Gastrodermal structure and feeding responses in the scleractinian Mycetophyllia reesi,a coral with novel digestive filaments

Mycetophyllia reesi Wells is a colonial scleractinian coral whose outer surface consists of a series of oral-pharyngeal openings that lack tentacles. The polyps also lack a column and cannot protrude from the colonial surface. Correspondingly, there is no central digestive cavity. Instead, the pharynx is directly connected to a series of radially arranged mesenterial ducts lying parallel to the skeleton. The ducts, composed primarily of ciliated cells with small mucus inclusions and large, compartmentalized mucocytes, house filaments that protrude through the oral apertures during feeding. The filaments may or may not be directly connected with or originate from the mesenterial ducts and are histologically distinct from them. They are therefore referred to as digestive, rather than mesenterial filaments. In contrast with other scleractinians, the digestive filaments are thin, unequally bilobed stalks with spatulate ends. The cnidoglandular (CG) lobe, the larger of the two, exhibits a distinct cellular zonation. Large mastigophore cnidae and elongated zymogen-like cells are clustered at its distal end. Neither of these cells appear to respond to particulate food material, suggesting that they may be employed in alternative modes of nutrition and/or competition. Behind the distal region, the CG lobe exhibits typical zymogen, mucus, and collar cells as well as numerous atrichous nematocysts. The atrichs and zymogen cells discharge during particulate feeding. Tracts of collar cells with particularly well-defined cilia, elongated rootlets, and mucus inclusions are found at the outer edge of the CG lobe. These cells disgorge their contents during feeding and appear to function in food transport. The smaller lobe of the filament is a muscular sheet containing well-defined fields of circular and longitudinal myofibrils along with associated neurons. Collar cells with lysosome-like inclusions and large, compartmentalized mucocytes are also characteristic of this region. There are no zooxanthellae in the filaments, but these endosymbionts are present as a thin layer in the oral-most portion of the gastrodermis. The cellular zonation and multi-functionality of these digestive filaments suggest another example of a cnidarian structure at the organ level of complexity.     

Studies on Stenostomum grande Child, 1902 (Platyhelminthes,Catenulida): fine structure of the digestive tract and the endocytotic activity of the gastrodermis

Abstract The digestive tract and its endocytotic activity in the catenulid Stenostomum grande were studied by electron microscopy. The pharynx was typical of the simplex type. At the mouth, between the integumental epithelium and the pharyngeal epithelium proper, was a transition zone. Among the epithelial cells of this transition were monociliated sensory cells and the necks of bucco-pharyngeal secretory cells of two types. The pharyngeal epithelium proper was densely ciliated, with long ciliary rootlets and mitochondria. It was surrounded by two layers of muscles. The gastrodermis consisted of phagocytes and typical secretory Minotian cells. It was underlain by a delicate basal lamina and muscle fibers. Distinctive of the phagocytes was the presence of differentiated cilia, cup-shaped mitochondria, and vacuoles with dense inclusions. Morphological differences between pharyngeal and gastrodermal cilia suggest functional differences. Experiments using latex beads as tracers and the identification of acid phosphatase in cytoplasmic vacuoles pointed to a high level of endocytotic and digestive activity in the phagocytes. Our data demonstrate that the basic structure of the digestive tract in S. grande conforms well to that of other free-living platyhelminths, but it does have ultrastructural peculiarities.     

The structure of cnidosacs in nudibranch mollusc <Emphasis Type="Italic">Aeolidia papillosa</Emphasis> (Linnaeus, 1761) and presumable mechanism of nematocysts release

The structure of cnidosacs in nudibranch mollusc Aeolidida papillosa (Linnaeus, 1761) before and after the discharging of kleptocnidae has been studied. In the apical zone of the cnidosac, the basal laminae of epidermis and gastrodermis are interrupted, and the muscle layers of the cnidosac and the epidermis are absent. We suggest the formation of a temporary channel during the discharging of the cnidosac. Through this channel, nematocysts move from the cnidosac to the cnidopore, which forms on the top of the ceras.     

