Retinoic Acid Stimulates the Dynamics of Mouse Gastric Epithelial Progenitors
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《干细胞学杂志》
a Department of Anatomy, Faculty of Medicine and Health Sciences, United Arab Emirates University, Al-Ain, United Arab Emirates;
b Department of Medicine, Washington University School of Medicine, St. Louis, Missouri, USA
Key Words. Gastric stem cell ? Cell dynamics ? Pit cell ? Zymogenic cell ? Vitamin A ? Stomach
Correspondence: Sherif M. Karam, M.D., Ph.D., Department of Anatomy, Faculty of Medicine and Health Sciences, UAE University, Al-Ain, P.O.Box17666, United Arab Emirates. Telephone:971-3-703-9493; fax:971-3-767-2033; e-mail: skaram@uaeu.ac.ae
ABSTRACT
In the mouse stomach, the oxyntic epithelium is organized to form numerous short pits populated by pit and parietal cells producing mucus and acid, respectively. The pits open into long tubular glands lined by mucous neck, parietal, enteroendocrine, and zymogenic cells. The latter secretes pepsinogen and intrinsic factor. Each gland also includes epithelial progenitors anchored in a narrow region next to the pit boundary . Normally, these progenitor cells proliferate actively to reproduce themselves and to differentiate during a spatially well-organized bipolar migration toward the luminal surface and the gland bottom and therefore produce all cells lining the pit-gland unit .
Similar epithelial progenitors have been defined recently in the human stomach and also found to be responsible for the continuous production of all cell types in the gastric epithelium . Although it is generally believed that these progenitor cells play an important role during gastric carcinogenesis and, in a transgenic mouse model, their transdifferentiation into neuroendocrine gastric cancer cells has been recently demonstrated , little information is available regarding the factors that control the dynamic features of these epithelial progenitors.
Vitamin A or retinoic acid is known to have profound effects on cell proliferation and differentiation in other renewing epithelia. It has been shown to play an important role in epidermal keratinization , mammary gland formation , and gene expression of tracheobronchial epithelium . Retinoic acid acts through specific receptors that are members of the nuclear steroid receptor superfamily of proteins. These receptors function as ligand-dependent transcription factors that are believed to control cell proliferation and differentiation .
In the human stomach, retinoic acid is thought to play a role in gastric cancer prevention and has long been known as a cytoprotective agent against gastric mucosal damage and ulcer formation by mechanisms that are not well understood . Mozsik et al. have shown that the action of retinoic acid depends on intact adrenals and vagal innervation; however, the possibility that it has direct effects on the gastric epithelium was not excluded. Recently, mRNA expression of the retinoid receptors was demonstrated in the gastric mucosa . However, still there is a debate whether retinoic acid can be used as a chemopreventive agent against gastric cancer. Although some investigators found that it prevents progression of atrophic gastritis to gastric cancer , others reported that it has no effect on cancer progression and may even increase the risk of other types of cancer, such as bronchogenic carcinoma .
This report examines whether retinoic acid has an effect on the proliferation of gastric epithelial progenitors. Also, it provides some insights into a possible role of retinoic acid in the control of differentiation program and turnover of the gastric epithelium.
MATERIALS AND METHODS
Retinoic Acid Enhances BrdU Labeling in the Gastric Glands
When a single injection of BrdU was given to control and retinoic acid–treated mice and stomach sections were immunostained using anti-BrdU antibody, S-phase cells were labeled and found to be localized in the upper portion of the gastric glands (Figs. 1A, 1B). When comparing probed sections of control and retinoic acid–treated mice, more S-phase cells were found in the latter. Counts conducted in the gastric glands (n = 16 per mouse) of three different pairs of control and retinoic acid–treated mice revealed that the average number of S-phase cells per gland was approximately two cells in the control mice versus four cells in the retinoic acid–treated mice. In each pair of mice, the difference was statistically significant (p .001). These dividing cells were found mainly in the isthmus region, with a small proportion in the low pit segment and the neck region (Fig. 2, left panel).
