Calcium channels in crayfish muscle fibre fragments studied by means of the Vaseline gap technique

Calcium currents in crayfish muscle fibres were studied by means of the vaseline gap voltage clamp technique. Overlapping potassium currents were fully suppressed using fibre fragments equilibrated in K+-free intracellular solution. The design of the recording chamber tailored to crayfish muscle fibres is described in detail. Ca currents recorded has a two-component time course. The transient (ICa, T) component (peaking in about 10 ms) attained, on average, maximal overall density of 26.4 microA/cm2 at depolarization to -4.6 mV from a holding potential of -80 mV. The steady (ICa, S) component attained 16.7 microA/cm2 (evaluated at the end of a 70 ms pulse) at +13.8 mV. The average overall surface area of the clamped membrane surface (including invaginated parts) was about 0.07 cm2. The ICa, S component could be separated from ICa, T using short inactivating prepulses. Voltage and time dependence of the transient component inactivation, as well as its recovery from inactivation, were in agreement with a Ca-dependent mechanism. Independent behaviour of the two Ca current components and differences in their properties support the hypothesis concerning the existence of two populations of Ca channels in the surface membrane of the crayfish muscle.     

Histochemical and immunohistochemical properties of skeletal muscle fibres fromRana andXenopus

Summary Modern histochemical and immunohistochemical techniques have been used to type skeletal muscle fibres from threeRana species andXenopus laevis.Differing myosin properties and metabolic capacities (representing various contractile properties) define a minimum of four fibre types inRana and five inXenopus. TheRana andXenopus types are sufficiently similar so that a single nomencclature can be applied to them. This nomenclature uses an initial letter indicating the probable contractile performance (F=fast-twitch, S=slow-twitch and T=tonic), and a number indicating rank order of presumed shortening velocity.The largest, fastest fibres-F1-have low oxidative and, at best, moderate glycolytic capacities. Commonly adjacent to them are smaller, F2 fibres with variable but at least moderate metabolic capacities. F3 fibres are rarer and have on average the highest oxidative capacity, and at least moderate glycolytic capacity. They usually occur in the reddest parts of the muscle and, inRana, only in the vicinity of tonic fibres.Metabolically weak, classical amphibian tonic fibres (T5) occur in bothXenopus andRana, but onlyXenopus also has an S4 fibre type. This has moderate metabolic capacity and myosin properties suggesting it is probably capable of slow shortening as well as tonic hold. Immunohistochemically, S4 fibres are most similar to avian slow-twitch fibres.     

U0ltrastructural description of the musculature,the intraepidermal nervous system,and their interrelation in Pseudochordodes bedriagae (Nematomorpha)

The ultrastructure of the body wall muscles and the intraepidermal nervous system of the Gordiida Pseudochordodes bedriagae are described. The body wall muscles are of the circomyarian type, since the sarcomeres constitute a system of continuous peripheral helices. The organisation of the sarcomeres follows a pattern that resembles that of the striated muscles. The muscle fibres are separated into areas by invaginations formed exclusively by the plasma membrane (T component), while the sarcoplasmic reticulum lies at the sides of the Z granules forming subsarcolemmal cisternae, and in the zone near the nucleus, like flattened vesicles, contributing with the T component to the formation of dyads and triads. The muscle fibres present two types of adaptations for their innervation: (1) cytoplasmic projections towards the epidermis, and (2) invaginations of the plasmalemma. The motor peripheral nervous system is conformed by the nerve fibres that run within the epidermis and their projections towards the basal membrane in order to contact the adaptations of the muscle fibres in a basi-epidermal synapsis. The presence of an intraepithelial peripheral nervous system in Gordiida confirms a structural pattern common to other taxa of Nemathelminthes.     

