C-cell follicles of canine thyroid glands studied by PAS reaction and electron microscopy

Summary Small follicles composed solely of C cells were occasionally observed in large C cell groups of dog thyroid glands. The lumina of C-cell follicles were filled with, or contained peripheral depositions of PAS-positive amorphous material, which was similar in ultrastructural features to thyroglobulin-containing colloid in typical thyroid follicles. This indicates that C cells, in addition to secreting calcitonin, produce a glycoprotein that can be stored in the lumina of the follicles.     

The fine structure of the terminal segment of the bovine seminiferous tubule

Summary The intratesticular excurrent duct system of the bull is composed of rete testis, tubuli recti, and the terminal segment of the seminiferous tubules. Each terminal segment is surrounded by a vascular plexus and may be subdivided into a transitional region, middle portion, and terminal plug. The modified supporting cells of the middle portion and the terminal plug no longer display the typical Sertoli-Sertoli junctions seen in the transitional region and the seminiferous tubule proper. In the region of the terminal plug a distinct central lumen is generally not observed: spermatozoa and tubular fluid must pass through an intricate system of communicating clefts between the apices of the closely attached modified supporting cells. Vacuoles in the supranuclear region of the cells in the middle portion indicate strong transepithelial fluid transport. In analogy to the epithelium of rete testis and tubuli recti, the supporting cells of the terminal segment are capable of phagocytosing spermatozoa. The vascular plexus investing the terminal segment serves a dual purpose: it is a regulatory device for fluid and sperm transport, as well as an area of increased diapedesis for white blood cells.Supported by a grant from the Deutsche Forschungsgemeinschaft     

Effects of high concentrations of secretagogues on the morphology and secretory activity of the pancreas: A role for microfilaments

Summary Cholecystokinin (CCK) and acetylcholine, at concentrations greater than those required for maximal pancreatic enzyme secretion, elicit a submaximal secretory response. The mechanism for this secretagogue-induced unresponsiveness is unknown. Using isolated pancreatic acini of the mouse, we now find that high concentrations of secretagogues also induce a profound alteration in acinar morphology, characterized by the formation of spherical protrusions on the basal surface of the cells. Since both the determination of cell shape and exocytosis may involve calcium and contractile proteins, we used a calcium-free medium and cytochalasin B (CB) to evaluate the importance of a contractile mechanism in the secretory and morphological effects of high concentrations of CCK-octapeptide (CCK₈). Incubation in a calcium-free medium partially blocked CCK-induced unresponsiveness, but brought about dissociation of the acini. CB at a concentration of 3 g/ml caused the disappearance of apical microfilaments and, most strikingly, completely prevented the morphological alteration induced by CCK₈. Furthermore, CB converted the biphasic dose-response curve for CCK₈-induced amylase release to a monophasic shape, such that the amylase release stimulated by a high concentration of CCK₈ (10 nM) was augmented. It is concluded, therefore, that a contractile process involving microfilaments may mediate secretagogue-induced unresponsiveness in pancreatic acinar cells.     

Ascidian larval tunic: Extraembryonic structures influence morphogenesis

Summary The larval tunic of Corella inflata is composed of two cuticular layers, extracellular filaments and ground substance. It lies outside the epidermis and most of it is known to be produced by the epidermis. The dorsal, ventral and caudal fins are specialized parts of the tunic that are essential for larval locomotion. The following hypothesis was tested: Morphogenesis of the larval fins is dependent upon the presence of extraembryonic structures (test cells, chorion or follicle cells) before completion of the late tail bud stage of development. We tested this by dechorionating embryos of Corella inflata and Ascidia paratropa. The operation removes all extraembryonic structures. It was performed mainly on neurula, early tail-bud and late tail-bud stages.Fin formation is inhibited when neurulae are dechorionated but not when late tail-bud or older embryonic stages are dechorionated. Dechorionated neurulae produce all of the major components of the tunic (cuticular layers, filaments and ground substance) but they are unable to form functional fins. At the time of dechorionation, in all experiments, the embryos had no fins.Removal of the follicle cells does not inhibit fin formation. The test cells are known to secrete granular ornaments that attach to the surface of the tunic. The fibrous, acellular chorion may serve to contain the test cells and their products or products of the embryo that are not firmly attached. The test cells may induce or control the morphogenesis of the larval fins in ascidians before the late tail-bud stage of development. We suggest ways of testing this hypothesis and an alternative hypothesis.     