Epidermal structure of the scleractinian coral Mycetophyllia ferox: light-induced vesicles,copious mucocytes,and sporadic tentacles

Three colony fragments of the scleractinian coral Mycetophyllia ferox Wells from Florida were observed in flow-through seawater aquaria under light and dark conditions. The colonies were then anesthetized and fixed for microscopic examination. Small vesicles formed across the epidermis in response to light as gastrodermis containing approximately 1.9 × 10⁶ zooxanthellae cm⁻² migrated into them. The vesicles flattened in the dark and the gastrodermis retreated to a clumped position. The epidermis is dominated by mucus cells with more than 6300 per mm². In contrast, there are very few epidermal cnidae. The polyps lack tentacles entirely, though small tentacles do occur, albeit sporadically, along the colline walls. Colline tentacles are expanded both day and night, and there is considerable intracolonial variability in the number of cnidae within them, ranging from as few as 316 to more than 3200 per mm² tentacle. There may be several small cnidocyst batteries containing both spirocysts and nematocysts (all microbasic p-mastigophores), but the principal battery is at the tentacle tip where cnidae are much more densely packed. There is considerable variation in the ratio of the two cnidae among tentacles in the same colony. Since the tentacles occur inconsistently and do not appear to expand, their functional role is unclear. Comparisons of epidermal characters are made with other members of the genus Mycetophyllia.     

Collar cells in planula and adult tentacle ectoderm of the solitary coral Balanophyllia regia (anthozoa eupsammiidae)

Summary The planula larva of the solitary coral Balanophyllia regia has an ectoderm of flagellate, diplosomal collar cells. The collar of these cells is composed of a ring of microvilli linked with mucus strands. Four types of flagellate gland cells, three types of nematocyst and spirocysts are present in the planula ectoderm. The function of these ectoderm cells is discussed. The mesogloeal muscular and packing tissues of the planula are briefly described. The tentacle of the adult coral, examined for comparison, has an ectoderm of flattened flagellate cells with a shallow collar. Collar cells similar to those of the planula are occasionally found on the tentacle and their function is not known. Independent sensory cells built on a modified collar cell plan with collar of thickened microvilli are common in the tentacle. These are quite separate from the three kinds of tentacular nematocyte. Distended glandular areas occur in the tentacle ectoderm. The flagellate tentacle gastrodermis, muscle and mesogloeal region are briefly described. The evolutionary significance of collar cell ectoderm in a planula is discussed and the occurrence of collar cells throughout the animal kingdom, reviewed.I am most grateful to Paul Tranter of the Plymouth Laboratory for providing material and to Gareth Morgan for assistance with electron microscopy.     

Comparative ultrastructure of catch tentacles and feeding tentacles in the sea anemone Haliplanella

TEM observations of catch tentacles revealed that the tentacle tip epidermis is filled with two size classes of mature holotrich nematocysts and a gland cell filled with electron-dense vesicles. Vesicle production is restricted to upper-middle and tentacle tip regions, whereas holotrich development occurs in the lower-middle and tentacle base regions. Thus, catch tentacles have a maturity gradient along their length, with mature tissues concentrated at the tentacle tip. Occasional feeding tentacle cnidae (microbasic p-mastigophores and basitrichs) and mucus gland cells occur in proximal portions of catch tentacles, but are phagocytized by amoeboid granulocytes and transported to the gastrodermis for further degradation. No feeding tentacle cnidae or mucus cells occur distally in catch tentacles. Unlike catch tentacles, feeding tentacles are homogeneous in structure along their length with enidocytes containing mature spirocysts, microbasic p-mastigophore or basitrich nematocysts distributed along the epithelial surface. Cnidoblasts are recessed beneath cnidocytes, occurring along the nerve plexus. Mucus gland cells and gland cells filled with electron-dense vesicles are present in feeding tentacles, distributed at the epithelial surface. Granular phagocytes are rare in the feeding tentacle tip, but common in the tentacle base.     

