediate stages in this modified keratinization process. In hyperkeratosis, small, opaque particles of about 150 N are seen in a very large number in the noncornified epidermis (4). They are less numerous in.the T-cells of types A 1 and A2 but still more abundant than in the normal T-cells (1). They do not occur in A a, B 1, and B 2 or in the stratum corneum (5). If these particles represent ribosomal particles (4), their absence in transition cells of types A 8, B1, and B2 could be interpreted as indicating little or no protein synthesis in these cells. This then would
EPIDERMIS IN PSORIASIS VULGARIS.
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suggest further that these cells represent more advanced intermediate stages in the keratinization process than A1 and A2. If the particles play a role in the formation of the keratohyalin regions of the tonofibrils, a possible transformation of the heterogeneous areas of the B-cells into keratohyalin of the A-cells would not occur. This would then speak against the possibility of a transformation of the B-cells into the A-cells. In psoriatic epidermis characterized by parakeratosis with keratohyalin, T-cells are observed showing both keratohyalin regions and heterogeneous areas of the tonofibrils in the same cell (6). In some parts of these heterogeneous areas a distinct keratin pattern is seen (6). These observations strongly favor the idea that the keratin pattern develops from the keratohyalin regions via the heterogeneous areas characteristic for the tonofibrils in the transition cells of type B. In this investigation the fibrillar substance in the transition cells of type B~ occasionally shows indications of a keratin pattern (6). The heterogeneous areas characterizing the fibrils in the transition cell of type B1 become smaller in Bz. The cells are then very similar to the stratum corneum cells. Preliminary electron microscopic results of the human foot skin show the occurrence of transition cells both of type A and B. Whereas in type A no keratin pattern is observed, it does occur in type B (6). These different observations might support the assumption that the transition cells of type B are more advanced intermediate stages than those of the type A and that the development of the stratum intermedium cells into the horny cells occurs via the different stages of types A and B in this order of progression. The preliminary electron microscopic studies of human foot skin have revealed the occurrence of T-cells both of types A and B (6). These cells form a continuous layer between the stratum intermedium and the stratum corneum. In normal human skin from sites other than the palmar and plantar regions T-cells only of type A have been observed in the electron microscope (1). In the parakeratotic psoriatic epidermis without keratohyalin no T-cells were found (2). In the present investigation of hyperkeratotic psoriatic epidermis an increased number of transition cells both of type A and B are seen, which form a continuous layer. The T-cells might correspond to the infrabasal horny layer demonstrated by Unna (7) in the normal human foot skin. However, further studies will be necessary to elucidate whether in the light microscope the T-cells of type A can be distinguished from the cells of the stratum intermedium or whether only the T-cells of type B form the infrabasal horny layer as described by Unna (7). According to Unna no keratohyaiin is present in this layer (7). The increased number of T-cells in hyperkeratotic psoriatic epidermis might be another expression for a slowing down of the maturation process of the tonofibrils in this type of keratinization.
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I. BRODY REFERENCES
1. BRODY, 1.,
J. Ultrastruct. Res.
6, 354 (1962). 3. - - - i b i d . 7, 346 (1962). 4. - i b i d . 8, 566 (1963). 5. - i b i d . 8, 595 (1963). 6. - to be published. 7. UNNA, P. G., B e r l i n . K l i n . 2. - - - -
4, 264 (1960).
ibid.
Wochschr.
58, 447 (1921).