Interrelations between auditory and vestibular receptors in the frog's labyrinth

Summary The influence of the efferent vestibular system being eliminated, the spontaneous activity of afferent fibres of the ampullary nerves of the horizontal and vertical anterior semicircular canals was recorded in the frog. By functionally eliminating either both papillae or all the vestibular receptors except for the papillae, and then using statistical methods, as well as by stimulating the papillae by sounds or the papillary nerve fibres by electrical stimulus, it has been shown that the auditory papillae have a facilitatory influence on the spontaneous afferent activity from the horizontal and vertical anterior canals. This influence is most likely mediated by receptor-receptor fibres arising from the auditory organs and innervating the semicircular canals.Abbreviations HC horizontal canal - VAC vertical anterior canal This research was supported by a grant from D.G.R.S.T. (Aide à la Recherche n 77.7.1127)     

A study of human root cementum surfaces as prepared for and examined in the scanning electron microscope

Summary The morphology of the cementum surface of human teeth was studied directly by scanning electron microscopy of 50 freeze-dried and 170 anorganic specimens. Extrinsic (Sharpey) fibres were found to occupy almost 100% of the surface in acellular cement, about 40% of the surface in cement with intrinsic fibres but no cells, and 15–40% in cellular cement, and averaged 6 m diameter. Some areas with no extrinsic fibres resembled adult lamellar bone. The mineral front of the intrinsic fibres generally was similar to forming and resting lamellar bone; that of the extrinsic fibres varied according to the activity of the surface, but was normal to the axis of the fibre, even where these entered the surface at a sharp angle. Resorption bays were either small and isolated, often seen in newly erupted teeth; or, more rarely, large and quite deep where excessive force must have occurred.This work has been supported by grants from the Medical Research Council and the Science Research Council.We would like to thank Mr. P. S. Reynolds and Mr. T. Brett for invaluable technical assistance and Mrs. J. Mills for secretarial assistance.     

A morphometric analysis of regional differences in myotomal muscle ultrastructure in the juvenile eel (Anguilla anguilla L.)

Summary Subpopulations of fast and slow fibres within the trunk musculature of elvers were examined using morphometric analysis of electron micrographs. Fibre regions were characterised by their histochemical staining characteristics, and individual fibres located using a coordinate mapping system utilising morphological features as reference points. Percentages of fibre volume occupied by mitochondria, myofibrils, sarcoplasmic reticulum (S.R.), and T-system were determined in each of the fibre groups, along a transect from the skin to the vertebral column (fibres 1–14, respectively).The fine structure of slow (red) fibres (1–2 fibres deep) is relatively homogeneous throughout its range, giving mean values for mitochondria, 21.4%; myofibrils, 61.0%; S.R., 2.10%; T-system, 0.31%. The fibres are relatively small (204 m²) and the mitochondrial cristae poorly developed.In contrast, there is a marked heterogeneity in the ultrastructure of fast (white) fibres, dependent on both position and size. The moderately small (333 m²) superficial fast fibres (3–4 fibres deep) have a significantly higher mitochondrial content (7.6%) than the larger deep fibres (1.2%) (6–12 fibres deep, 775 m²). The mean fractional volumes occupied by myofibrils, S.R., and T-system in the deep fibres are: 80.4%, 5.95%, and 0.38%, respectively. Fibres < 100 m² constitute up to 5% of the fast muscle and have a significantly higher mitochondrial volume (4.3%), more glycogen granules, and a slightly lower volume of S.R. (5.57%) than larger fibres.It is suggested that metabolic subpopulations of fast fibres correspond to different stages of fibre growth. The relatively poorly developed S.R. of eel fast muscle is thought to be correlated with the low frequency, high amplitude nature of the propagated waveform found in anguilliform locomotion.     

Parvalbumin in skeletal muscles of teleost (Tinca tinca L. and Misgurnus fossilis L.). Histochemical and immunohistochemical study.