Cell differentiation during the larval development of the ophiuroid <Emphasis Type="Italic">Amphipholis kochii</Emphasis> Lütken, 1872 (Echinodermata: Ophiuroidea)

The differentiation of the ectodermal, entodermal, and mesodermal cell lines in developing plutei of the ophiuroid Amphipholis kochii was examined using electron microscopy and the immunochemical staining technique. The ectodermal cells form the pseudostratified epithelium of the ciliary band, the flattened epithelium of the body wall, and the esophageal epithelium. The epithelium of the ciliary band consists of ciliated and mucous cells; at its base is an axonal tract formed of the processes of neurons. The serotoninergic neurons form two lateral ganglia located along the paraoral ciliary band and the posterolateral arms’ ciliary band. The prominent features of the neurons are large size, the presence of a cilium, an electron-light cytoplasm filled with microvesicles with neurotransmitters, and a large nucleus with a predominant euchromatin. The ectoderm cells (except mucous cells) are characterized by the presence of a cilium surrounded by a collar of microvilli and a thin layer of apical extracellular matrix. The entodermal cells form the digestive tract epithelium and differentiate into four cell types: type I and II cells probably function in the nutrient uptake and assimilation; type III cells perhaps secrete digestive enzymes; and myoepithelial cells that constitute the cardiac and pyloric sphincters and the anus. Sclerenchymatous cells, which are the descendants of the primary mesenchyme, form a syncytium around the developing spicules. The biomineralization process is intrasyncytial, the ophioplutei spicules retain the cytoplasmic covering throughout the period of larval development. The secondary mesenchyme gives rise to smooth muscle cells and amebocytes. Muscle cells compose the circumesophageal musculature, the cell processes of each “muscle band” seem to fuse together. At the base of the preoral band are two symmetrically located groups of muscles, viz., the anterior dilators. Amebocytes function in excretion either near the epidermis or are able to penetrate through the epidermis and excrete wastes into the external environment. The mesoderm formed by the enterocoely gives rise to three pairs of coeloms; their cells remain unspecialized during the entire period of larval development. Results of this study are compared with the micro- and neuroanatomy of the larvae of other echinoderms.     

Occurrence and Ultrastructure of Collar Cells in the Stomach Gastrodermis of Polypodium hydriforme Ussov (Cnidaria)

The existence of collar cells lining the stomach gastrodermis in free-living Polypodium hydriforme and their ultrastructure are described. The collar cells are provided with a collar consisting of 9–10 microvilli which encircles a central flagellum and forms a flagellar pit. At the bottom of the pit around the basal part of the flagellum there is fine crystalline material which extends also in the spaces between the microvilli and keeps them straight. The flagellum has a typical axoneme (9+2), its basal body is located below the apical surface of the collar cell and continues into a striated rootlet. An accessory centriole is situated close to the upper part of the rootlet. The cell nucleus is located in the basal part of the cell. Prominent mitochondria with tubular cristae, Golgi cisternae and fragments of rough endoplasmic reticulum are situated mostly in the basal part of the cytoplasm. Discoidal vesicles are abundant in the apical cytoplasm. The collar cells are connected to each other by septate junctions and interdigitations. The ultrastructure of collar cells described here is discussed in comparison to that of other Cnidarians and in connection with the problem of Polypodium's systematic position.     

Isolation of tissue layers in hermatypic corals by N-acetylcysteine: morphological and proteomic examinations

Corals are diploblastic in body pattern and include two tissue layers, the epidermis and gastrodermis, interconnected by an acellular matrix mesoglea. During development, cells in these tissue layers differentiate morphologically and functionally. In most hermatypic corals, the gastrodermis further develops an ability to associate with microalgae dinoflagellates. This endosymbiosis occurs inside specific host gastrodermal cells, and its mechanism still remains unclear notwithstanding decades of research. The delay in progress is partly due to the difficulty in separating the gastrodermis and its symbionts from the epidermis for detailed cellular and biochemical investigations. The present study reports a simple method to separate these two tissue layers in hermatypic corals using the reducing agent, N-acetylcysteine (NAC). Efficient tissue and proteomic isolations are demonstrated by microscopy and two-dimensional SDS polyacrylamide gel electrophoresis (2D SDS-PAGE). The NAC treatment was able to separate tissue layers without inducing protein degradation. Furthermore, the sensitivity of protein detection greatly increases in the isolated tissue layers. The application of the present technique provides future research on endosymbiosis and coral development with a tool for higher accuracy and sensitivity.     