J. ULTRASTRUCTURE RESEARCH 8, 580--594 (1963)
The Ultrastructure of the Epidermis in Psoriasis Vulgaris as Revealed by Electron Microscopy 6. The Transition Cells in Hyperkerotosis I. BRODYz
Department of Zoology, University of California; Department of Medicine, Division of Dermatology, Medical School, University of California, Los Angeles, California, U.S.A.; and Laboratory for Biological Ultrastructure Research, Department of Anatomy, Karolinska Institutet, Stockholm, Sweden Received February 6, 1963 A two to four cell thick, continuous layer of transition cells between the stratum intermedium and stratum corneum has been demonstrated in hyperkeratotic psoriatic epidermis. Two main types of T-cells are described: type A and type B. Type A is referred to those transition cells in which intensely stained regions appear in the tonofibrils. I n type B heterogeneous areas of an opaque material are found in. the fibrils. It has been assumed that these differences in the tonofibrillar morphology of the T-cells of types A and B indicate different intermediate stages of the fibrillar substance in its development into keratin. A further subclassification of the T-cells into A~, A s, A s, B~, and B~ is possible on the basis of the density and opacity of the cytoplasm, the presence or absence of a speckled pattern in the fibrils, and of small, opaque particles in the cytoplasm and the opacity and thickness of the dense layer at the cell surfaces. I n the cells of type A no keratin pattern is discerned, whereas in B2 occasionally a faint pattern is seen. The density of the cells is mainly due to the appearance of a nonfibrillar substance. Nuclei, mitochondria, polymorphic components, and round or oval granules are observed in various numbers in the transition cells. Different alternatives for a transformation of the stratum intermedium cells by the Way of the T-cells into the stratum corneum cells are discussed. The development of the stratum intermedium cells via the different stages of type A and B in this order of progression is proposed as the most.likely. I n the n o r m a l h u m a n epidermis from sites other t h a n the p a l m a r and p l a n t a r regions, the t r a n s i t i o n between s t r a t u m i n t e r m e d i u m a n d s t r a t u m c o r n e u m usually occurs abruptly. Single t r a n s i t i o n or T-cells are sometimes observed between these two layers (1). These cells exhibit features similar b o t h to those seen in the cells of the 1 Present address: Department of Dermatology, Karolinska Sjukhuset, Stockholm 60, Sweden.
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stratum intermedium and to those of the stratum cornemn. In the T-cells the tonofibrils show the most advanced intermediate stage in their development into keratin with indications of a keratin pattern (l). In parakeratotic psoriatic epidermis without keratohyalin, where pronounced disturbances in the differentiation process of the tonofibrils occur with failure of the development of the keratin pattern, T-cells also fail to appear (2). In hyperkeratosis, on the other hand, where the successive transformation stages of the tonofibrils in the noncornified layers are still more distinct than in the normal epidermis (4), numerous T-cells are present. Here they form a continuous layer. MATERIAL AND METHODS A detailed account of Material and Methods has been given in an earlier report (4). RESULTS In the material under investigation here the transition between the stratum intermedium and the stratum corneum occurs via a continuous layer of T-cells. This layer is two to four cells thick. The transition cells have elongate profiles with the long axes always oriented parallel to the skin surface. The cells attenuate into long processes which project between adjacent cells (Figs. 1 and 2). Their height is usually less than that of the cells of the stratum intermedium. Nuclei are sometimes seen in the T-cells (Fig. 1). A varying number of mitochondria occur. Their triple-layered membranes show negative contrast with unstained outer layers and an opaque interspace. Round or oval granules are scattered all over the cytoplasm. They usually occur singly (Figs. 1 and 2). A large number of polymorphic components of unknown nature are observed in the cells. They show negative contrast (Figs. 2 and 6). They are similar in ultrastructure to those observed in the horny layer of parakeratotic psoriatic epidermis without keratohyalin (3). The T-cells may show fairly even cell surfaces with few microvilli (Fig. 2) or there may appear numerous microvilli (Fig. 1). Between the frequent desmosomes the intercellular space appears mainly electron optically "empty." Sometimes the space contains a homogeneous substance of fairly low opacity. This substance does not fill out the space completely but is separated from the cell surface by a narrow, less opaque space. More frequently, however, when an intercellular substance occurs, it is seen only as a thin membrane along the cell surface. This substance occasionally parts the intercellular space into compartments (Figs. 1-5). On the basis of the ultrastructure of the tonofibrils, two main types of transition cells can be distinguished: types A and B. Differences of the cells with regard to density and opacity of the cytoplasm, the appearance of a speckled pattern in the
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tonofibrils, the presence or absence of small, opaque particles in the cytoplasm, a n d the opacity a n d thickness of the dense layer at the cell surfaces permit a further subclassification of the cells into A1, A~, A3, B1, a n d B2. The different t r a n s i t i o n cells are n o t seen at a c o n s t a n t level i n the sections, b u t appear very irregularly m r e l a t i o n to each other a n d to the cells of the s t r a t u m i n t e r m e d i u m a n d s t r a t u m c o r n e u m .
