The lungs of Polypterus senegalus and Erpetoichthys calabaricus: Insights into the structure and functional distribution of the pulmonary epithelial cells

The present article is a comparative, structural study of the lung of Polypterus senegalus and Erpetoichthys calabaricus , two species representative of the two genera that constitute the Polypteriformes. The lung of the two species is an asymmetric, bi‐lobed organ that arises from a slit‐like opening in the ventral side of the pharynx. The wall is organized into layers, being thicker in P. senegalus . The inner epithelium contains ciliated and non‐ciliated bands. The latter constitute the respiratory surface and are wider in E. calabaricus . The air‐blood barrier is thin and uniform in P. senegalus and thicker and irregular in E. calabaricus . In the two species, the ciliated areas contain ciliated cells, mucous cells and cells with lamellar bodies. Additionally, P. senegalus contains polymorphous granular cells (PGCs) and neuroendocrine cells (NECs) while E. calabaricus lacks PGCs but shows granular leukocytes and a different type of NEC. Interestingly, ciliated cells and secretory cells show a dual morphology in E. calabaricus indicating the presence of cellular subtypes and suggesting more complex secretory activity. Also in E. calabaricus , cilia show a novel doublet‐membrane interaction that may control the displacement of the microtubule doublets. The subepithelium is a connective layer that appears thicker in P. senegalus and contains, in the two species, fibroblasts and granulocytes. The outer layer contains bundles of richly innervated striated muscle. This layer is likely involved in the control of lung motion. In the two species, smooth muscle cells constitute a limiting layer between the subepithelium and the striated muscle compartment. The role of this layer is unclear.     

Structure of the digestive tract of the pluteus larva of Dendraster excentricus (Echinodermata: Echinoida)

Summary Histological and ultrastructural observations of the digestive tract of eight-armed plutei of Dendraster excentricus are reported. The esophagus is divided into two regions. The uppermost is a narrow tube comprised of ciliated cells that assist in transporting food to the more bulbous lower esophagus where food particles are formed into a bolus prior to entering the stomach. The esophagus is surrounded by a network of smooth muscle fibers that are predominantly oriented circumferentially in the upper esophagus, and longitudinally in the lower esophagus. The musculature of the upper esophagus produces peristaltic contractions, whereas contractions of the muscle of the lower esophagus open the cardiac sphincter and force food from the lower esophagus into the stomach. Axons are associated with the ciliated cells and the muscles of the upper esophagus. The cardiac sphincter consists of a ring of myoepithelium, with cross-striated myofibrils oriented around the bases of the cells. The gastric epithelium is comprised of two cell types. Type I cells, which predominate, absorb and store nutrients, and may be the source of secreted digestive enzymes. Type II cells apparently phagocytize and intracellularly digest whole algal cells. The intestine is comprised of relatively unspecialized cells and probably functions primarily as a conductive tube for the elimination of undigested materials.     

Ultrastructure of the tubular nephron of the lizard Podarcis (= Lacerta) Taurica

The proximal, intermediate, and distal convoluted tubules of the neprhon of Podarcis (= Lacerta) taurica were examined by electron microscopy. Proximal tubule cells have large, apical cytoplasmic protrusions and microvilli interpreted to function in urate secretion. Adjacent cells are bound apically by tight junctions and desmosomes but interdigitate in their basal region. This situation is repeated in the other tubules with significant differences in intercellular space width. The basal surfaces bear numerous cytoplasmic processes. The intermediate tubule has proximal and distal segments each with dark, ciliated, and light cells, the cuboidal dark cells with dense cytoplasm constituting the main bulk of the wall. As the cells of the proximal and distal segments resemble those of the proximal and distal convoluted tubules, respectively, the intermediate tubule is considered as a transition region. The ciliated cell body has two broad processes extending from the lumen, one to the basement membrane and one to a foot process of a light cell. The light cell is surrounded by dark and ciliated cells. It does not reach the lumen, but contacts the basement membrane through a process running below a ciliated cell to form a mushroom-shaped structure in tubule cross-section, the light cell process forming the stalk and a ciliated cell the cap. The cilia probably propel the glomerular filtrate towards the distal convoluted tubule. This latter tubule has initial, middle, and terminal zones, all nonciliated but with different lumen widths and cell shapes.     

