Histological observations and gene expression of the digestive system of the Chinses mitten crab (Eriocheir sinensis)

In order to elucidate the structural characteristics and gene expression of the tissues of the digestive system of adult Eriocheir sinensis, this study was conducted to systematically study the digestive system of E. sinensis using histological and transcriptomic techniques. The results showed that the digestive system of E. sinensis mainly consisted of the esophagus, stomach (including cardia stomach and pyloric stomach), midgut, intestinal bulb, hindgut and hepatopancreas. In the cardia stomach, calcification of the cuticle into cuticular ossicles, chitin tooth, ridge, bristles and other structures were observed at different sites, which together form the "Gastric mill" for grinding food. The pyloric stomach contains special comb-like structures and rows of pectinate bristles, which are important components of the gland filter for filtering and sifting food. The wall of the digestive tract of E. sinensis consists of four layers: the mucosal layer, the submucosal layer, the muscular layer and the outer membrane, of which the surface of the mucosal layer of the esophagus and hindgut has a well-developed cuticle with rows of setae, and the submucosal layer has special mucus glands (esophageal glands or hindgut glands); the surface of the mucosal layer of the midgut has no cuticle and has dense microvillus. The intestinal bulb has the morphological characteristics of the transition from midgut to hindgut. The hepatopancreas consists of many hepatic tubules, and four types of cells are often seen in the hepatic tubules, including absorptive cells (R cells), vesicular cells (B cells), fibroblasts (F cells), and embryonic cells (E cells). The transcriptome data of the digestive system of E. sinensis showed that Esophagus, Stomach, Midgut, Intestinal bulb, Hindgut and hepatopancreas expressed 13252, 13285, 13634, 11147, 13127, 12551 genes respectively; and they specifically expressed 1529, 549, 1199, 624,990, 1688 genes respectively. A total of 25 digestive enzyme genes were isolated from six tissues of the digestive system. Except for maltosidase-glucoamylase and phosphatase A2, which had small differences in expression in various tissues of the digestive system, all other digestive enzymes had the highest expression in the hepatopancreas. Digestive enzymes with high expression in the hepatopancreas are trypsin, pancreatic rennin, carboxypeptidase B, ɑ-amylase, and astacin.