Figure 1. One hour (A, B), 1-day (C, D), and 3-day (E, F) BrdU labeling of S-phase cells and their progeny in the oxyntic mucosae of control (A, C, E) and retinoic acid–treated (B, D, F) mice. Sections were incubated with polyclonal anti-BrdU and then with rabbit anti-goat IgG conjugated with peroxidase. Sites of antigen-antibody binding (S-phase nuclei of dividing cells or their progeny) were visualized with diaminobenzidine and appear brown. Sections were counter-stained with periodic acid Schiff to visualize the mucus of pit cells. Note that in the control tissue, 1 hour of BrdU injection results in a few labeled nuclei scattered in a region corresponding to the isthmus. BrdU infusion for 1 and 3 days (C, E) produces an increase in the number of labeled cells that extend upward to the luminal surface and downward to the neck region. In the retinoic acid–treated tissues (B, D, F), BrdU-labeled cells are more numerous and occupy a wider region than in the controls. Labeled cells are seen along the surface and free in the gastric lumen (F). They also expand deep into the basal region. Bar = 140 3m (AD) and 70 3m (E, F). Abbreviations: BrdU, 5-bromo-2'-deoxyuridine; IgG, immunoglobulin G.
Figure 2. Percentages of BrdU-labeled cells in three gland regions (isthmus, neck, and base) and two pit segments (high and low) of control (white bars) and retinoic acid–treated (black bars) mice that received BrdU as a single BrdU injection 1 hour before euthanasia (left panel) and as continuous infusion for 1 and 3 days (middle and right panels). Note that BrdU labeling starts mainly in the isthmus and then gradually increases with time and makes its appearance in upper and lower levels. Also note that retinoic acid treatment enhances BrdU labeling in the isthmus and the migration of labeled cells toward the high pit segment and the base region. Abbreviation: BrdU, 5-bromo-2'-deoxyuridine.
The increase in dividing progenitors was not restricted to the glandular epithelium of the stomach. The basal progenitor cells of the stratified nonglandular epithelium of the fundus region were also affected by retinoicacid. Counts in three pairs of control and retinoic acid–treated mice revealed that whereas 9% of the basal cells of control fundus were BrdU labeled, this labeling index was significantly increased up to 36% in retinoic acid–treated mice (Figs. 3A, 3B).
Figure 3. Labeling of S-phase basal cells of the fundus (A, B) pit and enteroendocrine cells (C, D), neck and zymogenic cells of the oxyntic mucosa (E, F), and mucous and G cells of the pyloric antrum (G, H) of normal (A, C, E, G) and retinoic acid–treated (B, D, F, H) mice. (A, B): Nuclei of S-phase cells appear brown and become more numerous after retinoic acid treatment. (C, D): TRITC-conjugated UEA-1 lectin was used as a marker of pit cells (red), and anti-ghrelin was used to label the cytoplasm of enteroendocrine cells (green). Note that the labeling and distribution of both cell types appear more or less similar in the retinoic acid–treated and control mucosae. (E, F): fluorescein isothiocyanate–conjugated Grifforia simplifolica II lectin was used as a marker of neck cells (green), and anti-intrinsic factor was used to label the cytoplasm of zymogenic cells (red). Note that more zymogenic cells are present in the retinoic acid–treated mucosa. (G, H): TRITC-conjugated UEA1 lectin labels luminal mucous and some mucus-secreting cells (red), and anti-gastrin antibodies label the enteroendocrine G cells (green). Note that no change is seen in the G cell labeling after retinoicacid infusion. Bar=303m (A, B), 90 μm (C, D), and 60 μm (EH). Abbreviations: TRITC, tetramethylrhodamine isothiocyanate; UEA-1, Ulex europaeus agglutinin type 1 lectin.
Labeling of all S-phase cells and their progeny in the oxyntic glands during the 1- and 3-day periods of BrdU infusion and comparing it with 1-hour labeling showed that, after their production, cells migrate upward to the luminal surface and downward to the gland bottom (Figs. 1C–1F). The area occupied by BrdU-labeled cells was wider in the retinoic acid–treated mice compared with the control (Fig. 1C versus 1D and Fig. 1E versus 1F). The mucosae of retinoic acid–treated mice showed many labeled cells in the pit segments and along the luminal surface, and some were even seen free in the stomach lumen (Fig. 1F).
To visualize and confirm the difference in cell production, BrdU labeling was estimated in the isthmus, neck, and base regions, in addition to the pit segments. Counts of the 1-day BrdU-infused control mice showed an increase in the percentage of labeled cells in the isthmus and the low pit segment and neck region. In addition, very few labeled cells made their appearance in the high pit segment and the base region. Labeled cells were more numerous in retinoic acid–treated mice compared with control ones (Fig. 2, middle panel). By 3-day BrdU infusion, counts revealed a further increase in the number of labeled cells, which was more enhanced in the high pit segment and neck and base regions of the retinoic acid–treated mice (Fig. 2, right panel).