Musculatures of two fish species belonging to the suborder Cyprinoidei were examined histochemically and immunohistochemically for demonstration of mATP-ase activity and parvalbumin content, respectively. The tench lateral musculature shows a typical "acid reversal" picture of mATP-ase activity--red fibres are alkali labile, acid stable and white and white fibres are alkali stable, acid labile. In the pond loach a superficial layer of musculature is composed of alkali--and acid stable red fibres. This alkaline stability of the red fibres caused the classical "acid reversal" picture impossible to obtain. For immunohistochemical reaction a monoclonal antibody specific for parvalbumin II component of the Carp muscles (Pv Carp II) was used. Immunohistochemical results gave evidence that lateral musculatures of the two species examined possessed parvalbumin II component entirely in the fast-contracting fibres--intermediate and white. This results correlate with mATP-ase activity in the tench musculature and did not correlate in the pond loach with respect to mATP-ase alkali stable red muscle fibres in the fish. We conclude that fish muscles mATP-ase activity not always correlates with parvalbumin content in the opposite to mammalian muscles where such correlation seems to be obvious.     

Ultrastructural Identification of gastrin-like immunoreactive nerve fibres in the brain of Xenopus laevis by means of colloidal gold or ferritin immunocytochemical methods

Summary By use of an anti-gastrin serum and colloidal gold- or ferritin-labelled sheep anti-rabbit -globulins, nerve fibres and nerve terminals containing a gastrin-like substance were characterized at the ultrastructural level in the median eminence of Xenopus laevis. These immunoreactive fibres contain neurosecretory granules displaying medium to high electron density and a mean diameter of 75 nm. Labelling intensity varies from granule to granule. This is the first demonstration at the ultrastructural level of the precise location of a gastrin-like hormone in the median eminence of a vertebrate.Supported by the D.G.R.S.T., Contrat no 80.7.0242     

Blockade of voltage-dependent 42K efflux from rat brain synaptosome by minaprine and tetrahydroaminoacridine

The effect of minaprine (3-(2-morpholinoethylamino)-4-methyl-6-phenylpyridazine) on the K+ channels was studied by means of 42K efflux from rat brain synaptosomes, comparing the effects of 4-aminopyridine and 9-amino-1,2,3,4-tetrahydroacridine (THA). 42K efflux from rat brain synaptosomes was classified into five components: a resting component (R), a rapidly inactivating, voltage-dependent component (T), a slowly inactivating, voltage-dependent component (S) and a voltage-dependent, Ca(2+)-dependent component which is divided into a fast phase (CT) and a slower phase (CS). 4-Aminopyridine selectively inhibited 42K efflux of component T. THA blocked both S and T components. The inhibitory effect of THA on the 42K efflux of component S was quite pronounced compared with that of component T. Minaprine inhibited the 42K efflux of components S and T but the inhibitory effect on component S was observed with a lower dose of minaprine than that needed for the effect on component T. Minaprine had no effect on the Ca(2+)-dependent component while THA blocked component CT. 42K efflux of the resting component was not changed by minaprine, THA or 4-aminopyridine. These results suggest that minaprine blocks Ca2+ independent voltage-dependent K+ channel is involved in the pharmacological actions of minaprine.     

Hints of a functional connection between the neuropeptidergic innervation of arteriovenous anastomoses and the appearance of epithelioid cells in the rabbit ear

