219
J. Anat. (1975), 120, 2, pp. 219-225 With 4 figures Printed in Great Britain
Ultrastructure of endocrine cells in metaplastic epithelium of human gall bladder MATTI LAITIO AND TIMO NEVALAINEN
Department of Pathological Anatomy and Laboratory of Electron Microscopy, University of Turku, Turku, Finland
(Accepted 1 January 1975) INTRODUCTION
In an earlier study of a surgical series of over 100 cases, enterochromaffin-like cells were found in 50 % of the metaplastic goblet cell areas and 47 % of the antral type glands of gall bladders removed for gall stones. There were no such cells in the normal columnar epithelium of glands of the gall bladder neck region (Laitio,
unpublished). Many kinds of endocrine cell have been described in the gastrointestinal mucosa (Solcia, Vassallo & Capella, 1969; Solcia, Capella & Vassallo, 1970; Rubin, 1972a). Such cells possess many cytochemical characteristics in common: they have been classified on the basis of their ultrastructure and the morphology of their secretory granules (Solcia et al. 1969; Solcia et al. 1970; Rubin, 1972 a). Although the presence of such cells in metaplastic areas of gall bladder mucosa has been known for a long time (Erspamer, 1936; Hospes, 1959; Christie, 1954; Jarvi, 1962; Delaquerriere, Tremblay & Riopelle, 1962) their ultrastructure has not been described so far. The present communication deals with the ultrastructure of two endocrine cell types found in the metaplastic epithelium of human gall bladder. METHODS
The specimens were fixed in 3 % glutaraldehyde in a phosphate buffer at pH 7-3 (Sabatini, Bensch & Barrnett, 1963) for 3 hours and post-fixed in 1 % osmium tetroxide (Millonig, 1961) for 1 hour. The tissues were dehydrated in ethanol and embedded in Epon 812 (Luft, 1961). Epon sections were stained with uranyl acetate and lead citrate and studied in a Siemens Elmiskop IA electron microscope. Study of adjacent semithin (1 ,um) Epon sections stained with the periodic acid-Schiff technique (Nevalainen, Laitio & Lindgren, 1972) facilitated location of the endocrine cells. RESULTS
Endocrine cells were found in the epithelium of 3 gall bladders processed for electron microscopy from 45 cases of cholecystectomies performed for gall stones. The endocrine cells were of two types, which differed in the morphology of the secretory granules. There were, however, certain similarities. Both rested on the basement membrane (Fig. 1) and tended to be close to a capillary. The cells were I5
ANA 120
220
A*
MATTI LAITIO AND TIMO NEVALAINEN
e -1
y
I Fig. 1. Basal part of gall bladder epithelium with two endocrine cells: I (type I) and II (type II) resting on the basement membrane (B). Note the slightly irregular outline of the nucleus in cell I and the round nucleus in cell II. The type II cell is closely associated with a columnar epithelial cell, which appears to surround it. Most of the secretory granules are in the basal cytoplasm, but in cell I some are located apically. In addition to secretory granules there are heterogeneous dense bodies (L) in the cytoplasm of cell I. Note nuclei (N) of columnar epithelial cells. x 7000.
Endocrine cells in human gall bladder
221
Fig, 2. The large secretory granules in type I cell (I) are homogeneous, slightly irregular in shape and markedly electron-opaque; whereas those in type II cell (II) are small and round. B, basement membrane. x 23000.
oval or slightly triangular in shape, with the shortest side resting on the basement membrane. The cell height was variable, but the apical cell surface was never seen to reach the lumen in the sections studied. Most of the secretory granules were found in the subnuclear cytoplasm. The endocrine cell type I contained relatively large secretory granules of slightly variable shape (Figs. 1 and 2). The granules were mainly located in the subnuclear cytoplasm, but some were seen also above the nucleus (Fig. 1). The granules contained markedly electron-opaque homogeneous material (Fig. 2). In addition to mitochondria of standard appearance and short profiles of endoplasmic reticulum, there were large heterogeneous, irregularly-shaped, lipid-like bodies, possibly of lysosomal origin, in the cytoplasm (Figs. 1 and 2). The nuclei were oval and slightly irregular in outline. The endocrine cell type II was smaller and round or oval in shape (Fig. 1). The type II cells were closely surrounded by epithelial cells except for the face which abuts a basement membrane (Fig. 1), in the way deseribed by Rubin (1972b) in the stomach mucosa. The nuclei showed a round profile (Figs. 1 and 4). The secretory granules were located almost exclusively in the subnuclear cytoplasm, and were 15-2
222
MATTI LAITIO AND TIMO NEVALAINEN i'
$
s.,G ;F
1',
.0
MF
il ;t.