The structures of the tentacles of Rhabdopleura compacta (Hemichordata) with special reference to neurociliary control

Summary The tentacle of Rhabdopleura compacta (Hemichordata) consists of two layers of cells surrounding a central coelomic cavity. The two layers of cells are separated by a cell free basement lamella.The tentacles on the arms of Rhabdopleura bear three longitudinal rows of cilia. The ciliated cells are closely associated with bundles of nerve fibres, and between some of the cells and nerve fibres there are synapses. The peripheral regions of the ciliated cells are joined to one another by desmosomes. Tonofibrils join some of these desmosomes to the kinetosomes of the cilia.The nerve fibres are confined to the ectodermal layer and the muscle cells to the layer of cells within the basement lamella. In the ectodermal layer besides ciliated cells there are mucus cells, densely pigmented cells, and green bodies. The function of these last two types of cells is secretory. Most of the epithelial cells have microvilli upon their free borders.I wish to thank Professor J. Z. Young F. R. S. for enthusiastic advice and encouragement. Dr. R. Bellairs generously provided the facilities for electron microscopy. Mr. R. Moss gave excellent technical and photographic assistance. Dr. A. Stebbing of the Plymouth Marine Biological Laboratory helped me to obtain and to identify the specimens. Professor D. W. James kindly allowed me to use his facilities for interference microscopy.     

Scanning and transmission electron microscopic study of the lung of the newt,Triturus alpestris Laur.

Summary The lungs of Triturus alpestris Laur. were investigated with the scanning and transmission electron microscopes. Dimensions of the cell bodies of pneumocytes and ciliated cells, as well as the thickness of the air-blood barrier, were determined. The lungs of the newt form two simple sacs without septa. A ciliated epithelium containing goblet cells lines the pulmonary vein and partially the pulmonary artery. The remainder of the lung surface is covered internally by respiratory epithelium consisting of one type of cell and only occasionally showing the presence of single ciliated cells. All cells, ciliated, goblet and pneumocytes, contain in their cytoplasm lamellar bodies. Multivesicular bodies and numerous vesicles of variable electron density also occur in the cytoplasm of pneumocytes. Atypical mitochondria can be found in all cell types of the lung. Fixation with addition of tannic acid reveals the surface lining film. Tubular myelin figures were not observed.     

Fine Structure of the Female Intoshia variabili(Alexandrov & Sljusarev) (Mesozoa: Orthonectida)

The morphology and fine structure of female Intoshia variabili, new combination for Rhopalura variabiliAlexandrov & Sljusarev, 1992, were studied with transmission electron microscopy. The body surface is covered with a 3-layered cuticula, under which is a layer of ciliated + non-ciliated cells arranged in alternating rings around the body. Ciliated cells have lateral extensions that intercalate with the non-ciliated cells. The kinetosome of each cilium has two longitudinally oriented cross-striated rootlets. The outer surface of the ciliated cells is covered with small tubercles, and the cytoplasm of these cells contains granules, vacuoles, mitochondria, fibrillar structures and lamellary bodies. A band of dense fibrils passes through the upper part of each ring of cells, going from one cell junction to another, encircling the entire body. Between the layer of ciliated + non–ciliated cells and the oocytes, elongated contractile cells from 4–5 longitudinal columns and 1 ring, the latter at the level of ciliated rings 7–9. The contractile cells contain thick and thin longitudinally oriented fibrils. The oocytes contain a large nucleus, numerous mitochondria, electron–dense granules and 1–2 spherical structures. An anteriorly situated, ciliated goblet–like receptor, not described for any other orthonectids, consists of three closely apposed cells, the upper part of which contains densely packed cilia. The genital pore opens through a non–ciliated cell and is surrounded by several cells with granules.     

