The fine structure of the myoneural junctions in the body wall muscles in Branchiobdella pentodonta Whit. (Annelida, Oligochaeta).

The ultrastructure of the myoneural junctions in the body wall muscles has been studied in Branchiobdella pentodonta Whit. A single junctional type has been found. Within the terminal axon there are two types of vesicles, which differ in number, size and electron opacity. The junctional gap contains the basement membrane. The post-junctional membrane displays concave patches on which slender projections arising from the outer lamina of the sarcolemma extend into the junctional gap. These concave patches overlie a cytoplasmic lamina made up of electron dense material.     

The fine structure of the body wall of Gyrocotyle urna

Summary The covering layer of Gyrocotyle, which is thought to occupy an intermediate position between the monogeneans and the cestodes, resembles that of the cestodes more than the epidermis of the ectoparasitic monogeneans. It is a cytoplasmic epidermis bearing densely arranged microvilli and is connected to parenchymally situated cell bodies. The microvilli of Gyrocotyle differ from the microtriches of tapeworms in lacking a thickened tip. Vesicles between the microvilli at the free edge of the epidermis are associated with disintegrating striated rod-bodies and are therefore probably secretory rather than concerned with micropinocytosis. The epidermal cell bodies have the characteristics of secretory cells and appear to elaborate the striated rod-bodies which occur throughout the outer epidermis and may consist of condensed mucoprotein. Cells with electron opaque cytoplasm which occur in the neighbourhood of the epidermal cell bodies have been identified as myoblasts. The posterior rosette appears to be specialized for adhesion as gland cells secreting PAS positive material line the ventral rosette surface and the epidermis of this region bears short bifurcate microvilli.

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Zusammenfassung Die Deckschicht von Gyrocotyle, die offenbar eine Zwischen-stellung zwischen der von Monogenen, Trematoden und Cestoden einnimmt, ist der von Cestoden mehr als der Epidermis ektoparasitischer Monogenea ähnlich. Sie stellt eine zytoplasmatische Epidermis dar, die aus dicht stehenden Mikrovilli besteht, und ist verbunden mit einem parenchymatösen Zellkörper. Die Mikrovilli von Gyrocotyle unterscheidet sich von den Mikrotrichen der Bandwürmer darin, daß ihnen eine verdickte Spitze fehlt. Bläschen zwischen der Mikrovilli am freien Rand der Epidermis hängt mit zerfallenden gestreiften stäbchenförmigen Körpern zusammen und sind eher sekretorischer Natur als Ausdruck einer Mikropinozytose. Die epidermalen Zellkörper haben die charakteristischen Eigenschaften von sekretorischen Zellen und scheinen die gestreiften länglichen Körper auszuscheiden, die über die ganze Epidermis verteilt vorkommen und aus einem kondensierten Mukoprotein bestehen. Zellen mit elektronendichtem Zytoplasma, die in der Nachbarschaft der epidermalen Zellkörper auftreten, sind als Myoblasten identifiziert worden. Die hintere Rosette erscheint spezialisiert zum Anheften wie Drüsenzellen; sie sezernieren PAS-positives Material entlang der ventralen Rosetten-Oberfläche. Die Epidermis dieser Region trägt kurze zweigallige Mikrovilli.

     

A quantitative study of cholinesterase in myoneural junctions from rat and guinea-pig extraocular muscles

1. Cholinesterase staining of rat and guinea-pig extraocular muscle shows focally and multiply innervated fibres.2. The distribution of cholinesterase activities in the populations of focal endings (from singly innervated fibres) and fine motor endings (from multiply innervated fibres) was determined by a sensitive radiochemical method.3. Focal endings had a greater cholinesterase activity than fine motor endings.4. It is suggested that this difference in enzyme activities is related to the functional difference between twitch and slow fibres in extraocular muscle.     

The fine structure of the Necturus (Amphibia) heart

The fiber diameter in the heart of Necturus maculosus is not significantly different from that of other amphibian myocardial cells, averaging approximately 5 μ over most of its length and about 10 μ in the nuclear region. No transverse (T) tubular system was found. Instead, there were shallow invaginations of the sarcolemma and many pinocytotic vesicles at the periphery of the myocytes. Other intracellular membrane systems such as the Golgi apparatus and sarcoplasmic reticulum were also sparse and poorly organized. However, subsarcolemmal cisterns, specialized segments of the sarcoplasmic reticulum in close apposition with the sarcolemma, were clearly demonstrated. Nexal junctions were recognized at points of contact between myocytes, although they occupied smaller areas than those of mammalian myocardial cells. Mitochondria were small and few. Light cells with few organelles or myofibrils, which resembled Purkinje fibers of mammalian and avian hearts, were often observed. A nerve ending in close contact with muscle cells was described.     