Retinoic Acid Expands the Area of Zymogenic Cells
Probing of normal and retinoic acid–treated tissue sections with cell markers specific for pit, neck, parietal, and enteroendocrine cells demonstrated no significant difference in the labeled area of these cell types. However, when anti-intrinsic factor was used to probe normal and treated tissues, an increase in the area of the zymogenic cells after retinoic acid treatment was observed (Figs. 3C–3F). Measurements of the areas of different cell types confirmed these observations and showed a significant increase in the area occupied by zymogenic cells in the retinoic acid–treated tissues (Fig. 4). Zymogenic cell counts in PAS/hematoxylin-stained sections, obtained from three pairs of control and retinoic acid–treated mice, showed that the average number of zymogenic cells in control mice was approximately 11 cells per gland. In retinoic acid–treated mice, the number of zymogenic cells was significantly increased to up to 17 cells per gland.
Figure 4. Measurements of the area of the cell types in the gastric glands of normal (white bars) and retinoic acid–treated (black bars) mice. The percentage of the area of pit, zymogenic, parietal, and isthmal-neck cells are expressed as mean ± standard error of the mean. *Statistically significant difference (p < .0004) between the values of zymogenic cells.
Retinoic Acid Enhances the Degeneration of Pit Cells
Because BrdU-labeled pit cells were frequently observed free in the gastric lumen of retinoic acid–treated mice (Fig. 1F), it was necessary to confirm this observation and to check for the viability of pit cells at the luminal surface of control versus retinoic acid–treated mice.
The TUNEL assay was used to detect apoptotic cells in the gastric mucosae of control and retinoic acid–treated mice. As previously reported, pit cells normally undergo degeneration at the luminal surface of the gastric mucosa . However, more pit cells at the luminal surface of retinoic acid–treated mice underwent apoptosis compared with control mice (Fig. 5). Counts conducted in oxyntic mucosal sections (n = four per mouse) revealed that the number of apoptotic cells was approximately one cell per five glands, whereas in retinoic acid–treated mice, it was approximately three cells per five glands.
Figure 5. Labeling of apoptotic cells in the gastric mucosa of control (A) and retinoic acid–treated (B) mice. Sections were processed for the TUNEL assay as mentioned in the Material and Methods section and then counterstained with periodic acid Schiff. (A, B): Nuclei of apoptotic cells appear brownish (arrows) and are found at the luminal surface (mucus-secreting pit cells). Note that apoptotic cells are more numerous in (B) compared with (A). Bar = 50 μm.
Electron microscopic examination of the oxyntic mucosae of normal and retinoic acid–treated mice confirmed the presence of few degenerated cells on the luminal surface of control stomach. In the retinoic acid–treated mice, the degenerated pit cells were numerous. These cells were located along the luminal surface of the mucosa and were characterized by accumulation of vacuolar spaces in the cytoplasm, dissolution of the nucleoplasm, and condensation of the chromatin (Fig. 6). Therefore, it seems that retinoic acid enhances the production of pit cells in the isthmus region and low pit segment and enhances their degeneration at the luminal surface.
Figure 6. Electron micrographs showing pit cells of normal (A) and retinoic acid–treated (B) mice as seen at the luminal surface of the oxyntic mucosa. Part of the stomach lumen is seen at the upper right corner, and part of the gland lumen (L) is seen at the left. (A): Pit cells appear elongated, and their cytoplasm contain dark mucous granules. Part of a degenerated pit cell with vacuolated cytoplasm is seen at the bottom (asterisk). (B): Only a few pit cells appear like the normal ones in (A), and six pit cells appear degenerated (asterisks). They have vacuolated cytoplasm, and their nuclei exhibit condensed chromatin. Bar = 5 μm.
DISCUSSION
The authors would like to thank Dr. Gerald Buzzell for his critical comments on the manuscript. This work was carried out with the support of grant NP/02/22 from the Faculty of Medicine and Health Sciences, United Arab Emirates University, to S.M.K.
REFERENCES
Karam SM, Leblond CP. Identifying and counting epithelial cell types in the "corpus" of the mouse stomach. Anat Rec 1992;232:231–246.
Karam SM, Leblond CP. Dynamics of epithelial cells in the corpus of the mouse stomach, I: identification of proliferative cell types and pinpointing of the stem cell. Anat Rec 1993;236:259–279.
Karam SM, Straiton T, Hassan WM et al. Defining epithelial cell progenitors in the human oxyntic mucosa. STEM CELLS 2003;21:322–336.
