# Developmental characteristics of aggregated lymphoid nodules area in the abomasum of fetal Bactrian camels (Camelus bactrianus)

## TL;DR

This study examines how immune structures in the stomach of fetal Bactrian camels develop during gestation, revealing unique characteristics compared to other ruminants.

## Contribution

The study provides the first detailed description of the developmental stages of aggregated lymphoid nodules in fetal Bactrian camels.

## Key findings

- Aggregated lymphoid nodules first appear at 5 months gestation and show increasing complexity by 13 months.
- Lymphoid follicles develop from the submucosa to the lamina propria, and no germinal centers are found at birth.
- The fetal abomasum's follicular-associated epithelium changes from columnar to cuboidal epithelium, opposite to patterns in cattle and sheep.

## Abstract

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 .

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.

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.