III.2
"*-:
-4
4
46
5,.
'6 r:r
B *
C-.-
W.W
L
IS 0 &Prf ,A.
t0t *Xs. W.%*a
Fig. 3. Type II cell secretory granules are small, round and contain slightly granular material. There is often a clear zone (halo) between the dense core and the limiting membrane of the granule. In addition to the secretory granules there are also bundles of microfilaments (MF) appearing longitudinally as well as in cross section in the cytoplasm. x 25000.
Endocrine cells in human gall bladder
223
.A
i
~
-~~~~~~~~~~~~~
y
~~~
~
~
AW
~
~
~
~
2N ~~~dA
f~~~~A
RS~~~~~~~~~~~~~R~ * ~~~~~~ &~~~~~~~~~~4~
wSz
3
3
r
v
v '@
tz
?4
Ar-
A?
Fig. 4. The type II cell in this electron micrograph contains only a few secretory granules, some of which are closely associated with the maturing (concave) face of the Golgi apparatus (G). ER, rough surfaced endoplasmic reticulum. The microfilaments (MF) in the cytoplasm are concentrated typically around the nucleus (N). The swelling of the mitochondria and of some of the Golgi cisternae most probably represent fixation artifacts. x 15 500.
often closely associated with the maturing (concave) face of a well developed Golgi apparatus (Fig. 4). The granules were smaller than those in type I cells (Fig. 2). Their number varied markedly from cell to cell (Figs. 1-4) At higher magnification the contents of the secretory granules appeared non-homogeneous and slightly granular. There was often a clear zone (halo) between the granular core and the limiting membrane (Fig. 3). In addition to mitochondria, and profiles of endoplasmic reticulum, there was a rich network of microfilaments in the cytoplasm (Figs. 3
MATTI LAITIO AND TIMO NEVALAINEN 224 and 4). The microfilaments were especially concentrated around the nucleus (Fig. 4). There were also a few pale lipid inclusions in the type II cells. DISCUSSION
Several types of epithelial cell with the morphological and cytochemical characteristics of endocrine cells have been found as a result of electron microscopic studies of human gastrointestinal mucosa (Solcia et al. 1969; Solcia et al. 1970; Rubin, 1972a). Terms such as argyrophil, argentaffin, enterochromaffin, and endocrine-like have been used to denote these cells. Rubin (1972a, 1973) has described the fine structure of four morphologic types - G (gastrin), ECL (enterochromaffin-like), EC (enterochromaffin), and D (D-like) - in human gastric mucosa. These cell types have been variously associated with the production of hormones and similar substances such as serotonin, gastrin, catecholamines, histamine, and enteroglucagon (Rubin, 1972a). No attempt was made in the present study to characterize the endocrine cells in the gall bladder cytochemically, and thus it is not possible to state their function categorically. The type I cells described in the present study resembled rather closely the EC (enterochromaffin) cells of the gastric mucosa which are believed to synthesize and store serotonin (Rubin, 1972a). The pleomorphism of secretory granules in these cells seems to be typical. The type II cells were similar in fine structure to the ECL (enterochromaffin-like) cells which form the majority of the endocrine cells in the body of the human stomach and may produce histamine (Vassallo, Solcia & Capella, 1969) as well as other humoral substances, such as serotonin (Erspamer & Asero, 1952; Rubin, Gershon & Ross, 1971). The round, small secretory granules were typical of such cells, as was the rich network of cytoplasmic microfilaments, especially around the nucleus. In a light microscopical study (Laitio, unpublished) argentaffin cells were found in the deepest parts of the antral type glands in the gall bladder. In this basal location the endocrine cells were associated closely with the capillaries, presumably to allow rapid transfer of secretory material to the blood stream. In the present study also the endocrine cells rested on the basement membrane and were located often close to capillaries. A similar topographical association was described in the stomach mucosa by Rubin (1972b). The two endocrine cells described in the present study are probably the commonest endocrine cells of metaplastic gall bladder epithelium. Other endocrine cells, such as D and G cells, were not positively identified in the present specimens, although there were some unidentified, possibly endocrine type, cells with a rather heterogeneous population of granules in the gall bladder epithelium. Thus it is impossible to rule out the possibility of the presence of endocrine cells other than those described in the gall bladder epithelium in the present study.