Del Buono R, Wright NA. The growth of human tumours. In: Peckham M, Pinedo MH, Veronesi U, eds. Oxford Textbook of Oncology. Oxford: Oxford University Press, 1995:3–12.
Syder AJ, Karam SM, Mills JC et al. A transgenic mouse model of metastatic carcinoma involving trans-differentiation of a gastric epithelial lineage progenitor to a neuroendocrine phenotype. Proc Natl Acad Sci U S A 2004;101:4471–4476.
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Jetten AM, Fujimoto W, Zhang L et al. Regulation of gene expression during squamous differentiation by multiple retinoic acid signaling pathways. In: Livrea MA, Vidali G, eds. Retinoids: From Basic Science to Clinical Applications. Basel: Birkhauser Verlag, 1994:243–252.
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b Department of Medicine, Washington University School of Medicine, St. Louis, Missouri, USA
Key Words. Gastric stem cell ? Cell dynamics ? Pit cell ? Zymogenic cell ? Vitamin A ? Stomach
Correspondence: Sherif M. Karam, M.D., Ph.D., Department of Anatomy, Faculty of Medicine and Health Sciences, UAE University, Al-Ain, P.O.Box17666, United Arab Emirates. Telephone:971-3-703-9493; fax:971-3-767-2033; e-mail: skaram@uaeu.ac.ae
ABSTRACT
In the mouse stomach, the oxyntic epithelium is organized to form numerous short pits populated by pit and parietal cells producing mucus and acid, respectively. The pits open into long tubular glands lined by mucous neck, parietal, enteroendocrine, and zymogenic cells. The latter secretes pepsinogen and intrinsic factor. Each gland also includes epithelial progenitors anchored in a narrow region next to the pit boundary . Normally, these progenitor cells proliferate actively to reproduce themselves and to differentiate during a spatially well-organized bipolar migration toward the luminal surface and the gland bottom and therefore produce all cells lining the pit-gland unit .
Similar epithelial progenitors have been defined recently in the human stomach and also found to be responsible for the continuous production of all cell types in the gastric epithelium . Although it is generally believed that these progenitor cells play an important role during gastric carcinogenesis and, in a transgenic mouse model, their transdifferentiation into neuroendocrine gastric cancer cells has been recently demonstrated , little information is available regarding the factors that control the dynamic features of these epithelial progenitors.
Vitamin A or retinoic acid is known to have profound effects on cell proliferation and differentiation in other renewing epithelia. It has been shown to play an important role in epidermal keratinization , mammary gland formation , and gene expression of tracheobronchial epithelium . Retinoic acid acts through specific receptors that are members of the nuclear steroid receptor superfamily of proteins. These receptors function as ligand-dependent transcription factors that are believed to control cell proliferation and differentiation .
In the human stomach, retinoic acid is thought to play a role in gastric cancer prevention and has long been known as a cytoprotective agent against gastric mucosal damage and ulcer formation by mechanisms that are not well understood . Mozsik et al. have shown that the action of retinoic acid depends on intact adrenals and vagal innervation; however, the possibility that it has direct effects on the gastric epithelium was not excluded. Recently, mRNA expression of the retinoid receptors was demonstrated in the gastric mucosa . However, still there is a debate whether retinoic acid can be used as a chemopreventive agent against gastric cancer. Although some investigators found that it prevents progression of atrophic gastritis to gastric cancer , others reported that it has no effect on cancer progression and may even increase the risk of other types of cancer, such as bronchogenic carcinoma .
This report examines whether retinoic acid has an effect on the proliferation of gastric epithelial progenitors. Also, it provides some insights into a possible role of retinoic acid in the control of differentiation program and turnover of the gastric epithelium.
MATERIALS AND METHODS
Retinoic Acid Enhances BrdU Labeling in the Gastric Glands
When a single injection of BrdU was given to control and retinoic acid–treated mice and stomach sections were immunostained using anti-BrdU antibody, S-phase cells were labeled and found to be localized in the upper portion of the gastric glands (Figs. 1A, 1B). When comparing probed sections of control and retinoic acid–treated mice, more S-phase cells were found in the latter. Counts conducted in the gastric glands (n = 16 per mouse) of three different pairs of control and retinoic acid–treated mice revealed that the average number of S-phase cells per gland was approximately two cells in the control mice versus four cells in the retinoic acid–treated mice. In each pair of mice, the difference was statistically significant (p .001). These dividing cells were found mainly in the isthmus region, with a small proportion in the low pit segment and the neck region (Fig. 2, left panel).
