The Journal of Dermatology Vol. 6:299-308,1979
SCANNING ELECTRON MICROSCOPIC STUDY OF THE HUMAN AXILLARY APOCRINE GLANDS TAEKO INOUE ABSTRACT
The human axillary apocrine glands were observed by scanning electron microscopy. 1. The axillary apocrine glands revealed three types of apocrine secretion: macroapocine, microapocrine and intermediate apocrine. In the macroapocrine type of secretion, a round projection, which lacked microvilli bulged up from the surface of the secretory cell. In the microapocrine type, the tips of microvilli which covered the large apocrine protrude were expanded and separated. In the intermediate apocrine type, several secretory protrubrances were formed at the luminal portion of the cell. Each protrubrance was composed of several swollen microvilli united. 2. Small round pores were observed on the luminal plasma membrane. They seemed to represent the state of eccrine secretion, but nothing definite was learned about their nature. 3. Some secretory cells had the luminal plasma membrane ruptured. They might not indicated a state of holocrine secretion, but were assumed to exhibit a deciduous phenomenon in the metabolism of secretory cells. 4. The cracked surfaces of secretory cells were observed to confirm the presence of nuclei and cell organelles.
In the field of dermatology, transmission electron microscopic studies have frequently been made on the axillary apocrine gland, as one of the accessory organs of the skin. As a result, the presence of secretory granules in the secretory cells and the mechanism of formation ofthese granules have been revealed (1-5). The mode of secretion in the apocrine glands of the human axilla has been examined three stages in the type of apocrine secretion have been found (6). Moreover, it was elucidated that the secretory cells of the human axillary apocrine gland were related not only to apoBy Invitation: Received and accepted for publication May 1,1979. From the Department of Anatomy (Director: Prof. Keiichi Tanaka), and the Department of Dermatology (Director: Prof. Shuhei Shimao), Faculty of Medicine, University of Tottori. Reprint requests to: T. Inoue, Department of Dermatology, Faculty of Medicine, University of Tottori, 86, Nishimachi, Yonago 683. Japan.
crine secretion but also to other types of secretion (7-10). Scanning electron microscopy has come to be used for the observation of biological specimens thanks to the development of a method of critical point drying. Furthermore, a scanning electron microscopy of the field emission type has been devised, which has improved resolving power to such a remarkable extent that it has come to be efficiently applied to cytological research. In addition, the application of various newly developed methods of cracking, as well as the conventional ion etching method, has made it possible to observe not only the cell surface but also intracellular structures (11, 12). The present investigation was carried out with a scanning electron microscope of the field emission type to examine the luminal surface and intracellular structures of the apocrine glands.
300
I:\'OLIE
MATERIALS AND METHODS
Pieces of axillary skin from four women with osmidrosis axillae (17 to 44 years of age) were surgically removed under 1 % lidocaine anesthesia. The specimens were immediately cut into small pieces approximately 1 x 1 x5 mm in size and fixed in 1 % glutaraldehyde in phosphate buffer (pH 7.2) overnight. They were subsequently dipped in 60% DMSO solution or isoamyl acetate, and then cracked in a frozen state with liquid nitrogen. The specimens were further fixed in 1 % OS04 in cacodylate buffer for one hour and dehydrated in a graded ethanol series. After critical point drying, the cracked surface of the specimen was coated with evaporated platinum. Observations and photography were performed with a Hitachi HFS-2 field emission scanning electron microscopy. The acceleration voltage was 25KV and the angle of inclination 30 degrees. RESULTS
The apocrine gland had a wide lumen and was generally larger in size than the eccrine gland. Its secretory portion was composed of secretory glandular cells and myoepithelial cells. The secretory cells were arranged regularly to, form a single-cubic or columnar epithelium.
1. Low magnification of the gland
apocrine
In the cracking surface of the skin, the secretory portion was frequently found in an area extending from the corium to the subcutaneous fat tissue. The secretory portion was clearly separated from the surrounding tissue by the presence of a hyaline basement membrane. The luminal surface of the secretory cell bulged out. The secretory cells were generally 10-15 P. in height, although they varied in size with the stages of the secretory cycle (Fig. 1).
