الفهرس 
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Abstract
The present study was carried out on 48 New Zealand White rabbit. The fetuses were collected on gestational days 10, 12, 18, 21, 25 and 29. All used animals were obtained from the laboratory animal house in Faculty of Medicine, Assiut University.
- Specimens obtained from fetal rabbits were prepared for histological and histochemical investigations.
- Gross examination of pregnant rabbit uterus showed that each uterine horn contained three to five gestational sacs. Each sac contained one embryo which was surrounded by its fetal membranes.
- At 10th gestational day, the fetal rabbit stomach was represented by dilatation in the foregut just cranially to septum transversum that contained hepatic primordial. The primordial stomach was lined by one layer of undifferentiated cuboidal cells that is surrounded by undifferentiated highly vascular mesenchymal tissue and surrounded from outside by a single layer of cuboidal cells.
- At 12th gestational day, the fetal stomach was considerably dilated and lined with undifferentiated stratified columnar epithelium that showed a pale staining vacuolated cytoplasm and contained large irregular nuclei. The lining epithelium was surrounded by highly cellular densely arranged mesenchymal connective tissue containing blood vessels and covered by single layer of cuboidal cells representing the future visceral peritoneum.
- At 18th gestational day, large clear intracellular vacuoles appeared within the epithelial cells and interrupted bands of ill-developed smooth muscle fibers were recognized representing the primitive muscularis layer.
- At 21st gestational day, the different parts of the stomach including, cardia, fundus, body and pylorus could be easily distinguished. The gastric mucosa was thrown into distinct folds and consisted of lamina epithelialis, lamina propria and muscularis mucosa. The gastric epithelium was variable from stratified to simple columnar and their cytoplasm contained PAS positive granules. The gastric pits were first recognized and they leaded to shallow primitive tubular or alveolar gastric glands. These primitive glands were lined by undifferentiated columnar cells. The lamina propria and tunica submucosa were consisted of loose highly vascular connective tissue that contained fine elastic and collagen fibers and separated by a thin layer of ill-developed smooth muscle fibers representing the first appearance of muscularis mucosa. Tunica muscularis was more developed than the previous age and was formed of longitudinally oriented smooth muscle fibers. The serosa was formed of loose connective tissue containing numerous blood capillaries, nerve fibers and covered by mesothelium.
- At 25th developmental day, the gastric mucosal folds were more prominent. The gastric surface epithelial cells appeared columnar with basally located nuclei and acidophilic cytoplasm. The surface epithelium was interrupted by the openings of the gastric pits which were more prominent and numerous than previous age. PAS and alcian blue positive intracellular apical granules and thick mucous layer covering the mucosa were demonstrated in the all gastric regions.
- The present study revealed that parietal and chief cells were firstly recognized at 25th developmental day within the lining epithelium of the gastric glands at the all gastric regions. Parietal cells occupied a peripheral position at the body and base of the gland and appeared as sporadic, large cells bearing deeply acidophilic cytoplasm and relatively small and rounded nuclei. While chief cells were located near the bottom of the gland and appeared rounded or polyhedral cells with centrally located nuclei. The lamina propria and tunica submucosa consisted mainly of elastic and collagen fibers which were more prominent than the previous age. The Muscularis layer appeared thicker and more developed than previous age. The serosa was consisted of highly vascular loose connective tissue covered by mesothelium.
- At 29th developmental day, tunica mucosa and tunica submucosa of the fetal stomach were laid in longitudinally oriented folds known as rugae. The gastric mucosal folds were more prominent than the previous gestational age and more prominent in the cardiac region than in fundus, body and pyloric regions. The gastric epithelial cells appeared columnar with oval basally located nuclei and acidophilic vacuolated cytoplasm. PAS and alcian blue positive intracellular granules were demonstrated at the apical portions of these cells.
- The gastric pits were relatively wider than previous age, and were invaginated to various extends in the lamina propria and leading to the branched tubular gastric glands. At cardiac region, gastric glands appeared as highly coiled, highly branched tubular glands, while in fundus and body, the gastric glands appeared as branched tubular glands, however, in pylorus the gastric glands still appeared as simple tubular glands. Mucous neck cells were recognized in groups or clusters in the neck of the gastric gland and appeared cuboidal with spherical nuclei. The parietal and chief cells in the fetal rabbit gastric glands were more numerous than that observed in the previous age. The lamina propria and tunica submucosa were consisted of well-developed network of elastic and collagen fibers. The muscularis layer appeared well developed and its thickness increased significantly with advancement of age. It was of the highest thickness in cardia and of the least thickness in fundus and body in fetal stomach at all investigated ages. The serosa was consisted of highly vascular loose connective tissue containing blood capillaries and nerve fibers and covered by mesothelium.
- Immunohistochemical staining of the fundus and body of fetal rabbit stomach was performed for demonstration of gastric enteroendocrine cells. Enteroendocrine cells expressing, chromogranin A , gastrin and somatostatin were recognized within the gastric glands in fetal rabbits at 25th and 29th gestational day. It was observed that, immunoreactive cells were more numerous in gastric glands of fetal rabbit stomach at 29th gestational day than 25th day. On the other hand, numerous telocytes were recognized within the lamina propria of gastric mucosa and tunica muscularis of fetal rabbit stomach at 25th gestational day. These cells expressed a strong chromogranin A positive reaction and a strong somatostatin positive reaction.
- Scanning electron microscope was applied to elucidate the three dimensional structural features of the gastric epithelium in fetal rabbit The ultrastructural features of the gastric mucosal surface of the fetal stomach showed that, the gastric mucosal foldness (the rugae) first appeared at 21st gestational day. Gastric pits appeared as shallow openings leading to shallow gastric glands. At 25th gestational day, the rugae were more distinct and the gastric pits were deeper than the previous age. At 29th gestational day, the gastric rugae were relatively more prominent and higher while the gastric pits were much deeper than previous ages.