Structural features associated with movement and ‘catch’ of sea-urchin spines

Spine catch ligaments of a sea urchin Arbacia punctulata were extended under constant load. Ligaments from an undisturbed animal may show any extension rate from zero (catch state) to rapid extension to failure. Replacing the preparation bath with Ca²⁺- and Mg²⁺-free sea water reversibly abolishes the catch state. The fine structure of the outer muscle layer and inner ligament cone associated with the spine base is described. The unstriated paramyosin muscles bear thin flanges and form compact interlocking rows. Subsurface cisternae are associated with the plasma membrane. The muscles are innervated by glia-free axons ending in bulbous terminals containing lucent synaptic vesicles. The ligament comprises cylindrical bundles of collagen fibrils: one or more minute muscle fibers (paramyosin) lie parallel with and closely adjoining each bundle. The mean diameter of these muscles is 0.3 μg and they occupy 2–3 % of the ligament's cross-sectional area. Axons containing electronopaque secretory droplets accompany the muscles between the collagen bundles: the cell bodies of these neurones generally lie on the outer surface of the ligament. When an urchin points a spine, the ligament on the side of the contracting spine muscle shortens but does not buckle. A function of the intraligamental muscles is to effect this non-buckled shortening. The catch mechanism (which resides entirely within the ligament) may be due either to the intraligamental muscles and/or to a locked polymer mechanism in which matrix molecules between collagen fibrils are reversibly crosslinked by divalent cations.     

The cytology of submandibular glands of the opossum

Tooth attachment in the majority of the bony fish is by ankylosis or fibrous membrane. However, in one group of the osteichthys, the trigger-fish or balistids, tooth attachment is by means of a periodontium composed of a shallow alveolar socket, a periodontal ligament and acellular cementum. Histologically, the balistid periodontal ligament is composed of a dense fibro-cellular connective tissue possessing an abundance of typical fibrocytes, collagen fiber bundles, and oxytalan fibers. The collagen fiber bundles which resemble the principal fiber bundles of the mammalian periodontal ligament are inserted into the bone of the shallow alveolar sockets and are anchored to the teeth by means of a layer of amorphous acellular cementum that covers the radicular dentin. No cementoblasts were found in functional teeth, and epithelial rests are lacking. The mid-central zone of the balistid periodontal ligament is occupied by small blood vessels.     

Stereom morphogenesis and differentiation during regeneration of adambulacral spines of Asterias rubens (Echinodermata,Asteroida)

Summary The very first mineral deposits appearing in regenerating fractured adambulacral spines of Asterias rubens are minute polyhedrons that cover the surface of fractured trabeculae. Polyhedrons fuse together forming a fold from which a microspine differentiates. Microspines develop into long linear trabeculae which send out lateral processes at regular length intervals. Lateral processes from adjacent trabeculae fuse together, bridging the trabeculae and giving the regenerate the typical meshwork structure of stereom. Most of the regenerate is built up according to this growth pattern which ensures its longitudinal growth. Simultaneously, the initial fascicular stereom of the stub sends out short radial processes which branch into upward and downward directed subprocesses. The latter fuse with their equivalents located above or below, building up longitudinal rows of stereom meshes. These rows then bridge together by additional branched or unbranched lateral processes, so forming a new stereom layer which progressively covers the whole stub. Up to three new layers of stereom are formed in this way at the stub periphery. These become continuous with the stereom layers of the regenerate by fusion of reciprocal subprocesses, so ensuring the continuity between the stub and the regenerate. In both structures the first stage of mineralization results in an open stereom. Stereom thickening occurs in a second stage of mineralization (that is chronologically separated from the formation of the open stereom) and results in the differentiation of the original stereom fabrics (i.e. fascicular stereom). Regeneration of removed spines starts with the formation of a new spine base made of labyrinthic stereom. The development of the latter mostly relies on short branched and unbranched processes which fuse with each other or with predifferentiated meshes. After completion of its base, the regenerating spine lengthens and thickens similarly to the regenerating fractured spines. The diversity of the stereom growth processes observed in the present work may be reduced to the combination of one to three elementary events, viz. the development of long linear processes, of short unbranched processes and of short branched processes. A survey of the literature allows the suggestion that the implementation of these elementary events is sufficient to describe most types of stereom morphogenesis.Senior research assistant NFSR (Belgium)     

Structure of the periodontal ligament during tooth eruption in the dog: descriptive aspects

