Ultrastructure of the pharynx of Prorhynchus (Platyhelminthes, Lecithoepitheliata)

The pharynx variabilis of Prorhynchus is strongly muscular, with a small pharyngeal fold and a thin surrounding sheath. There is one row of inner longitudinal musclcs, up to six rows of inner circular muscles, many radial muscles, one row of outer circular and one row of outer longitudinal muscles, with no sphincter muscle groups. Three kinds of secretion, produced in a cluster of gland cell bodies posterior to the pharynx, enter the pharynx wall. They travel anteriorly in ducts and two kinds unite in a common duct just prior to discharging into the anterior region of the pharynx lumen. The perikarya of lumen epithelial cells lie within the pharynx musculature and, at the anterior and posterior margins of the pharynx, external to the pharynx. Bundles of ciliated receptors are numerous at the anterior and posterior constrictions. Similarities in the ultrastructure of flame bulbs of Rhabdocoela and Lecithoepitheliata suggest a relationship between these groups. However, the usefulness of pharynx ultrastructure for platyhelminth phylogeny cannot be assessed until complete ultrastructural studies of various groups of Rhabdocoela have been made.     

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.     

The pharynx of Caenorhabditis elegans.

The anatomy of the pharynx of Caenorhabditis elegans has been reconstructed from electron micrographs of serial sections. The pharynx is used for pumping food into the gut, and is composed of 34 muscle cells, 9 marginal cells, 9 epithelial cells, 5 gland cells and 20 neurones. Three regions of specialization in the cuticle lining of the pharyngeal lumen may aid in the accumulation of food particles. A basement membrane isolates the pharynx from the rest of the animal, making the pharyngeal nervous system a nearly self-contained unit which is composed primarily of five classes of motor neurones and six classes of interneurones. Three other classes have also been described, which by their morphology appear to be neurosecretory and motor, motor and interneuronal, and lastly one pair that only innervates three of the marginal cells. Some classes of neurone have free endings just under the cuticle lining the lumen of the pharynx, suggesting that these are mechano- or proprio-receptive endings. The connectivity of these neurones has been described at the level of individual synaptic regions, and after combining this information with video taped observations of the pharynx pumping, some interpretations of how these neurones function have been offered.     

The role of epithelial remodelling in tooth eruption in larval zebrafish

Based on light and transmission electron-microscopic observations on erupting first-generation teeth in the zebrafish, Danio rerio, we propose a biphasic mechanism for tooth eruption: (1). formation of an epithelial crypt prior to eruption of the tooth, possibly as a result of constraints in the epithelium resulting from the growth of adjacent tooth germs, and (2). detachment of cellular interdigitations both within the pharyngeal epithelium, at the pharyngeal epithelium/enamel organ boundary, and between the outer and inner dental epithelium, resulting in the exposure of the tooth tip in the crypt, immediately after tooth ankylosis. Later, further detachment of interdigitations between the inner and the outer enamel epithelium unfolds the epithelium even more and leads to a more pronounced exposure of the tooth tip. The presence of small patches of non-collagenous matrix on the outer surface of the tooth close to where it merges with the attachment bone is interpreted as a device to prevent complete detachment of the enamel organ. The biphasic nature of the mechanism for tooth eruption is supported by observations on in vitro cultured heads. First-generation teeth develop normally and crypts are formed, as under in vivo conditions, but the teeth fail to erupt. Taken together, our observations suggest that epithelial remodelling plays a crucial role in eruption of the teeth in this model organism.     

The embryonic development of the rhabdocoel flatworm Mesostoma lingua (Abildgaard, 1789)

The embryonic development of the flatworm Mesostoma lingua was studied using a combination of life observation and histological analysis of wholemount preparations and sections (viewed by both light and electron microscopy.) We introduce a series of stages defined by easily recognizable morphological criteria. These stages are also applicable to other platyhelminth taxa that are currently under investigation in our laboratory. During cleavage (stages 1 and 2), the embryo is located in the center of the egg, surrounded by a layer of yolk cells. After cleavage, the embryo forms a solid, disc-shaped cell cluster. During stage 3, the embryo migrates to the periphery of the egg and acquires bilateral symmetry. The side where it contacts the egg surface corresponds to the future ventral surface of the embryo. Stage 4 is the emergence of the first organ primordia, the brain and pharynx. Gastrulation, as usually defined by the appearance of germ layers, does not exist in Mesos-toma; instead, organ primordia emerge ”in situ” from a mesenchymal mass of cells. Organogenesis takes place during stages 5 and 6. Cells at the ventral surface form the epidermal epithelium; inner cells differentiate into neurons, somatic and pharyngeal muscle cells, as well as the pharyngeal and protonephridial (excretory) epithelium. A junctional complex, consisting initially of small septate junctions, followed later by a more apically located zonula adherens, is formed in all epithelial tissues at stage 6. Beginning towards the end of stage 6 and continuing throughout stages 7 and 8, cytodifferentiation of the different organ systems takes place. Stage 7 is characterized by the appearance of eye pigmentation, brain condensation and spindle-shaped myocytes. Stage 8 describes the fully dorsally closed and differentiated embryo. Muscular contraction moves the body in the egg shell. We discuss Mesostoma embryogenesis in comparison to other animal phyla. Particular attention is given to the apparent absence of gastrulation and the formation of the epithelial junctional complex. Received: 10 February 2000 / Accepted: 10 April 2000     