Figure 1. One hour (A, B), 1-day (C, D), and 3-day (E, F) BrdU labeling of S-phase cells and their progeny in the oxyntic mucosae of control (A, C, E) and retinoic acid–treated (B, D, F) mice. Sections were incubated with polyclonal anti-BrdU and then with rabbit anti-goat IgG conjugated with peroxidase. Sites of antigen-antibody binding (S-phase nuclei of dividing cells or their progeny) were visualized with diaminobenzidine and appear brown. Sections were counter-stained with periodic acid Schiff to visualize the mucus of pit cells. Note that in the control tissue, 1 hour of BrdU injection results in a few labeled nuclei scattered in a region corresponding to the isthmus. BrdU infusion for 1 and 3 days (C, E) produces an increase in the number of labeled cells that extend upward to the luminal surface and downward to the neck region. In the retinoic acid–treated tissues (B, D, F), BrdU-labeled cells are more numerous and occupy a wider region than in the controls. Labeled cells are seen along the surface and free in the gastric lumen (F). They also expand deep into the basal region. Bar = 140 3m (AD) and 70 3m (E, F). Abbreviations: BrdU, 5-bromo-2'-deoxyuridine; IgG, immunoglobulin G.
Figure 2. Percentages of BrdU-labeled cells in three gland regions (isthmus, neck, and base) and two pit segments (high and low) of control (white bars) and retinoic acid–treated (black bars) mice that received BrdU as a single BrdU injection 1 hour before euthanasia (left panel) and as continuous infusion for 1 and 3 days (middle and right panels). Note that BrdU labeling starts mainly in the isthmus and then gradually increases with time and makes its appearance in upper and lower levels. Also note that retinoic acid treatment enhances BrdU labeling in the isthmus and the migration of labeled cells toward the high pit segment and the base region. Abbreviation: BrdU, 5-bromo-2'-deoxyuridine.
The increase in dividing progenitors was not restricted to the glandular epithelium of the stomach. The basal progenitor cells of the stratified nonglandular epithelium of the fundus region were also affected by retinoicacid. Counts in three pairs of control and retinoic acid–treated mice revealed that whereas 9% of the basal cells of control fundus were BrdU labeled, this labeling index was significantly increased up to 36% in retinoic acid–treated mice (Figs. 3A, 3B).
Figure 3. Labeling of S-phase basal cells of the fundus (A, B) pit and enteroendocrine cells (C, D), neck and zymogenic cells of the oxyntic mucosa (E, F), and mucous and G cells of the pyloric antrum (G, H) of normal (A, C, E, G) and retinoic acid–treated (B, D, F, H) mice. (A, B): Nuclei of S-phase cells appear brown and become more numerous after retinoic acid treatment. (C, D): TRITC-conjugated UEA-1 lectin was used as a marker of pit cells (red), and anti-ghrelin was used to label the cytoplasm of enteroendocrine cells (green). Note that the labeling and distribution of both cell types appear more or less similar in the retinoic acid–treated and control mucosae. (E, F): fluorescein isothiocyanate–conjugated Grifforia simplifolica II lectin was used as a marker of neck cells (green), and anti-intrinsic factor was used to label the cytoplasm of zymogenic cells (red). Note that more zymogenic cells are present in the retinoic acid–treated mucosa. (G, H): TRITC-conjugated UEA1 lectin labels luminal mucous and some mucus-secreting cells (red), and anti-gastrin antibodies label the enteroendocrine G cells (green). Note that no change is seen in the G cell labeling after retinoicacid infusion. Bar=303m (A, B), 90 μm (C, D), and 60 μm (EH). Abbreviations: TRITC, tetramethylrhodamine isothiocyanate; UEA-1, Ulex europaeus agglutinin type 1 lectin.
Labeling of all S-phase cells and their progeny in the oxyntic glands during the 1- and 3-day periods of BrdU infusion and comparing it with 1-hour labeling showed that, after their production, cells migrate upward to the luminal surface and downward to the gland bottom (Figs. 1C–1F). The area occupied by BrdU-labeled cells was wider in the retinoic acid–treated mice compared with the control (Fig. 1C versus 1D and Fig. 1E versus 1F). The mucosae of retinoic acid–treated mice showed many labeled cells in the pit segments and along the luminal surface, and some were even seen free in the stomach lumen (Fig. 1F).