Developmentally regulated localization of endosymbiotic dinoflagellates in different tissue layers of coral larvae

In adult cnidarians, symbiotic dinoflagellate Symbiodinium are usually located in the gastrodermis. However, the onset of this endosymbiotic association and its regulation during larval development are unclear. This study examined the distribution of the Symbiodinium population in tissue layers of planula larvae released from the stony coral Euphyllia glabrescens. Symbiodinium were redistributed from the epidermis to the gastrodermis, at a rate that was fastest during early planulation and then decreased prior to metamorphosis. This process indicates that the endosymbiotic activity of coral tissues is developmentally regulated. During the early larval stage, both the epidermis and gastrodermis contained Symbiodinium; then, as the larvae developed toward metamorphosis, the numbers in the epidermis gradually diminished until they were only found in the gastrodermis. The mechanism of redistribution remains unknown, but may be due to a direct translocation and/or change in the proliferation of symbionts in different tissue layers.     

Histological and ultrastructural studies of the basal disk of Hydra

Summary The gastrodermis and mesoglea of the basal disk of Hydra were investigated to conclude a three-part series of papers. The gastrodermis is composed of digestive cells (most predominant cell type), mucous and nerve cells (both immature and fully differentiated). The principal function of the digestive cells appears to be storage of protein, lipid and glycogen reserves which are utilized by neighboring cells. Mucous cells apparently use some of the reserves to synthesize their secretions which lubricate cells and prevent cell damage during egestion of waste through the aboral pore. The function of the gastrodermal nerve cells is uncertain.The mesoglea of the basal disk, contains the same structural components as seen in other regions of the polyp. It is reasonable to assume that it maintains the same function of cell adhesion and migration. As the mesoglea converges on the aboral pore, it loses its structural integrity and cells are sloughed off the column.This investigation was supported by The National Science Foundation, Grant Number GB-27395.     

Ultrastructure and histochemistry of the foregut in<Emphasis Type="Italic"> Wirenia argentea</Emphasis> and<Emphasis Type="Italic"> Genitoconia rosea</Emphasis> (Mollusca,Solenogastres)

Wirenia argentea and Genitoconia rosea feed on Cnidaria like most representatives of the molluscan taxon Solenogastres (Aplacophora, Neomeniomorpha sensu Scheltema). The structure and histochemistry of the foregut are described based on histologic, semithin, and ultrathin section series. The ultrastructure was analyzed by means of transmission electron microscopy. There are two sets of unicellular glands: a narrow row of preoral gland cells opening to the preoral area, and pharyngeal gland cells in high numbers. Preoral gland cells produce serous secretions in W. argentea, but mucosubstances in G. rosea, whereas pharyngeal gland cells are similar in structure and histochemistry in both species. Based on the size and electron density of gland vesicles, five distinct types of pharyngeal gland cells can be defined. In contrast to earlier assumptions, all types of pharyngeal gland cells produce serous secretions, most probably representing digestive ferments, but no mucosubstances.     

Anatomy and histology of the digestive system of the paddlefish (Polyodon spathula)

The paddlefish (Polyodon spathula) is one of the most primitive and unique freshwater fishes of North America. It is adapted as a plankton filter-feeder. The wide mouth and greatly expansible pharyngeal cavity allows it to process a large quantity of water. Numerous setiform gillrakers, which operate against the flattened surfaces of the gill arches and opercula, are capable of filtering out small food particles. The abundance of taste buds and mucus secreting cells in the buccal cavity and pharynx probably aids in selecting and entangling the food. The first segment of the stomach is provided with peculiar, large longitudinal rods of fat which may help in selecting food from water by causing occlusion of the small lumen when the striated fibers of the muscularis contract. Teeth, unnecessary for microphagous fishes, are absent in adults. The digestive tube is one of the most complex present in fishes. It has two distinct divisions of the stomach and four distinct divisions of the intestine besides possessing a large caecum. Ciliated epithelial cells occur in many parts of the gut and may be an aid in moving or processing the small items of food. Besides the presence of cilia, other primitive features of the digestive tract include the lack of distinction between the esophagus and stomach, the presence of a spiral valve, and the presence of some of the diffuse pancreatic tissue within the wall of the gut. The numerous Peyer's patches in the wall of much of the intestine may be an immunological response to the heavy parasitic infestation.     