Endocrine cells in human gall bladder
225
SUMMARY
Two endocrine cell types were found in the metaplastic epithelium of gall bladders removed for gall stones. The endocrine cell type I resembled the EC (enterochromaffin) cell of the normal stomach mucosa. The homogeneous and electron-dense secretory granules were variable in size and shape and mainly located in the subnuclear cytoplasm. The endocrine cell type II was similar to the ECL (enterochromaffin-like) cell of the human stomach. The secretory granules were almost exclusively present in a subnuclear location, and were round and homogeneous in size. The secretory material was slightly granular and often there was a clear zone (halo) between the granule core and the limiting membrane. A rich network of microfilaments in the cytoplasm, especially around the nucleus, was typical of the type II endocrine cell. Both cell types were closely related to the basement membrane and the capillaries underneath. REFERENCES CHRISTIE, A. C. (1954). Three cases illustrating the presence of argentaffin (Kultschitzky) cells in the human gallbladder. Journal of Clinical Pathology 7, 318-321. DELAQUERRIERE, L., TREMBLAY, G. & RIOPELLE, J. L. (1962). Argentaffin cells in chronic cholecystitis. Archives of Pathology 74, 142-152. ERSPAMER, V. (1936). Die enterochromaffinen Zellen der Gallenwege in normalen und pathologischen Zustanden. Virchows Archiv fur pathologische Anatomie und Physiologie und fur klinische Medizin
297, 70-92. ERSPAMER, V. & ASERO, B. (1952). Identification of enteramine, the specific hormone of the enterochromaffin cell system, as 5-hydroxytryptamine. Nature 169, 800. HosPEs, K. (1959). Uber die argyrophile Cholecystitis. Virchows Archiv fur pathologische Anatomie und Physiologie undfur klinische Medizin 332, 236-244. JARVI, 0. (1962). A review of the part played by gastro-intestinal heterotopias in neoplasmogenesis. Proceedings of the Finnish Academy of Science and Letters 151-187. LuFr, J. H. (1961). Improvements in epoxy resin embedding methods. Journal of Biophysical and Biochemical Cytology 9, 409-414. MILLONIG, G. (1961). Advantages of a phosphate buffer for OS4 solutions in fixation. Journal of Applied Physiology 32, 1937. NEVALAINEN, T. J., LAITIO, M. & LINDGREN, 1. (1972). Periodic acid-Schiff (PAS) staining of Eponembedded tissues for light microscopy. Acta histochemica 42, 230-233. RUBIN, W. (1972a). Endocrine cells in the normal human stomach. A fine structural study. Gastroenterology 62, 784-800. RUBIN, W. (1972 b). An unusual intimate relationship between endocrine cells and other types of epithelial cells in the human stomach. Journal of Cell Biology 52, 219-227. RUBIN, W. (1973). A fine structural characterization of the proliferated endocrine cells in atrophic gastric mucosa. American Journal of Pathology 70, 109-118. RUBIN, W., GERSHON, M. D. & Ross, L. L. (1971). Electron microscope autoradiographic identification of serotonin-synthesizing cells in the mouse gastric mucosa. Journal of Cell Biology 50, 399-415. SABATINI, D. D., BENSCH, K. & BARRNETT, R. J. (1963). Cytochemistry and electron microscopy. The preservation of cellular ultrastructure and enzymatic activity by aldehyde fixation. Journal of Cell Biology 17, 19-58. SOLCIA, E., CAPELLA, C. & VASSALLO, G. (1970). Endocrine cells of the stomach and pancreas in states of gastric hypersecretion. Rendiconti R. Gastroenterologia 2, 147-158. SOLCIA, E., VASSALLO, G. & CAPELLA, C. (1969). Studies on the G cells of the pyloric mucosa, the probable site of gastrin secretion Gut 10, 379-388. VASSALLO, G., SOLCIA, E. & CAPELLA, C. (1969). Light and electron microscopic identification of several types of endocrine cells in the gastrointestinal mucosa of the cat. Zeitschrift fur Zellforschung und mikroskopische Anatomie 98, 333-356.