To visualize and confirm the difference in cell production, BrdU labeling was estimated in the isthmus, neck, and base regions, in addition to the pit segments. Counts of the 1-day BrdU-infused control mice showed an increase in the percentage of labeled cells in the isthmus and the low pit segment and neck region. In addition, very few labeled cells made their appearance in the high pit segment and the base region. Labeled cells were more numerous in retinoic acid–treated mice compared with control ones (Fig. 2, middle panel). By 3-day BrdU infusion, counts revealed a further increase in the number of labeled cells, which was more enhanced in the high pit segment and neck and base regions of the retinoic acid–treated mice (Fig. 2, right panel).
Retinoic Acid Expands the Area of Zymogenic Cells
Probing of normal and retinoic acid–treated tissue sections with cell markers specific for pit, neck, parietal, and enteroendocrine cells demonstrated no significant difference in the labeled area of these cell types. However, when anti-intrinsic factor was used to probe normal and treated tissues, an increase in the area of the zymogenic cells after retinoic acid treatment was observed (Figs. 3C–3F). Measurements of the areas of different cell types confirmed these observations and showed a significant increase in the area occupied by zymogenic cells in the retinoic acid–treated tissues (Fig. 4). Zymogenic cell counts in PAS/hematoxylin-stained sections, obtained from three pairs of control and retinoic acid–treated mice, showed that the average number of zymogenic cells in control mice was approximately 11 cells per gland. In retinoic acid–treated mice, the number of zymogenic cells was significantly increased to up to 17 cells per gland.
Figure 4. Measurements of the area of the cell types in the gastric glands of normal (white bars) and retinoic acid–treated (black bars) mice. The percentage of the area of pit, zymogenic, parietal, and isthmal-neck cells are expressed as mean ± standard error of the mean. *Statistically significant difference (p < .0004) between the values of zymogenic cells.
Retinoic Acid Enhances the Degeneration of Pit Cells
Because BrdU-labeled pit cells were frequently observed free in the gastric lumen of retinoic acid–treated mice (Fig. 1F), it was necessary to confirm this observation and to check for the viability of pit cells at the luminal surface of control versus retinoic acid–treated mice.
The TUNEL assay was used to detect apoptotic cells in the gastric mucosae of control and retinoic acid–treated mice. As previously reported, pit cells normally undergo degeneration at the luminal surface of the gastric mucosa . However, more pit cells at the luminal surface of retinoic acid–treated mice underwent apoptosis compared with control mice (Fig. 5). Counts conducted in oxyntic mucosal sections (n = four per mouse) revealed that the number of apoptotic cells was approximately one cell per five glands, whereas in retinoic acid–treated mice, it was approximately three cells per five glands.
Figure 5. Labeling of apoptotic cells in the gastric mucosa of control (A) and retinoic acid–treated (B) mice. Sections were processed for the TUNEL assay as mentioned in the Material and Methods section and then counterstained with periodic acid Schiff. (A, B): Nuclei of apoptotic cells appear brownish (arrows) and are found at the luminal surface (mucus-secreting pit cells). Note that apoptotic cells are more numerous in (B) compared with (A). Bar = 50 μm.
Electron microscopic examination of the oxyntic mucosae of normal and retinoic acid–treated mice confirmed the presence of few degenerated cells on the luminal surface of control stomach. In the retinoic acid–treated mice, the degenerated pit cells were numerous. These cells were located along the luminal surface of the mucosa and were characterized by accumulation of vacuolar spaces in the cytoplasm, dissolution of the nucleoplasm, and condensation of the chromatin (Fig. 6). Therefore, it seems that retinoic acid enhances the production of pit cells in the isthmus region and low pit segment and enhances their degeneration at the luminal surface.
Figure 6. Electron micrographs showing pit cells of normal (A) and retinoic acid–treated (B) mice as seen at the luminal surface of the oxyntic mucosa. Part of the stomach lumen is seen at the upper right corner, and part of the gland lumen (L) is seen at the left. (A): Pit cells appear elongated, and their cytoplasm contain dark mucous granules. Part of a degenerated pit cell with vacuolated cytoplasm is seen at the bottom (asterisk). (B): Only a few pit cells appear like the normal ones in (A), and six pit cells appear degenerated (asterisks). They have vacuolated cytoplasm, and their nuclei exhibit condensed chromatin. Bar = 5 μm.
DISCUSSION
The authors would like to thank Dr. Gerald Buzzell for his critical comments on the manuscript. This work was carried out with the support of grant NP/02/22 from the Faculty of Medicine and Health Sciences, United Arab Emirates University, to S.M.K.
REFERENCES
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