Cellular structure and ultrastructure of the black coral Antipathes aperta: 1. Organization of the tentacular epidermis and nervous system

The tentacular epidermis of the black coral Antipathes aperta is organized into three distinct regions, containing at least nine different types of cells. The outermost region is dominated by spirocytes along with two types of nematocytes, organized into discrete wart-like batteries. The two nematocyte types both contain microbasic b-mastigophore nematocysts. The outer boundary of the wart is marked by the presence of both spumous and vesicular mucus cells. The ciliation of the wart is contributed principally by the spirocytes. Warts are enveloped and separated from one another by an unusual neurosensory cell complex that extends from the tentacular surface to the mesogleal connective tissue foundation. Funnel-like, flagellated cells composing the complex connect with ganglion cells composing the dominant portion of the nerve net system. Branches of this complex also penetrate the central portion of the wart, making direct contact with the cnidae. The tentacular mid-region is composed of nematocytes and spirocytes in various stages of maturation, along with epitheliomuscular cell (EMC) somata. The EMC's narrow apically extend toward the tentacle surface, forming contacts with the cnidae. The basal end of the EMC expands to form the larger portion of the tentacular musculature. The inner region of tentacular epidermis is marked by a neuromuscular complex sheathed by extensions of mesoglea. The ganglion cells occur as a plexus deep within the tentacle and form polarized junctions with the EMC's, but neuromuscular synapses are not well enough defined for documentation. Polarized synapses lacking well-defined membrane thickenings characterize the interneuronal junctions. Granular cells lining the mesogleal surface appear to be responsible for mesogleal fibrillogenesis.     

Numbers, distribution, and types of neurons in the pedal disk of Hydra based on a serial reconstruction from transmission electron micrographs

The numbers, distribution, and types of neurons in a pedal disk of Hydra littoralis were determined from electron micrographs of 567 serial sections approximately 0.12 micron thick. Of 248 neurons counted, we found 234 ganglion cells in the epidermis and 14 in the gastrodermis. No sensory cells with surface projecting cilia were observed in either epithelial layer of the foot region. We found ciliary structures in 196 (84%) of the epidermal neurons: 55 had a well defined cilium-stereociliary complex, 30 had a cilium lacking stereocilia, and 111 could not be classified. In contrast, 38 epidermal neurons lacked evidence of ciliary structures; 10 of the 14 gastrodermal neurons had one or more centrioles, some with an elaborate pericentriolar rootlet system, but no cilium or stereocilia. Neuronal perikarya could be classified into those with dense heterochromatic nuclei and those with light granular nuclei; often these two nuclear variations were observed in paired or triad arrangements of epidermal neurons. In addition, 68 (29%) of the epidermal neurons were characterized by the presence of small dense granules (115-178 nm in diameter) in the cytoplasm around the periciliary space. Although 32 pairs and 5 triads of contiguous neuronal perikarya were present in the epidermis, only two paired neuronal perikarya were present in the gastrodermis. The major concentration of neurons was approximately midway between the basal surface and the region of transition of epitheliomuscular cells into glandulomuscular cells. There was no evidence of large neuronal aggregations suggestive of ganglia in the pedal disk.     

Pharyngeal and Gastrodermal Ultrastructure of Prorhynchus stagnalis Schultze, 1851 (Turbellaria,Lecithoepitheliata)

The digestive tract of a freshwater Lecithoepitheliata turbellarian, Prorhynchus stagnalis, has been studied at the ultrastructural level. The buccal tube connecting the mouth opening to the pharynx is lined by an insunk epithelium, and is provided with two kinds of secretory cells. The bulbous pharynx itself, typical of the genus, is a highly muscular organ, also lined internally by an insunk epithelium; it also bears at least two different types of gland cells. Externally the pharynx is enveloped by a thin, flat epithelium. The gastrodermis consists of phagocytes and Minotian gland cells, as typical for most Tricladida. No ciliary covering was observed along the whole digestive system; instead, phagocytic cells have long microvillar projections. Preliminary attempts to follow the digestive process using external markers and cytochemical reaction for acid phosphatase were successful in demonstrating endocytic activity in the phagocytes. These data, besides being a contribution to morphology and systematics, also speak in favour of the general digestive theory of Turbellaria proposed by Jennings and serve as a starting point for experimental studies on intracellular digestion within the Lecithoepitheliata.     