2. Surface of secretory cells of the apocrine gland When the luminal surface of the secretory cells was observed, it was found to consist of
pentagonal or hexagonal secretory cells which showed a smooth boundary line with the adjacent secretory cells. Microvilli were arranged densely all over the free surface of each secretory cell. They were particularly dense on the boundary line between secretory cells as if they had formed a barrier on this line (Fig. 2). They were generally rod-shaped and occasionally ladleshaped with a swollen tip (Fig. 4). They sometimes were very long (600mp.). The appearance of the luminal surface of the secretory cell characteristicly varied with the stage of secretion. The following structures were observed on the luminal surface. (1) Cytoplasmic processes of the apocrine type (a) Cytoplasmic processes of macroapocrine type: These processes ~ried in size with the stage of the secretory cycle. In stages considered to be very early, it was observed as a hemispherical protuberance about 2 p. in diameter in the middle of the surface of the cell (Fig. 2). This protuberance had no microvilli. With the advance in the stage of secretion, it increased in size to a linguiform process about 2p. in height (Fig. 3). (b) Cytoplasmic processes of microapocrine type: The cytoplasmic processes of this type were always seen on the process of the apocrine type. The secretory process of the abovementioned apocrine type had no microvilli on its surface. However, the whole surface of one secretory cell was covered with a large apocrine process, from which microvilli projected densely. Each microvillus had a swollen tip with a fig-like appearance. This picture seemed to be identical with that of the cytoplasmic processes of the microapocrine type observed by the transmission electron microscope (Fig. 4). (c) Cytoplasmic processes of intermediate type between macroapocrine and micro-
SEM OF THE APOCRINE GLANDS
Fig. 1.
Low magnification micrograph of the secretory portion of the axillary apocrine gland. The secretory portion is separated from the surrounding tissue by the presence of a hyaline basement membrane. The luminal surface of the secretory cell bulges out. x700.
Fig. 2.
The luminal surface consists of pentagonal or hexagonal secretory cells. Microvilli are arranged densely all over the free surface of each secretory cell. Cytoplasmic processes of macroapocrine type are observed in the middle of the cell (arrow). X6,250.
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INOUE
Fig. 3.
Cytoplasmic process of macroapocrine type. With an advance in stage of secretion 'from Fig. 2, it has increased to a linguiform process about 2)L in height. x 16,000.
Fig. 4.
Cytoplasmic process of microapocrine type. The whole surface of one secretory cell is covered with a large apocrine process, from which microvilli project densely. x 10,000.
SEM OF THE APOCRINE GLANDS
apocrine types: There were secretory processes of an intermediate type which were classified as being between secretory processes of macroapocrine type and of microapocrine type. Cytoplasmic processes of intermediate type were seen in a relatively large gland. In this gland each secretory cell had several cytoplasmic processes about 800 mlJ in diameter on the luminal surface (Fig. 5, 6). These processes were smaller than cytoplasmic processes of macroapocrine type, but larger than those of microapocrine type. They has not yet been clarified by transmission electron microscopy. They seemed to belong to an intermediate type between macroapocrine and microapocrine types. They were presumed to have been formed by a union of swollen microvilli. A picture showing such a union of microvilli could be seen, although very infrequently (Fig. 7). (2) Destruction of secretory cell: The
Fig. 5.
303
luminal surfaces were destroyed and the nucleus and intracellular secretory granules were exposed. This phenomenon had previously been reported to indicate holocrine secretion. It seems reasonable, however, to interpret that such a phenomenon may present a stage of metabolism in the secretory cell (Fig. 8). (3) Small round pores on the luminal surface: High magnification of the surface of a secretory cell in the process of apocrine secretion revealed the presence of small round pores 200-300 A in diameter scattered among microvilli on the cytoplasmic membrane (Fig. 9).
3. Cracked surface of secretory cell of apocrine gland The secretory cells of the apocrine gland were columnar. An essentially round nucleus was situated near the basement membrane. A round nucleolus was located close to the edge of the nucleus. The supranuclear portion of the
Cytoplasmic process of intermediate type. x 1,000.
304
INOUE
Fig. 6.
Fig. 7.
A higher magnification of intermediate type of secretion. x l 0,000.
Cytoplasmic process of intermediate type is a union of swollen microvilli. x28, 000.
SEM OF THE APOCRINE GLANDS
Fig. 8.
The luminal surface is destroyed and the nucleus (N) and intracellular secretory granules are exposed. x4,000.