Serially stained uncalcified sections of young dog mandibles were examined to study the structure of the periodontal ligament of the erupting first right molar. The periodontal ligament around tooth crown presents three zones. The first, near the dental follicle, is a tick layer of parallel collagen bundles with numerous flattened fibroblasts. The second, intermediate, contains a blood vessels network, particularly veins and capillaries. The third, outer, is occupied by a continuous layer of osteoclasts and osteoblasts. Also the periodontal ligament around the tooth presents three layers, the outer and the intermediate rich of cells more than the inner. Particularly, the outer layer shows numerous osteoblasts surrounding the developing trabeculae of the alveolar bone and the collagen fiber bundles of the periodontal ligament. These penetrate into the trabeculae and appear similar to the osteoid layer. These results indicate that the alveolar bone increases by ossification of the connective tissue of the periodontal ligament.     

Structure and development of the ctenial spines on the scales of a teleost fish,the cichlid Cichlasoma nigrofasciatum

Sire, J.‐Y. and Arnulf, I. 2000. Structure and development of the ctenial spines on the scales of a teleost fish, the cichlid Cichlasoma nigrofasciatum. — Acta Zoologica (Stockholm) 81 : 139–158 Numerous teleost species possess ctenoid scales characterized by the presence of ctenial spines arranged in rows (the cteni) along their posterior, free margin. Whilst the morphology and function of the ctenial spines are similar to those of odontodes (extra‐oral teeth), e.g. in armored catfish, their homology is questionable. To address this problem, we have studied ctenial spine development, structure, attachment to a bony support, and replacement with the aim of comparing these features to those described for odontodes. The ctenial spines have been studied in a growth series of the cichlid Cichlasoma nigrofasciatum, using light, scanning and transmission electron microscopy. Ctenial spines are entirely constituted of a collagen matrix. They lack a pulp cavity and, although their distal end can be in contact with the epidermal basal layer cells, they are not covered by an enameloid‐like tissue. They are attached to the scale by means of a narrow strand of unmineralized collagen matrix acting as a ligament and allowing spines to be movable. The ctenial spines develop as prolongations of the external layer of the scale, a woven‐fibroid collagen matrix, and subsequently grow by addition of parallel‐fibred collagen matrix. New ctenial spines are added at the posterior scale border in waves that follow the same rhythm as the deposition of circuli in the anterior region. From the focus region to the scale border, the ctenial spines constitute lines in which only the most posterior ctenial spine is functional. The other spines that are no longer functional are not shed but resorbed from the top, and their attachment region mineralizes and thickens by deposition of new material. The remnants of spines constitute the main part of the superficial layer of the scale in which anchoring bundles attach; this region is covered afterwards by the limiting layer, a tissue devoid of collagen fibrils. Because of their tooth‐like morphology (shape and size), their posterior orientation and their attachment to the scale surface, the ctenial spines resemble odontodes. Moreover, both elements perform a similar hydrodynamic function. Nevertheless, the structure and development of the ctenial spines differ completely from those of odontodes and consequently, they cannot be considered homologous elements. Ctenial spines and odontodes in teleosts provide us with a beautiful example of homoplasy; they share shape and function, but have a different origin as evidenced by their different structure and process of development.     

Phosphatidylinositol-dependent bond between alkaline phosphatase and collagen fibers in the periodontal ligament of rat molars

Alkaline phosphatase (ALP) is anchored to the outer leaflet of the lipid bilayer via phosphatidylinositol (PI) and ALP activity has been localized in the plasma membrane of numerous tissues. In the periodontal ligament ALP activity is found in the collagen fibers in addition to the plasma membrane of the osteoblasts and fibroblasts. In this study, we examined the distribution of ALP activity in the periodontal ligament of rat molars and also examined whether the bond between ALP and collagen fibers is dependent on PI by using phosphatidylinositol-specific phospholipase C (PI-PLC). ALP activity was distributed in the periodontal ligament. The activity mirrored the distribution of collagen fibers in the periodontal ligament. Cytochemical analysis also demonstrated that ALP activity was located not only in the plasma membrane of fibroblasts, but also in the collagen fiber bundles and fibrils in the periodontal ligament. After treatment with PI-PLC, the loss of ALP activity in the periodontal ligament was observed histochemically, and the loss of ALP activity in the fibroblasts as well as in the collagen fiber bundles and fibrils was observed cytochemically. These results strongly indicate that the bond between ALP and the collagen fibers is also dependent on PI.     

