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The adaptation changes in the rumen mucosa in response to concentrate feeding were studied using 24 sheep. Beside the hay-fed group (control-group), these animals were assigned to 7 groups according to the duration of concentrate feeding (2, 4 days, 1, 2, 4, 6 and 12 weeks). Macro- and mesoscopic examinations, routine light microscopical staining and immunohistochemical techniques as well as scanning and transmission electron microscopical methods were employed for characterization of rumen epithelium of the different groups. Morphometrical analysis of the ruminal mucosa was carried out with a computerized image analysis program (Lucia 32-G Corona 4.11) (Nikom) for measurements of thickness of different epithelium strata and length of papillary bodies, and calculation of the total surface of papillae. The total absorptive surface of the rumen-epithelium increased with increasing the duration of intake of concentrate feeding and showed marked structural differences from that of hay-fed sheep. The morphological adaptation of the rumen to concentrate feeding occurred rapidly within 2 days. However, 4 and 6 weeks concentrate-fed groups revealed the most significant structural differences among the different concentrate-fed groups, which can be summarized as follows: 1. Scanning electron microscopy (S.E.M) examination showed a time dependent complete sequence of transitional forms under the influence of the change of diet from small, tongue shaped papillae present in the rumen of hay-fed group to large, heavily cornified, finger and foliate shaped papillae present in the rumen of 4 and 6 weeks concentrate-fed groups, respectively. 4-12 weeks concentrate-fed groups showed deep and numerous grooves present on the surface of the papillae compared to the few shallow grooves present on the surface of papillae of hay-fed group. These deep primary and secondary grooves formed the irregular sides of papillae and the secondary papillae. High magnification of the papillary surface revealed squamous cells with cytoplasmic protrusions, which were well developed and had different shape and arrangement (foliate structure or forming an arrangement of complex cytoplasmic flaps) in sheep fed concentrate for 4-12 weeks compared to the less developed nipple-like projections, which are characteristic for hay fed-sheep. 2. Using quantitative morphological analysis, a remarkable effect of the diet and the duration of the concentrate feeding on the development of the ruminal papillae was observed. Among concentrate-fed groups, the mean length and width of the papillae of 4 weeks concentrate-fed group was strikingly elevated (4.67 mm, 2.75 mm), respectively, compared to that of hay-fed group (2.21 mm, 1.77 mm). The mean length and width of the papillae of 4 weeks concentrate-fed group were more than 2 and 1.5 times the papillary length and width, respectively, of that of the hay-fed group. The number of papillae of 4 weeks concentrate-fed group (48 papillae / cm2 mucosa) was higher than that of hay-fed group (38 papillae / cm2 mucosa); however, these differences were not significant. The enlargement of papillae leads to a significant increase in the total surface of papillae and reached the peak value in the 4 weeks concentrate-fed group (1248.44 mm2 / cm2 mucosa), which is about 4 fold that of the hay-fed group (308.21 mm2 / cm2 mucosa). 3. 4 and 6 weeks concentrate-fed groups exhibited hypertrophic changes in the ruminal papillae and showed well developed epithelial pegs and papillary bodies. They showed also well developed connective tissue of the lamina propria concomitant with an increasing number of blood vessels, which extended close to the subepithelial surface in-between the epithelial pegs in the form of sinusoidal capillaries. 4. Qualitative morphological analysis showed that concentrate feeding for different time intervals might affect size of the epithelial cells more than their number. The number of epithelial cell layers was slightly increased from 6-8 cell layers in hay-fed animals to 7-10 cell layers in 4-12 weeks concentrate-fed groups. However, the mean thickness of the epithelium was increased significantly in 4 weeks concentrate-fed group (109 µm thick) and reached a maximum value (129 µm thick) in 6 weeks concentrate-fed group compared to that of hay-fed group (74 µm thick). 5. Transmission electron microscopy (T.E.M.) revealed that feeding concentrate diet for 4-6 weeks increased the intercellular spaces in all epithelial strata, particularly at the level of stratum basale and spinosum. Furthermore, it revealed also a well developed desmosome-rich stratum spinosum, a mitochondria-rich stratum basale and spinosum, the appareance of membrane coating granules, which were more concentrated at the upper surface of the most superficial spiny cells and the deep granular cells, and the appearance of thick tonofilaments in the stratum basale and spinosum (in 6 weeks concentrate-fed group). It showed also the development of extensive proximal projections of the basal cells accompanied by deeply folded basal lamina and short distances-located blood capillaries (well developed in 4 and 6 weeks concentrate-fed groups), which allowed to increase the surface area of the proximal portion of the basal cells and improved nutrient exchange processes between epithelial tissue and the circulatory system. 6. α-smooth muscle actin-immunoreactive cells were detected in the rumen mucosa as a condensed layer at the position equivalent to the Lamina muscularis mucosae. Strong degree of antibody reaction (thick layer) was seen in 2-6 weeks concentrate-fed sheep. Plasma membrane connexin 43 immunostaining was most intense at the stratum basale and stratum spinosum (deep layers) and decreased in staining intensity through stratum spinosum (superficial layers) to stratum granulosum. However, the stratum corneum was negative for connexin 43. The reaction around the cells gave a syncytial appearance with increased apical-immunostaining concentration. The degree of antibody reaction was weak in hay fed-sheep and in 2 days concentrate-fed sheep, moderate in 4 days and 1 week concentrate-fed sheep, strong in 2 weeks concentrate-fed sheep and very strong in 4 to 12 weeks concentrate-fed sheep. NHE3-immunostaining exists in all strata of the epithelium except stratum corneum, with increased intenseness at both stratum granulosum (deep layer) and stratum spinosum (superficial layer), predominantly at the apical surface of the cells. The degree of antibody reaction was weak in hay fed-sheep and in all concentrate-fed groups, except in 2 and 4 weeks concentrate-fed groups, in which the degree of the antibody reaction was strong. On the other hand, some abnormalities were seen in the rumen mucosa of 12 weeks concentrate-fed sheep, which could be summarized in the following: (a) branching of the lamina propria, with resultant formation of dark brown, mushroom-shaped papilla (b) misshaped epithelial pegs (c) evidence of inflammation and parakeratosis in the stratum corneum; the corneal cells formed a cap over the apex of the papilla and showed accumulation of lymphocyte-like cells in addition to the presence of foreign body lesion and leucocytes within the stratum spinosum (d) very wide intercellular spaces (e) appearance of a very thick tonofibrils bundles in both stratum basale and spinosum (f) appearance of keratohyaline granules in stratum spinosum and lipid infiltration within the cytoplasm of all epithelial strata.