Fig. 9.
Small round pores 200-S00 A in diameter on the luminal surface (arrows). X50,000.
30"
306
INOUE
Fig. 10.
Cracked surface of secretory cell of apocrine gland. The secretory cells of the apocrine gland are columnar. A round nucleus (N) is situated near the basement membrane. A nucleolus (No) is located close to the edge of the nucleus. The supranuclear portion of the cytoplasm contains endoplasmic reticulumn (ER) and secretory granules (G). X3.000.
cytoplasm was wide and contained endoplasmic retic~lumn and a number of rounded secretory granules varying in size. Numerous microvilli were present on the cell surface facing the glandular lumen. Such portions of the cytoplasm as were situated immediately below the luminal surface contained neither mitochondria nor secretory granules, were apparently homogeneous, and formed a crust layer (Fig. 10). DISCUSSION
The sweat glands of the human axilla are divided into apocrine and eccrine glands by light microscopy. The former have been considered to be active in apocrine secretion. Recently, on the basis of observation with the transmission electron microscope, it has been reported that the' apocrine gland of the axilla
carried out not only apocrine secretion but also merocrine, diacrine, and holocrine secretion (6, 8, 9). The present investigation using scanning electron microscopy revealed not only secretion of the apocrine type but also of a microapocrine type and an intermediate type. The results of the present observation were then compared with the types of secretion previously distinguished by transmission electron microscopy. 1. Macroapocrine type secretion: The apocrine gland of the human axilla was examined light microscopically for secretion type. When the apocrine secretion takes place, the secretory cells change their shape from cubic to columnar, and their apical surface bulges up towards the lumen. Cytoplasmic processes are gradually transformed into drops to be de-
SEM OF THE .\POCRINE GLANDS
tached from the cytoplasm. It has been reported, however, that light microscopy of frozen sections failed to find such transformation of cytoplasmic process into drops. With the advances in transmission electron microscopy, the mechanism of apocrine secretion could be investigated in detail. In the apocrine type of secretion three stages were observed: (1) formation of an apical cap; (2) formation of a dividing membrane at the base of the apical cap; and (3) formation of tubules above the dividing membrane that extended parallel to the membrane and led to a separation of the apical cap from the underlying cell. Then apical cap possesses microvilli (6). When the axillary apocrine gland was examined by the scanning electron microscopy, the apocrine secretion was found to initiate with the appearance of a small hemisphere from the middle of the surface of the secretory cell. Microvilli disappeared from the top of the hemisphere and became scanty at the foot of the hemisphere. When completed, the apocrine process was large and linguiform, and had no microvilli on its surface. Each secretory cell had only one apocrine process. They were usually few or no microvilli on the surface of this process. Occasionally, scanning electron microscopy revealed the presence of a drop-like cytoplasmic process with a narrow stalk as has been observed in histological sections stained with hematoxylin and eosin. Further investigation may be necessary to determine whether this process is an artifact or not. 2. Microapocrine type secretion: Kurozumi has reported microapocrine secretion in the apocrine gland of the human external auditory meatus (13). According to him, the apical surface of the secretory cell bulges up towards the lumen and small projections are abundant on the luminal surface. They correspond to microvilli, but are often irregular in shape. In the present studies on the apocrine gland of the
307
human axilla, no pictures were obtained which suggested the direct polyp-like projection of any microvillus on the surface of the secretory cell. However, often observed was a huge convex process of the apocrine type which covered the whole secretory cell. The tips of microvilli which projected from this process were swollen. The author could not find any secretory cell performing the microapocrine secretion alone; secretory cells always exhibited microapocrine secretion simultaneously with macroapocrine secretion. 3. Intermediate apocrine type secretion: Secretion of this type has not as yet been demonstrated by transmission electron microscope. What is tentatively called apocrine secretion of intermediate type differs from secretion of macroapocrine and microapocrine types in the pattern of producing a cytoplasmic process. In it, more than one microvillus is swollen at the same time, fused together, and projected into a secretory process. It is characteristic of secretion of intermediate type that at least two processes project simultaneously from the surface of one secretory cell, and that the size of each process is between that of a microapocrine process and that of a macroapocrine process. 4. Eccrine secretion: From their observations with the transmission electron microscope, G. Schaumburg-Lever et al. reported that eccrine secretion was also performed by the apocrine gland of the human axilla (6). According to them, granular material covered by a limiting membrane approaches the surface of the cell. This membrane is then fused with the luminal plasma membrane and, subsequently, the vesicles discharge their granular contents into the lumen of the gland. After the discharge the limiting membrane of the granules becomes a part of the plasma membrane, and the initial condition of the cell is restored. In the present investigation, small pores