T-cell o f type A 1 This type of t r a n s i t i o n cell seems to differ f r o m the cells in the s t r a t u m i n t e r m e d i u m only with respect to the appearance of a dense layer at the surface of the cells. This layer has the same thickness ( a b o u t 80 A) as, b u t shows o n the whole a m u c h higher opacity than, the p l a s m a m e m b r a n e s of the s t r a t u m i n t e r m e d i u m cells (Figs. 1 a n d 2). The triple-layered p a t t e r n characterizing the p l a s m a m e m b r a n e s in the n o n cornified epidermis (4) is n o t discerned here. The cytoplasm exhibits the same appearance of looseness as the subjacent cells of the s t r a t u m i n t e r m e d i u m . The intensely stained parts of the tonofibrils seem to have the same opacity as the corresponding regions in the upper cells of the s t r a t u m i n t e r m e d i u m (Figs. 1 a n d 2). I n the entire fibrillar substance a speckled p a t t e r n is distinctly observed. I n Figs. 1 a n d 2, showing this type of T-cell, the tonofibrils are fairly few in n u m b e r a n d aggregate into larger b u n d l e s only to a m i n o r extent. H o w ever, the n u m b e r of tonofibrils as well as the aggregation of the fibrils vary in the cells of the different sections. Between a n d close to the tonofibrils particles a b o u t 150 A in diameter are seen (Fig. 2). They show the same opacity a n d ultrastructure as those occurring i n the cells of the n o n c o r n i f i e d epidermis (4). The particles seem to be less n u m e r o u s t h a n in the cells of the s t r a t u m i n t e r m e d i u m .
FI6. 1. Survey of upper cells in the stratum intermedium (/), T-cells of type At (/:11) and Bt (B1) and of the basal (b) and intermediate (i) layers of the stratum corneum. In the stratum intermedium cell on the right the tonofibrils are aggregated into larger bundles than in the cell on the left. Similarly as in the stratum intermedium cells the cytoplasm of A1 displays an appearance of looseness. The keratohyalin in At seems to have the same opacity as that in the stratum intermedium cells. The only difference in the electron micrograph between the At cell and the cells in the subjacent layer appears to be the appearance of a highly opaque, dense layer at the cell surface of At. The plasma membranes of the stratum intermedium cells are very faintly seen in this magnification. The cytoplasm of B1 is more dense than that of At owing to the existence of a homogeneous, nonfibrillar substance between the fibrils and in the perinuclear zone. This substance is less opaque than the fibrils. The cytoplasm of the cells in the basal layer of the stratum corneum is dense and of a fairly even opacity. In the intermediate layer cells with a lesser density usually occur but cells of the same density as in the basal layer are also seen. Nuclei (N) are observed in the B1 cell and in one of the basal horny cells. The nuclear membrane in the cornified cell has a negative contrast, x, oval or round granules. Ultraviolet irradiation on the whole block for 40 hours. × 14,000.
584
I. BRODY
T-cell of type A2 The dense layer at the cell surface of the transition cells of type A2 (Fig. 3) is similar in opacity to that of A1 but is about 100 A in thickness. The cytoplasm of the cells as a whole is denser and more opaque than that of type A1 and of the stratum intermedium cells. The difference in density and opacity may, in the main, be due to the distinct appearance of a nonfibrillar substance between the fibrils. This substance varies conspicuously in amount in the cells. It has a lower opacity than that of the fibrils and appears homogeneous. The tonofibrils in type Az (Fig. 3) usually conform in ultrastructure with those in t y p e A 1. However, cells of the Az type are also seen (Fig. 4), where the speckled pattern of the tonofibrils is less apparent than in A~. The intensely stained regions of the tonofibrils appear somewhat less opaque than the corresponding areas in the subjacent stratum intermedium cells [see Fig. 1, ref. (4)]. In the transition cells the tonofibrils usually seem to be more extensively packed than those in the stratum intermedium cells, but still great variations are seen in this respect. Particles about 150 Jk in diameter seem to occur in reduced number in the T-cells of type A2 as compared with those in the stratum intermedium cells. It is difficult to tell whether they are less numerous than in A1. The particles are usually arranged in groups of different sizes. They are mainly found between and close to the tonofibrils (Fig. 3). Only a few are seen in the perinuclear zone.