Scanning electron microscopy of neurons isolated from the pedal disk and body column of Hydra

Segments of pedal disk and body column were cut from specimens of Hydra littoralis and separated into epidermis and gastrodermis, then macerated to isolate neurons for scanning electron microscopy. Bipolar and multipolar ganglion cells were present in both tissue layers, whereas sensory cells were found only in the gastrodermis. A single cilium projected from the perikaryon of some bipolar and multipolar ganglion cells; the cilium was long in the pedal disk ganglion cells and short in those from the body column. Ganglion cells from the pedal disk had short, thick processes, whereas those from the body column had long, thin neurites. Gastrodermal sensory cells were characterized as unipolar by the presence of an apical cilium near the perikaryon or as asymmetrical bipolar by the presence of a narrow neck region between the perikaryon and cilium. The axon was short in pedal disk sensory cells and long in those from the body column.     

家蚕消化管内桑叶的银染法研究

通过银染法对家蚕整体染色,结果表明:家蚕消化管内的桑叶由叶表皮、叶肉和叶脉组成。叶表皮包括上表皮和下表皮。上表皮细胞可分为三种:钟乳体细胞、绿色表皮细胞和黄色表皮细胞;下表皮内含有气孔;叶肉组织内含有晶体,其中海绵组织内的最多。家蚕消化管由前向后可分为前肠、中肠和后肠,由外向内依次为肌层、底膜、上皮细胞层、内膜。中肠最为发达,其发达的上皮细胞向内表面突起形成许多大的指突形皱褶;上皮细胞层内有圆筒形细胞、杯形细胞两种细胞,两者在形状、功能以及嗜银性等方面有所差异。家蚕消化管对桑叶不同组织的消化吸收效率有差异,上表皮吸收效率最高,下表皮和栅栏组织次之,最低的是海绵组织。采用动物细胞染色方法对植物细胞进行染色,并与常规植物学染色方法进行了比较;依据细胞嗜银性的不同,可将桑叶的上表皮细胞分为两种亚型。     

Ultrastructure and histochemistry,of the stomach of Asplanchna sieboldi

Stomach cells of female Asplanchna sieboldi are specialized for absorption and intracellular digestion of nutrients. Evidence is presented to show that electron-opaque colloidal substances, present in the medium and within digestive vacuoles of the prey (Paramecium), are taken up by the stomach cells at the apical cell membrane and sequestered within food vacuoles which contain hydrolases working in both the acid and alkaline pH range. The stomach cells are also implicated in the absorption of molecules below the resolving power of the electron microscope. In rotifers possessing a complete digestive tract, this task is presumed to be handled by the intestine.     

Chemical, histochemical, and histopathological studies on corals. Porites spp., parasitized by trematode metacercariae

The occurrence of the metacercaria of Plagioporus sp. encysted in two species of corals, Porites compressa and P. lobata, from Kaneohe Bay, Oahu, Hawaii, is reported. This parasite is covered with a bilayered metacercarial cyst the chemical compositions of which have been ascertained by histochemistry. Both of these cyst walls are believed to be secreted by the parasite. In addition, the cytochemical properties of the metacercaria and the adjacent host cells are reported.Histo- and cytopathological alterations in the epidermis and gastrodermis of the coral hosts were studied and such changes have been interpreted to have resulted from mechanical pressure exerted by the parasite.Because gross observations had revealed the presence of elevated nodules at the sites where the metacercariae occurred, chemical analysis was conducted to ascertain whether this larval trematode had induced hypercalcification in the hosts. However, calcium determinations of Decal in which pieces of parasitized coral tissue had been decalcified revealed that hypercalcification had not occurred.Examination of the cellular reactions in both species of Porites revealed that if the gastrodermis had been ruptured, partial encapsulation of the parasite occurs. The reaction cells include free cells of the mesoglea that become fused, accidentally trapped cnidoblasts and nutritive-muscular cells, and gastrodermal interstitial cells. Despite the partial encapsulation, the encysted metacercariae did not appear to be injured.