308
200-300 A in diameter were observed on the luminal surface of secretory cells actively performing the apocrine secretion. They looked like the pores of the limiting membrane shown in the case of eccrine secretion. It was not determined, however, whether these pores indicated eccrine secretion or of pinocytosis. 5. Holocrine secretion: Transmission electron microscopy has revealed the process of holocrine secretion as follows. Secretory cells forming a layer around the glandular lumen disintegrated to a varying degree and separated from one another (6). Only rarely were there secretory cells with a ruptured luminal plasma membrane through which organelles had escaped into the lumen of the gland (3, 14). Scanning electron microscopy revealed that the luminal plasma membrane had been lost at some sites of the cell layer around the lumen and that nuclei and cell organelles had been exposed at these sites. These changes were seen collectively in some glands. In the holocrine secretion of the sebaceous gland, karyopyknosis is induced with advancing maturity of glandular cells. The nucleus often becomes stellate in shape due to the pressure of lipid droplets. At last, the cells are decomposed by themselves, and the nucleus, mitochondria, and endoplasmic reticulum, as well as the lipid droplets contained in the cells, are released outside (15). On the other hand, in the apocrine gland of the human axilla, it is reasonable to assume that the disintegration of cells may not indicate such processes of maturation of cells as has been observed in the sebaceous gland, but may rather illustrate the metabolic shedding of cells.
INOL'F
ACKNOWLEDGEMENTS The author is grateful to Prof. T. Tanaka and to Prof. S. Shimao for their kind guidance, and also to Miss H. Fukudome, and to Mr. H. Osatake for their technical assistance. REFERENCES 1) Charles, A.: An electron microscopic study of the human axillary apocrine gland,]. Anat., 93: 226-232, 1959. 2) Yamada, H.: Electron microscopic observations on the secretory processes of the axillary apocrine glands, Actapath.jap., 10: 173-187, 1960. 3) Hibbs, R.G.: Electron microscopy of human apocrine sweat glands,]. Invest. Derm., 38: 77-84,1962. 4) Biempica, L. and Montes, L.F;: Secretory epithelium of the large axillary sweat glands. A cytochemical and electron microscopic study, Am.]. Anat., 117: 47 -72, 1965. 5) Montagna, W.: The apocrine sweat glands, The Structure and Function of Skin, 2nd ed., Academic Press, New York, 1962, pp. 374-424. 6) Schaumburg· Lever, G. and Lever, W.F.: Secretion from human apocrine glands. An electron microscopic study,]. Invest. Derm., 64: 38-41,1975. 7) Kurosumi, K., Kitamura, T. and Iijima, T.: Electron microscope studies on the human axillary apocrine sweat glands, Arch. histol. jap., 16: 523-566,1959. 8) Hashimoto, K.: Apocrine sweat gland (1), Hifuka no Rinsho, 17: 552-555,1975. (in Japanese) 9) Hashimoto, K.: Apocrine sweat gland (2), Hihuka no Rmsho, 17: 638-641, 1975. (in Japanese) 10) Ito, T. and Shibazaki, S.: Sweat Gland, Hifuka no Rinsho, 8: 68-82,1966. (in Japanese) 11) Naguro, T. and Iino, A.: Pancrease, Daishya, 13: i-ii, 1976. (in Japanese) 12) Tanaka, K.: Intracellular structure, Daishya, 13: i-ii, 1976. (in Japanese) 13) Kurosumi, K. and Kawabata, I.: Transmission and scanning electron microscopy of the human cerumirous apocrine gland I. Secretory glandular cells, Arch. histol. jap., 39: 207 -229, 1976. 14) Hashimoto, K., Gross, B.G. and Lever, W.F.: Electron microscopic study of apocrine secretion,]. Invest. Derm., 46: 378-390,1966. 15) Morohashi, M.: Fine structure and function of sebaceusgland, Saibou, 4: 18-28,1972. (in Japanese)
SCANNING ELECTRON MICROSCOPIC STUDY OF THE HUMAN AXILLARY APOCRINE GLANDS TAEKO INOUE ABSTRACT
The human axillary apocrine glands were observed by scanning electron microscopy. 1. The axillary apocrine glands revealed three types of apocrine secretion: macroapocine, microapocrine and intermediate apocrine. In the macroapocrine type of secretion, a round projection, which lacked microvilli bulged up from the surface of the secretory cell. In the microapocrine type, the tips of microvilli which covered the large apocrine protrude were expanded and separated. In the intermediate apocrine type, several secretory protrubrances were formed at the luminal portion of the cell. Each protrubrance was composed of several swollen microvilli united. 2. Small round pores were observed on the luminal plasma membrane. They seemed to represent the state of eccrine secretion, but nothing definite was learned about their nature. 3. Some secretory cells had the luminal plasma membrane ruptured. They might not indicated a state of holocrine secretion, but were assumed to exhibit a deciduous phenomenon in the metabolism of secretory cells. 4. The cracked surfaces of secretory cells were observed to confirm the presence of nuclei and cell organelles.