Renal architecture of the dogfish Scyliorhinus caniculus (Chondrichthyes,Elasmobranchii)

Summary The excretory portion of the opisthonephric kidney of Scyliorhinus caniculus displays a mesial zone that is supplied with venous blood by the renal portal system and with arterial blood from the efferent arterioles of the glomeruli, and a zone of lateral bundles that is irrigated with arterial blood via arterioles in parallel to the afferent arterioles of the glomeruli. Each single nephron performs two large convolutions in the mesial tissue and two hairpin loops in the bundle. The nephron is differentiated into renal corpuscle (located between the two zones), neck segment (in the bundle), proximal segment I (beginning in the bundle, major convolution between the zones), proximal segment II (exclusively in the mesial zone), intermediate segment (beginning in the mesial tissue and ending in the bundle), distal segment (exclusively in the bundle) and collecting tubule (beginning in the bundle, with a large convolution in the mesial tissue and ending in the bundle) that joins the collecting duct-ureter system. In the bundles proximal and distal nephron segments, the end of the renal tubule and a central bundle vessel are arranged together and form a complex countercurrent system that is enclosed in a sheath of connective tissue. The bundles provide the structural basis for the creation of an environment with low urea concentration around the final portion of the renal tubules, which is consistent with previous experimental evidence of a significantly lower urea content of the bundles as compared with the blood and the mesial tissue in another marine elasmobranch, Raja erinacea. This condition is thought to lead to passive reabsorption of urea from the fluid of the end of the renal tubule. Separation of individual nephrons in the bundle zone appears to be correlated with the peculiar secondary structure that results from the folding of the bundles and may be in addition a requirement in conjunction with intermittent function of the glomeruli. The zonation of the renal tissue with formation of bundles with counter-current systems is characteristically found in marine Elasmobranchs and is considered to be the morphological correlate to the physiological ability of the marine Elasmobranchii to use urea for osmoregulation.     

Composition and ultrastructure of the byssus of Mytilus edulis

Three regions of the byssus of the marine mussel Mytilus edulis L. are distinct in structural organization at the macroscopic and microscopic level and in amino acid composition. The threads that emanate from the stem at the base of the foot are divided into two regions. The proximal, elastic region has a crimped, densely staining cortex enclosing an interior matrix of spiral fibers, and its amino acid composition reflects protein heterogeneity. The more distal, rigid region has a straight, tubular cortex surrounding an inner matrix of linearly arranged bundles of fibrils and has a composition approximating pure collagen. The plaque, or disc-shaped portion, which mediates attachment to various substrates, is distinguished by a surface matrix of collagen-like fibers similar to those of the thread region and anchored on an inner spongy matrix. Compositional evidence exists for a collagenous component, a catechol-rich protein, and at least one other accessory protein in the plaque.     

Sensitivity of insertion locations on length patterns of anterior cruciate ligament fibers

A technique is demonstrated, employing an instrumented spatial linkage, for the determination of the length patterns of discrete fiber bundles within a ligament under controlled loading conditions. The instrumented spatial linkage was used to measure the three-dimensional joint motion. The linkage was also used as a three-dimensional coordinate digitizer to determine the spatial location of bony landmarks and the ligament's insertion areas. The length of pseudo fiber bundles was determined as the straight line distance between bone attachments. A comparison is presented, showing good agreement, between elongation patterns obtained from this method and those measured using an instrumented fine wire cable fiber. A sensitivity analysis was performed to evaluate the influence of tibial and femoral attachment location on the length pattern of fiber bundles of the anterior cruciate ligament. It was found that the relationship between fiber elongation and knee flexion depended strongly on the fibers femoral attachment location but not on its tibial attachment location.     

Immunoelectron microscopic localization of collagens type I, V, VI and of procollagen type III in human periodontal ligament and cementum

We examined the ultrastructural localization of collagens Type I, V, VI and of procollagen Type III in decalcified and prefixed specimens of the periodontal ligament and cementum, by immunoelectron microscopy using ultra-thin cryostat sections. Immunostaining for collagen Type I was pronounced on the major cross-striated fibrils entering cementum and in cementum proper, whereas staining for procollagen Type III was almost exclusively observed on the major fibrils in the periodontal ligament situated more remote from cementum. Reactivity for collagen Type V was limited to aggregated, unbanded filamentous material of about 12 nm diameter that was found mainly in larger spaces between bundles of cross-striated collagen fibrils and occasionally on single microfibrils that apparently originated from the ends of the major collagen fibrils, which may support the concept of this collagen as a component of core fibrils. Collagen Type VI was present as microfilaments appearing to interconnect single cross-striated fibrils. In the densely packed fibril bundles of the periodontal ligament, no collagen type VI was detected. Neither Type V or Type VI collagen was observed in cementum.     