T-cell of type A3 A particularly striking feature in the electron micrographs of the T-cell of type A 3 (Fig. 2) is the lower opacity of the dense layer at the cell surface as compared with that of the transition cells of types A 1 and A2. However, it does not appear to show as low opacity as the dense layer of a stratum corneum cell usually does. The dense layer is about 100 A thick.
FIG. 2. Survey of a stratum intermedium cell (I), T-ceUs of type A~ (A1), A3 (As), and B2 (B~) and of cells in the basal (b) and intermediate (i) layers of the stratum corneum. The tonofibrils in A1 are few in number. The keratohyalin has the same opacity as that in the stratum intermedium cell. Small, opaque particles are fairly numerous. They may appear close to the keratohyalin regions and are similar to them in opacity. A highly opaque, dense layer at the cell surf~tce of A~ is discernible. The plasma membrane of the stratum intermedium cell is only faintly seen; it shows a triple-layered pattern (arrow). The cells attenuate into long processes that project between the adjacent cells. The Tcell of type As deviates conspicuously in ultrastructure from that of A1 The cytoplasm is dense and consists here mainly of closely packed fibrils. The keratohyalin as well as the dense layer at the cell surface of Aa are less opaque than the corresponding components in Ax. The basal layer of the stratum corneum is here two cells thick. The karyoplasm of the nucleus (N) has a higher opacity than the cytoplasm of the horny cell. The nuclear membrane has a negative contrast. Close to the nucleus a mitochondrion (m) is seen, Polymorphic components, are scattered throughout the cytoplasm, x, round granules of low opacity; IS, intercellular space; de, desmosome. Ultraviolet irradiation on the whole block for 45 hours, x 42,000.
....'iili~
586
I. BRODY
The cytoplasm shows a high opacity and density with a varying amount of a nonfibrillar substance. In Fig. 2 the fibrils seem to form the main part of the cytoplasm. Here they mainly aggregate into large bundles. The intensely stained regions of the fibrils show a lesser opacity than in type A 1. The fibrillar substance does not show any speckled pattern or keratin pattern. Particles do not appear in the cytoplasm in this type of transition cell.
T-cell of type B1 The cytoplasm of these cells has a high density and opacity (Figs. 1 and 4.6). The fibrils do not show a speckled pattern or intensely stained regions, but heterogeneous areas of an opaque material. These areas display as a whole a lower opacity than the intensely stained regions of the fibrils in the T-cells of type A (Figs. 1 and 4). They may occupy small parts in the center or at the periphery of the fibrils or they may make up the main parts of the fibrillar substance (Figs. 4-6). In the cells a varying amount of a nonfibrillar substance occurs. This has a lower opacity than the fibrils (Figs. 1 and 4-6). Small, opaque particles are not seen in the cytoplasm. The dense layer at the cell surface has a fairly high opacity and a thickness of about 100/~ (Figs. 4-6).
T-cell of type B2 A type of transition cell is discernible in which the dense layer at the cell surface shows about the same opacity as that of the cornified cells. The layer is about 100
FIG. 3. T-cell of type A2. The fibrils (t) occur in a large number, they anastomose and aggregate into bundles of different widths. The fibrils exhibit a distinct speckled pattern. They are mainly longitudinally sectioned but transversally cut fibrils are also seen (--~). The keratohyalin regions of the tonofibrils appear very irregularly. Between the tonofibrils is observed a homogeneous, nonfibrillar substance (+-->) of less opacity than the fibrils. In the space between the fibrils groups of small, opaque particles (p) are also present. The dense layer at the cell surface is opaque. In the intercellular space (IS) a less opaque substance appears, separated from the dense layer by a narrow space. In the upper left corner of the picture a part of a T-cell of type B~ (B2) is seen, showing small, heterogeneous areas of an opaque material (q~--~). The dense layer at the surface of this cell has a low opacity as compared to that of the A2 cell. de, desmosome, x 65,000. FIG. 4. Parts of T-cells of type B1 (B1) and As (A2). In B1 at the bottom of the picture the tonofibrils (t) display heterogeneous areas of an opaque material (---~),whereas in As the intensely stained regions of the tonofibrils on the whole have a high opacity (+--~). A speckled pattern is not observed in the tonofibrillar substance in either T-cell. Between the fibrils in As and B1 a less opaque, nonfibrillar material exists (+~-~). In As groups of small, opaque particles (p) are seen. The dense layers at the cell surfaces of the T-cells are highly opaque. IS, intercellular space containing a small amount of a substance. The intercellular disc of the desmosome (de) indicates a layered structure, x 97,000. Fla. 5. Part of a transition cell of type B1 (B1). The opaque fibrils (t) display heterogeneous areas of a highly opaque material (---~). The areas have different shapes and sizes. The tonofibrils in the Tcells do not show any speckled pattern. A large part of the cytoplasm is formed by a less opaque, nonfibrillar substance (+-+). Whereas the plasma membrane of the stratum intermedium cell (/)is only faintly seen (-H--~), the dense layers at the cell surfaces of the B1 and A2 (As)cells show a high opacity. Groups of small, opaque particles (p). de, desmosome, x 52,000.
