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Abstract Background Bactrian camel is one of the important economic animals in northwest China. They live in arid desert, and their gestation period is about 13 months, which is longer than other ruminants (such as cattle and sheep). The harsh living conditions have made its unique histological characteristics a research focus. Aggregated lymphoid nodules area (ALNA) in the abomasum of Bactrian camels, as one of the most important sites for the induction of the immune response, provide a comprehensive and effective protective role for the organism, and their lack of information will affect the feeding management, reproduction and epidemic prevention of Bactrian camels. In this study, the histological characteristics of the fetal ALNA in the abomasum of Bactrian camels at different developmental gestation have been described by using light microscopy and histology . Results The ALNA in the abomasum of the Chinese Alashan Bactrian camel is a special immune structure that was first discovered and reported by Wen-hui Wang. To further establish the developmental characteristics of this special structure in the embryonic stage, the abomasum ALNA of 8 fetuses of Alashan Bactrian camels with different gestational ages (5~13 months) were observed and studied by anatomy and histology. The results showed that the aggregation of reticular epithelial cells (RECs) surrounded by a very small number of lymphoid cells was detected for the first time in the abomasum of fetal camel at 5 months gestation, which was presumed to be primitive ALNA. At 7 months gestation, the reticular mucosal folds region (RMFR) appeared, but the longitudinal mucosal folds region (LMFR) was not significant, and histological observations showed that there were diffusely distributed lymphocytes around the RECs. At 10months gestation, RMFR and LMFR were clearly visible, lymphoid follicles appeared in histological observation, lymphocytes proliferated vigorously. By 13 months, the volume of lymphoid follicles increased, forming the subepithelial dome (SED), and there was a primitive interfollicular area between the lymphoid follicles, which contained high endothelial vein (HEV), but no germinal center (GC) was found. In summary, ALNA of Bactrian camels is not fully mature before birth. Conclusions Generally, the small intestine PPs of ruminants (such as cattle and sheep) is already mature before birth, while the ALNA in the abomasum of Bactrian camels is not yet mature in the fetal period. During the development of ALNA in Bactrian camel, the development of lymphoid follicles extends from submucosa to Lamina propria. Interestingly, the deformation of FAE changes with age from simple columnar epithelium at the beginning of pregnancy to Simple cuboidal epithelium, which is opposite to the FAE deformation characteristics of PPs in the small intestine of fetal cattle and sheep. These results are the basis of further research on the specificity of ALNA in the abomasum of Bactrian camels.

Waterborne ammonia is one of the crucial issues that limited production and animal health in aquaculture. Ammonia-tolerant varieties are highly desired in intensive fish farming. Screening for the key regulatory genes of ammonia tolerance is essential for variety breeding. According to the previous hypothesis, Rh glycoproteins play an important role in ammonia excretion in teleosts. However, the ammonia defensive mechanisms are not well described at present for large-scale loach (Paramisgurnus dabryanus), a typical air-breathing and commercially important fish in East Asia. Here we show that the transcription of Rh glycoprotein-related genes was significantly affected by ammonia exposure in this species. Probit analysis showed that 96 h-LC50 of NH4Cl at 23 ℃ and pH 7.2 was 92.64 mmol/L. A significant increase of Rhcg expression in gills was observed after 48 h of 60 mmol/L and 36 h of 80 mmol/L NH4Cl exposure, suggesting that Rhcg present on the apical side of the branchial epithelium facilitates NH3 excretion out of gills. A high concentration of acute ammonia exposure induced elevated Rhbg transcript in the gills of large-scale loaches, while a slight change in Rhbg expression was observed in response to lower ammonia, suggesting that transcriptions of Rhbg genes are activated by a considerably high level of ambient ammonia to eliminate excessive endogenous nitrogen. The Rhag mRNA level in gills of large-scale loaches increased markedly with the prolonging of exposure time from 0 to 36 h of ammonia loading, suggesting Rhag localized in gills may be primarily associated with ammonia handling. During 7–21 days of ammonia exposure, the expression of most Rh glycoproteins-related genes in the gills decreased, indicating that the functional role of Rh glycoproteins is not primarily associated with ammonia defense over a long period (more than 7 days). Although a significant transcript of Rhbg was found in the skin of a large-scale loach, the lack of Rhcg and down-regulation of Rhag may indicate that the skin is not an essential location of ammonia excretion, at least when submerged to high levels of ammonia in the environment. In conclusion, Rh glycoproteins localized in gills as ammonia transporters play a momentous role in ammonia detoxification in this species during acute ammonia loading. However, it does not show a positive function during long-term ammonia exposure. Furthermore, the physiological function of Rh glycoproteins localized in the skin is still unclear and deserves further study.

Aquaporins (Aqp)1 and Aqp3 are the representatives of two types of aquaporins (classical aquaporin and aquaglyceroporin) and have been proven to potentially facilitate ammonia detoxification in air-breathing fishes. Paramisgurnus dabryanus is one of East Asia’s most important commercially farmed species and can breathe air directly through its hindgut. Besides, excellent ammonia tolerance was reported in this species. However, the mechanisms of its tolerance of high ammonia are still unclear. To explore the physiological roles of Aqp1 and Aqp3 in Paramisgurnus dabryanus, we detected the expression pattern of these two Aqps in several tissues and their transcriptional changes in response to different levels (30, and 70 mmol L-1 NH4Cl) and periods (0, 12, 48, and 96 h) of ammonia exposure and different periods (0, 12, 48, and 96 h) of drought stress. The aqp1 mRNA showed a high abundance in the brain and the osmoregulatory tissues, such as the intestine and kidney. While aqp3 was mainly expressed in the epithelium of skin and gills that were directly exposed to the environment. The expression level of aqp3 in gills was more responsive than aqp1 to ammonia challenges, although both of them were up-regulated. Considering the wide difference between aqp1 and aqp3 in the skin, aqp3 increased significantly after 12 h of air exposure. The changes of the two aqps in the intestine of P. dabryanus were relatively stable after ammonia stress. After drought stress, intestinal aqp1 increased significantly, while aqp3 decreased. Combined with the effects of tissue location and environmental stress on Aqps, it was suggested that Aqp1 was mainly involved in osmotic regulation, providing water as raw material for body metabolism. Accordingly, our results suggest that Aqp3 has the positive function of assisting ammonia transportation and epidermal moisturizing.

Elucidating the mechanisms of alkaline tolerance in freshwater teleosts will help in the development of commercial saline-alkaline aquaculture. The large-scale loach (Paramisgurnus dabryanus) is a viable species for such aquaculture, but the mechanisms of its tolerance of alkaline water are unclear. Large-scale loach was exposed to 40, 50, and 60 mmol L-1 NaHCO3 for 12, 48, and 96 h to evaluate the transcriptional changes of branchial Rhesus (Rh) glycoproteins, and aquaporins (Aqp)1 and Aqp3. Rhag transcript levels increased with longer exposure times. Rhag expression also rose considerably at higher carbonate alkalinities. Rhbg mRNA levels declined significantly under carbonate alkalinity exposure. A marked up-regulation of Rhcg was observed in the gills of the loach. Exposure to 60 mmol L-1 NaHCO3 also induced a significant up-regulation of aqp1. By contrast, aqp3 expression was significantly lower after 48 h exposure. The current findings reveal that the large-scale loach up-regulates Rhag and Rhcg to enhance ammonia efflux from the gills when exposed to high alkalinity. It is proposed that this species maintains appropriate osmolality when adapting to an alkaline environment by down-regulating aqp3 (to impede urea removal) and up-regulating aqp1 in the gills (to excrete excessive internal water).