The Distribution of Transient Receptor Potential Melastatin-8 in the Rat Soft Palate, Epiglottis, and Pharynx

Immunohistochemistry for transient receptor potential melastatin-8 (TRPM8), the cold and menthol receptor, was performed on the rat soft palate, epiglottis and pharynx. TRPM8-immunoreactive (IR) nerve fibers were located beneath the mucous epithelium, and occasionally penetrated the epithelium. These nerve fibers were abundant in the posterior portion of the soft palate and at the border region of naso-oral and laryngeal parts of the pharynx. The epiglottis was free from such nerve fibers. The double immunofluorescence method demonstrated that TRPM8-IR nerve fibers in the pharynx and soft palate were mostly devoid of calcitonin gene-related peptide-immunoreactivity (CGRP-IR). The retrograde tracing method also demonstrated that 30.1 and 8.7 % of sensory neurons in the jugular and petrosal ganglia innervating the pharynx contained TRPM8-IR, respectively. Among these neurons, the co-expression of TRPM8 and CGRP-IR was very rare. In the nodose ganglion, however, pharyngeal neurons were devoid of TRPM8-IR. Taste bud-like structures in the soft palate and pharynx contained 4–9 TRPM8-IR cells. In the epiglottis, the mucous epithelium on the laryngeal side had numerous TRPM8-IR cells. The present study suggests that TRPM8 can respond to cold stimulation when food and drinks pass through oral and pharyngeal cavities.     

On the median pharyngeal valve of the American alligator (Alligator mississippiensis)

The middle ear cavities of crocodilians have complex connections with the pharyngeal lumen, including lateral and median components which both open into a single chamber located on the dorsal midline of the pharynx. This chamber and the surrounding soft-tissue is herein termed the median pharyngeal valve. In the American alligator (Alligator mississippiensis) this valve opens, for a duration of 0.3 s, approximately every 120 s; the patency of the median pharyngeal valve was not influenced by either auditory stimuli or by submersing the alligator underwater. The median pharyngeal valve has an outer capsule of dense connective tissue and fibrocartilage and an inner “plug” of loose connective tissue. These opposing surfaces are lined by respiratory epithelium and separated by a cavity that is continuous with the middle ear cavities and the pharyngeal lumen (through a central opening in the capsule termed the pore). The inner plug of the median pharyngeal valve is contacted by skeletal muscles positioned to serve as both elevators/retractors (which would open the valve) and elevators/protractors (which, in conjunction with gravity, would close the valve). Unlike other vertebrate valve systems, the median pharyngeal valve appears to function as a deformable ball check valve.     

Anatomy and ultrastructure of the ventral pharyngeal organs of Saccocirrus(Saccocirridae) and Protodriloides(Protodriloidae fam. n.) with remarks on the phylogenetic relationships within the Protodrilida (Annelida: Polychaeta)

General anatomy and ultrastructure of the ventral pharyngeal organs were investigated in Succocirrus krusudensis, Protodriloides chaetifer and P. symbioticus. Succocirrus papillocercus, a species without a ventral pharyngeal organ, was included for comparison. The two genera show homologous similarities in their pharynges: bulbus muscle composed of transverse muscle fibres and interstitial cells, those cells with small cell bodies and processes containing prominent tonofilaments which are orientated dorsoventrally and connect the bulbus epithelium with the investing muscle; bulbus muscle fibres circomyarian with nuclei and mitochondria located basally, investing muscle also with interstitial cells, which lack tonofilaments; oesophagus surrounded by gland cells opening into the pharynx. Most likely, a ventral pharynx with these characters was already present in the stem species of Saccocirridae, Protodriloidae fam. n. and Protodrilidae and evolutionary processes led to several changes: a tongue-like organ with a prominent tip and supporting elements is a synapomorphic character of Protodrilus and Succocirrus; the pharyngeal organ is reduced to stomodeal pouches and salivary glands in S. papillocercus and completely lost in Asromus raenioides. These results and data from previous studies are summarized in a tentative phylogenetic dendrogram and allow the introduction of a new family, Protodriloidae.     