In the field of dermatology, transmission electron microscopic studies have frequently been made on the axillary apocrine gland, as one of the accessory organs of the skin. As a result, the presence of secretory granules in the secretory cells and the mechanism of formation ofthese granules have been revealed (1-5). The mode of secretion in the apocrine glands of the human axilla has been examined three stages in the type of apocrine secretion have been found (6). Moreover, it was elucidated that the secretory cells of the human axillary apocrine gland were related not only to apoBy Invitation: Received and accepted for publication May 1,1979. From the Department of Anatomy (Director: Prof. Keiichi Tanaka), and the Department of Dermatology (Director: Prof. Shuhei Shimao), Faculty of Medicine, University of Tottori. Reprint requests to: T. Inoue, Department of Dermatology, Faculty of Medicine, University of Tottori, 86, Nishimachi, Yonago 683. Japan.
crine secretion but also to other types of secretion (7-10). Scanning electron microscopy has come to be used for the observation of biological specimens thanks to the development of a method of critical point drying. Furthermore, a scanning electron microscopy of the field emission type has been devised, which has improved resolving power to such a remarkable extent that it has come to be efficiently applied to cytological research. In addition, the application of various newly developed methods of cracking, as well as the conventional ion etching method, has made it possible to observe not only the cell surface but also intracellular structures (11, 12). The present investigation was carried out with a scanning electron microscope of the field emission type to examine the luminal surface and intracellular structures of the apocrine glands.
300
I:\'OLIE
MATERIALS AND METHODS
Pieces of axillary skin from four women with osmidrosis axillae (17 to 44 years of age) were surgically removed under 1 % lidocaine anesthesia. The specimens were immediately cut into small pieces approximately 1 x 1 x5 mm in size and fixed in 1 % glutaraldehyde in phosphate buffer (pH 7.2) overnight. They were subsequently dipped in 60% DMSO solution or isoamyl acetate, and then cracked in a frozen state with liquid nitrogen. The specimens were further fixed in 1 % OS04 in cacodylate buffer for one hour and dehydrated in a graded ethanol series. After critical point drying, the cracked surface of the specimen was coated with evaporated platinum. Observations and photography were performed with a Hitachi HFS-2 field emission scanning electron microscopy. The acceleration voltage was 25KV and the angle of inclination 30 degrees. RESULTS
The apocrine gland had a wide lumen and was generally larger in size than the eccrine gland. Its secretory portion was composed of secretory glandular cells and myoepithelial cells. The secretory cells were arranged regularly to, form a single-cubic or columnar epithelium.
1. Low magnification of the gland
apocrine
In the cracking surface of the skin, the secretory portion was frequently found in an area extending from the corium to the subcutaneous fat tissue. The secretory portion was clearly separated from the surrounding tissue by the presence of a hyaline basement membrane. The luminal surface of the secretory cell bulged out. The secretory cells were generally 10-15 P. in height, although they varied in size with the stages of the secretory cycle (Fig. 1).