Peripheral blood flow can be regulated by specialized vessel segments, the arteriovenous anastomoses. Their wall consists of a relatively thick layer of smooth muscle cells and so-called epithelioid cells. The epithelioid cell is a specialized myogenic cell phenotype expressing nitric oxide synthase. We studied the innervation of the different segments of arteriovenous anastomoses in the rabbit ear using antisera against neuropeptide Y, tyrosine hydroxylase, calcitonin gene-related peptide and substance P, as well as neuron-specific enolase, calbindin D and neurotubulin. The participation was especially examined of neuropeptidergic innervation and a possible morphological connection to the occurrence of epithelioid cells and a paracrine function. The NADPH diaphorase reaction and -smooth muscle actin immunoelectron microscopy served to distinguish epithelioid cells from smooth muscle cells. Using conventional fluorescence microscopy and confocal laser scanning microscopy, we found the most dense innervation pattern of pan-neuronal markers (neurotubulin, neuron-specific enolase), tyrosine hydroxylase-immunor eactive nerve fibres and neuro-peptidergic nerve fibres (neuropeptide Y, calcitonin gene-related peptide, substance P) around the intermediate segment in arteriovenous anastomoses, whereas the venous segment was barely marked. Single nerve fibres penetrated into the medial layer and reached the epithelioid cells. Using immunoelectron microscopy, we found intercellular contacts between epithelioid cells, but not the gap junction protein connexin 43. Here, we report for the first time a correlation of the innervation pattern with epithelioid cell type in arteriovenous anastomoses. Our findings suggest that epithelioid cells of the arteriovenous anastomoses are controlled by a dense network of neuropeptidergic nerve fibres in functional connection to their paracrine role as a nitric oxide producer. © 1998 Chapman & Hall     

Quantitative analysis of histochemical and immunohistochemical reactions in skeletal muscle fibres ofRana andXenopus

Summary Intensities of histochemical and immunohistochemical reactions in muscle fibres ofRana andXenopus have been estimated microphotometrically, and the data from serial sections statistically analysed. Quantitative validities of reactions and measurements have also been assessed against independent published evidence. It is concluded that NADH—tetrazolium reductase overestimates tonic-fibre aerobic capacities and the actomyosin ATPase reaction overestimates their contraction speeds. However, it appears that succinate dehydrogenase, despite being a near-equilibrium enzyme of particulate distribution, indicates the relative aerobic capacities of fibres with acceptable accuracy when lightly reacted. Capacities for aerobic and anaerobic metabolism are positively correlated over all types of fibre (r typically 0.6 for 200 fibres), perhaps as an adaptation to environmental hypoxia.Multivariate clusters (indicating fibre types) have been sought, using Ward's method with optimizing procedures (iterative relocation and multivariate-normal modelling). Cluster analysis confirms the subjective identifications of two slow/tonic types inXenopus (labelled T5 and S4) but of only one (T5) inRana. Division of the fast family twitch fibres into three types (F1–F3) in both genera, with metabolic capacity related inversely to apparent shortening velocity, is highly supportable by objective criteria. However, statistically significant subdivisions also present themselves.Rana F2 andXenopus F1 clusters can be bisected according to metabolic capacity; andXenopus F2 fibres fall into three subtypes reflecting different isomyosin contents. In the different types of twitch fibre, ratios of myofibrillar ATP consumption rate to aerobic capacity increase up to 30-fold with contraction speed, but anaerobic/aerobic ratios do so only 5-fold.     

A C-terminal region of Yersinia pestis YscD binds the outer membrane secretin YscC

YscD is an essential component of the plasmid pCD1-encoded type III secretion system (T3SS) of Yersinia pestis. YscD has a single transmembrane (TM) domain that connects a small N-terminal cytoplasmic region (residues 1 to 121) to a larger periplasmic region (residues 143 to 419). Deletion analyses established that both the N-terminal cytoplasmic region and the C-terminal periplasmic region are required for YscD function. Smaller targeted deletions demonstrated that a predicted cytoplasmic forkhead-associated (FHA) domain is also required to assemble a functional T3SS; in contrast, a predicted periplasmic phospholipid binding (BON) domain and a putative periplasmic "ring-building motif" domain of YscD could be deleted with no significant effect on the T3S process. Although deletion of the putative "ring-building motif" domain did not disrupt T3S activity per se, the calcium-dependent regulation of the T3S apparatus was affected. The extreme C-terminal region of YscD (residues 354 to 419) was essential for secretion activity and had a strong dominant-negative effect on the T3S process when exported to the periplasm of the wild-type parent strain. Coimmunoprecipitation studies demonstrated that this region of YscD mediates the interaction of YscD with the outer membrane YscC secretin complex. Finally, replacement of the YscD TM domain with a TM domain of dissimilar sequence had no effect on the T3S process, indicating that the TM domain has no sequence-specific function in the assembly or function of the T3SS.     