The Tentacle Cilia of Aglantha digitale (Hydrozoa: Trachylina) and their Control

The tentacles of Aglantha have ciliary bands along the sides. Metachronal waves pass along these bands. The strong ciliary currents produced propel water past the tentacles, increasing the probability of prey capture. The ciliated cells are unusual in having many (up to about 500) cilia per cell, where most cnidarian ciliated cells have only one. The cells are also peculiar in containing numerous axonemes without membrane coverings, lying loose in the cytoplasm. Tentacles show independent, rhythmic, slow flexions in the oral direction and groups of tentacles show coordinated, slow flexions as part of a regularly repeated fishing cycle. In both cases, these slow, graded movements are mediated by a slowly conducting system, probably the network of small neurons present in the ectoderm, and are accompanied by ciliary arrests. Much faster, more powerful, coordinated contractions of the tentacles occur in the context of escape behaviour; these are mediated by giant axons which run down the tentacles and are also accompanied by ciliary arrest. Ciliary and muscle effectors evidently share a common motor innervation. Electron microscopy shows that the giant and non-giant nerves both synapse with muscle cells. The latter are joined to the ciliated cells by gap junctions, and it is suggested that whenever the muscles are excited depolarizations spread to the ciliated cells through the gap junctions and cause ciliary arrests. Neuronal control of ciliary activity has not previously been reported in the Hydrozoa.     

On the structure of the pulmonate osphradium

Summary The osphradium of Planorbarius consists of a blindly-ending ciliated canal, formed by an infolding of the mantle epithelium, and a basal ganglion of nerve cells which is comparable in complexity with ganglia of the central nervous system. The distribution of cell types in the osphradial epithelium is specialised so that three regions can be recognised; the ciliated, the secretory and the sensory regions. The basal sensory region of the canal epithelium consists of ciliated cells and is innervated by sensory neurones of the osphradial ganglion. The middle secretory region contains mainly of mucus-secreting cells and the epithelium adjacent to the osphradial aperture of ciliated cells and secretory cells of a second type. The sensory neurones of the osphradial ganglion are bipolar or of a modified monopolar type. Other monopolar neurones, similar to those common in the central nervous system are of non-sensory function. The osphradium of Paludina, although of typical prosobranch form, possesses ciliated pits similar to the single canal of Planorbarius, which may indicate a shared modality of receptor function. A definite function cannot be ascribed to the pulmonate osphradium based on morphological evidence alone.     

Ciliogenesis in cryopreserved mammalian tracheal epithelial cells cultured at the air–liquid interface

To determine air–liquid interface (ALI) culture derived from cryopreserved mammalian tracheal ciliated cells is a viable ciliated cell model for the investigations of regulatory mechanisms of ciliary beat frequency (CBF), two studies were performed using ovine and porcine tracheae obtained from local slaughterhouses. The protease-digested tracheal ciliated cells were harvested and cultured at the ALI using collagen-coated, porous membrane inserts. In study 1, the ALI culturing protocols were established using non-cryopreserved ovine tracheal ciliated cells. Ciliogenesis was documented with immuno-histology and electron micrographs. Vigorous beating cilia were video-recorded. CBF was measured by laser light scattering. The functional integrity of the autonomic receptors of the ciliated cells was confirmed with the stimulatory responses of CBF using luminal methacholine and basolateral terbutaline. In study 2, porcine tracheal ciliated cells stored in liquid nitrogen for a minimum of 4 weeks were used. The cryopreserved cells were thawed and cultured using the ALI protocol established in study 1. After two months, cilia outgrowths were confirmed using video microscopy and scanning electron micrograph (SEM). The trans-epithelial resistances were 28.5 kΩ (n = 4). Luminal applications of 1 μM and 10 μM methacholine stimulated CBF from a baseline of 7.4 ± 0.2 Hz to 8.4 ± 0.8 Hz and 7.7 ± 0.4 Hz, respectively (n = 5). Basolateral applications of 1 μM and 10 μM terbutaline stimulated CBF from a baseline of 7.5 ± 0.3 Hz to 8.2 ± 0.4 Hz and 8.0 ± 0.4 Hz, respectively (n = 5). These data demonstrated that a ciliated cell bank can be established using cryopreserved ciliated cells for pulmonary drug discovery and toxicological screening.     