Macroscopic and microscopic analyses in flexor tendons of the tarsometatarso‐phalangeal joint of ostrich (Struthio camelus) foot with energy storage and shock absorption

Flexor tendons function as energy storage and shock absorption structures in the tarsometatarso‐phalangeal joint (TMTPJ) of ostrich feet during high‐speed and heavy‐load locomotion. In this study, mechanisms underlying the energy storage and shock absorption of three flexor tendons of the third toe were studied using histology and scanning electron microscopy (SEM). Macroscopic and microscopic structures of the flexor tendons in different positions of TMTPJ were analyzed. Histological slices showed collagen fiber bundles of all flexor tendons in the middle TMTPJ were arranged in a linear‐type, but in the proximal and distal TMTPJ, a wavy‐type arrangement was found in the tendon of the M. flexor digitorum longus and tendon of the M. flexor perforans et perforatus digiti III, while no regular‐type was found in the tendon of the M. flexor perforatus digiti III. SEM showed that the collagen fiber bundles of flexor tendons were arranged in a hierarchically staggered way (horizontally linear‐type and vertically linear‐type). Linear‐type and wavy‐type both existed in the proximal TMTPJ for the collagen fiber bundles of the tendon of the M. flexor perforatus digiti III, but only the linear‐type was found in the distal TMTPJ. A number of fibrils were distributed among the collagen fiber bundles, which were likely effective in connection, force transmission and other functions. The morphology and arrangement of collagen fiber bundles were closely related to the tendon functions. We present interpretations of the biological functions in different positions and types of the tendons in the TMTPJ of the ostrich feet.     

Sensitivity of an equine distal limb model to perturbations in tendon paths, origins and insertions

As equine musculoskeletal models become common, it is important to determine their sensitivity to the simplifications used. A subject-specific distal forelimb model was created using bones extracted from CT scans to examine movement from in-vivo invasive-marker motion capture. The movements of the sesamoid bones were simulated using the constraints of maintaining an isometric virtual ligament and maintaining contact between the appropriate articular surfaces, creating a variable moment arm for the tendons. The simulation of the proximal sesamoid bones was compared to movement recorded in-vitro. The paths and origins used for the deep digital flexor tendon (DDFT), superficial digital flexor tendon (SDFT) and suspensory ligament (SL) were altered and the effects on their calculated strains during trot stance were examined. The most influential alteration tested was the dorsopalmar changes to the tendon paths at the level of the proximal sesamoid bones, which led to a maximum length reduction of 4 and 2 mm for the SDFT and DDFT, respectively. Alterations to the virtual origins of the SDFT and DDFT were not influential leading to up to a 0.01% effect on strains for a 1cm dorsopalmar shift. In the SL, the choice of the proximal or distal edge of the proximal attachment site varied the strains calculated by up to 1% (3 mm). These results show that within the anatomically realistic spectrum, changes to tendon paths can have an appreciable effect on calculated strains; however the origin sites chosen are not as influential as changes to paths at the metacarpo-phalangeal joint.     

Variable tensility of the oral arm plate ligaments of the brittlestar Ophiura ophiura (Echinodermata: Ophiuroidea)

The oral arm plates of the brittlestar Ophiura ophiura L. are connected to lateral arm plates at distal and proximal ligamentous junctions. The distal junction is mobile and is disrupted during arm autotomy; the proximal junction is more rigid and does not participate in autotomy. Aspects of the morphology and mechanical properties of the distal and proximal oral arm plate ligaments have been investigated in order to determine if their tensility is under physiological control. By means of creep tests it was found that elevation of the external potassium (K⁺) ion concentration causes a decrease in the viscosity of the distal ligament which is either transient or continues until rupture intervenes. In forced vibration tests the distal ligament often shows a biphasic stiffening then softening response to excess K ^- ions. Anaesthetics block the softening phase but enhance the stiffening component of this response. This ligament is also softened by repetitive electrical stimuli but stiffened by excess calcium ions and by acetylcholine. The proximal ligament appears to have the capacity for only transient changes in mechanical properties. Both ligaments are penetrated by the processes of juxtaligamental cells whose perikarya are arranged in clusters innervated by hyponeural nerves. These cells are thought to modulate the interfibrillar cohesion of the ligaments. It is concluded that the distal and proximal ligaments are mutable collagenous structures which in their stiffened condition help to maintain arm posture without the need for continuous muscular activity, and that at autotomy the distal ligament undergoes a profound loss of tensile strength which facilitates arm detachment.     