38--' 631827 J . Ultrastructure Research
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n BROOY
thick. These cells are sometimes found in several layers. They contain a variable amount of fibrillar and nonfibrillar substance. There are few, small heterogeneous areas o~ the fibrils as described in type B1. The fibrils occasionally display a faint keratin pattern (6). DISCUSSION
Classification of the T-cells In this investigation of the psoriatic epidermis the transition cells form a continuous layer. They exhibit differences in ultrastructure, on the basis of which two main types of transition cells have been described. The classification of the T-cells into the main types A and B has been made without any reference to the mutual relationship between these types of cells with respect to stage of development in the keratinization process. This investigation does not permit a clear interpretation in this respect. The type A transition cells show tonofibrillar substance with intensely stained regions. These regions are referred to as keratohyalin for practical reasons until a more suitable name can be given on the basis of their biochemistry. The T-cells of type B are characterized by tonofibrils showing heterogeneous areas of an opaque material. The keratohyalin in type A~ seems to show the same opacity as that in the cells of the stratum intermedium, whereas that in A 3 is less opaque. The keratohyalin in A2 seems to be somewhat less opaque than that in the subjacent stratum intermedium cells. It is not possible to tell whether the keratohyalin in A2 differs in opacity from that in A3, as these cells have not yet been observed in the same sections. The tonofibrils in A1 also agree in ultrastructure with those in the subjacent cells of the stratum intermedium with regard to the distinct speckled pattern of the fibrils. This pattern is mostly clearly observed also in the T-cells of type A2. In A 3 it is not seen at all. The tonofibrils of the transition cells of type A thus show differences which could be interpreted as representing successive changes. Differences can also be seen in respect to the occurrence of a nonfibrillar substance, the decrease in number of the small, opaque particles in the cytoplasm, and the opacity and thickness of the dense layer at the cell surfaces. These observations further support the hypothesis earlier suggested on the basis of ultrastructural studies of the normal human epidermis that the transition cells represent the most advanced intermediate stage in the development of the stratum corneum cells (1).
FIG. 6. Parts of T-cells of type B1 (B1) and B2 (B~) and a cell in the basal layer of the stratum corneum (b). The heterogeneous areas (--~) of the fibrils (t) of B1 appear here very distinctly. They vary as to shapes and sizes. Between the fibrils a nonfibrillar substance appears (+--~). Ultraviolet irradiation on the whole block for 45 hours, x 69,000.
592
L RRODY
The heterogeneous areas of the tonofibrillar substance in type Bx differ in opacity in its whole from the keratohyalin regions in type A. However, with regard to their shape and size and the fact that they appear only in the fibrils, these areas are similar to the keratohyalin regions of the tonofibrils in type A. it therefore seems justifiable to assume that the keratohyalin regions of the T-cells of type A and the heterogeneous areas of type B only represent different intermediate stages of the tonofibrillar substance in its development into keratin.