On the ultrastructure of the abdominal sense organ of the giant scallop,Placopecten magellanicus (Gmelin)

Summary The abdominal sense organ of the giant scallop, Placopecten magellanicus, is composed of two cell types. The presumed receptor cells bear a single cilium 60 to 70 m in length. The microvilli at the apices of these cells may also be modified into microvillous whorls. From eight to twelve ciliated cells are associated with a single mucous cell. The mucous cells have a much greater diameter than the ciliated cells and contain many electron-dense mucous granules. No function has yet been determined for the ciliated cells, although a number of theories are presented.This research was supported by National Research Council of Canada Operating Grant No. A-6444 to Dr. V.C. Barber. Additional support came from the Department of Biology and the School of Graduate Studies, Memorial University. Contribution no. 250 from the Marine Sciences Research Laboratory, Memorial University of Newfoundland     

Ultrastructure of cell types of the olfactory epithelium in a catfish,Heteropneustesfossilis (Bloch)

Transmission electron microscopical study of olfactory epithelium of a mud-dwelling catfish,Heteropneustes fossilis (Bloch) shows receptor, supporting, goblet and basal cells. The receptor cells are of ciliated and microvillous type. Both ciliated and microvillous receptor cells are provided with olfactory knob. The dendrite of all the receptor cells bears many longitudinally arranged microtubules. Occurrence of the rod cell and its function is quite debatable. Specialized juctional complexes between the receptor and adjacent cells are clearly noted. The supporting cells are both ciliated and nonciliated. The ciliated supporting cells are responsible for water ventilation in the olfactory chamber as well as in the inter-lamellar spaces. This facilitates better perception of odours by the receptor cells. In addition to providing mechanical support to other cells, the nonciliated supporting cells also have a secretory function which is evident from the present study. The different stages of maturity of goblet cells are well documented. The presence of white cells in the olfactory epithelium is a very rare finding.     

Ultrastructure of the Penicillate Podia of the Spatangoid Echinoid Echinocardium cordatum (Echinodermata) with Special Emphasis on the Epidermal Sensory-Secretory Complexes

The spatangoid echinoid Echinocardium cordatum possesses specialized penicillate podia that handle sediment particles during burrowing and feeding. Epidermal complexes, which occur on podial surfaces directly contacting the sediment, each comprise four cells: a non-ciliated secretory cell containing granules rich in mucopolysaccharides (NCS cell), a ciliated secretory cell containing granules of unknown composition (CS cell), and two ciliated non-secretory cells (CNS cells). The cilium of the CS cell is subcuticular whereas that of each CNS cell traverses the cuticle. We propose that these four cells constitute a sensory-secretory complex wherein the ciliated cells are sensory cells and the secretory cells function for adhesion and de-adhesion. More exactly, an NCS cell adhesive and a CS cell de-adhesive would be sequential and would be initiated by two successive stimulations transduced by cilia when the podium touches the sediment. Cilia that first contact the sediment are those protruding through the cuticle from the CNS cells. Their stimulation would result in the secretion of an adhesive material by the NCS cells. Subsequently, the subcuticular cilia of CS cells would be stimulated when the podial digitations closely squeeze the substrate, and this would induce the secretion of a de-adhesive. These two antagonistic secretions would allow the podium to pick up and discharge sediment repetitively during burrowing and feeding.     

