Morphology and ultrastructure of digestive system in pre-zoea and zoea I larvae of red king crab, Paralithodes camtschaticus (Tilesius, 1815).

The digestive system structure in pre-zoea and zoea I larvae of the red king crab Paralithodes camtschaticus has been examined. During this development period, the digestive system consists of an esophagus, a stomach, a midgut (where the hepatopancreas ducts open), and a hindgut. The esophagus begins from the oral slit on the animal's ventral side and extends vertically up to the junction with the cardiac stomach. The latter is followed by the pyloric stomach. At the stages under study, crabs have a cardiac-pyloric valve and a pyloric filter in the stomach already developed. The midgut begins with an expansion in the cephalothorax, enters the pleon, grows narrower there, and extends to somite 3 of pleon. The hepatopancreas is represented by a symmetrical paired gland which occupies almost the entire cephalothorax space and opens with its ducts at the junction of the pyloric stomach with the midgut. The hepatopancreas is divided into the anterior and posterior lobes. At the pre-zoea stage, the anterior lobes are large and filled with yolk. At the zoea I stage, the anterior lobes are smaller relative to the entire hepatopancreas, and the posterior lobes increase and form tubular outgrowths. It has been shown that during the transition from pre-zoea to zoea I, the number of mitochondria in enterocytes increases and a peritrophic membrane forms in the midgut. These changes are probably associated with the transition to independent living and feeding.

Expression of Piwi, MMP, TIMP, and Sox during Gut Regeneration in Holothurian Eupentacta fraudatrix (Holothuroidea, Dendrochirotida).

Mesodermal cells of holothurian Eupentacta fraudatrix can transdifferentiate into enterocytes during the regeneration of the digestive system. In this study, we investigated the expression of several genes involved in gut regeneration in E. fraudatrix. Moreover, the localization of progenitor cells of coelomocytes, juvenile cells, and their participation in the formation of the luminal epithelium of the digestive tube were studied. It was shown that Piwi-positive cells were not involved in the formation of the luminal epithelium of the digestive tube. Ef-72 kDa type IV collagenase and Ef-MMP16 had an individual expression profile and possibly different functions. The Ef-tensilin3 gene exhibited the highest expression and indicates its potential role in regeneration. Ef-Sox9/10 and Ef-Sox17 in E. fraudatrix may participate in the mechanism of transdifferentiation of coelomic epithelial cells. Their transcripts mark the cells that plunge into the connective tissue of the gut anlage and give rise to enterocytes. Ef-Sox9/10 probably controls the switching of mesodermal cells to the enterocyte phenotype, while Ef-Sox17 may be involved in the regulation of the initial stages of transdifferentiation.

Regeneration of the digestive system in the crinoid Himerometra robustipinna occurs by transdifferentiation of neurosecretory-like cells.

The structure and regeneration of the digestive system in the crinoid Himerometra robustipinna (Carpenter, 1881) were studied. The gut comprises a spiral tube forming radial lateral processes, which gives it a five-lobed shape. The digestive tube consists of three segments: esophagus, intestine, and rectum. The epithelia of these segments have different cell compositions. Regeneration of the gut after autotomy of the visceral mass progresses very rapidly. Within 6 h after autotomy, an aggregation consisting of amoebocytes, coelomic epithelial cells and juxtaligamental cells (neurosecretory neurons) forms on the inner surface of the skeletal calyx. At 12 h post-autotomy, transdifferentiation of the juxtaligamental cells starts. At 24 h post-autotomy these cells undergo a mesenchymal-epithelial-like transition, resulting in the formation of the luminal epithelium of the gut. Specialization of the intestinal epithelial cells begins on day 2 post-autotomy. At this stage animals acquire the mouth and anal opening. On day 4 post-autotomy the height of both the enterocytes and the visceral mass gradually increases. Proliferation does not play any noticeable role in gut regeneration. The immersion of animals in a 10-7 M solution of colchicine neither stopped formation of the lost structures nor caused accumulation of mitoses in tissues. Weakly EdU-labeled nuclei were observed in the gut only on day 2 post-autotomy and were not detected at later regeneration stages. Single mitotically dividing cells were recorded during the same period. It is concluded that juxtaligamental cells play a major role in gut regeneration in H. robustipinna. The main mechanisms of morphogenesis are cell migration and transdifferentiation.