The fine structure of the lymphatics in the rumen wall of the goat]

Ruminal lymphvessels from the goat were investigated with light- and electronmicroscope after blockade of the lymphdrainage and vascular perfusion by means of yellow fix solution. The lymphvessels of the ruminal wall do not originate directly from below the epithelium but from the deeper layers of the Propria mucosae. As far as ruminal papillae are concerned, lymphatic vessels are only to be found at their base. All lymphvessels consist of a basement membrane and an endothelium. Only the ones which are situated in the Subserosa or in the musculature of the wall are provided with lymphatic valves.     

The fine structure of innervated myotendinous cylinders in extraocular muscles of rhesus monkeys

Summary Many of the myelinated nerve fibres of the distal myotendinous region of rectus muscles terminate on muscle fibre tips. The terminal expansions contain aggregated, small clear vesicles and mitochondria. Neuromuscular clefts at the contacts measure 20–40 nm and are uninterrupted by a basal lamina; the sarcoplasm opposite the contacts is unmodified. Some terminals invaginate the muscle fibre tips and others contact the sides of processes formed by splitting of the tips. The muscle fibre termination, its tendon and the nerve fibre branches are encapsulated to form an end-organ averaging 125 m in length and described as a myotendinous cylinder.Approximately 350 innervated myotendinous cylinders were estimated to be present in the horizontal recti with rather fewer in the vertical rectus muscles. Many of them occur shortly before the main myotendinous junction. All muscle fibres contributing to myotendinous cylinders were identified as the compact, felderstruktur, multi-innervated variety with directly apposed myofibrils that are known to be non-twitch fibres. All felderstruktur fibre terminations examined were encapsulated but 19% of them were not innervated.The nerve terminals of myotendinous cylinders are similar to those described by Dogiel (1906) as palisade endings and it is argued that they meet the morphological criteria of sensory neuromuscular endings. Their disposition suggests a capacity to monitor felderstruktur muscle fibre contraction.     

Photoreceptor fine structure in the archerfish (Toxotes jaculatrix)

The fine structure and arrangement of the photoreceptor cells of the archerfish (Toxotes jaculatrix) have been studied by electron microscopy. Rods, twin cones, and single cones are present. In the light-adapted state, the rods are very tall cells reaching almost to the base of the retinal epithelial cells. The outer segment is composed of membraneous discs of uniform diameter displaying a single incisure. The rod inner segment displays a distal small ellipsoid and an extremely thin myoid region. The nuclei of rods are electron-dense, and the synaptic spherule displays two or three invaginated sites. The single cone is similar to the individual members of a twin cone and displays a tapering outer segment and accompanying accessory outer segment. The wider cone inner segment contains a large, centrally loated ellipsoid and a peripheral region rich in endoplasmic reticulum, polysomes, and microfilaments. Twin cones display subsurface cisternae along their entire contiguous surfaces. The cone nuclei are large and vesicular and located vitread to the external limiting membrane. The synaptic pedicle of cones is lager and moe electron-lucent and contains more invaginated “ribbon” synaptic sites (ten to 12) than do rods. In addition, small “coated” invagination and larger synaptic vesicle-filled processes are also seen within cone pedicles. In the light-adapted state the cone photoreceptors are arranged in a repeating square mosaic pattern with one single cone surrounded by four twin cones.     

Sperm fine structure of the pikeperch, Sander lucioperca (Percidae, Teleostei)

The spermatozoon of the pikeperch, Sander lucioperca represents a new type of sperm organization. It differs in several parameters from Perca fluviatilis, i.e., the cell dimensions, the organization of the midpiece, and the structure of nucleus, mitochondria and centriolar complex.     

Fine structure of the epidermal cuticle of some Brazilian earthworms (Oligochaeta: Annelida)

Electron microscopic techniques were adopted to study the epidermal cuticle of 4 species of brazilian earthworms. Basically this structure consists of collagenous fibers arranged in alternate layers and embedded in an amorphous and fibrillar matrix. Of characteristics, the ellipsoidal projections of the cuticle of P. hawayana were found to be enclosed between 2 parallel layers, where each layer consists of 2 electron dense lines, separated by a space filled with an electron dense material.