Proteoglycan synthesis by fibroblast cultures initiated from regions of adult bovine tendon subjected to different mechanical forces

Fibroblast cultures were initiated from two distinct regions of the adult bovine deep flexor tendon and synthesis of 35S-labeled proteoglycans by these cultures was investigated. The proximal/tensional region of the tendon was composed of linearly arranged dense collagen bundles, and its glycosaminoglycan hexosamine content was only 0.2% of the dry weight of the tissue. The proteoglycans of this region were predominantly small (Kav = 0.5 on Sepharose CL-4B). Cells placed into culture from this region attached to the substratum readily, and the radiolabeled proteoglycans from these cultures were 90% small proteoglycans. In a more distal region of the tendon that is subjected to compressive forces, the collagen was arranged as a network of fibrils separated from each other by a matrix that stained intensely with Alcian blue. The glycosaminoglycan content of this compressed region was up to 5-fold higher than in the proximal region, and as much as 50% of the proteoglycans were large molecules (eluted from Sepharose CL-4B in the Vo). Cells placed into culture from the distal/compressed region did not attach to the substratum as readily as those from the proximal region and were characterized by the presence of numerous cytoplasmic lipid inclusions. The [35S]proteoglycans synthesized by the distal tendon fibroblast cultures were divided into two approximately equal populations of large and small proteoglycans having elution characteristics similar to the proteoglycans extracted from this tissue. The distinct profiles of proteoglycan production were maintained by the cells in culture for several weeks, although eventually the amount of large proteoglycan synthesized by the distal tendon fibroblast cultures diminished. Both regions of tendon contained predominantly type I collagen, and collagen production was about 10% of the total protein synthesized by both cell cultures. These observations indicate that adult tendon fibroblasts in culture express stable synthesis of proteoglycan populations similar to those found in the region of tendon from which they were derived.     

Spermiogenesis and sperm ultrastructure of Cyathocephalus truncatus (Pallas, 1781) Kessler, 1868 (Cestoda: Spathebothriidea)

Spermiogenesis and sperm ultrastructure of adult Cyathocephalus truncatus, a member of presumably basal group of "true" cestodes (Eucestoda), have been examined for the first time by using transmission electron microscopy. The process of sperm formation corresponds in basic pattern to that of the Pseudophyllidea. In addition, the 2 pairs of electron-dense attachment zones are present in median cytoplasmic process of C. truncatus. However, mature spermatozoa of C. truncatus differ significantly from those of the pseudophyllideans, especially in the morphology of the proximal and distal spermatozoon extremities. The proximal extremity of the mature spermatozoon lacks a crested body, which is present in more derived cestodes and some pseudophyllideans. The distal end of the mature spermatozoa exhibits different morphology in the gametes from testes and those from receptaculum seminis. New for the Eucestoda is a finding that a lateral cytoplasmic extension creates the distal end of the spermatozoa from testes, resembling sperm of some Monogenea and Digenea. In contrast, the distal extremity of the spermatozoa from receptaculum seminis contains only a nucleus. Despite the above-mentioned peculiarities, the ultrastructural data on sperm/spermiogenesis suggest close relationships of the Spathebothriidea and Pseudophyllidea.     

Mechanical properties and structure of the spine-joint central ligament of the sea urchin, Diadema setosum (Echinodermata, Echinoidea)

In the regular echinoid Diadema setosum Leske the central ligament that connects the spine to its tubercle is mainly composed of closely-packed collagen fibres which are arranged in the longitudinal axis of the ligament. The mechanical properties of the ligament are quite different from ligament to ligament: the viscosity determined by creep tests ranges from 0·02 to 6 GPa·s. The viscosity of a ligament changes reversibly in response to stimulation. Adrenaline and noradrenaline (10^-6-10^-3 M) decrease the viscosity. Acetylcholine (10^-8-10^-3 M), artificial sea water containing high (100 MM) potassium concentration, and electrical stimulation increase the viscosity. As the ligament contains no muscle cells in it, the viscosity change cannot be attributable to muscle activities. The functional significance of having a central ligament with variable and controllable viscosity is that it binds the spine base onto the articulation surface of the tubercle, so as to reduce the possibility of spine dislocation, while being flexible enough to allow the spine considerable freedom of movement when necessary.     