J. Anat. (1975), 120, 2, pp. 219-225 With 4 figures Printed in Great Britain
Ultrastructure of endocrine cells in metaplastic epithelium of human gall bladder MATTI LAITIO AND TIMO NEVALAINEN
Department of Pathological Anatomy and Laboratory of Electron Microscopy, University of Turku, Turku, Finland
(Accepted 1 January 1975) INTRODUCTION
In an earlier study of a surgical series of over 100 cases, enterochromaffin-like cells were found in 50 % of the metaplastic goblet cell areas and 47 % of the antral type glands of gall bladders removed for gall stones. There were no such cells in the normal columnar epithelium of glands of the gall bladder neck region (Laitio,
unpublished). Many kinds of endocrine cell have been described in the gastrointestinal mucosa (Solcia, Vassallo & Capella, 1969; Solcia, Capella & Vassallo, 1970; Rubin, 1972a). Such cells possess many cytochemical characteristics in common: they have been classified on the basis of their ultrastructure and the morphology of their secretory granules (Solcia et al. 1969; Solcia et al. 1970; Rubin, 1972 a). Although the presence of such cells in metaplastic areas of gall bladder mucosa has been known for a long time (Erspamer, 1936; Hospes, 1959; Christie, 1954; Jarvi, 1962; Delaquerriere, Tremblay & Riopelle, 1962) their ultrastructure has not been described so far. The present communication deals with the ultrastructure of two endocrine cell types found in the metaplastic epithelium of human gall bladder. METHODS
The specimens were fixed in 3 % glutaraldehyde in a phosphate buffer at pH 7-3 (Sabatini, Bensch & Barrnett, 1963) for 3 hours and post-fixed in 1 % osmium tetroxide (Millonig, 1961) for 1 hour. The tissues were dehydrated in ethanol and embedded in Epon 812 (Luft, 1961). Epon sections were stained with uranyl acetate and lead citrate and studied in a Siemens Elmiskop IA electron microscope. Study of adjacent semithin (1 ,um) Epon sections stained with the periodic acid-Schiff technique (Nevalainen, Laitio & Lindgren, 1972) facilitated location of the endocrine cells. RESULTS
Endocrine cells were found in the epithelium of 3 gall bladders processed for electron microscopy from 45 cases of cholecystectomies performed for gall stones. The endocrine cells were of two types, which differed in the morphology of the secretory granules. There were, however, certain similarities. Both rested on the basement membrane (Fig. 1) and tended to be close to a capillary. The cells were I5
ANA 120
220
A*
MATTI LAITIO AND TIMO NEVALAINEN
e -1
y
I Fig. 1. Basal part of gall bladder epithelium with two endocrine cells: I (type I) and II (type II) resting on the basement membrane (B). Note the slightly irregular outline of the nucleus in cell I and the round nucleus in cell II. The type II cell is closely associated with a columnar epithelial cell, which appears to surround it. Most of the secretory granules are in the basal cytoplasm, but in cell I some are located apically. In addition to secretory granules there are heterogeneous dense bodies (L) in the cytoplasm of cell I. Note nuclei (N) of columnar epithelial cells. x 7000.
Endocrine cells in human gall bladder
221
Fig, 2. The large secretory granules in type I cell (I) are homogeneous, slightly irregular in shape and markedly electron-opaque; whereas those in type II cell (II) are small and round. B, basement membrane. x 23000.
oval or slightly triangular in shape, with the shortest side resting on the basement membrane. The cell height was variable, but the apical cell surface was never seen to reach the lumen in the sections studied. Most of the secretory granules were found in the subnuclear cytoplasm. The endocrine cell type I contained relatively large secretory granules of slightly variable shape (Figs. 1 and 2). The granules were mainly located in the subnuclear cytoplasm, but some were seen also above the nucleus (Fig. 1). The granules contained markedly electron-opaque homogeneous material (Fig. 2). In addition to mitochondria of standard appearance and short profiles of endoplasmic reticulum, there were large heterogeneous, irregularly-shaped, lipid-like bodies, possibly of lysosomal origin, in the cytoplasm (Figs. 1 and 2). The nuclei were oval and slightly irregular in outline. The endocrine cell type II was smaller and round or oval in shape (Fig. 1). The type II cells were closely surrounded by epithelial cells except for the face which abuts a basement membrane (Fig. 1), in the way deseribed by Rubin (1972b) in the stomach mucosa. The nuclei showed a round profile (Figs. 1 and 4). The secretory granules were located almost exclusively in the subnuclear cytoplasm, and were 15-2
222
MATTI LAITIO AND TIMO NEVALAINEN i'
$
s.,G ;F
1',
.0
MF
il ;t.