Morphogenesis of rat experimental pulmonary emphysema induced by intratracheally administered papain: changes in elastic fibres

The ultrastructural changes of elastic fibres in emphysematous lungs have been studied in men, but few works exist on this topic in experimental emphysematous animals. In this paper, the morphogenesis of emphysema and alterations of the elastic fibres produced by the instillation of papain are described by light and electron microscopy. Wistar rats were instilled through the trachea with papain at a rate of 3 mg/100 g animal weight. The animals were sacrificed 12 h, 3 days, 10 days and 60 days after enzyme instillation. The "Mean Linear Intercept" (MLI), the "Number of fenestrations/respiratory units" (NF) the "Number of macrophages per mm of alveolar wall" (NM) and the "Number of respiratory unit/mm2" (RU), both in the control and experimental groups were studied. Two months after treatment, the experimental group showed a strong increase in the MLI (p<0.001) and NF (p<0.001), and a diminished number of RU (p<0.05) compared with the control group. Partial correlation analysis showed a positive correlation only between MLI and NF. Twelve hours after papain instillation an inflammatory response was observed, the elastic fibres were ruptured, while the microfibrilar component remained. New formations of eulanin elastic fibres were observed three days post papain instillation. After ten days the interalveolar oedema had disappeared and the elastic fibres were of normal morphology although irregular groups of strips of elastic fibres were evident. A mixed pattern of panlobular, centrilobular and normal lung zones were observed. Two months after papain instillation abundant accumulations of elastic fibres of irregular outline were observed associated to collagen fibres. In conclusion, the morphometric parameters studied showed a significant progression of the emphysema. The strong correlation between NF and MLI suggested that papain-induced emphysema is principally caused by breaches of the alveolar walls. The results seem to point to a very abnormal remodelling process associated with elastic fibre regeneration, although there were no signs of destruction of these new fibres formed in emphysematous rat lung induced by papain.     

Polymorphisms in the Human Tropoelastin Gene Modify In Vitro Self-Assembly and Mechanical Properties of Elastin-Like Polypeptides

Elastin is a major structural component of elastic fibres that provide properties of stretch and recoil to tissues such as arteries, lung and skin. Remarkably, after initial deposition of elastin there is normally no subsequent turnover of this protein over the course of a lifetime. Consequently, elastic fibres must be extremely durable, able to withstand, for example in the human thoracic aorta, billions of cycles of stretch and recoil without mechanical failure. Major defects in the elastin gene (ELN) are associated with a number of disorders including Supravalvular aortic stenosis (SVAS), Williams-Beuren syndrome (WBS) and autosomal dominant cutis laxa (ADCL). Given the low turnover of elastin and the requirement for the long term durability of elastic fibres, we examined the possibility for more subtle polymorphisms in the human elastin gene to impact the assembly and long-term durability of the elastic matrix. Surveys of genetic variation resources identified 118 mutations in human ELN, 17 being non-synonymous. Introduction of two of these variants, G422S and K463R, in elastin-like polypeptides as well as full-length tropoelastin, resulted in changes in both their assembly and mechanical properties. Most notably G422S, which occurs in up to 40% of European populations, was found to enhance some elastomeric properties. These studies reveal that even apparently minor polymorphisms in human ELN can impact the assembly and mechanical properties of the elastic matrix, effects that over the course of a lifetime could result in altered susceptibility to cardiovascular disease.     