On the Fine Structure of the Alimentary Tract of Cephalodiscus gracilis (Pterobranchia,Hemichordata)

The U-shaped alimentary tract of Cephalodiscus is of exclusively epithelial structure; on the basis of fine structural criteria the entire tract can be divided into two large subdivisions: an anterior one with mouth, mouth cavity, pharynx and oesophagus, and a posterior one with stomach and intestine. The anterior subdivision is built up of a relatively uniform, innervated, pseudostratified, ciliated epithelium with mucus cells which are concentrated in the initial parts of the mouth cavity. Cilia and mucus presumably constitute a mechanism transporting food particles into the stomach. In the area of the gill slits specific vacuolated cells occur which may lend rigidity to the walls of the slits. The gastric epithelium consists of prismatic cells characterized by, among others, large inclusion bodies, which may represent digestive vacuoles, small dense rod-shaped granules and an elaborate system of microridges, at the base of which abundant endocytotic vesicles occur. The dorsal gastric pouch contains cells rich in rough ER and secretory granules, probably containing digestive enzymes. Thus morphological evidence points both to intra- and extracellular digestion. The intestinal epithelium resembles that of the stomach, however, it is lower, its organelles are fewer and it bears, beside cilia, mainly microridges, which towards its distal end become sparse. Both in the gastric and intestinal epithelium small granulated cells have been found, which presumably represent endocrine cells.     

SCHIZOGENY AND ULTRASTRUCTURE OF DEVELOPING LATEX DUCTS IN MAMMILLARIA GUERRERONIS (CACTACEAE)

The schizo-lysigenous latex ducts of Mammillaria guerreronis, sect. Subhydrochylus, were examined by optical and electron microscopy. These ducts are complex having a distinct outer epithelium without intercellular spaces and an inner epithelium in which schizogenous spaces arise. Schizogeny begins with formation of bulbous pockets and/or production of dark streaks in certain walls. Spaces formed by these processes ultimately contain a combination of electron dense materials, vesicles, and numerous thin, convoluted wall layers. Schizogeny may be responsible for initial formation of lumen and latex and may also separate some inner epithelial cells from the surrounding layers. Lysigeny of the inner epithelial cells contributes materials to the latex and allows enlargement of the duct. The inner epithelial cells contain mitochondria, dictyosomes, apparently functional nuclei, and numerous vesicles. The outer epithelium has fewer vesicles than the inner epithelium. Schizo-lysigeny in the members of sect. Subhydrochylus is considered to be ancestral to the strict lysigeny of the members of sect. Mammillaria. The inner and outer epithelia of M. guerreronis are thought to be homologous to the lumen and epithelium respectively of M. heyderi.     

Biogenesis of myeloid bodies in regenerating newt (Notophthalmus viridescens) retinal pigment epithelium

Summary Myeloid bodies are believed to be differentiated areas of smooth endoplasmic reticulum membranes, and they are found within the retinal pigment epithelium in a number of lower vertebrates. Previous studies demonstrated a correlation between phagocytosis of outer segment disc membranes and myeloid body numbers in the retinal pigment epithelium of the newt. To test the hypothesis that myeloid bodies are directly involved in outer segment lipid metabolism and to further characterize the origin and functional significance of these organelles, we examined the effects on myeloid bodies of eliminating the source of outer segment membrane lipids (neural retina removal) and of the subsequent return of outer segments (retinal regeneration) in the newt Notophthalmus viridescens. Light- and electron-microscopic analysis demonstrated that myeloid bodies disappeared from the pigment epithelium within six days of neural retina removal. By week 6 of regeneration, rudimentary photoreceptor outer segments were present but myeloid bodies were still absent. However, at this time, the smooth endoplasmic reticulum in some areas of the retinal pigment epithelial cells had become flattened, giving rise to small (0.5 m long), two-to-four layer-thick lamellar units, which are myeloid body precursors. Small myeloid bodies were first observed one week later at week 7 of retinal regeneration. This study revealed that newt myeloid bodies are specialized areas of smooth endoplasmic reticulum. It also showed that a contact between functional photoreceptors and the retinal pigment epithelium is essential to the presence of myeloid bodies in the epithelial cells.     