III.2
"*-:
-4
4
46
5,.
'6 r:r
B *
C-.-
W.W
L
IS 0 &Prf ,A.
t0t *Xs. W.%*a
Fig. 3. Type II cell secretory granules are small, round and contain slightly granular material. There is often a clear zone (halo) between the dense core and the limiting membrane of the granule. In addition to the secretory granules there are also bundles of microfilaments (MF) appearing longitudinally as well as in cross section in the cytoplasm. x 25000.
Endocrine cells in human gall bladder
223
.A
i
~
-~~~~~~~~~~~~~
y
~~~
~
~
AW
~
~
~
~
2N ~~~dA
f~~~~A
RS~~~~~~~~~~~~~R~ * ~~~~~~ &~~~~~~~~~~4~
wSz
3
3
r
v
v '@
tz
?4
Ar-
A?
Fig. 4. The type II cell in this electron micrograph contains only a few secretory granules, some of which are closely associated with the maturing (concave) face of the Golgi apparatus (G). ER, rough surfaced endoplasmic reticulum. The microfilaments (MF) in the cytoplasm are concentrated typically around the nucleus (N). The swelling of the mitochondria and of some of the Golgi cisternae most probably represent fixation artifacts. x 15 500.
often closely associated with the maturing (concave) face of a well developed Golgi apparatus (Fig. 4). The granules were smaller than those in type I cells (Fig. 2). Their number varied markedly from cell to cell (Figs. 1-4) At higher magnification the contents of the secretory granules appeared non-homogeneous and slightly granular. There was often a clear zone (halo) between the granular core and the limiting membrane (Fig. 3). In addition to mitochondria, and profiles of endoplasmic reticulum, there was a rich network of microfilaments in the cytoplasm (Figs. 3
MATTI LAITIO AND TIMO NEVALAINEN 224 and 4). The microfilaments were especially concentrated around the nucleus (Fig. 4). There were also a few pale lipid inclusions in the type II cells. DISCUSSION
Several types of epithelial cell with the morphological and cytochemical characteristics of endocrine cells have been found as a result of electron microscopic studies of human gastrointestinal mucosa (Solcia et al. 1969; Solcia et al. 1970; Rubin, 1972a). Terms such as argyrophil, argentaffin, enterochromaffin, and endocrine-like have been used to denote these cells. Rubin (1972a, 1973) has described the fine structure of four morphologic types - G (gastrin), ECL (enterochromaffin-like), EC (enterochromaffin), and D (D-like) - in human gastric mucosa. These cell types have been variously associated with the production of hormones and similar substances such as serotonin, gastrin, catecholamines, histamine, and enteroglucagon (Rubin, 1972a). No attempt was made in the present study to characterize the endocrine cells in the gall bladder cytochemically, and thus it is not possible to state their function categorically. The type I cells described in the present study resembled rather closely the EC (enterochromaffin) cells of the gastric mucosa which are believed to synthesize and store serotonin (Rubin, 1972a). The pleomorphism of secretory granules in these cells seems to be typical. The type II cells were similar in fine structure to the ECL (enterochromaffin-like) cells which form the majority of the endocrine cells in the body of the human stomach and may produce histamine (Vassallo, Solcia & Capella, 1969) as well as other humoral substances, such as serotonin (Erspamer & Asero, 1952; Rubin, Gershon & Ross, 1971). The round, small secretory granules were typical of such cells, as was the rich network of cytoplasmic microfilaments, especially around the nucleus. In a light microscopical study (Laitio, unpublished) argentaffin cells were found in the deepest parts of the antral type glands in the gall bladder. In this basal location the endocrine cells were associated closely with the capillaries, presumably to allow rapid transfer of secretory material to the blood stream. In the present study also the endocrine cells rested on the basement membrane and were located often close to capillaries. A similar topographical association was described in the stomach mucosa by Rubin (1972b). The two endocrine cells described in the present study are probably the commonest endocrine cells of metaplastic gall bladder epithelium. Other endocrine cells, such as D and G cells, were not positively identified in the present specimens, although there were some unidentified, possibly endocrine type, cells with a rather heterogeneous population of granules in the gall bladder epithelium. Thus it is impossible to rule out the possibility of the presence of endocrine cells other than those described in the gall bladder epithelium in the present study.