Fine structure and metabolic adaptation of red and white muscles in tuna

Synopsis An electron microscopic study of the red and white muscle fibres in the trunk musculature of the Kawakawa tuna (Euthynnus affinis) was carried out with a view to correlating their structure with metabolic adaptation. The red fibres which are considerably smaller in diameter (34.58 m ± 6.16 S.D.) are characterized by their high content of myoglobin, mitochondria, lipid droplets and glycogen granules. The white fibres which are relatively larger in diameter (66.03 m ± 11.59 S.D.) are characterized by their lack of myoglobin, low mitochondria) density, high content of glycogen granules and the conspicuous absence of lipid droplets. The characteristics in fine structure of the two fibre types are discussed in the light of their metabolic adaptation, the red fibres as being adapted for long term cruising movement utilizing lipid as the main source of energy and the white fibres for short bursts of activity metabolizing glycogen as the chief fuel.The tuna, with the acquisition of the counter-current heat exchange system which provides for the retention of the heat generated from high substrate oxidation in the red muscle and an efficient respiratory system, it is postulated, is well adapted for high speed sustained swimming.     

A matrix of connections of mossy fibers with glandular cells in the cerebellum

An analytical formulation of known structural data and succeeding calculation are used to show that non-zero elements of the mossy-granule connection matrix are located near the main matrix diagonal. The width of this "non-zero" matrix strip is 10 fibres and 1000 cells in mossy and granula coordinates respectively. Depending on this result estimates of mossy-granula mapping informational properties are presented.     

Fine structure of muscles in the tickHyalomma (Hyalomma) dromedarii (Ixodoidea: Ixodidae)

Skeletal and visceral muscles are distinguished in the unfed nymphHyalomma (Hyalomma) dromedarii according to position, structure and function. The skeletal muscles include the capitulum, dorsoventral and leg oblique muscles. Their muscle fibres have the striated pattern of successive sarcomeres whose thick myosin filaments are surrounded by orbitals of up to 12 thin actin filaments. The cell membrane invaginates into tubular system (T) extending deeply into the sarcoplasm and closely associated to cisternae of sarcoplasmic reticulum (SR). The T and SR forming two-membered dyads are considered to be the main route of calcium ions whose movements are synchronized with the motor impulse to control contraction and relaxation in most muscles. Two types of skeletal muscle fibres are recognized, and are suggested as representing different physiological phases.In the visceral-muscle fibres investing tick internal organs, the actin and myosin filaments are slightly interrupted, and the T and SR are well demonstrated. Both skeletal and visceral muscles are invaginated by tracheoles and innervated by nerve-axons containing synaptic vesicles.     

The localization of oxytocin,vasopressin, somatostatin and luteinizing hormone releasing hormone in the rat neurohypophysis

Summary The hypothalamic hormones arginine-vasopressin (AVP), oxytocin (OXT), somatostatin (SOM), and luteinizing hormone-releasing hormone (LHRH) were localized in the rat neurohypophysis by the use of semithin serial sections and the unlabeled antibody enzyme method. Clusters of AVP fibres are present within the central region of the neural lobe, clusters of OXT fibres mainly in the peripheral part. The AVP fibres enter bilaterally into the neural lobe.The results call into question previous reports on the presence of AVP on receptors in the pars intermedia cells, since incubation with anti-AVP resulted in similar staining in the pars intermedia of the Wistar and homozygous Brattleboro rat, a mutant strain deficient in AVP. The same intermediate lobe cells are stained after incubation of serial sections with anti-AVP and anti--melanocyte-stimulating hormone (-MSH). This staining of anti-AVP could be removed by solid phase absorption to -MSH and is thus most probably due to cross reaction with -MSH. SOM fibres appear to be present in the peripheral parts of the proximal neurohypophysial stalk and mainly lateral in its more distal parts. In the neural lobe they rapidly decrease in number, although some fibres continue into the distal part of the neural lobe, running bilaterally and situated adjacent to the pars intermedia. The SOM staining within magnocellular elements, which has been reported in the literature, can most probably be explained by cross reaction of anti-SOM with neurophysins. LHRH fibres are very scarce in the neurohypophysial stalk and absent in the neural lobe.Supported by the Foundation for Medical Research FUNGOThe authors wish to thank Drs. J. De Mey (Beerse, Belgium), A. Arimura (New Orleans, U.S.A.), M.P. Dubois (Nouzilly, France), B.L. Baker (Ann Arbor, U.S.A.) and A.G.E. Pearse (London, U.K.) for their gifts of anti-somatostatin serum, Dr. B. Kerdelhué (Gif-sur-Yvette, France) for anti-LHRH serum, and Dr. F. Vandesande (Ghent, Belgium) for anti-neurophysin I and II serum and bovine neurophysin I and II. Dr. J.G. Streefkerk (Free University, Amsterdam) is acknowledged for critical comments and Mr. A.T. Potjer and Miss J. van der Velden for their skilled assistance     