Ultrastructure of the Olfactory Epithelium in the Australian Lungfish Neoceratodus forsteri

Four cell types are present in the olfactory epithelium of Neoceratodus forsteri, i.e., olfactory receptor cells, supporting cells, non-sensory ciliated cells, and basal cells. Only microvilli and no cilia were observed on the receptor cells. The neurotubules pass out into these microvilli. Conspicuous arrays of agranular endoplasmic reticulum are present in the nuclear region of the receptor cells. The supporting cells are provided with microvilli. These cells may be secretory. The non-sensory ciliated cells produce secretory granules containing acid mucopolysaccharides. A discontinuous zonula occludens appears to be present.     

Surface ultrastructure of the olfactory rosette of an air-breathing catfish,Heteropneustes fossilis (Bloch)

The surface architecture of the olfactory rosette ofHeteropneustes fossilis (Bloch) has been studied by scanning electron microscopy. The olfactory rosette is an oval structure composed of a number of lamellae arranged pinnately on a median raphe. The raphe is invested with epithelial cells and pits which represent goblet cell openings. On the basis of cellular characteristics and their distribution the lateral surface of each olfactory lamella is identified as sensory, ciliated non-sensory and non-ciliated non-sensory epithelium. The sensory epithelium is provided with receptor and supporting cells. The ciliated non-sensory epithelium is covered with dense cilia obscuring the presence of other cell types. The non-ciliated non-sensory epithelium is with many polygonal areas containing cells.     

Functional morphology of the tentacles and tentilla of Coeloplana bannworthi (Ctenophora, Platyctenida), an ectosymbiont of Diadema setosum (Echinodermata, Echinoida)

 The tentacular apparatus of Coeloplana bannworthi consists of a pair of tentacles which bear, on their ventral side, numerous tentilla. Each tentacle extends from and retracts into a tentacular sheath. Tentacles and tentilla are made up of an axial core covered by an epidermis. The epidermis includes six cell types: covering cells, two types of gland cells (mucous cells and granular gland cells), two types of sensory cells (ciliated cells and hoplocytes), and collocytes, this last cell type being exclusively found in the tentilla. The core is made up of a fibrillar matrix, the mesoglea, which is crossed by nerve processes and two kinds of smooth muscle cells. Regular muscle cells are present in both the tentacles and tentilla while giant muscle cells occur exclusively in the tentilla. The retraction of the tentacular apparatus is an active phenomenon due to the contraction of both types of muscle cells. The extension is a passive phenomenon that occurs when the muscle cells relax. Tentacles and tentilla first extend slightly due to the rebound elasticity of the mesogleal fibers and then drag forces exerted by the water column enable the tentacular apparatus to lengthen totally. Once the tentacles and tentilla are extended, gland cells, sensory cells, and collocytes are exposed to the water column. Any swimming planktonic organism may stimulate the sensory cilia which initiates tentillum movements. Pegs of hoplocytes can then more easily contact the prey which results in a slight elevation of the nearby collocytes, the last being responsible for gluing the prey to the tentilla. Accepted: 1 April 1997     

TGFβ1 induces growth arrest and apoptosis but not ciliated cell differentiation in rat tracheal epithelial cell cultures

Summary We are studying the regulation of ciliated cell differentiation using an in vitro model of tracheal regeneration. Previously, we reported that removal of growth stimulating compounds such as epidermal growth factor (EGF) and cholera toxin reduced DNA synthesis and cell number while increasing ciliated cell differentiation (Clark et al., 1995). This result suggested that the induction of growth arrest may stimulate terminal differentiation of airway epithelial cells into ciliated cells. Transforming growth factor βs (TGFβs) inhibit epithelial cell proliferation and have also been shown to stimulate epithelial cell differentiation. In this study, the effect of TGFβ₁ on growth and ciliated cell differentiation of rat tracheal epithelial (RTE) cells was examined. TGFβ₁ inhibited [³H]thymidine incorporation by RTE cells in a dose-dependent manner. A 40% inhibition was observed after a 24-h incubation with 10 pM TGFβ₁. Continuous treatment with TGFβ₁ (1–50 pM) also reduced cell number during the time when ciliogenesis occurs. This reduction resulted in part from a loss of cells through exfoliation, in addition to the inhibition of proliferation. The exfoliated cells exhibited several morphological features characteristic of apoptosis, including shrunken cells, condensed and fragmented nuclei, and intact organelles. In addition, electrophoretic analysis of genomic DNA analysis isolated from exfoliated cells demonstrated the presence of a nucleosomal ladder. However, in contrast to the removal of EGF, treatment with TGFβ₁ for 7 d did not increase ciliated cell differentiation. TGFβ1 is, therefore, capable of inhibiting proliferation and increasing apoptosis in RTE cells without stimulating ciliated cell differentiation.     