The ontogeny of shell secretion in Terebratalia transversa (Brachiopoda, Articulata). I. Development of the mantle

The morphology of the mantle in free-swimming and metamorphosing larvae of the articulate brachiopod Terebratalia transversa has been examined by scanning and transmission electron microscopy. The mantle begins to form approximately 2 days after fertilization and subsequently develops into a skirtlike lobe that encircles the middle region of the larval body. A simple epithelium covers both the outer surface of the mantle lobe and the inner side situated next to the pedicle lobe of the larva. During metamorphosis, the mantle lobe is everted over the anterior end of the larva. Thus, the epithelium covering the outer part of the mantle lobe in the larva subsequently becomes the inner epithelium of the juvenile mantle. Similarly, the inner epithelium of the larval mantle lobe represents the future outer epithelium of the juvenile mantle. In free-swimming larvae, the prospective outer mantle epithelium contains two types of cells, called "lobate" and "vesicular" cells. Lobate cells initially deposit a thin layer of amorphous material, and vesicular cells produce ovoid multigranular bodies. Following settlement at about 5 days postfertilization, the vesicular cells secrete an electron-dense sheet that constitutes the basal layer of the developing periostracum. Within several hours to a day thereafter, reversal of the mantle lobe is rapidly effected, apparently by contractions of the pedicle adjustor muscles.     

Reconstruction of the pharyngeal corpus of Aphelenchus avenae (Nematoda: Tylenchomorpha), with implications for phylogenetic congruence

The corpus of the pharynx in the nematode Aphelenchus avenae (Nematoda: Tylenchomorpha) was three‐dimensionally reconstructed to address questions of phylogenetic significance. Reconstructed models are based on serial thin sections imaged by transmission electron microscopy. The corpus comprises six classes of radial cells, two classes of marginal cells, and 13 neurones belonging to eight classes. Between the arcade syncytia and isthmus cells, numbers of cell classes along the pharyngeal lumen and numbers of nuclei per cell class correspond exactly between A. avenae and Caenorhabditis elegans. The number of radial cell classes between the arcade syncytia and the dorsal gland orifice (DGO) in A. avenae is also identical with outgroups. Proposed homologies of the pharynx imply that expression of the anterior two cell classes as epithelial or muscular differs within both Rhabditida and Tylenchomorpha. Numbers of neurone cell bodies within the corpus correspond exactly to C. elegans, other free‐living outgroups, and other Tylenchomorpha. Neurone polarity and morphology support conserved relative positions of cell bodies of putative neurone homologues. The configuration of cells in the procorpus, including the length of individual cell classes along its lumen, differs across representatives of three deep Tylenchomorpha lineages. Nonhomology of the procorpus challenges the homology of DGO position within the metacorpus, the primary taxonomic character for circumscribing ‘Aphelenchoidea’. Comparison of A. avenae with Aphelenchoides blastophthorus shows that, despite gross pharynx similarity, these nematodes have several differences in corpus construction at a cellular level. The possibility of convergent evolution of an ‘aphelenchid’ pharynx in two separate lineages would be congruent with molecular‐based phylogeny. Putative homologies and conserved arrangement of pharyngeal neurones in Tylenchomorpha expand the experimental model of C. elegans. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010.     

Origin and Differentiation of the Inner Follicular Cells during Oogenesis in Molgula pacifica (Urochordata), an Ascidian without Test Cells

In Molgula pacifica small previtellogenic oocytes are found between cells of the ovarian epithelium. Each oocyte subsequently grows within a compartment of the epithelium known as a primary follicle. The wall of the primary follicle is composed of outer follicular epithelial cells. While growing from about 15–70 μm in diameter, each oocyte gradually recruits a set of about 950 non-epithelial inner follicular cells. These cells co-differentiate in sets with each oocyte, but test cells never appear. The first filamentous components of the vitelline coat appear on the surface of an oocyte in places where it is in contact with undifferentiated (stage 2) inner follicular cells. Each fully differentiated inner follicular cell stores adhesive precursors in a large compartment of the endoplasmic reticulum and probably secretes components of the vitelline coat. There is no evidence that the outer follicular epithelial cells transform into inner follicular cells by dedifferentiation as has often been assumed. Inner follicular cells, in stage 1, are nearly identical to hemoblasts. Hemoblasts may form the inner follicular cells, but to do this they would have to cross the outer follicular epithelium and this phenomenon has not yet been seen.     