A systematic genetic analysis in Neisseria meningitidis defines the Pil proteins required for assembly, functionality, stabilization and export of type IV pili

Although type IV pili (Tfp) are among the commonest virulence factors in bacteria, their biogenesis by complex machineries of 12-15 proteins, and thereby their function remains poorly understood. Interestingly, some of these proteins were reported to merely antagonize the retraction of the fibres powered by PilT, because piliation could be restored in their absence by a mutation in the pilT gene. The recent identification of the 15 Pil proteins dedicated to Tfp biogenesis in Neisseria meningitidis offered us the unprecedented possibility to define their exact contribution in this process. We therefore systematically introduced a pilT mutation into the corresponding non-piliated mutants and characterized them for the rescue of Tfp and Tfp-mediated virulence phenotypes. We found that in addition to the pilin, the main constituent of Tfp, only six Pil proteins were required for the actual assembly of the fibres, because apparently normal fibres were restored in the remaining mutants. Restored fibres were surface-exposed, except in the pilQ/T mutant in which they were trapped in the periplasm, suggesting that the PilQ secretin was the sole Pil component necessary for their emergence on the surface. Importantly, although in most mutants the restored Tfp were not functional, the pilG/T and pilH/T mutants could form bacterial aggregates and adhere to human cells efficiently, suggesting that Tfp stabilization and functional maturation are two discrete steps. These findings have numerous implications for understanding Tfp biogenesis/function and provide a useful groundwork for the characterization of the precise function of each Pil protein in this process.     

On degeneration of peptidergic neurosecretory fibres in the albino rat

Three types of degenerating peptidergic neurosecretory fibres have been found in the posterior pituitary of chronically dehydrated albino rats. "Dark" neurosecretory fibres and their swellings contain neurosecretory granules, neurotubules, shrunken mitochondria and diffusely distributed fine dense material. Some swellings are filled with synaptic vesicles and/or conglomerations of dense membranes. The transitional forms exist between these fibres and extracellular accumulations of electron dense material. Synaptic vesicles, single neurosecretory granules, lipid-like droplets and lamellar bodies occur in the latter. Some neurosecretory fibres and swellings have numerous polymorphous inclusions arising due to degradation of secretory inclusions and organelles, mitochondria and neurotubules in particular. "Dark" neurosecretory elements and those with numerous polymorphous inclusions are enveloped by pituicyte cytoplasm. Sometimes the plasma membranes both of the pituicytes and neurosecretory fibres are destroyed or transformed into a multi-membrane complex. It is assumed that pituicytes may phagocytize degenerating neurosecretory elements. N urosecretory fibres with a locally dissolved neuroplasm and/or large lucent vacuoles seem to be due to axonal degeneration by the "light" type. These neurosecretory elements, the largest of them in particular, may transform into large cavities bordered by a membrane and containing flake-like material and single-membrane vacuoles. Degeneration of neurosecretory elements seems to occur mainly due to hyperfunction of the hypothalamo-hypophysial neurosecretory system.