Lidia T. Frolova

Structure of the Digestive Tube in the Holothurian Eupentacta fraudatrix (Holothuroidea: Dendrochirota)

In the holothurian Eupentacta fraudatrix,the gut wall exhibits trilaminar organization. It consists of an inner digestive epithelium, a middle layer of connective tissue, and an outer mesothelium (coelomic epithelium). The pharynx, esophagus, and stomach are lined with a cuticular epithelium composed of T-shaped cells. The lining epithelium of the intestine and cloaca lacks a cuticle and consists of columnar vesicular enterocytes. Mucocytes are also encountered in the digestive epithelium. The connective tissue layer is composed of a ground substance, which houses collagen fibers, amoebocytes, morula cells, and fibroblasts. The gut mesothelium is a pseudostratified epithelium, which is dominated by peritoneal and myoepithelial cells and also includes the perikarya and processes of the neurons of the hyponeural plexus and vacuolated cells.

The morphology of the digestive tract and respiratory organs of the holothurian Cladolabes schmeltzii (Holothuroidea, Dendrochirotida).

The microanatomy of the digestive and respiratory systems of the holothurian Cladolabes schmeltzii was studied. The digestive tube of C. schmeltzii is divided into seven parts. The pharynx, esophagus, and stomach are lined with cuticular immersed epithelium. In these regions, the epithelial cells are connected via desmosomes, septate junctions, and rivet-like structures. The presence of the cuticle and rivet-like structures suggests an ectodermal origin for these parts of the digestive tube. The luminal intestinal epithelium is formed by vesicular enterocytes, which have different structures in different intestinal regions. Moreover, the epithelium of the first descending part of the intestine contains the granular enterocytes. The respiratory system consists of paired respiratory trees lined by a luminal epithelium that is formed by cells of irregular shape. The apical surface of these epithelial cells has few lamellae. The cells are connected to each other through a system of intercellular junctions, consisting of both desmosomes and well-developed septate junctions. The coelomic epithelium of the intestine and the respiratory trees consists of peritoneal and myoepithelial cells.

Metamorphosis and definitive organogenesis in the holothurian Apostichopus japonicus

Abstract The structure of the late doliolaria, pentactula and 1-month-old juvenile of the holothurian Apostichopus japonicus was studied using light microscopy and 3D reconstruction methods. It was shown that metamorphosis in this species consists in the reorganization of the shape of the body and the destruction of provisional organs. The late doliolaria has a spindle-like form, ciliary rings and hyaline spheres shifted relative to the anterior-posterior axis of the body. Some provisional organs (ciliary rings, hyaline spheres) are destroyed during settlement, and others (hydropore and hydroporic canal) remain after metamorphosis. Definitive organogenesis in A. japonicus begins long before metamorphosis. The late doliolaria already has well-developed water-vascular and digestive systems, and the ectoneural part of the nervous system. Muscle and hemal systems begin to form in the pentactula. Moreover, the calcareous ring and connective tissue part of the body wall develop at this stage. The pentactula has anlages of the hyponeural part of the nervous system, which form in the mid-ventral and dorsal nerve cords. The hemal ring of the pentactula is located on the inner wall of the water-vascular ring. It remains unclosed in the left ventral radius. One-month-old juveniles have all the major organ systems except respiratory and reproductive systems. The hemal vessels of the intestine are well developed and begin to form the rete mirabile. Differentiation of the intestine into regions due to differential specialization of the enterocytes begins in 1-month-old juveniles. Obviously, emergence of new types of enterocytes enables the animal to consume a wider range of food items and indicates its increased feeding activity.

Microscopic anatomy of the digestive system in normal and regenerating specimens of the brittlestar Amphipholis kochii.

The morphology and regeneration of the digestive system of the ophiuroid Amphipholis kochii were investigated. The epithelia of the esophagus and stomach of A. kochii were composed of typical enterocytes and mucous cells. The digestive epithelium of the stomach contained two types of granular secretory cells. After autotomy of the disk, the animals retained the esophagus and a small part of the stomach. The dedifferentiation of enterocytes and mucous cells began on the first day after autotomy. On day 3 the cells formed an anlage of stomach around the mouth opening. Later, the stomach anlage grew as a result of cell proliferation. The opening on the aboral side of the body was closed by day 7. By this time differentiating cells were already observed in the stomach lining. The stomach mesothelium was formed by peritoneocytes and myoepithelial cells, which migrated from other coelomic epithelia of the body. Our study showed that the formation of the digestive system in A. kochii during regeneration depended on cells from the esophagus and the stomach remnant. Both enterocytes and mucous cells were able to dedifferentiate, migrate, and proliferate to give rise to the luminal epithelium. The basic mechanism of stomach formation was epithelial morphogenesis.