Structural reactions to polarized light of microvilli in photoreceptor cells of the moth Spodoptera

Summary Intact armyworm moths (Spodoptera exempta, Farn. Noctuidae) were illuminated by polarized monochromatic light to induce structural changes in the rhabdomeres of the compound eyes. The degree of distortion of their microvilli depends on the light energy absorbed per time unit. Under polarized light, the number of quanta absorbed varies with the position of the plane of polarization relative to the axis of the microvilli (intrinsic dichroism). Therefore, in Spodoptera, different degrees of deformations could be demonstrated in differently oriented rhabdomeres of both types of ommatidia. Moreover, in rhabdoms of the lobed type with fan-like arranged microvilli, different reactions were regularly seen in differently oriented microvilli of one rhabdomere. This indicates that microvilli may react to light individually.Supported by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 114 (Bionach)     

The effects of carbon monoxide and hypoxic hypoxia on amine-containing cells in the tracheal epithelium of young rabbits

Summary Endogenously fluorescent, singly occurring, amine-containing cells in tracheal epithelium were examined in 3-, 10-, and 28-day-old rabbits. These cells are pyramidal in shape with the apex projected toward the tracheal lumen. The cytoplasm exhibits a yellow fluorescence which is predominantly supranuclear. Occasional, infranuclear, fluorescent cytoplasmic processes project from the cells. The numbers of fluorescent cells per unit length of trachea increase with age. Acute exposure of 10-day-old rabbits to 13% O₂ decreases the number of detectable fluorescent cells in the trachea compared to controls exposed to room air. Similarly, exposure to 750 ppm carbon monoxide decreases the number of fluorescent epithelial cells appearing in tracheas of 10- and 28-day-old rabbits. These results suggest that the amine-containing epithelial cells of the trachea respond to tissue hypoxia and that decreased airway pO₂ is not necessary to elicit a response.Supported by a grant from The Council for Tobacco Research, U.S.A., Inc. We are grateful to Margaret Hogan and Scott Pine for technical assistance     

Morphological studies on neuroglia

Summary Electron-microscopic survey of selectively stained microglial cells in the cerebral cortex of the rat reveals that the processes of this cell type often encircle axo-dendritic synapses. Enzyme-histochemical methods for thiamine pyrophosphatase (TPPase) or nucleoside diphosphatase (NDPase) were used for the selective marking of the microglial cells; TPPase and NDPase activities were observed in the plasma membrane of microglial cells. The synapses encircled by microglial processes displayed presynaptic structures containing round clear vesicles (50 nm in diameter) and a prominent thickening of the postsynaptic membrane. In vitro, the above-mentioned enzymatic activities were completely suppressed by neuroactive agents such as catecholamines and phenothiazine derivatives. Examination using enzyme-histochemical techniques suggests that a single enzyme may be responsible for both above-mentioned enzymatic reactions. The functional significance of microglial cells in the normal central nervous tissue is discussed.This work was supported by grant No. 437002 from the Ministry of Education, Science and Culture, Japan     