2. Surface of secretory cells of the apocrine gland When the luminal surface of the secretory cells was observed, it was found to consist of
pentagonal or hexagonal secretory cells which showed a smooth boundary line with the adjacent secretory cells. Microvilli were arranged densely all over the free surface of each secretory cell. They were particularly dense on the boundary line between secretory cells as if they had formed a barrier on this line (Fig. 2). They were generally rod-shaped and occasionally ladleshaped with a swollen tip (Fig. 4). They sometimes were very long (600mp.). The appearance of the luminal surface of the secretory cell characteristicly varied with the stage of secretion. The following structures were observed on the luminal surface. (1) Cytoplasmic processes of the apocrine type (a) Cytoplasmic processes of macroapocrine type: These processes ~ried in size with the stage of the secretory cycle. In stages considered to be very early, it was observed as a hemispherical protuberance about 2 p. in diameter in the middle of the surface of the cell (Fig. 2). This protuberance had no microvilli. With the advance in the stage of secretion, it increased in size to a linguiform process about 2p. in height (Fig. 3). (b) Cytoplasmic processes of microapocrine type: The cytoplasmic processes of this type were always seen on the process of the apocrine type. The secretory process of the abovementioned apocrine type had no microvilli on its surface. However, the whole surface of one secretory cell was covered with a large apocrine process, from which microvilli projected densely. Each microvillus had a swollen tip with a fig-like appearance. This picture seemed to be identical with that of the cytoplasmic processes of the microapocrine type observed by the transmission electron microscope (Fig. 4). (c) Cytoplasmic processes of intermediate type between macroapocrine and micro-
SEM OF THE APOCRINE GLANDS
Fig. 1.
Low magnification micrograph of the secretory portion of the axillary apocrine gland. The secretory portion is separated from the surrounding tissue by the presence of a hyaline basement membrane. The luminal surface of the secretory cell bulges out. x700.
Fig. 2.
The luminal surface consists of pentagonal or hexagonal secretory cells. Microvilli are arranged densely all over the free surface of each secretory cell. Cytoplasmic processes of macroapocrine type are observed in the middle of the cell (arrow). X6,250.
301
302
INOUE
Fig. 3.
Cytoplasmic process of macroapocrine type. With an advance in stage of secretion 'from Fig. 2, it has increased to a linguiform process about 2)L in height. x 16,000.
Fig. 4.
Cytoplasmic process of microapocrine type. The whole surface of one secretory cell is covered with a large apocrine process, from which microvilli project densely. x 10,000.
SEM OF THE APOCRINE GLANDS
apocrine types: There were secretory processes of an intermediate type which were classified as being between secretory processes of macroapocrine type and of microapocrine type. Cytoplasmic processes of intermediate type were seen in a relatively large gland. In this gland each secretory cell had several cytoplasmic processes about 800 mlJ in diameter on the luminal surface (Fig. 5, 6). These processes were smaller than cytoplasmic processes of macroapocrine type, but larger than those of microapocrine type. They has not yet been clarified by transmission electron microscopy. They seemed to belong to an intermediate type between macroapocrine and microapocrine types. They were presumed to have been formed by a union of swollen microvilli. A picture showing such a union of microvilli could be seen, although very infrequently (Fig. 7). (2) Destruction of secretory cell: The
Fig. 5.
303
luminal surfaces were destroyed and the nucleus and intracellular secretory granules were exposed. This phenomenon had previously been reported to indicate holocrine secretion. It seems reasonable, however, to interpret that such a phenomenon may present a stage of metabolism in the secretory cell (Fig. 8). (3) Small round pores on the luminal surface: High magnification of the surface of a secretory cell in the process of apocrine secretion revealed the presence of small round pores 200-300 A in diameter scattered among microvilli on the cytoplasmic membrane (Fig. 9).
3. Cracked surface of secretory cell of apocrine gland The secretory cells of the apocrine gland were columnar. An essentially round nucleus was situated near the basement membrane. A round nucleolus was located close to the edge of the nucleus. The supranuclear portion of the
Cytoplasmic process of intermediate type. x 1,000.
304
INOUE
Fig. 6.
Fig. 7.
A higher magnification of intermediate type of secretion. x l 0,000.
Cytoplasmic process of intermediate type is a union of swollen microvilli. x28, 000.
SEM OF THE APOCRINE GLANDS
Fig. 8.
The luminal surface is destroyed and the nucleus (N) and intracellular secretory granules are exposed. x4,000.
