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.

Regeneration of the digestive system in the crinoid Lamprometra palmata (Mariametridae, Comatulida).

The morphology and regeneration of the digestive system and tegmen after autotomy of the visceral mass in the crinoid Lamprometra palmata (Clark 1921) was studied. The gut has a five-lobed shape and is covered by a tegmen. The tegmen consists of epidermis and underlying connective tissue. The digestive tube can be divided into three parts: esophagus, intestine, and rectum. At 6 h post-autotomy, the calyx surface is covered by a layer of amoebocytes and juxtaligamental cells (JLCs). At 14-18 h, post-autotomy transdifferentiation of JLCs begins and give rise to the epidermis and cells of digestive system. On days 1-2 post-autotomy, JLCs undergo the mesenchymal-epithelial transition. Some JLCs turn into typical epidermal cells, while other JLCs form small closed epithelial structures that represent the gut anlage. On day 4 post-autotomy, the animals have a mouth opening and a small anal cone. On day 7 post-autotomy, the visceral mass and the digestive system become fully formed but are smaller than normal. A 24-h exposure of L. palmata individuals to a 10-7 M colchicine solution did not slow down regeneration, and the timing of gut formation was similar to that in the control animals. We conclude that JLCs are the major cell source for gut and epidermis regeneration in L. palmata. The main mechanisms of morphogenesis are cell migration, mesenchymal-epithelial transition, and transdifferentiation.

Retinal ganglion cell topography and spatial resolution in the Japanese smelt Hypomesus nipponensis (McAllister, 1963).

Vision plays a crucial role in the life of the vast majority of vertebrate species. The spatial arrangement of retinal ganglion cells has been reported to be related to a species' visual behavior. There are many studies focusing on the ganglion cell topography in bony fish species. However, there are still large gaps in our knowledge on the subject. We studied the topography of retinal ganglion cells (GCs) in the Japanese smelt Hypomesus nipponensis, a highly visual teleostean fish with a complex life cycle. DAPI labeling was used to visualize cell nuclei in the ganglion cell and inner plexiform layers. The ganglion cell layer was relatively thin (about 6-8 µm), even in areas of increased cell density (area retinae temporalis), and was normally composed of a single layer of cells. In all retinal regions, rare cells occurred in the inner plexiform layer. Nissl-stained retinae were used to estimate the proportion of displaced amacrine cells and glia in different retinal regions. In all retinal regions, about 84.5% of cells in the GC layer were found to be ganglion cells. The density of GCs varied across the retina in a regular way. It was minimum (3990 and 2380 cells/mm2 in the smaller and larger fish, respectively) in the dorsal and ventral periphery. It gradually increased centripetally and reached a maximum of 14,275 and 10,960 cells/mm2 (in the smaller and larger fish, respectively) in the temporal retina, where a pronounced area retinae temporalis was detected. The total number of GCs varied from 177 × 103 (smaller fish) to 212 × 103 cells (larger fish). The theoretical anatomical spatial resolution (the anatomical estimate of the upper limit of visual acuity calculated from the density of GCs and eye geometry and expressed in cycles per degree) was minimum in the ventral periphery (smaller fish, 1.46 cpd; larger fish, 1.26 cpd) and maximum in area retinae temporalis (smaller fish, 2.83 cpd; larger fish, 2.75 cpd). The relatively high density of GCs and the presence of area retinae temporalis in the Japanese smelt are consistent with its highly visual behavior. The present findings contribute to our understanding of the factors affecting the topography of retinal ganglion cells and visual acuity in fish.

Anterior regeneration after fission in the holothurian Cladolabes schmeltzii (Dendrochirotida: Holothuroidea).

The regeneration of the anterior portion of the body after fission was studied in the holothurian Cladolabes schmeltzii using electron microscopy methods. Following fission, the posterior portion of the digestive tube, cloaca, and respiratory trees remain in the posterior fragment of the body. The regeneration comprises five stages. In the first stage, connective-tissue thickening (an anlage of the aquapharyngeal bulb) occurs on the anterior end between the torn-off ends of the ambulacra. Most of the lost anterior organs developed in the second and third stages. The structures of water-vascular system and nerve ring form through dedifferentiation, proliferation, and migration of cells of the radial water-vascular canals and the radial nerve cords, correspondently. The lost digestive system portion is restored through the formation and merging of two anlagen. The digestive epithelium of the esophagus and pharynx develops from lining cells of microcavities near the central portion of the connective-tissue thickening, which probably migrate from the epidermis. The second gut anlage develops through transformation of the anterior gut remnant portion. The enterocytes partly dedifferentiate, but the epithelium retains integrity. The gut anlage grows down the mesentery and joins the regenerating aquapharyngeal bulb. In the fourth and fifth stages, all lost organs are formed and have nearly normal structure. The regeneration was concluded to occur through morphallactic rearrangements of the remaining parts of organs. Epithelial morphogenesis is the key development mechanism of the digestive, water-vascular, and nervous systems.

Posterior regeneration following fission in the holothurian Cladolabes schmeltzii (Dendrochirotida: Holothuroidea).

The regeneration of the posterior portion of the body after fission was studied in the holothurian Cladolabes schmeltzii using electron microscopy methods. Following fission, the aquapharyngeal complex, gonad and anterior portion of the first descending part of the intestine remain in the anterior fragment of the body. The entire regeneration process is divided into five stages. In the first three stages, the digestive system and damaged ends of the longitudinal muscle bands regenerate. The intestine is formed through the rearrangement and growth of the remaining portion of the first descending part of the intestine. The gut anlage grows down the mesentery and joins the regenerating cloaca. The cloaca is formed from two sources: its posterior portion appears as a result of immersion of the epidermis, while the anterior portion develops from the terminal segment of the growing intestine. Regeneration of muscles progresses in the typical manner for echinoderms: through immersion and myogenic transformation of the coelomic epithelium. Respiratory trees appear in animals when the growth of the external part of the body has begun (fourth stage). They are formed as an outgrowth of the dorsal wall of the anterior portion of the cloaca. It was concluded that regeneration of the posterior portion of the body in the holothurian C. schmeltzii following fission is realized through morphallactic rearrangements of the remaining parts of organs. The main mechanism through which the digestive, respiratory, and contractile systems are formed is epithelial morphogenesis.