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.

Muscle Regeneration in Holothurians without the Upregulation of Muscle Genes.

The holothurian Eupentacta fraudatrix is capable of fully restoring its muscles after transverse dissection. Although the regeneration of these structures is well studied at the cellular level, the molecular basis of the process remains poorly understood. To identify genes that may be involved in the regulation of muscle regeneration, the transcriptome of the longitudinal muscle band of E. fraudatrix has been sequenced at different time periods post-injury. An analysis of the map of biological processes and pathways has shown that most genes associated with myogenesis decrease their expression during the regeneration. The only exception is the genes united by the GO term "heart valve development". This may indicate the antiquity of mechanisms of mesodermal structure transformation, which was co-opted into various morphogeneses in deuterostomes. Two groups of genes that play a key role in the regeneration have been analyzed: transcription factors and matrix metalloproteinases. A total of six transcription factor genes (Ef-HOX5, Ef-ZEB2, Ef-RARB, Ef-RUNX1, Ef-SOX17, and Ef-ZNF318) and seven matrix metalloproteinase genes (Ef-MMP11, Ef-MMP13, Ef-MMP13-1, Ef-MMP16-2, Ef-MMP16-3, Ef-MMP24, and Ef-MMP24-1) showing differential expression during myogenesis have been revealed. The identified genes are assumed to be involved in the muscle regeneration in holothurians.

Development of respiratory trees in the holothurian Apostichopus japonicus (Aspidochirotida: Holothuroidea).

The development of respiratory trees in the holothurian Apostichopus japonicus has been studied using light and electron microscopy. Primordial respiratory trees emerge in 2-3-mm-long animals (2 months after fertilization). They arise as two independent outgrowths from the dorsal wall of the anterior part of the cloaca. Upon first emerging and throughout the course of development, the left respiratory tree is longer than the right one. A common base develops in 4-mm-long animals (2-3 months after fertilization). In yearlings, the left respiratory tree grows into gaps between the loops of the intestinal tube interlaced with intestinal hemal vessels. The developing coelomic and luminal epithelia have ultrastructural peculiarities. The luminal epithelium of respiratory trees has been shown for the first time to comprise nerve cells and their processes. Characteristic structural features of the epithelia are shown to begin developing as early as in 4-mm-long animals (2-3 months after fertilization). In yearlings, the respiratory trees demonstrate definitive structural patterns and are entirely functional.

Post-autotomy regeneration of respiratory trees in the holothurian Apostichopus japonicus (Holothuroidea, Aspidochirotida).

Specialised respiratory organs, viz. the respiratory trees attached to the dorsal part of the cloaca, are present in most holothurians. These organs evolved within the class Holothuroidea and are absent in other echinoderms. Some holothurian species can regenerate their respiratory trees but others lack this ability. Respiratory trees therefore provide a model for investigating the origin and evolution of repair mechanisms in animals. We conducted a detailed morphological study of the regeneration of respiratory trees after their evisceration in the holothurian Apostichopus japonicus. Regeneration of the respiratory trees occurred rapidly and, on the 15th day after evisceration, their length reached 15-20 mm. Repair involved cells of the coelomic and luminal epithelia of the cloaca. Peritoneocytes and myoepithelial cells behaved differently during regeneration: the peritoneocytes kept their intercellular junctions and migrated as a united layer, whereas groups of myoepithelial cells disaggregated and migrated as individual cells. Although myoepithelial cells did not divide during regeneration, the peritoneocytes proliferated actively. The contractile system of the respiratory trees was assumed to develop during regeneration by the migration of myoepithelial cells from the coelomic epithelium of the cloaca. The luminal epithelium of the respiratory trees formed as a result of dedifferentiation, migration and transformation of cells of the cloaca lining. The mode of regeneration of holothurian respiratory trees is discussed.