Ultrastructural Observations on the Dwarf Male of Bonellia viridis (Echiura)

The organization of the dwarf male of Bonellia viridis was studied by electron microscopy. The epidermis is formed by two types of epithelial cells: the majority are multiciliated cells; highly vacuolated, non-ciliated cells are less abundant. The body wall musculature consists of an outer circular, a diagonal, and a longitudinal layer. As a unique feature in coelomate spiralians it was found that the perikarya of all muscle cells are located internal to the entire contractile muscular layer. The muscles are solitary myocytes embedded in extracellular matrix. Masses of secretory and indifferent cells occur inside the muscles. Two types of secretory cells were distinguished. Both of them apparently undergo holocrine secretion. A complete lining of thin peritoneal cells delimits the body cavity. Also, the gut and sperm sac have a complete peritoneal lining. The coelomic lining of the gut is a single-layered myoepithelium, that of the sperm sac a pseudo-stratified myoepithelium. The vas deferens was seen to be ciliated. The entrance of the sperm sac is formed by a ciliated funnel that leads into the reservoir by means of a thin, ciliated canal. The existence of repeated transverse nerves and of four longitudinal nerve cords is described for the first time.     

Fate of ciliated epidermal cells during early development of Xenopus laevis using whole-mount immunostaining with an antibody against chondroitin 6-sulfate proteoglycan and anti-tubulin: transdifferentiation or metaplasia of amphibian epidermis

Xenopus embryonic epidermis changes its cellular composition during development: the appearance of ciliated epidermal cells before hatching is a remarkable characteristic. In this study, the functional change of ciliated cells to mucus-secreting cells was examined with immunocytochemistry using anti-tubulin and anti-chondroitin 6-sulfate (C6S). Before hatching, most epidermal cells were labeled with anti-C6S in a granular fashion. Immunoelectron microscopy revealed that the anti-C6S-positive structure was the mucus granule. Ciliated epidermal cells lacked anti-C6S staining, but were strongly labeled with anti-tubulin. After hatching, most ciliated cells in the surface of the embryo disappeared. During their disappearance, some ciliated cells exhibited anti-C6S-positive granular labeling. This strongly suggests that the disappearance of ciliated cells is a functional conversion to mucus-secreting cells instead of shedding through cell death.     

Differentiation of ciliated cells in the terminal bronchioles of neonatal calves

The bronchiolar ciliated cells are exquisitely sensitive to injury caused by infection or irritation of the airways. The mechanism by which bronchiolar ciliated cells are renewed following injury or during the normal course of differentiation is still debated. The present study aimed at recognizing the progenitor cell population for bronchiolar ciliated cells during early neonatal life of calves and to demonstrate the course of events occurs during its differentiation into ciliated cells. Scanning electron microscopy of the terminal bronchiolar epithelium revealed two distinct cell types namely ciliated and non-ciliated cells. Transmission electron microscopy revealed ciliated, non-ciliated (Clara), intermediate and basal cells. At least two categories of intermediate cells could be distinguished: intermediate cells with abundant glycogen and variable numbers of organelles; intermediate cells with little glycogen, large numbers of polyribosomes, and variable numbers of basal bodies. We conclude that: (1) both bronchiolar non-ciliated and basal cells serve as progenitors for the bronchiolar ciliated cells; (2) differentiation of ciliated cell from the non-ciliated one involves a transitional cell in which glycogen is lost, polyribosomes are synthesized before the synthesis of basal bodies and cilia.