Morphofunctional changes of the digestive gland in the bivalve mollusk Crenomytilus grayanus (Dunker, 1853) in normal conditions and after parasitic invasion by trematodes

Specifics of the digestive cycle were studied in normal individuals of Gray’s mussel Crenomytilus grayanus and in those infested by trematodes from an unpolluted area of the sublittoral zone of Peter the Great Bay in May (during the day), June, July, and August. Four types of tubules corresponding to four phases of the digestive cycle were identified, and three varieties of destructing tubules were distinguished. It was shown that normally phases of absorption and digestion prevailed in the digestive gland. The portion of absorptive tubules (type II) was 95% in spring and 79% in summer. The total portion of destructing and restoring tubules (types III and IV) did not exceed 10% in spring and 20% in summer, and portion of tubules at the initial condition (type I) was 4% in spring and 2% in summer. During the day, insignificant variations in the ratio of different type tubules were recorded. It was revealed that, normally, granulocytomes were formed during utilization of necrotic tubules. Parasitic invasion of trematodes entailed swelling of the gland and a change in the proportion of tubules: the number of absorptive tubules decreased to 20%, the number of destructing and restoring tubules increased up to 60% and 12%, respectively.

Digestive system formation during the metamorphosis and definitive organogenesis in Apostichopus japonicus

We studied the ultrastructure of the digestive system in late doliolaria, pentactula, and 1-month-old juvenile of the holothurian Apostichopus japonicus. In late doliolaria and pentactula, the digestive system is divided into three segments: pharynx, gut, and cloaca. Pharynx and the posterior part of cloaca are lined by cuticular epithelium and apparently of ectodermal origin. The anterior part of cloaca and the entire gut is not differentiated histologically and is lined by a single type of digestive cells, vesicular enterocytes I. These cells are characterized by large secretory granules, containing an acidic substance, found in cytoplasm. As the anterior part of the cloaca is lined by cells typical of the endodermal segment of digestive tube (vesicular enterocytes I), we suggest this part to be of endodermal origin and probably formed from the larval stomach. In 1-month-old juveniles, the structure of the digestive system grows more complicated. In addition to vesicular enterocytes I, three more types of enterocytes appear in luminal epithelium. The specific distribution of the four types of digestive cells divides the intestine into three parts, each probably performing its own function. All enterocytes develop long microvilli, which indicate the intensification of the extracellular digestion processes and an increased absorption of dissolved nutrients.

Regeneration of the Epithelial Lining of the Stomach after Autotomy of a Disk in the Brittle Star Amphipholis kochii (Lütken) (Echinodermata: Ophiuroidea)

The regeneration of the epithelial lining of the stomach of the brittle star Amphipholis kochii after autotomy of the aboral part of the disk was studied. It was shown that a part of the stomach epithelium remained after autotomy. Its cells participated in regeneration of the epithelium during restoration of the lost digestive system. There was a partial dedifferentiation of the epithelium cells of stomach, their migration and proliferation; cells of the stomach retained some specialized cytoplasmic structures, secretory vacuoles, and pinocytic vesicles. Migration of ectodermal cells of the esophagus in the damaged area was also recorded.

Features of biology of the sea anemone Charisea saxicola Torrey, 1902 (Actiniaria: Condylanthidae) from the northwest Pacific

The first data on features of the biology of the sea anemone Charisea saxicola, which is widespread in the Northwest Pacific, were obtained. That species, inhabiting the littoral of Shikotan Isl. (the Minor Kurile Ridge), belongs by its trophological attributes to nonselective deposit feeders. Animals swallow soil together with the organisms in it, not separating mineral particles from organic ones. Populations of Ch. saxicola are presented by individuals of mail and female sex, however the females prevailed in number. Hermaphrodites and evidence of sex change were not revealed in that species. Females were in the postspawning condition in all settlements. Spermatogenic cells of the new generation at two stages of development were recorded in males.

Formation of the ectodermal organs during the metamorphosis and definitive organogenesis in the holothurian Apostichopus japonicus

The fine structure of the ectodermal organs (epidermis and nervous system) in late doliolaria, pentactula, and 1-month-old juveniles of the holothurian Apostichopus japonicus was studied. The definitive structure of the epidermis is shown to start forming prior to metamorphosis. The cell types of late doliolaria epidermis are the same as those in adult holothurian epidermis, consisting of support, secretory, and granular cells. The rivet-like structures are also present already at this stage of development. The ciliary rings of late doliolaria represent ribbon-like aggregations of tall ciliary cells. The destruction of ciliary rings is due to the programmed death of ciliary cells, which are then utilized by mesenchymal cells. Changes to the epidermis during metamorphosis are linked to the sinking of the cells into the underlying extracellular matrix, resulting in the formation of the typical sunken epidermis of the body and appendages. The nervous system in late doliolaria consists of a nerve ring, five radial nerve cords, and tentacle and tube foot nerves. The nerve ring and the radial nerve cords represent epithelial tubes. First hyponeural cells appear in the mid-ventral nerve cord during the late doliolaria stage. In the rest of the nerve cords, the hyponeural part begins developing only after the settlement. Functioning of the adult nervous system in holothurians begins only after metamorphosis, following the complete loss of provisory organs and the switch to benthic life.

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.