Fig. 9.
Small round pores 200-S00 A in diameter on the luminal surface (arrows). X50,000.
30"
306
INOUE
Fig. 10.
Cracked surface of secretory cell of apocrine gland. The secretory cells of the apocrine gland are columnar. A round nucleus (N) is situated near the basement membrane. A nucleolus (No) is located close to the edge of the nucleus. The supranuclear portion of the cytoplasm contains endoplasmic reticulumn (ER) and secretory granules (G). X3.000.
cytoplasm was wide and contained endoplasmic retic~lumn and a number of rounded secretory granules varying in size. Numerous microvilli were present on the cell surface facing the glandular lumen. Such portions of the cytoplasm as were situated immediately below the luminal surface contained neither mitochondria nor secretory granules, were apparently homogeneous, and formed a crust layer (Fig. 10). DISCUSSION
The sweat glands of the human axilla are divided into apocrine and eccrine glands by light microscopy. The former have been considered to be active in apocrine secretion. Recently, on the basis of observation with the transmission electron microscope, it has been reported that the' apocrine gland of the axilla
carried out not only apocrine secretion but also merocrine, diacrine, and holocrine secretion (6, 8, 9). The present investigation using scanning electron microscopy revealed not only secretion of the apocrine type but also of a microapocrine type and an intermediate type. The results of the present observation were then compared with the types of secretion previously distinguished by transmission electron microscopy. 1. Macroapocrine type secretion: The apocrine gland of the human axilla was examined light microscopically for secretion type. When the apocrine secretion takes place, the secretory cells change their shape from cubic to columnar, and their apical surface bulges up towards the lumen. Cytoplasmic processes are gradually transformed into drops to be de-
SEM OF THE .\POCRINE GLANDS
tached from the cytoplasm. It has been reported, however, that light microscopy of frozen sections failed to find such transformation of cytoplasmic process into drops. With the advances in transmission electron microscopy, the mechanism of apocrine secretion could be investigated in detail. In the apocrine type of secretion three stages were observed: (1) formation of an apical cap; (2) formation of a dividing membrane at the base of the apical cap; and (3) formation of tubules above the dividing membrane that extended parallel to the membrane and led to a separation of the apical cap from the underlying cell. Then apical cap possesses microvilli (6). When the axillary apocrine gland was examined by the scanning electron microscopy, the apocrine secretion was found to initiate with the appearance of a small hemisphere from the middle of the surface of the secretory cell. Microvilli disappeared from the top of the hemisphere and became scanty at the foot of the hemisphere. When completed, the apocrine process was large and linguiform, and had no microvilli on its surface. Each secretory cell had only one apocrine process. They were usually few or no microvilli on the surface of this process. Occasionally, scanning electron microscopy revealed the presence of a drop-like cytoplasmic process with a narrow stalk as has been observed in histological sections stained with hematoxylin and eosin. Further investigation may be necessary to determine whether this process is an artifact or not. 2. Microapocrine type secretion: Kurozumi has reported microapocrine secretion in the apocrine gland of the human external auditory meatus (13). According to him, the apical surface of the secretory cell bulges up towards the lumen and small projections are abundant on the luminal surface. They correspond to microvilli, but are often irregular in shape. In the present studies on the apocrine gland of the
307
human axilla, no pictures were obtained which suggested the direct polyp-like projection of any microvillus on the surface of the secretory cell. However, often observed was a huge convex process of the apocrine type which covered the whole secretory cell. The tips of microvilli which projected from this process were swollen. The author could not find any secretory cell performing the microapocrine secretion alone; secretory cells always exhibited microapocrine secretion simultaneously with macroapocrine secretion. 