A labile,Ca2+-dependent cytoskeleton in rhabdomeral microvilli of blowflies

Summary Rhabdomeral microvilli of photoreceptors of the blowfly Lucilia are shown to contain a cytoskeleton. An axial filament ( 6–11 nm) in each microvillus is inserted into a terminal cap distally, and into a plug filling the narrow neck of the microvillus proximally. In some states, the axial filament projects beyond the neck; within the microvillus it is surrounded by amorphous material. Together, they form an axial complex, which supports side-arms linking it to the plasma membrane. Conventional fixation for examination with the electron microscope destroys the cytoskeleton. To preserve it, retinae are pre-treated with a Ringer's solution buffered with 20 mM imidazole and containing, minimally, the following components: (i) a protease inhibitor, usually phenylmethylsulphonyl-fluoride (PMSF); (ii) either the Ca²⁺-chelator EGTA, or the calmodulin-blocking agent trifluoperazine (TFP); and (iii) a source of divalent cations to preserve the side-arms. When EGTA is used, Mg²⁺, Sr²⁺, Ba²⁺, Mn²⁺ and Co²⁺ are effective, Ba²⁺ giving the most satisfactory contrast, and Mg²⁺ and Co²⁺ the best preservation. It is inferred that the cytoskeletal complex includes at least one Ca²⁺-activated protease, and possibly calmodulin. Microvilli are bonded together by intermicrovillar bridges with a periodicity of 11–17nm. The cytoskeleton is destroyed by pretreatment with 1 mM dithiothreitol (DTT), possibly by the activation of a thiol protease. It does not survive osmication unless treated with low molecular weight tannic acid (LMWT). The evidence does not discriminate between actin and intermediate filaments as the basis of the cytoskeleton. Attention is drawn to similarities and differences between the rhabdomeral cytoskeleton and that of vertebrate intestinal brush-borders. The extreme lability of the rhabdomeral cytoskeleton to conventional methods of fixation is attributed in part to the Ca²⁺ fluxes experienced by invertebrate photoreceptors, and in part to the effects of osmication.The authors thank Dr. Lindsay Barton-Brown and Tom Van Gerwen for supplying flies from CSIRO cultures: Smith Kline and French Laboratories Ltd., French's Forest, N.S.W. for a generous gift of trifluoperazine, and Mallinckrodt, Inc., St. Louis, Missouri, USA, for a gift of low molecular weight tannic acid. Many colleagues, especially Richard Payne, Steve Shaw and Gert De Couet helped by discussing the results. George Weston and the staff of the Electron Microscope Unit provided support and advice. Sandy Smith prepared Table 1     

The distribution and ultrastructure of sensory elements in the baroreceptor region of the truncus arteriosus of the lizard Trachydosaums rugosus

Summary The proximal truncus arteriosus of the lizard Trachydosaurus rugosus was studied with light-, fluorescence and electron-microscopical techniques. Three vessels comprised the truncus: the pulmonary, left aortic, and caroticoaortic arteries. Right and left truncal nerves, each derived from the ipsilateral vagus nerve, innervated the truncus, particularly its proximal 3 mm.Ultrastructurally, the nerves had a variety of appearances: some were clearly adrenergic, c-type or p-type. A number of profiles contained large numbers of mitochondria and were classified as sensory. Some profiles defied exact classification, having characteristics common to two different types of profile.Within the outer medial layers, profiles up to 7 m in diameter were found. These contained large numbers of mitochondria, myelin bodies and structures intermediate between the two. In addition, the profiles contained large amounts of glycogen and small numbers of vesicles. These nerve fibres were classified as baroreceptors, since they closely resemble carotid sinus and aortic arch baroreceptors in mammals.Large numbers of chromaffin cells were found, particularly in the common wall of the pulmonary and left aortic arteries. Many of these cells emitted a long tapering process, which sometimes entered a nearby nerve bundle. Sensory, p-type and c-type profiles, but not adrenergic profiles, made extensive close contacts with chromaffin cells.     

Exocytotic release of secretory granules from endocrine cells in the midgut of insects

Summary Exocytotic release of the secretory granules of the endocrine cells in the midgut of a cockroach, Periplaneta americana, was studied by means of fixation with tannic acid in combination with glutaraldehyde and osmium tetroxide. A sequence of images indicative of exocytosis suggests the following steps in this process: (1) A delicate connection appears between the granule-limiting membrane and the plasma membrane. (2) The plasma membrane approaches the granule, forming a concave indentation. (3) The granule-limiting membrane fuses with the plasma membrane and opens to give rise to an omega profile. (4) The granule content is voided into extracellular space. Exocytosis occurs not only at the base of the cell but occasionally at its side facing adjacent cells. (5) The exocytotic invagination after release becomes smaller and narrower; sometimes a coated pit with bristles appears. Multiple exocytosis, and exocytosis in the endocrine cells of the nidus, i.e., the regenerative cell mass, are also described.