The fine structure of the pigment epithelium and photoreceptor cells of the newt,Triturus viridescens dorsalis (Rafinesque)

The morphology of the retinal pigment epithelium and photoreceptor cells has been studied in the common newt Triturus viridescens dorsalis by light, conventional transmission and scanning electron microscopy. The pigment epithelium is formed by a single layer of low rectangular cells, separated by a multilayered membrane (Bruch's membrane) from the vessels of the choriocapillaris. The scleral border of the pigment epithelium is highly infolded and each epithelial cell contains smooth endoplasmic reticulum, myeloid bodies, mitochondria, lysosomes, phagosomes and an oval nucleus. Inner, pigment laden, epithelial processes surround the photoreceptor outer and inner segments. The three retinal photoreceptor types, rods, single cones and double cones, differ in both external and internal appearance. The newt, rod, outer segments appear denser than the cones in both light and electron micrographs, due to a greater number of rod lamellae per unit distance of outer segment and to the presence of electron dense intralamellar bands. The rod outer segments possess deep incisures in the lamellae while the cone lamellae lack incisures. Both rod and cone outer segments are supported by a peripheral array of dendritic processes containing longitudinal filaments which originate in the inner segment. The inner segment mitochondria, forming the rod ellipsoid, arelong and narrow while those in the cone are spherical to oval in shape. The inner segments of all three receptor cell types also contain a glycogen-filled paraboloid and a myoid region, just outside the nucleus, rich in both rough and smooth endoplasmic reticulum. The elongate, cylindrical nuclei differ in density. The rod nuclei are denser than those of the cones, contain clumped chromatin and usually extend further vitreally. Similarly, the cytoplasm of the rod synaptic terminal is denser than its cone counterpart and contains synaptic vesicles almost twice as large as those of the cones. Photoreceptor synapses in rods and cones are established by both superficial and invaginated contacts with bipolar or horizontal cells.     

Placentation in garter snakes. II. Transmission EM of the chorioallantoic placenta of Thamnophis radix and T. sirtalis

Transmission electron microscopy was used to examine the ultrastructure of the allantoplacenta of garter snakes during the last half of gestation. This placenta occupies the dorsal hemisphere of the egg and is formed through apposition of the chorioallantois to the inner lining of the uterus. The uterine epithelium consists of flattened cells with short, irregular microvilli and others that bear cilia. The lamina propria is vascularized and its capillaries lie at the base of the uterine epithelial cells. The chorionic epithelium consists of a bilayer of squamous cells that are particularly thin superficial to the allantoic capillaries. Neither the chorionic epithelium nor the uterine epithelium undergoes erosion during development. Although a thin remnant of the shell membrane intervenes between fetal and maternal tissue at mid-gestation, it undergoes fragmentation by the end of gestation. Thus, uterine and chorionic epithelial are directly apposed in some regions of the allantoplacenta, forming continuous cellular boundaries at the placental interface. During development, capillaries proliferate in both the uterine and chorioallantoic tissues. By late gestation, the interhemal diffusion distance has thinned in some areas to less than 2 microm through attenuation of the uterine and chorionic epithelia. Morphologically, the allantoplacenta is well adapted for its function in gas exchange. However, the presence of cytoplasmic vesicles, ribosomal ER, and mitochondria in the chorionic and uterine epithelial cells are consistent with the possibility of additional forms of placental exchange.     

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.     

Formation of a Polarised Primitive Endoderm Layer in Embryoid Bodies Requires Fgfr/Erk Signalling

The primitive endoderm arises from the inner cell mass during mammalian pre-implantation development. It faces the blastocoel cavity and later gives rise to the extraembryonic parietal and visceral endoderm. Here, we investigate a key step in primitive endoderm development, the acquisition of apico-basolateral polarity and epithelial characteristics by the non-epithelial inner cell mass cells. Embryoid bodies, formed from mouse embryonic stem cells, were used as a model to study this transition. The outer cells of these embryoid bodies were found to gradually acquire the hallmarks of polarised epithelial cells and express markers of primitive endoderm cell fate. Fgf receptor/Erk signalling is known to be required for specification of the primitive endoderm, but its role in polarisation of this tissue is less well understood. To investigate the function of this pathway in the primitive endoderm, embryoid bodies were cultured in the presence of a small molecule inhibitor of Mek. This inhibitor caused a loss of expression of markers of primitive endoderm cell fate and maintenance of the pluripotency marker Nanog. In addition, a mislocalisation of apico-basolateral markers and disruption of the epithelial barrier, which normally blocks free diffusion across the epithelial cell layer, occurred. Two inhibitors of the Fgf receptor elicited similar phenotypes, suggesting that Fgf receptor signalling promotes Erk-mediated polarisation. This data shows that primitive endoderm cells of the outer layer of embryoid bodies gradually polarise, and formation of a polarised primitive endoderm layer requires the Fgf receptor/Erk signalling pathway.