Endocrine cells in human gall bladder
225
SUMMARY
Two endocrine cell types were found in the metaplastic epithelium of gall bladders removed for gall stones. The endocrine cell type I resembled the EC (enterochromaffin) cell of the normal stomach mucosa. The homogeneous and electron-dense secretory granules were variable in size and shape and mainly located in the subnuclear cytoplasm. The endocrine cell type II was similar to the ECL (enterochromaffin-like) cell of the human stomach. The secretory granules were almost exclusively present in a subnuclear location, and were round and homogeneous in size. The secretory material was slightly granular and often there was a clear zone (halo) between the granule core and the limiting membrane. A rich network of microfilaments in the cytoplasm, especially around the nucleus, was typical of the type II endocrine cell. Both cell types were closely related to the basement membrane and the capillaries underneath. REFERENCES CHRISTIE, A. C. (1954). Three cases illustrating the presence of argentaffin (Kultschitzky) cells in the human gallbladder. Journal of Clinical Pathology 7, 318-321. DELAQUERRIERE, L., TREMBLAY, G. & RIOPELLE, J. L. (1962). Argentaffin cells in chronic cholecystitis. Archives of Pathology 74, 142-152. ERSPAMER, V. (1936). Die enterochromaffinen Zellen der Gallenwege in normalen und pathologischen Zustanden. Virchows Archiv fur pathologische Anatomie und Physiologie und fur klinische Medizin
297, 70-92. ERSPAMER, V. & ASERO, B. (1952). Identification of enteramine, the specific hormone of the enterochromaffin cell system, as 5-hydroxytryptamine. Nature 169, 800. HosPEs, K. (1959). Uber die argyrophile Cholecystitis. Virchows Archiv fur pathologische Anatomie und Physiologie undfur klinische Medizin 332, 236-244. JARVI, 0. (1962). A review of the part played by gastro-intestinal heterotopias in neoplasmogenesis. Proceedings of the Finnish Academy of Science and Letters 151-187. LuFr, J. H. (1961). Improvements in epoxy resin embedding methods. Journal of Biophysical and Biochemical Cytology 9, 409-414. MILLONIG, G. (1961). Advantages of a phosphate buffer for OS4 solutions in fixation. Journal of Applied Physiology 32, 1937. NEVALAINEN, T. J., LAITIO, M. & LINDGREN, 1. (1972). Periodic acid-Schiff (PAS) staining of Eponembedded tissues for light microscopy. Acta histochemica 42, 230-233. RUBIN, W. (1972a). Endocrine cells in the normal human stomach. A fine structural study. Gastroenterology 62, 784-800. RUBIN, W. (1972 b). An unusual intimate relationship between endocrine cells and other types of epithelial cells in the human stomach. Journal of Cell Biology 52, 219-227. RUBIN, W. (1973). A fine structural characterization of the proliferated endocrine cells in atrophic gastric mucosa. American Journal of Pathology 70, 109-118. RUBIN, W., GERSHON, M. D. & Ross, L. L. (1971). Electron microscope autoradiographic identification of serotonin-synthesizing cells in the mouse gastric mucosa. Journal of Cell Biology 50, 399-415. SABATINI, D. D., BENSCH, K. & BARRNETT, R. J. (1963). Cytochemistry and electron microscopy. The preservation of cellular ultrastructure and enzymatic activity by aldehyde fixation. Journal of Cell Biology 17, 19-58. SOLCIA, E., CAPELLA, C. & VASSALLO, G. (1970). Endocrine cells of the stomach and pancreas in states of gastric hypersecretion. Rendiconti R. Gastroenterologia 2, 147-158. SOLCIA, E., VASSALLO, G. & CAPELLA, C. (1969). Studies on the G cells of the pyloric mucosa, the probable site of gastrin secretion Gut 10, 379-388. VASSALLO, G., SOLCIA, E. & CAPELLA, C. (1969). Light and electron microscopic identification of several types of endocrine cells in the gastrointestinal mucosa of the cat. Zeitschrift fur Zellforschung und mikroskopische Anatomie 98, 333-356.