3. Intermediate apocrine type secretion: Secretion of this type has not as yet been demonstrated by transmission electron microscope. What is tentatively called apocrine secretion of intermediate type differs from secretion of macroapocrine and microapocrine types in the pattern of producing a cytoplasmic process. In it, more than one microvillus is swollen at the same time, fused together, and projected into a secretory process. It is characteristic of secretion of intermediate type that at least two processes project simultaneously from the surface of one secretory cell, and that the size of each process is between that of a microapocrine process and that of a macroapocrine process. 4. Eccrine secretion: From their observations with the transmission electron microscope, G. Schaumburg-Lever et al. reported that eccrine secretion was also performed by the apocrine gland of the human axilla (6). According to them, granular material covered by a limiting membrane approaches the surface of the cell. This membrane is then fused with the luminal plasma membrane and, subsequently, the vesicles discharge their granular contents into the lumen of the gland. After the discharge the limiting membrane of the granules becomes a part of the plasma membrane, and the initial condition of the cell is restored. In the present investigation, small pores
308
200-300 A in diameter were observed on the luminal surface of secretory cells actively performing the apocrine secretion. They looked like the pores of the limiting membrane shown in the case of eccrine secretion. It was not determined, however, whether these pores indicated eccrine secretion or of pinocytosis. 5. Holocrine secretion: Transmission electron microscopy has revealed the process of holocrine secretion as follows. Secretory cells forming a layer around the glandular lumen disintegrated to a varying degree and separated from one another (6). Only rarely were there secretory cells with a ruptured luminal plasma membrane through which organelles had escaped into the lumen of the gland (3, 14). Scanning electron microscopy revealed that the luminal plasma membrane had been lost at some sites of the cell layer around the lumen and that nuclei and cell organelles had been exposed at these sites. These changes were seen collectively in some glands. In the holocrine secretion of the sebaceous gland, karyopyknosis is induced with advancing maturity of glandular cells. The nucleus often becomes stellate in shape due to the pressure of lipid droplets. At last, the cells are decomposed by themselves, and the nucleus, mitochondria, and endoplasmic reticulum, as well as the lipid droplets contained in the cells, are released outside (15). On the other hand, in the apocrine gland of the human axilla, it is reasonable to assume that the disintegration of cells may not indicate such processes of maturation of cells as has been observed in the sebaceous gland, but may rather illustrate the metabolic shedding of cells.
INOL'F
ACKNOWLEDGEMENTS The author is grateful to Prof. T. Tanaka and to Prof. S. Shimao for their kind guidance, and also to Miss H. Fukudome, and to Mr. H. Osatake for their technical assistance. REFERENCES 1) Charles, A.: An electron microscopic study of the human axillary apocrine gland,]. Anat., 93: 226-232, 1959. 2) Yamada, H.: Electron microscopic observations on the secretory processes of the axillary apocrine glands, Actapath.jap., 10: 173-187, 1960. 3) Hibbs, R.G.: Electron microscopy of human apocrine sweat glands,]. Invest. Derm., 38: 77-84,1962. 4) Biempica, L. and Montes, L.F;: Secretory epithelium of the large axillary sweat glands. A cytochemical and electron microscopic study, Am.]. Anat., 117: 47 -72, 1965. 5) Montagna, W.: The apocrine sweat glands, The Structure and Function of Skin, 2nd ed., Academic Press, New York, 1962, pp. 374-424. 6) Schaumburg· Lever, G. and Lever, W.F.: Secretion from human apocrine glands. An electron microscopic study,]. Invest. Derm., 64: 38-41,1975. 7) Kurosumi, K., Kitamura, T. and Iijima, T.: Electron microscope studies on the human axillary apocrine sweat glands, Arch. histol. jap., 16: 523-566,1959. 8) Hashimoto, K.: Apocrine sweat gland (1), Hifuka no Rinsho, 17: 552-555,1975. (in Japanese) 9) Hashimoto, K.: Apocrine sweat gland (2), Hihuka no Rmsho, 17: 638-641, 1975. (in Japanese) 10) Ito, T. and Shibazaki, S.: Sweat Gland, Hifuka no Rinsho, 8: 68-82,1966. (in Japanese) 11) Naguro, T. and Iino, A.: Pancrease, Daishya, 13: i-ii, 1976. (in Japanese) 12) Tanaka, K.: Intracellular structure, Daishya, 13: i-ii, 1976. (in Japanese) 13) Kurosumi, K. and Kawabata, I.: Transmission and scanning electron microscopy of the human cerumirous apocrine gland I. Secretory glandular cells, Arch. histol. jap., 39: 207 -229, 1976. 14) Hashimoto, K., Gross, B.G. and Lever, W.F.: Electron microscopic study of apocrine secretion,]. Invest. Derm., 46: 378-390,1966. 15) Morohashi, M.: Fine structure and function of sebaceusgland, Saibou, 4: 18-28,1972. (in Japanese)