Ultrascale and microscale growth dynamics of the cidaroid spine of Phyllacanthus imperialis revealed by 26Mg labeling and NanoSIMS isotopic imaging

Growth dynamics of the primary spine of the cidaroid sea urchin Phyllacanthus imperialis was assessed for the first time using pulsed ²⁶Mg‐labeling and NanoSIMS isotopic imaging. The sea urchin was incubated twice (for 48 h) in artificial seawater with elevated level of ²⁶Mg. After each labeling event, the sea urchin was returned for 72 h to seawater with natural isotopic abundance of ²⁶Mg. NanoSIMS ion microprobe was subsequently used to visualize the labeled regions of the spine with submicrometer lateral resolution. The growth of the new skeleton was restricted to the distalmost and peripheral portions of the spine. Skeletogenesis involved mostly the deposition of continuous thickening layers and lateral growth involving bridges between previously formed trabeculae. The timescale of formation of individual thickening layers (ca. 1 µm in width) on the stereom trabeculae was on the order of 1 day. Longitudinal growth occurred mainly at the periphery in the form of small portions of the thickening deposits or more massive microspines that appeared to branch and fuse with those above and below. These microspines were found to grow at about 10 µm/day. These results reveal that the skeletal growth of a juvenile cidaroid spine is complex and highly heterogeneous, with different extension rates depending on the stage of the stereom development and/or direction of the growth fronts. The growth pattern observed here at the submicrometer scale provides direct evidence supporting the earlier suggestions that the lamellar structure of echinoderm stereom is formed by periodic deposition of continuous mineral layers. J. Morphol. 275:788–796, 2014. © 2014 Wiley Periodicals, Inc.     

Optimal replacement of the cruciate ligaments from the functional-anatomical point of view

By means of a combined technique of dissection and radiography, the function of cruciate ligament fibers was analyzed for motions in the sagittal plane, and different functional fiber bundles were reconstructed in cadaver knees. In order to grant permanent stability, crucial replacement must be concentrated on the reconstruction of the constantly taut 'guiding bundle' of each cruciate ligament. The femoral attachment point of each guiding bundle was found to be in a constant position within the femoral attachment of the respective cruciate ligament. The femoral attachment areas of different functional fibers are also defined geometrically, and the consequences of improper guiding bundle replacement are discussed.     

Architecture and functional aspects of the distal airways of Antarctic seals with different habits,the Weddell Seal (Leptonychotes weddellii) and the Crabeater Seal (Lobodon carcinophagus)

Summary The lung of the deep diving Weddell Seal is characterized by an unusually well developed periacinar dense collagenous connective tissue, and a thick coat of smooth musculature particularly in the distal bronchioli. Both, collagen and smooth musculature appear to be functionally interrelated, the first serving presumably as site of origin or attachment for the latter. The orientation of the bronchiolar smooth muscle cells is complex: there exists a basic pattern of two crisscrossing helical bundles that wind in opposite direction. In addition, longitudinal bundles are frequent both at the inside and the outside of the muscular coat. Furthermore, more or less complete ringshaped bundles occur as well as groups of muscle fibres running radially into the collagenous tissue of the surroundings of a bronchiolus. This complex architecture presumably allows active adjustment to various physiological needs of the Weddell Seal including as extremes both closing and widening of the bronchiolar lumen. Isometric contractions of the smooth musculature may stiffen the wall of the distal airways while diving. In the Crabeater Seal which dives for shorter durations and by far less deeply than the Weddell Seal, both periacinar collagen and bronchiolar smooth musculature are of similar arrangement, however, occur in considerably reduced amounts. A rich supply of autonomie nerve fibres with abundant varicosities controls the smooth muscle cells, which are interconnected by gap junctions and receive their innervation par distance (visceral type of smooth musculature). The majority of varicosities contains small clear vesicles, as is typical for cholinergic nerves, suggesting a strong parasympathetic influence. Other varicosities are presumably of peptidergic type. Mast cells and epithelial endocrine cells may exert additional influence on the musculature.