The development of the stratum intermedium cells into horny cells The question now arises: by what route do the stratum intermedium cells develop into horny cells via T-cells? At the present time it is not possible to tell whether the cells of type A are transformed into type B or vice versa and furthermore, whether the stratum intermedium cells can be directly transformed into the T-cells both of type A and B. Three alternatives are thus open for discussion: (a) transformation independently by way of types A and B; (b) transformation via the different stages of types of A into the types of B; (c) transformation via the types of B through the various types of A, i.e., the reverse of alternative (b). In alternative (a) the two types of transition cells would represent different lines of development in the keratinization process. However, any morphologic evidence on the basis of the tonofibrillar picture of the cells of the stratum intermedium suggesting a direct transformation of the stratum intermedium cells into the T-cells of type B has not yet been obtained. In normal h u m a n epidermis it was observed that the speckled pattern distinctly seen in the fibrils of the cells of the stratum spinosum became less apparent in the cells of the stratum intermedium. In the T-cells the speckled pattern was no longer seen (1). The demonstration of a distinct speckled pattern in the cells of the stratum intermedium (4) and in the transition cells of types A1 and A2 may indicate a slowing down of the keratinization process in the hyperkeratotic psoriatic epidermis. The absence of this pattern in the T-cells of type Aa, B1, and B2 favors the assumption that these cells represent the more advanced intermediate stages in this modified keratinization process. In hyperkeratosis, small, opaque particles of about 150 N are seen in a very large number in the noncornified epidermis (4). They are less numerous in.the T-cells of types A 1 and A2 but still more abundant than in the normal T-cells (1). They do not occur in A a, B 1, and B 2 or in the stratum corneum (5). If these particles represent ribosomal particles (4), their absence in transition cells of types A 8, B1, and B2 could be interpreted as indicating little or no protein synthesis in these cells. This then would
EPIDERMIS IN PSORIASIS VULGARIS.
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suggest further that these cells represent more advanced intermediate stages in the keratinization process than A1 and A2. If the particles play a role in the formation of the keratohyalin regions of the tonofibrils, a possible transformation of the heterogeneous areas of the B-cells into keratohyalin of the A-cells would not occur. This would then speak against the possibility of a transformation of the B-cells into the A-cells. In psoriatic epidermis characterized by parakeratosis with keratohyalin, T-cells are observed showing both keratohyalin regions and heterogeneous areas of the tonofibrils in the same cell (6). In some parts of these heterogeneous areas a distinct keratin pattern is seen (6). These observations strongly favor the idea that the keratin pattern develops from the keratohyalin regions via the heterogeneous areas characteristic for the tonofibrils in the transition cells of type B. In this investigation the fibrillar substance in the transition cells of type B~ occasionally shows indications of a keratin pattern (6). The heterogeneous areas characterizing the fibrils in the transition cell of type B1 become smaller in Bz. The cells are then very similar to the stratum corneum cells. Preliminary electron microscopic results of the human foot skin show the occurrence of transition cells both of type A and B. Whereas in type A no keratin pattern is observed, it does occur in type B (6). These different observations might support the assumption that the transition cells of type B are more advanced intermediate stages than those of the type A and that the development of the stratum intermedium cells into the horny cells occurs via the different stages of types A and B in this order of progression. The preliminary electron microscopic studies of human foot skin have revealed the occurrence of T-cells both of types A and B (6). These cells form a continuous layer between the stratum intermedium and the stratum corneum. In normal human skin from sites other than the palmar and plantar regions T-cells only of type A have been observed in the electron microscope (1). In the parakeratotic psoriatic epidermis without keratohyalin no T-cells were found (2). In the present investigation of hyperkeratotic psoriatic epidermis an increased number of transition cells both of type A and B are seen, which form a continuous layer. The T-cells might correspond to the infrabasal horny layer demonstrated by Unna (7) in the normal human foot skin. However, further studies will be necessary to elucidate whether in the light microscope the T-cells of type A can be distinguished from the cells of the stratum intermedium or whether only the T-cells of type B form the infrabasal horny layer as described by Unna (7). According to Unna no keratohyaiin is present in this layer (7). The increased number of T-cells in hyperkeratotic psoriatic epidermis might be another expression for a slowing down of the maturation process of the tonofibrils in this type of keratinization.
594
I. BRODY REFERENCES
1. BRODY, 1.,
J. Ultrastruct. Res.
6, 354 (1962). 3. - - - i b i d . 7, 346 (1962). 4. - i b i d . 8, 566 (1963). 5. - i b i d . 8, 595 (1963). 6. - to be published. 7. UNNA, P. G., B e r l i n . K l i n . 2. - - - -
4, 264 (1960).
ibid.
Wochschr.
58, 447 (1921).